Materials Selection and Design (2010) Part 14 docx

The composite strength may be lower than a monolithic specimen of pure matrix material, as it frequently is not possible to process the matrix to as high a quality with fibers present..

The high costs of continuous-fiber composites have driven the development of discontinuous-whisker and even particulate-reinforced composites The potential performance of these discontinuous composites is much poorer, and the mechanism of strengthening may be only partially by fiber reinforcement with the dispersion strengthening being the major effect Yet silicon-carbide-reinforced aluminum has a significantly higher modulus and strength, especially at higher temperatures Although the normal compressive forming processes can shape the composite, the lower ductility and fracture toughness have limited its general application (Ref 36) It is competitive for applications such as snow tire studs, which should be wear and corrosion resistant, as well as low cost

Properties of Metal-Matrix Composites

Metal-matrix composites offer significantly better mechanical properties than polymer-matrix composites for dominated properties, such as greater shear, compressive, and transverse tensile strengths Reinforcement can increase the maximum-use temperature over that of the monolithic material However, matrix behavior may not be as good as for the unreinforced matrix For example, the transverse tensile strength of a metal-matrix composite, modeled by a matrix with poorly bonded fibers, is only about one-third of the strength of the unreinforced matrix Furthermore, the microstructure

matrix-of the matrix may not be as desirable, and together with the physical constraints placed on the fibers by the matrix, ductility of the matrix is generally much reduced Metal-matrix composites also can have good electrical and thermal conductivity

The stress/strain behavior of metal-matrix composites is more complicated than resin-matrix composites because of work hardening and the change in the yield surface with different or multiple loading (Ref 37) (The same considerations make thermal expansion show hysteresis that changes with cycling.) If a residual stress-free metal-matrix composite is loaded parallel to the fiber axis, both the fiber and matrix will be elastically loaded initially (Fig 13) Upon further loading, the matrix will finally be loaded to its yield stress and then plastically deform, but the fibers will still be loaded elastically The effective modulus is decreased in this region Then, substantial fiber failures occur and composites with high fiber volume fraction fail catastrophically At low fiber volume fractions, fibers break up into critical lengths causing a substantial loss in modulus, and finally matrix failure occurs These effects have been successfully included in describing the behavior of metal-matrix composites, but design of stable structures that are subjected to temperature cycles is more difficult The details are beyond the scope of this article

Fig 13 Stress/strain curve for nicalon silicon carbide fiber in aluminum (1100) matrix The material has an

initial modulus (E1) of 87 GPa, which is representative of both fiber and matrix elastically deforming The

secondary modulus (E2) of 70 GPa is indicative of fiber elastic deformation and matrix micro-yielding For a relatively low-ductility matrix, failure often occurs at the end of the secondary modulus straight line, when the

first fibers begin to fail For the higher-ductility 1100 aluminum matrix, fiber fractures accumulate during the curved part of the stress/strain plot until final failure occurs Source: Ref 38

The plastic deformation that can occur in a metal-matrix composite is more apparent for off-axis and transverse loading to the fiber axis (Fig 14) Loading at 30° produces a stress/strain curve that looks much the same as for the pure matrix except that the strain scale is reduced The reason is that local matrix deformations around the fibers are much higher than the overall global strains, and failure finally occurs when local strains are similar to the failure strain in the unreinforced matrix (Triaxial tensile strains are also present in the matrix, which also tends to reduce ductility.)

Fig 14 Effect of loading direction on uniaxial boron fiber/aluminum (7075) Results are for fibers oriented at 0,

30, and 90° with respect to the load and pure matrix in the T6 and O conditions Source: Ref 39

Fatigue properties in the axial direction of the fibers is excellent, but fatigue for off-axis loading, which relies on the fatigue behavior of the matrix, can be worse than for the unreinforced matrix The accumulation of failed fibers and matrix cracks in fatigue testing results in loss of modulus This may continue until composite failure occurs However, in some laminate constructions or composites with low fiber volume fraction, damage may develop to a stable condition (shakedown), whereupon further fiber or matrix damage occurs slowly The drop in modulus may be as great as a factor

of two during shakedown, which may require part removal because of reduced stiffness

References cited in this section

31 M McLean, Directionally Sloughed Materials for High Temperature Service, The Metals Society, London,

34 P.R Smith and F.H Froes, J Met., Vol 36, 1984, p 19

35 P.A Selmers, M.R Jackson, R.L Mehan, and J.R Rairden, Production of Composite Structures by

Low-Pressure Plasma Deposition, Ceram Eng Sci Proc., Vol 6, 1985, p 896

36 W.A Logsdon and P.K Liaw, Eng Fract Mech., Vol 24, 1986, p 737

37 Y.A Bahei-El-Din and G.J Dvorak, Plastic Deformation Behavior of Fibrous Composite Materials,

Proceedings of the 4th Japan U.S Conference on Composite Materials, Technomic Publishing, 1989, p 118

38 J Tanaka, H Ishikawa, T Hayase, K Okamura, and T Matsuzawa, Mechanical Properties of SiC Fiber

Reinforced Al Composites, Progress in Science and Engineering of Composites, ICCM-IV, Japan Society

of Composite Materials, Tokyo, 1982, p 1410

39 G.D Swanson and J.R Hancock, Off-Axis and Transverse Tensile Properties of Boron Reinforced

Aluminum Alloys, Composite Materials: Testing and Design, STP 497, ASTM, 1971, p 472

Design with Composites

R.J Diefendorf, Clemson University

The mechanical behavior of these composites, which have similar constituent moduli, is quite different from resin-matrix composites, especially because the strain to failure of the matrix is typically smaller than that for the fiber The fibers are added not only to provide high-temperature creep resistance, but more importantly to provide damage tolerance and a more graceful failure at room temperature When a ceramic-matrix composite with uniaxially oriented fibers is loaded in tension parallel to the fibers, the load is relatively evenly distributed between the fiber and matrix, the exact ratio being determined by the relative moduli and volume fractions For relatively strong coupling between the fiber and the matrix,

the composite fractures when the first crack occurs in the matrix (Fig 15a) Often, there may already be surface cracks

from specimen preparation The composite strength may be lower than a monolithic specimen of pure matrix material, as

it frequently is not possible to process the matrix to as high a quality with fibers present The strength and toughness of commercially available composite products will generally be higher than for monolithic materials Better processing eliminates many of the defects by filling in voids with more and better matrix material Matrices with higher fracture toughness would help increase the strain at which first cracking occurs, as would lower modulus However, the fracture toughness decreases as the refractoriness (bond strength) of the material increases Low matrix fracture toughness is the likely result for the highest temperature applications However, combining several techniques to improve fracture toughness, such as adding whiskers to the matrix and placing multiple crack-stopping interfaces within the matrix can increase the strain for first matrix failure

Fig 15 Illustration of ceramic-matrix composite failure process (a) The crack, which initiates in the matrix,

propagates through both matrix and fibers in the composite when strong interfacial bonding exists (b) For intermediate or low interfacial bonding, the matrix crack runs through the matrix but around the fibers Multiple cracks accumulate if the fibers bridging the cracks can sustain the load (c) After attaining multiple matrix cracks with an equilibrium spacing, fibers fail at flaws with increasing load, and not necessarily in the bridged regions of highest stress (d) Fibers pull out of their matrix sockets with further extension

Decreasing the coupling between the fiber and matrix sufficiently allows the first crack in the matrix to deflect around the fibers such that the fibers will bridge the cracked matrix and sustain the load if the fiber strength and volume fraction are high enough (Fig 15b) As the load is increased, more matrix cracks form and finally yield a specimen with a relatively uniformly spaced set of cracks in the matrix, all bridged by the fibers The spacing of the cracks is determined by the fiber/matrix coupling: the poorer the coupling, the wider the spacing Finally, there is insufficient length between cracks

to transfer the load, which only the fibers are carrying in the bridging regions, to the matrix to cause further cracking A large drop ( 50%) in modulus can occur with poor interfacial bonding when the matrix cracks The design may be

limited by this decrease in modulus or by resonant frequency changes Further loading causes failure of fibers, and finally

at one bridging section the remaining fibers can no longer support the load and the specimen fails The probability for failure of the fibers is highest in the bridges, because the matrix carries no load However, there is a distribution of strength-reducing flaws along the length of the fibers, and there will be fracture of fibers away from the bridged matrix crack, albeit with a decreasing frequency with distance from the crack (as the matrix picks up load) (Fig 15c) The last step of the process is the pullout of the fractured fibers from their sockets in the matrix (Fig 15d) Much energy can be absorbed, as the apparent strain can be quite high Studies have shown that this pseudometallic, stress-strain behavior can

be achieved in ceramic composites

Constituent Selection

Ceramic-matrix composites are likely to be used at high temperature and are usually fabricated at high temperature In addition to the usual considerations of physical properties such as specific modulus and strength, creep and stress rupture become the limiting mechanical properties for high-temperature applications (Ref 40) The most creep-resistant ceramic is graphite, followed by silicon carbide, titanium diboride, and silicon nitride Yttrium-aluminum-garnet (YAG), mullite,

and sapphire appear to be the most attractive oxides (Ref 41) Both fibers and matrices must be creep resistant For

lower-temperature applications, glass-matrix or glass/ceramic-matrix provide a combination of thermal expansion coefficient, fracture toughness, and low modulus that allow very good composites to be produced The selection of fiber and matrix material combinations is very much limited by thermochemical and thermomechanical incompatibilities For example, carbon fibers in any oxide matrix are thermodynamically unstable above 1500 °C Similarly, a mismatch of thermal expansion coefficients much above 2 m/m · °C causes microcracking of the composite

The problem of selection of a ceramic composite system is compounded because a weak interface is necessary to control the fracture A third interfacial phase between the fiber and matrix is often added, especially when the matrix and fiber have the same chemical composition, such as with a silicon-carbide-fiber/silicon-carbide-matrix composite The interfacial materials generally have a layer structure, although highly porous interfacial phases have also been used However, no environmentally stable layers have been found for the higher-temperature composites (>1200 °C) Model systems usually have used graphite Boron nitride, which has some oxidation resistance to 1100 °C, has been found to be

an effective crack-stopping interfacial material also Oxide-layer compounds, such as synthetic micas, are useful for temperatures as high as 1100 °C However, the interface between fiber and matrix remains a problem, and present interfacial coatings add substantially to the composite cost

Ceramic-matrix composites are not as well developed as polymer-matrix composites, with the exception of fiber/carbon-matrix composites Carbon/carbon composites are used extensively for aircraft brakes, shuttle tiles, rocket motor nozzles, and reentry vehicles For other ceramic-matrix composites, the designer must select one of the few systems that are available Much stronger interaction with the materials supplier is required, because the processing and properties are very much producer dependent

carbon-Properties of Ceramic-Matrix Composites

The elastic properties can be calculated using the equations described in the sections "Calculation of Lamina Properties" and "Symmetric In-Plane and Through-Thickness Laminates" in this article The major difference with ceramic-matrix composites is that the moduli of the fiber and matrix are frequently similar, so that the elastic properties are more isotropic

The fracture behavior of ceramic-matrix composites differs from that of polymer- or metal-matrix composites in that the failure strain of the matrix is less than that for the reinforcement The stress/strain curve for a uniaxially aligned fiber specimen, loaded parallel to the fibers is shown in Fig 16 (Ref 42) Both matrix and fibers are being loaded initially, until the failure strain of the matrix is reached, and the matrix starts fracturing There are three different limiting behaviors depending on the interfacial bonding: (1) high interfacial bonding causes catastrophic failure, (2) intermediate bonding causes a change in slope, (3) poor bonding causes a load drop with a subsequent slope proportional to just the modulus and volume fraction of the fibers For low or intermediate interfacial bonding and a volume fraction and fiber strength sufficiently high to support the applied load after matrix fracture, composite fracture does not occur, and multiple matrix cracks will accumulate In the limit, as bond strength goes to zero, the ceramic matrix behaves as if it has holes These

"holes" act as stress raisers The final crack spacing is determined by the strength of the matrix and inversely on the maximum shear stress that can be sustained at the fiber/matrix interface The relationships for multiple matrix cracking, and the final matrix crack spacing are (Ref 43):

CMu < FuVF (Eq 12)

where CMu is the composite stress when first matrix cracking occurs; Mu and Fu are the matrix and fiber ultimate

strengths, respectively; R is the radius of the fiber; is the shear stress at the fiber/matrix interface; and L is the spacing

between matrix cracks

Fig 16 Stress/strain curve for a typical uniaxial ceramic-matrix composite loaded parallel to the fibers The

solid line (A) shows the behavior for strong interfacial bonding and catastrophic failure with the first matrix crack The dotted line (B) indicates intermediate bonding behavior such that the fibers bridge the matrix cracks, but with short fiber/matrix debonding near the cracks The dashed line (C) illustrates the stress/strain curve for very weakly coupled fiber and matrix In the limit, the matrix contribution to the modulus is completely lost after matrix fracture

In ceramic-matrix composite systems that retain high frictional or chemical bonding interaction between the fiber and the matrix after matrix fracture, a simple change in slope occurs in the stress/strain curve as the matrix begins fracturing (Fig 16) In other systems, there is loss of the matrix stiffness if coupling between fiber and matrix is lost when matrix fracture occurs The stress/strain curve under displacement control may show a load drop much like yielding, or become horizontal, until the load increases again with strain, but this time only with the modulus contribution from the fibers (The actual curve may differ because of residual stresses between the fiber and matrix.)

The microcracking of the matrix not only results in loss of modulus, but also allows internal oxidation to occur Parts could be designed to stress or strain levels below which matrix cracking occurs While it might be expected that the matrix would crack at similar strains as for the unreinforced ceramic matrix (0.05 to 0.10%), the strain for matrix cracking was shown to be enhanced (Ref 43):

Mu = [24 O M /EC DF(1 - VF]1/3 + EFVF T/EC (Eq 14)

where Mu is the reinforced matrix strain at failure, O is the interfacial shear strength, M is the matrix fracture energy,

DF is the fiber diameter, is the difference in thermal expansion coefficient between fiber and matrix, and T is the

difference between the stress-free temperature and the use temperature The stress-free temperature is often assumed to be the processing temperature

Although the fractional exponent on the first term minimizes the effect of the parameters, the wide range that some of the parameters can have produces significant changes in the strain High matrix fracture energy, high fiber modulus and volume fraction, small fiber diameter, and low matrix modulus all increase the matrix failure strain Increasing the interfacial shear strength also raises the matrix failure strain, but the value must not exceed that which causes brittle

failure One approach, which has doubled the matrix microcracking strain, is reinforcing the matrix with about 15% of fine whiskers

The second term in the equation is the residual stress that arises from the mismatch in the coefficients of thermal expansion between the fiber and matrix An axial compressive residual stress would be present in the matrix at room temperature if the fiber thermal expansion coefficient is larger than that for the matrix, because the composite is generally processed at high temperature Unfortunately, the thermal expansion coefficient of the fiber is likely to be smaller than the matrix, placing the matrix in residual axial tension

The average matrix strain-to-failure in a composite can frequently be increased to values of 0.4% or more, which combined with the 1% or better strain to failure of the fiber, can produce a stress/strain curve mimicking a ductile metal, albeit with very limited strain capability A problem is that the first few matrix failures are observed at strains only slightly higher than the unreinforced matrix Therefore, a prudent assumption is to assume that matrix microcracks will always be present that may allow internal oxidation and embrittlement

The ultimate tensile strength of uniaxially aligned composite, loaded parallel to the fibers, is given by the bundle strength and volume fraction of the fibers bridging the matrix cracks:

where Cu and Fu are the ultimate tensile strengths of the composite and fiber bundles, respectively

The bundle strength of the fibers cannot be measured on a gage length that approximates the short gage length in the cracked matrix region of the composite, but might be approximated from resin- bonded strand tests Because fiber volume fractions often are about 50% in a uniaxially aligned ceramic composite, the axial ultimate tensile strength will be about one-half of the fiber bundle strength, if the interfacial coupling has been properly adjusted to prevent brittle fracture If a fabric is used, somewhat less than 25% of the tensile strength of a bundle can be obtained because of the over/under construction in a fabric and the effective fiber volume fraction in the load direction

The toughness of a ceramic-matrix composite can be caused by at least five different mechanisms, which can act independently or in a combined manner However, the work to fracture, the area under the stress/strain curve, is generally dominated by the "pull out" of broken fibers from their matrix sockets Although it is generally desirable to have little scatter (high Weibull modulus) in the strengths of fibers in order to attain the highest composite, ultimate tensile strength,

no pullout occurs when there is no variation in fiber strength All fibers would fail simultaneously in the highest-loaded regions at the matrix cracks Pullout requires that fibers fail at flaws away from the matrix cracks, and then the broken

fibers drawn out from the matrix sockets The work of pullout (WP) for a multiple matrix cracked composite (in ft · lbf) is (Ref 44):

The optimal Weibull modulus for maximizing work of fracture is about 4 (There will probably not be any development

of fibers optimized for maximum work of pullout in the near future due to the limited market.) A high volume fraction of large-diameter fibers with high strength, but with a low interfacial strength produces a maximum work of pullout A comparison with the matrix microcrack strain equation shows that high fiber volume fraction and strength are beneficial

to both However, the effects for fiber diameter and interfacial shear strength are opposite A ceramic composite cannot be optimized for both the work of pullout and the matrix microcrack strain simultaneously

The strength of a uniaxial ceramic-matrix composite that is loaded off-axis is low This result is a consequence of the poor interfacial shear strength between the fiber and matrix that is required to control fracture Multiple-ply laminates with off-axis oriented plies must be used to sustain off-axis loads

The materials selected depend very much on the application For inert atmosphere or very short time applications (tens of minutes), unprotected carbon fibers in a carbon matrix provide the best mechanical properties, especially at very high temperature Creep is low until 2200 °C Coated carbon/carbon composites with internal additives can have lifetimes in oxidizing environments up to 1700 °C for as long as 100 h, but not very reproducibly Silicon carbide or nitride provide much better oxidation resistance, but would probably be creep limited at temperatures above 1500 °C for the carbide and somewhat lower for the nitride, even anticipating future improvements At present, oxide systems appear limited to 1200

°C and up to 1300 °C in the future The exact values all depend on stresses, time, and temperature, and the development

of suitable interfacial materials

References cited in this section

40 A Kelly, Design of a Possible Microstructure for High Temperature Service, Ceram Trans., Vol 57, 1995,

p 117

41 W.B Hillig, A Methodology for Estimating the Mechanical Properties of Oxides at High Temperatures, J

Am Ceram Soc., Vol 76 (No 1), 1993, p 129

42 A.G Evans and F.W Zok, The Physics and Mechanics of Fibre Reinforced Brittle Matrix Composites, J Mater Sci., Vol 29, 1994, p 3857

43 J Aveston, G.A Cooper, and A Kelly, Single and Multiple Fracture, Properties of Fiber Composites, IPC

Science and Technology, 1971, p 15

44 M Sutco, Weibull Statistics Applied to Fiber Failure in Ceramic Composites and Work of Fracture, Acta Metall., Vol 37 (No 2), 1989, p 651

Design with Composites

R.J Diefendorf, Clemson University

Large Composite Structures: Joints, Connections, Cutting, and Repair

A major advantage of composites is that large integrated structures can often be designed with a major reduction in parts count and assembly time The number of mechanical fasteners is often a small fraction of the number used in metal structures While it is sometimes possible to design a large composite in which loads are predominantly carried by the fibers, usually there will be regions that rely only on the matrix for load transfer from one major volume to another Several parts may be cocured together so that the matrix resin acts as the adhesive This area behaves much as a bonded joint Mechanical fasteners have been added to increase the reliability, but surprisingly performance has sometimes been decreased because of the stress concentrations and delamination damage at the free edge of holes Extensive use of mechanical fasteners is costly with carbon-fiber/resin composites, because the fasteners are generally made of titanium to minimize galvanic corrosion; use of coated fasteners would avoid corrosion and lower cost

Stitching has also been applied successfully using a tough fiber such as an aramid as the stitching yarn, as a method of improving interlaminar properties and minimizing edge delamination Stitching pierces the laminate with a minimum of laminate fiber damage and displacement Optimization of stitch spacing and yarn denier and tension has been performed for a number of different laminates and structures

While composite materials are especially amenable for making large integrated structures, it should always be realized that damage is likely to occur during the lifetime of the structure The structure must be designed for easy and efficient repair Consideration of where cuts should be for removal of damaged material, and flanges for attachment or bonding of repaired sections, and so forth, is required

Composite materials also offer the possibility for in situ structural-integrity monitoring by incorporating sensing fibers within the composite A number of studies have shown that optical and strain-sensitive fibers can be used to measure loads and structural integrity However, these techniques have yet to achieve wide application

Variance and Scaling Considerations

Composites traditionally have used brittle fibers, because of their otherwise superior properties For simplicity, consider a composite with the fibers all aligned parallel to the load direction While the most probable state of the composite would

be to have the worst flaws in each fiber randomly distributed along the fiber length, there is a small chance that all the worst flaws are located at the same distance along each fiber (If there are periodic processing-induced defects, the odds become worse that the flaws correlate.) The question is how does the variability, as measured by the Weibull parameter,

of composites compare to that of engineering metals? For well-developed composite systems, made in simple, flat laminates, the Weibull parameter for small specimens can be as high (20 to 24) as for aluminum sheet in the rolling direction, and even better than aluminum in the transverse direction However, strengths of longer laminates have often been lower than predicted by simple Weibull scaling (Ref 45) There are two problems Firstly, a large composite laminate may not be made in the same way as the smaller test laminates and more and worse flaws are introduced Secondly, the small test specimens do not sample the worst flaws very well, which actually cause failure of the long laminate By contrast, Weibull scaling of strength, when the width is increased, underpredicts the strength of the composite (The strength does not drop off as fast as predicted.) The designer must be aware of the effect of scale on strength Additional discussion of Weibull statistics is provided in the article "Design with Brittle Materials" in this Volume

Load, Heat, and Electrical Current Transfer

The large difference in properties that often exist between fiber and matrix make transfer of load, heat, and electricity to a composite part very inefficient The problem is easily illustrated in resin-matrix composites, because of the low modulus, and low thermal and electrical conductivity of most polymers Most of the load, heat, or electrical current is usually transferred to a composite through the resin to the fibers from the sides and not directly to the fibers from the end (Fig 17) In the case of mechanical loads, the load is transferred through shear stresses at the surface, and a tensile stress builds

up in the composite from the end A gradient in tensile stress also is present through the thickness, but disappears with sufficient length as the load is diffused throughout the composite The distance along the composite that is required to reach approximately a constant stress is much longer in a composite than in a metal While the stress can be considered constant in an isotropic metal within approximately two thicknesses for a sheet, the value for composites can vary from 6

to 40, the exact value depending on the composite architecture and materials Hence, a designer must be more careful in transferring load to a composite to prevent overloading outer fibers Load-transfer distances are much greater than in metals, and joints are less efficient

Fig 17 Transfer of load into a uniaxial composite The transfer of load through shear produces high stresses in

the outer ply fibers at the grips The distance along the composite that it takes for the load to diffuse

throughout the cross section is much greater than in a metal The stress profiles through the thickness are illustrated at several positions along the length of the composites

Fiber-reinforced composites have also been considered for heat-transfer applications, because of the development of very high thermal conductivity carbon fibers (five times the thermal conductivity of copper at room temperature) The thermal conductivity is at its maximum value somewhat below room temperature and decreases to low values near 0 K Above the maxima, the thermal conductivity decreases approximately inversely with temperature Hence, the very high thermal conductivity can only be achieved in near-room-temperature applications These high thermal conductivity carbon fibers have been used in resin, metal (particularly aluminum), and carbon matrices Flash laser thermal diffusivity experiments have shown that high thermal conductivity can be achieved in resin-matrix composites, when the laser beam directly impinges on the fiber ends in a uniaxial composite However, the effective thermal conductivity in many real designs is much lower, because the heat must be transferred through the resin matrix, and the heat flow never becomes uniform through the cross section of the part In the corresponding case for electrical conduction, uniform current flow is even more difficult to achieve Adding an electrically or thermally conducting powder to the matrix resin can help alleviate the problem

Metal-matrix composites allow better transfer of heat to the high thermal conductivity fibers, provided good fiber/matrix interfacial thermal contact can be maintained Although aluminum can galvanically corrode with carbon fibers, the low density and high thermal conductivity makes this combination attractive especially for space and aeronautical applications Protective coatings are available for carbon fibers to minimize corrosion

Joint Design

Only one-half of the potential weight savings is often achieved when using composites because of the necessity of joints The inherently inefficient load transfer into most composites, especially polymer-matrix composites, means that joint design is often a driving factor in composite design Frequently, sections are thick and load paths complicated, which requires careful, and three-dimensional, analysis Design of a joint is often more difficult than that of the rest of the composite part A primary decision is whether to use bonded or mechanically fastened joints Studies have shown that bonded joints can be more efficient than mechanically fastened joints However, the questionable reliability and longevity

of these bonded joints, especially if off-axis peeling stresses are present, have often resulted in the use of mechanical fasteners Mechanical fasteners must be used if easy disassembly or access is required

Mechanically Fastened Joints. The behavior of mechanically fastened joints is similar to but more complicated than the behavior of similar joints in metal Not only is the strength dependent on geometry as with a metal, but also on the fiber orientations Numerous composite joint configurations have been tested and evaluated Only the major conclusions are presented The important factors affecting joint strength are joint type, fastener, geometry, and failure mode

Several types of mechanical joints have been used with composite materials The choice of joint type depends on the application Single- or double-lap designs with rivet (pinning) or bolting are frequently used Riveted joints often provide adequate strength in carbon- or glass-fiber- reinforced resin-matrix composites, but bolts offer the greatest strength Higher-bearing-strength bolted joints result if the clamping pressure is increased to an optimum by using a higher bolt tightening torque Additionally, washers, and a carefully reamed hole that closely fits the bolt, produce higher-strength joints

Mechanically fastened joints in composites display the same failure modes as observed in metals: net-section tension, edge-section shear, end-section bearing, and combined modes (Fig 18) Geometrical factors as well as fiber orientations determine the failure mode The geometrical factors of width (pitch), end distance, and diameter influence the behavior of composite joints, just as they do in metals (Ref 47, 48) (Fig 19) End distance is defined as the distance from the end of the joint to the center of the closest hole, and width is the distance from the sides of the joint to the center of the nearest hole End distance and width must be above a certain minimum if full bearing strength is to be obtained Below these minima, tensile failure occurs if the width is too small, and shear-out or cleavage if the end distance is too small (Fig 18)

While all materials follow the same trend, the full strength occurs at differing w/d and e/d ratios for different materials and stacking sequences However, values of w/d and e/d of 5 appear adequate to achieve a bearing failure in most resin- matrix composites Adding fabric plies for reinforcement around holes is a very efficient way to reduce w/d and e/d

values to below 5, although the thickness of the composite will be increased in the local region.The hole diameter

compared to the laminate thickness, d/t, has a negligible effect if the hole is optimally clamped, but should be below 1 if

not

Fig 18 Failure modes in bolted joints Failure not only depends on the geometrical dimensions, but also on the

laminate construction Arrows show direction of tensile loading Source: Ref 46

Fig 19 Definition of bolted-joint geometry terms Source: Ref 46

Bonded Joints. A bonded joint consists of three components with different mechanical properties: the adhesive or joining material, and the two parts that may be the same or different material A large number of variables influence the

load-carrying capabilities of an adhesive joint Manufacturing variables include composition, preparation of the adhesive and adherends, and the joint geometry Joints may also involve use of brazing with or without chemical reaction in metal

or ceramic composites The behavior of a bonded composite joint, as with mechanically fastened joints, is more complicated than its metal counterpart The anisotropic nature of the material is significant, as are the low values of interlaminar and through-thickness modulus and strength for resin-matrix composites

Various types of adhesively bonded joints are used in composites (Ref 49) (Fig 20) The selection of joint shape is influenced by the type of assembly (thick/thin, etc.), bending moments, tooling requirements, and costs While the scarf and beveled lap joint provide more uniform stress distributions and high strength, they present several fabrication problems, especially in thin sections They are expensive to machine, the feather edges tear easily, the bonding jigs are expensive, and the pressure must be applied with care

Fig 20 Types of bonded joints Many variations are possible, but difficult machining and assembly often

eliminate more complex designs The best bonded joints are those with the largest bond areas and minimum peeling stresses Source: Ref 46

Compared to failure in metal joints, a larger number of failure modes can be identified in composites, due to their anisotropic nature Failure can be tensile, interlaminar, or transverse in the adherends (Fig 21) In the latter two cases, failure may either be in the resin or at the fiber/resin interface in a resin-matrix composite A cohesive failure mode can also occur in the adhesive

Fig 21 Failure modes in bonded joints Arrows show direction of tensile loading Source: Ref 46

Most of the adhesives used for structural purposes are relatively strong in shear and weak in cleavage or peel Therefore, the designer should attempt to place the adhesive in shear and minimize peel and cleavage stresses The stress distribution depends on the joint geometry and the mechanical properties of the adhesive and the adherends The strength of a simple, straight lap joint is directly proportional to the width of the bond, but increases to an asymptotic value with overlap length The length required to reach the asymptotic strength depends on the particular material system, but values in excess of 40 are typical Joint strengths are improved if identical adherends are used (if not, the in-plane and bending stiffness should be equalized), adherend stiffness is increased and adhesive moduli decreased, and as large an overlap as possible is used

As a general rule, bonded joints are more suitable for lightly loaded joints where their high efficiency can be fully realized Mechanically fastened joints are invariably used in high risk, highly loaded joints that are subjected to severe fatigue and environmental conditions The major penalties involved in the use of mechanical joints are the increased weight, part count, and associated cost However, there are many notable exceptions to this generality in which careful design has allowed the use of bonded structures in complex but critical parts A combination of fasteners and bonding is the most structurally efficient and reliable arrangement in many cases

Cut Edges

Resin-matrix composites are often sawed and then routed to final dimension, or cut by water jet While the cut surface may appear smooth, interlaminar cracks may be introduced because of the poor interlaminar strength of resin-matrix composites Even if no cracks are present, the free edge can act as an initiation site for cracking as high interlaminar stresses are often present at the free edge Good design should minimize the number of free edges in the design if possible Because free edges are often sites for failure in fatigue applications, a thin coating of a tough resin, even on the very smooth edges, has been found to increase fatigue life by a factor of ten

A damaged surface often results when an abrasive wheel is used to cut metal or ceramic-matrix composites The surface cracks may act as failure sites, especially in fatigue applications A more important problem is that the cut edges expose reactive fibers and interfacial coatings to the environment

Surface Coatings

Paints and wear-resistant coatings often have to be stripped and recoated The similarity of paints to resins in resin-matrix composites often makes stripping difficult and usually more expensive than the application cost However, the total life-cycle cost is often not minimized, because different groups manufacture and refurbish the composite structures A goal of

"paint for life" may be attractive for many applications

References cited in this section

45 C Zweben, Simple Design Oriented Composite Failure Criteria Incorporating Size Effects, Tenth International Conference on Composite Materials, Vol 1, Woodhouse Publishing, Cambridge, England,

48 E.W Godwin and F.L Matthews, A Review of the Strength of Joints in Fiber-Reinforced Plastics: Part 1,

Mechanically Fastened Joints, Composites, Vol 11 (No 3), July 1980, p 155

49 F.L Matthews, P.F Kilty, and E.W Godwin, A Review of The Strength of Joints in Fiber-Reinforced

Plastics: Part 2, Adhesively Bonded Joints, Composites, Vol 13 (No 1), Jan 1982, p 29

Design with Composites

R.J Diefendorf, Clemson University

Design for Manufacturing

Processing and fabrication costs could be reduced if structures were designed for manufacturing as well as performance Composite materials are generally designed to maximize performance, and much touch labor is often used to manufacture these designs Many composite laminates are fabricated by laying the laminate plies against a tooling surface, vacuum bagging, and then autoclave curing While the surface molded against the tool can be good, the laminate surface nearest to the vacuum bag is often quite rough and the thickness is not well controlled because of variances from ply thickness and processing The local roughness, often produced from "peel ply," can act as a beneficial spacer in bonding, but the variable thickness often requires shimming to maintain tolerances Use of large closed metal molds can eliminate this problem, but they are too expensive to use for limited production runs The use of high throughput, but limited geometry technology, such as pultrusion, filament winding, and braiding should be considered to reduce touch labor Novel technology such as fast winding of a simple shape, with subsequent deformation to a more complex shape, should be considered for adding more versatility to these otherwise more limited processes

High-temperature ceramic- and carbon-matrix composites often use chemical vapor infiltration to achieve very high performance The process is lengthy as a precursor gas must be transported into and product gases out of the interior of a fibrous "preform" to form the matrix A preform design that provides rapid transport of gases would drop costs and speed

up design cycles without a large decrease in mechanical properties Independent of the constituent materials, composites manufacture is complicated by the very tailorability that is the hallmark of these materials While high performance and manufacturability are by no means mutually exclusive, it is critical that the designer include processing considerations as part of the design cycle

Design with Composites

R.J Diefendorf, Clemson University

References

1 R.J Diefendorf and E.W Tokarsky, High Performance Carbon Fibers, Polym Sci Eng., Vol 15 (No 3),

1975, p 151

2 B.W Rosen, Tensile Failure of Fibrous Composites, AIAA J., Vol 2, 1964, p 1985

3 R.J Diefendorf, The Chemical Nature of the Fiber/Resin Interface, Tough Composite Materials, NASA

Langley, Noyes Publishing, 1985, p 192

4 C Muser and N.J Hoff, Stress Concentrations in Cylindrically Orthotropic Plates with Radial Variation of

the Compliances, Progress in Science and Engineering of Composites, T Hayashi, K Kawata, and S

Umekawa, Ed., Japan Society for Composite Materials, 1982, p 389

5 C Zweben and B.W Rosen, A Statistical Theory of Materials Strength with Application to Fiber

Composites, J Mech Phys Solids, Vol 15, 1970, p 189

6 D.W McKee and V.J Mimeault, Surface Preparation of Carbon Fibers, Chemistry and Physics of Carbon,

Vol 8, Marcel Dekker, 1973, p 201

7 H Schuerch, Prediction of Compressive Strength in Uniaxial Boron Fiber-Metal Matrix Composite

Materials, AIAA J., Vol 4, 1966

8 S.J DeTeresa, S.R Allen, R.J Farris, and R.S Porter, Compressive and Torsional Behavior of Kevlar 49

Fibre, J Mater Sci., Vol 19, 1984, p 57

9 Z Hashin, Analysis of Properties of Fibrous Composites with Anisotropic Constituents, J Appl Mech., Vol

46, Sept 1979

10 W Voight, Lehrbuch der Kristallphysik, Teubner, Leipzig, 1910

11 D.L McDanels, R.W Jech, and J.W Wheeton, Metals Reinforced with Fibers, Met Prog., Vol 78 (No 6),

Dec 1960, p 118-121

12 R.A Schapery, J Compos Mater., Vol 2, 1968, p 311

13 B.W Rosen, Mechanics of Composite Strengthening, Fiber Composite Materials, American Society for

16 S.W Tsai and H.T Hahn, Introduction to Composite Materials, Technomic Press, 1980

17 R.M Jones, Mechanics of Composite Materials, Scripta Book, 1975, p 55-56

18 R.E Rowlands, Failure Mechanics of Composites, Handbook of Composite Materials, Vol 3,

North-Holland, 1985, p 71

19 K.K Chawla, Composite Materials Science and Engineering, Springer-Verlag, 1987, p 287

20 H Edwards and N.P Evans, A Method for the Production of High Quality Aligned Short Fibre Mats and

Their Composites, Advances in Composites (ICCM3), Vol 2, Pergamon Press, 1980, p 1620

21 D.W Radford, "Shape Stability in Composites," Ph.D thesis, Rensselaer Polytechnic Institute, Troy, NY, May 1987, p 10

22 S.W Tsai and H.T Hahn, Introduction to Composite Materials, Vol 1, Technical Report

AFML-TR-78-201, Air Force Materials Laboratory, 1978, p 138

23 "Structural Design Guide for Advanced Composite Applications," AFML Advanced Composites Division, Air Force Materials Laboratory, 1971

24 S.W Tsai and H.T Hahn, Introduction to Composite Materials, Technomic Press, 1980, p 221

25 R.J Diefendorf, D.W Radford, and S.J.Winckler, Asymmetric Composites-Hygrothermal Stability in Flat

Plates, paper D4, Verbundwerk '91 3rd International Conference on Reinforced Materials and Composite Technologies, Demat, Frankfurt, Germany, 1991

26 M.W Hyer, Calculations of the Room-Temperature Shapes of Unsymmetric Laminates, J Compos Mater.,

Vol 15, July 1981

27 D.W Radford and R.J Diefendorf, Shape Instabilities in Composites Resulting From Laminate Anisotropy,

Reinf Plast Compos., Vol 12, Jan 1993, p 66

28 W.T Freeman and G.C Kuebeler, Mechanical and Physical Properties of Advanced Composites,

Composite Materials: Testing and Design (Third Conference), STP 546, ASTM, 1974, p 435

29 H.T Hahn and L Lorenzo, Advances in Fracture Research, International Conference on Fracture No 6,

New Delhi, Vol 1, Pergamon, Oxford, 1984, p 549

30 A Johnson, Modeling the Crash Response of a Composite Airframe, Proceedings of the Tenth International

Conference on Composite Materials, Vol 6, Woodhouse Publishing, Cambridge, England, 1995, p 71

31 M McLean, Directionally Sloughed Materials for High Temperature Service, The Metals Society, London,

34 P.R Smith and F.H Froes, J Met., Vol 36, 1984, p 19

35 P.A Selmers, M.R Jackson, R.L Mehan, and J.R Rairden, Production of Composite Structures by

Low-Pressure Plasma Deposition, Ceram Eng Sci Proc., Vol 6, 1985, p 896

36 W.A Logsdon and P.K Liaw, Eng Fract Mech., Vol 24, 1986, p 737

37 Y.A Bahei-El-Din and G.J Dvorak, Plastic Deformation Behavior of Fibrous Composite Materials,

Proceedings of the 4th Japan U.S Conference on Composite Materials, Technomic Publishing, 1989, p 118

38 J Tanaka, H Ishikawa, T Hayase, K Okamura, and T Matsuzawa, Mechanical Properties of SiC Fiber

Reinforced Al Composites, Progress in Science and Engineering of Composites, ICCM-IV, Japan Society

of Composite Materials, Tokyo, 1982, p 1410

39 G.D Swanson and J.R Hancock, Off-Axis and Transverse Tensile Properties of Boron Reinforced

Aluminum Alloys, Composite Materials: Testing and Design, STP 497, ASTM, 1971, p 472

40 A Kelly, Design of a Possible Microstructure for High Temperature Service, Ceram Trans., Vol 57, 1995,

p 117

41 W.B Hillig, A Methodology for Estimating the Mechanical Properties of Oxides at High Temperatures, J

Am Ceram Soc., Vol 76 (No 1), 1993, p 129

42 A.G Evans and F.W Zok, The Physics and Mechanics of Fibre Reinforced Brittle Matrix Composites, J Mater Sci., Vol 29, 1994, p 3857

43 J Aveston, G.A Cooper, and A Kelly, Single and Multiple Fracture, Properties of Fiber Composites, IPC

Science and Technology, 1971, p 15

44 M Sutco, Weibull Statistics Applied to Fiber Failure in Ceramic Composites and Work of Fracture, Acta Metall., Vol 37 (No 2), 1989, p 651

45 C Zweben, Simple Design Oriented Composite Failure Criteria Incorporating Size Effects, Tenth International Conference on Composite Materials, Vol 1, Woodhouse Publishing, Cambridge, England,

48 E.W Godwin and F.L Matthews, A Review of the Strength of Joints in Fiber-Reinforced Plastics: Part 1,

Mechanically Fastened Joints, Composites, Vol 11 (No 3), July 1980, p 155

49 F.L Matthews, P.F Kilty, and E.W Godwin, A Review of The Strength of Joints in Fiber-Reinforced

Plastics: Part 2, Adhesively Bonded Joints, Composites, Vol 13 (No 1), Jan 1982, p 29

Introduction to Manufacturing and Design

Henry W Stoll, Northwestern University

Introduction

THIS ARTICLE introduces and describes general concepts and practices related to manufacturing and design It is intended to:

• Place the activities of design and manufacturing in the context of the business system that they support

• Present an overview of the manufacturing technology field from a design and material selection perspective

• Provide insight into the complex relationship among design, material selection, and manufacturing

• Summarize modern design for manufacture practices being widely used in industry today

The main focus is on how design and manufacturing practices influence the properties and cost of engineered designs Engineered designs are products, equipment, devices, and hardware that have been designed to meet specific end-user needs In this context, nuts and bolts, computers, electrical transformers, portable phones, automobiles, machine tools, construction equipment, consumer products, and aircraft are all engineered designs In engineered designs, materials and methods of manufacture are typically selected to meet functionality, performance, cost, and reliability objectives

Manufacturing can be defined as the conversion of starting materials into finished parts, products, and goods that have value to end users Starting materials for engineered designs are very often semifinished products such as coil steel or pelletized plastic as well as "off-the-shelf" components and hardware The specific starting material used for a particular part or product depends on the manufacturing process and on the types, variety, and availability of semifinished products that are acceptable for use in the process

Manufacturing is both a technical activity and an economic activity As a technical activity, manufacturing involves the design, development, implementation, control, operation, and maintenance of a large variety of manufacturing processes that facilitate and perform the conversion of starting materials into finished products having greater value Processes used

in manufacture can be grouped into two basic types: (1) physical and chemical processes that transform the shape, properties, and/or appearance of starting materials into parts, and (2) assembly and joining processes that combine multiple parts into finished products In addition, there are myriad ancillary processes and operations that support the basic manufacturing processes in a variety of ways such as material handling, quality control, testing, and so on

In the economic sense, manufacturing is a commercial activity performed by companies that sell products to customers Hence, economic considerations are generally the overriding constraint in manufacturing decision making The costs of energy, material, purchased components, labor, tooling, and capital equipment must be minimized and properly controlled

if the firm is to make a profit Producing and selling products is generally a very complex activity involving a convoluted mix of people skills and disciplines, machines and equipment, tooling, computers, and automation working together to form a manufacturing system A manufacturing system comprises a large number of distinct functions and activities (Fig 1) that interact in a variety of ways to ultimately produce and sell the product On the business level, therefore, the challenge is to organize, optimize, and operate the manufacturing system in a way that ensures both long-term customer satisfaction and economic viability and success of the manufacturing enterprise

Fig 1 The variety of ways the many different functions of a manufacturing system can interact

Introduction to Manufacturing and Design

Henry W Stoll, Northwestern University

The Manufacturing Enterprise

Manufacturing usually is associated with a business activity or enterprise To understand the relationship between manufacturing and business, consider the basic actions that occur when conducting a manufacturing-related business Four groups of "main-line" work are generally necessary: decide what customers want, set up the factory to make it, produce it, and sell and service it Tracing the "main-line" generic work flows that connect these activities yields two major cycles on which most design and manufacturing practices focus: the order-to-delivery cycle that involves the receipt of orders and production of existing products, and the new product introduction cycle that involves planning and introducing new products and changes into production

The order-to-delivery cycle is the operations side of the business It generates the day-to-day cash flow for the company

by producing and distributing products, ensures product quality, and provides the interface between the company and its customers in the form of sales and service The new product introduction cycle, on the other hand, is the product development, engineering, and new technology side of the business For many businesses, product development and engineering is the life blood of the company Effective new product development increases the company's competitive edge in everchanging markets

At first glance, it may appear that these two cycles are independent of each other, but in fact they are very interrelated and closely coupled This is because the new products introduced into production by product development must map into the production environment with the least amount of disruption and expense and in a way that ensures high product quality This requires close coordination and cooperation between the product development and operations activities

This mapping is one of the principal motivations for design for manufacture (DFM) Most engineered designs are assemblages of parts Direct and indirect cost as well as quality of assemblies are determined by the number of parts; the ease with which the parts and components are handled, assembled, procured, and inspected; and the functionality, serviceability, maintainability, reliability, and durability achieved by the finished device or product

All of these factors are in turn dependent on the material, manufacturing processes, and detail design of the individual parts and components that make up the finished assembly Therefore, each part must be designed so that it not only meets functional requirements, but also can be manufactured and assembled economically and with relative ease in the production environment of the company Effective part design requires early consideration of the characteristics, capabilities, and limitations of materials, manufacturing processes, and related operations, machinery, and equipment (including consideration of part-to-part variability, dimensional accuracy, surface finish, processing time, and so forth) These issues are discussed in greater detail in the article "Design for Manufacture and Assembly" in this Volume

The type of production environment the design must map into drives many product design, material selection, and manufacturing decisions This often leads to different manufacturing strategies For example, one possible strategy is to assemble products using parts and components purchased from suppliers and vendors In this approach, the manufacturer might pay more for individual parts and components, but avoid the direct and long-term indirect cost of making them internally and gain the advantage that the design team is relatively free to select materials and processes that best meet the functional needs of the new product

On the other hand, if a manufacturer chooses to make many of the parts used in its products, that is, if it is "vertically integrated," then new products may need to be designed to be compatible with available "in-house" tooling and equipment This can impose constraints on material and manufacturing process selection For example, the cost and difficulty of introducing a spot-welded structure may be overwhelming for a company that has historically manufactured arc-welded structures Similarly, switching to an all-plastic housing may prove difficult for a company that has traditionally made sheet metal housings and, as a consequence, has invested heavily in sheet metal forming equipment and expertise In many product developments, "make or buy" decisions can strongly influence design direction; for many businesses, they can form the basis for business and manufacturing strategy

New Product Introduction Cycle

The new product introduction cycle involves two major activities: tactical planning and product development Tactical planning determines the "what" and "when" for product design and development and quantifies the needs that guide design and manufacturing Product development translates the "what" into "how" by defining the new product designs to

be introduced into production and providing the physical hardware, systems, information, training, and guidance to make new processes, tooling, and equipment work in production Tactical planning is typically performed by marketing, advanced product planning, and technology development groups, while product development is typically performed by product engineering and manufacturing engineering groups

Designing new products and introducing product changes takes time and costs money If a new product takes too long to design and launch, the manufacturing enterprise can lose financially because of lost sales and market share Even if the product gets to market in a timely manner or if time-to-market is irrelevant, the firm can lose financially if the cost of developing the design is excessive The need to develop new products efficiently has led to several major trends in design and manufacturing practice

Design Process Reengineering. Organizational and procedural changes have been made to enhance communication between functions, encourage DFM, and simplify and optimize work flow Many companies have adopted concurrent engineering practices and are using the team approach effectively In some cases, design and manufacturing engineering have been located on the same premises, while in other situations technologies such as teleconferencing, E-mail, and the World Wide Web are used to overcome wide geographical separations Many companies are also working to improve their product realization process so that the design team knows exactly what to do during each step of the process Gate or design reviews have been instituted to ensure economic viability of design projects and to facilitate simultaneous achievement of product manufacturability and tight schedule commitments See Ref 1 for a comprehensive treatment of management aspects of product design

Integrated Design Systems. Computer-aided design (CAD), aided manufacturing (CAM), and aided engineering (CAE) systems have been integrated so that product and manufacturing engineers work on compatible systems that share information in a seamless environment In addition, a wide variety of new CAD tools and methods

computer-have been introduced (see the Section "Design Tools" in this Volume), and their use has been made an integral part of the product realization process Instead of computerizing manual practices such as drafting, design organizations are now using the computer to eliminate these practices where possible and to replace them with new computer-aided practices that reduce design time and effort For example, parametric and feature-based CAD/CAM tools now enable design changes to be made and then propagated through the model with a few high-level commands Using these tools, engineering analysis, manufacturing, and inspection can be automated and integrated with design to a higher degree than ever before

Science Base. Traditionally, many engineered products have evolved and have been incrementally improved by using experimental or "test-and-fix" procedures involving costly and time-consuming construction and testing of many prototypes This practice is being eliminated by developing a "science base" for the product technology involved Once an appropriate science base is available, computer simulations and other analysis techniques can be used to more quickly select the best new design

Product architectures are evolving that take advantage of commonalities present in different product models and variants By modularizing and standardizing components and subsystems, creating building block parts, and rationalizing the variety of choices available for purchased components (such as threaded fasteners and ball bearings), many firms have greatly reduced design time for new products, especially those similar to existing products These practices are also leading to significant savings of scope and scale High-volume manufacturing processes and automation alternatives become economically feasible, and purchasing effort and the per part cost of supplied parts is reduced

Taking time out of the product design cycle yields numerous benefits Customer satisfaction is enhanced because improved product quality and short cycles give the customer what he wants when he wants it Cost is reduced because waste and nonvalue activity is eliminated Improved profitability results from increased market share combined with the ability to respond and adapt quickly to changing market conditions with minimum cost impact

Order-to-Delivery Cycle

The order-to-delivery cycle typically involves three major activities: order fulfillment, production, and service Order fulfillment is the sales arm of the business, responsible for bringing in orders for products and forecasting production requirements Production converts sales orders into products It encompasses all of the personnel, operations, processes, tooling, and equipment involved in the day-to-day manufacture of products including procurement, raw material and supplied component receiving and inspection, part manufacture, assembly, testing, and distribution of manufactured products Service provides product-related maintenance, repair, disposal services, and support to the customer

All of these activities influence design and manufacturing practices For example, design for service and design for disassembly are important strategies in many companies As another example, consider companies that sell specialized or customized products and services These companies must often "quote" the job first, then manufacture and deliver the product or equipment after the order has been received The time this process takes can contribute significantly to manufacturing lead time as well as strongly influencing customer satisfaction Modular designs or designs that allow customization at the end of the production line help reduce order-to-delivery time for many of these types of customized products and equipment

Marketing, manufacturing, and design strategy can also play an important role Consider a company whose marketing strategy is to sell personal computers that are customized according to specifications provided by the customer via a phone order Such a marketing strategy needs to be supported by a manufacturing strategy that facilitates easy manufacture and delivery of a customized product, together with a design strategy that makes the product easy to customize at remote distribution sites

Another approach is to use computerization to take time out of the ordering process One vision is that of a sales representative in the office of a customer with, for example, his or her portable computer having all product and pricing data available on disk, so that a complete purchase order or release can be negotiated on the spot and then inserted into the plant's production schedule by a modem and confirmed before the representative leaves the office

An important measure of production efficiency and effectiveness is manufacturing lead time (MLT) or the time required

to process the product through the plant Manufacturing lead time is directly related to manufacturing cost and customer satisfaction A short MLT implies less manufacturing time and labor and therefore lower manufacturing cost Also, the

shorter the MLT, the sooner the product can be sold and the company reimbursed for its investment in raw material and labor Most importantly, a short MLT means the customer gets the product when needed

Reference 2 calculates MLT as,

where n m is the number of machines or operations and Tsu, Q, To, and Tno are the setup time, batch quantity, operation time, and nonoperation time for each machine and/or operation, respectively Nonoperation time includes handling, storage, inspections, and other nonvalue-added activities Some manufacturing strategies given in Ref 2 that help reduce MLT and the order-to-delivery cycle are summarized in Table 1 These practices are often facilitated or made possible by design practices such as design for manufacture and assembly

Table 1 Manufacturing strategies to reduce manufacturing lead time and the order-to-delivery cycle

1 Specialization of operations Reduce To

2 Combined operations Reduce nm, Th, Tno

3 Simultaneous operations Reduce nm, To, Th, Tno

4 Integration of operations Reduce nm, Th, Tno

5 Increase flexibility Reduce Tsu, MLT, WIP; increase U

6 Improve material handling and storage Reduce Tno, MLT, WIP

7 On-line inspection Reduce Tno, q

8 Process control and optimization Reduce To, q

9 Plant operation control Reduce Tno, MLT; increase U

10 Computer-integrated manufacturing Reduce MLT, design time, production planning time; increase U

(a) Th, work-handling time; WIP, work in process; q, scrap rate or fraction defect; U, utilization

References cited in this section

1 J.E Ettlie and H.W Stoll, Managing the Design-Manufacturing Process, McGraw-Hill, 1990

2 M.P Groover, Automation, Production Systems, and Computer Integrated Manufacturing, Prentice-Hall,

1987, p 40

Introduction to Manufacturing and Design

Henry W Stoll, Northwestern University

Manufacturing Processes

A general appreciation for the different manufacturing processes that are commonly used and for the design and manufacturing practices that are associated with them can be gained by considering the fundamental nature of manufacturing processes A process can be defined as the change of properties of an object Changes may concern geometry, hardness, strength, chemical composition, information content, and so forth In general, three essential agents must be available to cause a change: energy, material, and information Therefore, a process may be an energy process, a material process, or an information process Most practical manufacturing processes are material processes

A material process consists of a material flow on which shape information is impressed (information flow) and an energy flow that carries out the transformation of information through the tool/die and the pattern of movement for the tool/die and the material (Fig 2) The energy flow includes both the energy necessary to carry out the process and the energy

output (loss) that is produced by the process The information flow, which includes both shape and property information, depends on the nature of the material, the type of process (e.g., mechanical, thermal, chemical), the characteristics of the tool/die, and the pattern of movement of the material and the tool/die The final geometry or shape information of the part

is the sum of the initial shape information of the starting material and the shape information impressed on it by the process Similarly, the final material properties of the part or product is the result of the property information of the starting material and the property change caused by the process In general, the nature of a material process depends on whether the mass of the material is conserved, decreased, or increased

Fig 2 Material process (a) Model (b) Schematic of one type of material process, bulk deformation by

extrusion 1, original material condition (shape and microstructure); 2, final material condition (shape and microstructure); 3, plastic deformation zone; 4, tooling effects; 5, friction effects; 6, equipment characteristics

Mass-Conserving Processes

In mass-conserving processes, the mass of the starting material is approximately equal to the mass of the final part or product During the process, the starting material is "forced" to change its shape or properties in some way Shape-replication processes are mass-conserving processes in which the part replicates the shape information stored in the die or mold by being forced to assume the shape of the surface of the tool cavity Near-net-shape processes are shape-replication processes that produce parts requiring little or no subsequent processing (e.g., machining) to obtain the finished part The nature and material requirements of shape-replication processes are generally determined by the state of the starting material

Liquid or semifluid material is forced to flow into a mold cavity under pressure where it assumes the shape of the cavity Hence, information is impressed primarily by the tool However, most materials shrink as they solidify so the final geometry of the part is the sum of the tool shape plus distortions and dimensional changes due to material shrinkage Representative processes include sand casting, die casting, and plastic injection molding Process-related material considerations typically include melting temperature, flow behavior, ability of the material to assume the shape of the mold cavity, existing thermal and mechanical properties during solidification, resulting properties of the solidified material, and flaws such as porosity or voids that can occur as a result of the process

Granular materials (metal powders, molding sand, etc.) are formed as a result of a flow process (filling, flow, placing) and a stabilization process (packing, plastic deformation, hardening, sintering, etc.) The geometry is primarily determined by the tool with some secondary geometry changes occurring due to shrinkage or forming occurring during stabilization Representative processes include powder metallurgy and ceramics processing Material considerations are process specific, but generally involve strength at various stages of the process, ability of the granular material to assume the desired shape, and density and final properties of the finished part

Ductile Solid Materials. Forming takes place by plastic or elastic deformation of the starting material The part shapes that are possible depend on the specific material, starting shape, and process to be used In general, the material properties, the process parameters (state of stress, force, velocity, temperature, etc.) and the surface creation method must

be analyzed together in order to establish the shape of the starting material, the tool design, and the process design Representative processes include bulk deformation processes such as forging, extrusion, and rolling and sheet metal working processes such as stretching and drawing Material considerations typically revolve around the ability of the material to flow plastically into the desired shape, property changes caused by plastic deformation, possible failure modes such as tears and splits, and dependency of mechanical properties on temperature and strain rate

Near-net-shape processes are very desirable because little or no material is wasted and the final part can be produced quickly and consistently However, large production volumes are often required to offset tooling cost Design and manufacturing practice for mass-conserving processes generally focus on reducing tooling cost and development time and

on setting process parameters to produce high-quality parts with low cycle times Detail design of the part and material selection are of particular importance in these processes because the part features must be compatible with both the material properties and the manufacturing process

Conventional Machining Processes. In these processes, material is removed by fracture and the formation of chips

created by relative movements between the workpiece material and the tool In most conventional machining processes, the contour content of the tool is relatively small so the patterns of movement of the tool and workpiece material play a large role in the surface creation The cutting tool can have a single cutting edge, such as the tool used in turning (lathe) or shaping operations, or it may have multiple cutting edges Multiple-cutting-edge tools have either well-defined edge geometry such as milling cutters, drills, and saws, or randomly oriented cutting edges such as those in grinding wheels and abrasive paper

Other Machining Processes. In these processes, material is removed by mechanical, thermal, electrical, and/or chemical action and often involve the use of granular, liquid, or gaseous media In general, the surface creation is the result of the material removal mechanism, the geometry of the tool or cross section of the energy source, and the pattern

of movement of the tool or energy source relative to the workpiece material Examples include ultrasonic machining, electrical discharge machining (EDM), electrochemical machining (ECM), waterjet cutting, laser cutting, thermal cutting, and chemical etching

Shearing processes involve separating adjacent parts of a sheet through controlled fracture In some cases, such as punching, the material that is removed is scrap In other cases, such as blanking, the material that remains is scrap By varying the geometry of the tool and the pattern of movement, a large number of different processes are possible Examples include punching, blanking, shearing, and slitting

The complex patterns of movement required in machining processes typically result in excessive setup and cycle times Hence, these processes are generally avoided if possible when large production quantities are involved On the other hand, machining processes are capable of high precision and often they are the only option if tight tolerances are required Also, because shape information is impressed through the pattern of movement rather than through the contour of the tool, machining is often the process of choice for low-volume manufacture where the cost of tooling must be kept low

Design and manufacturing practices associated with mass-reducing processes generally focus on eliminating setup and processing time and on controlling process variation For example, the use of numerical control (NC) makes it possible to change over from one part to another simply by downloading a different tool path program CAD/CAM systems allow the rapid generation of the NC program directly from the solid model database Also, by using modern sensor technology and feedback control techniques, NC systems can be designed to give extremely high accuracy and repeatability

Assembly Processes

In mass-increasing processes, change is the result of the assembly or joining of components into a whole Typically, the mass of the final part or product is approximately equal to the sum of the masses of the components Assembly can be described as a series of joining processes in which parts are oriented and added to the build The macrogeometry of the assembly is established by the positioning of the components Joining processes are either permanent or nonpermanent and can be classified as shown in Fig 3 Material properties and property changes produced by the process can be an important consideration in some joining processes such as solid-state and liquid-state welding

Fig 3 Classification of joining processes (adapted from Fig 9.1, in Ref 3) MIG, metal inert gas; TIG, tungsten

inert gas

Assembly imposes constraints on the design Not only must the parts be designed so that they can be assembled and joined together to provide the needed function, they must also be designed so that they are easy to handle, insert, retain, and verify that they have been assembled correctly Because assembly is an integrative process, problems with detail part designs often surface when they are assembled Parts do not fit together properly, tools cannot reach in the space provided, parts can be incorrectly assembled, and so forth These problems can often require extensive rework resulting in costly schedule slippage and undesirable design compromises

The importance of assembly as a design constraint has resulted in a greatly increased emphasis on assembly in the design process Design and manufacturing practice now focuses on ensuring that parts conform to specifications, on eliminating variability and randomness from the process, and on making nonvalue-added operations such as orienting and handling as simple and easy to perform as possible Many product design departments now use design for assembly techniques to improve the ease with which products are assembled Design for assembly, which is discussed in detail in the article

"Design for Manufacture and Assembly," which follows in this Volume, seeks to ensure ease of assembly by developing designs that are easy to assemble

Many excellent textbooks on manufacturing processes and methods are available See, for example, Ref 3, 4, and 5 Design issues related to specific processes are discussed in detail in the other articles in this Section of the handbook

References cited in this section

3 M.P Groover, Fundamentals of Modern Manufacturing, Prentice-Hall, 1996

4 J.A Schey, Introduction to Manufacturing Processes, McGraw-Hill, 2nd ed., 1987

5 S Kalpakjian, Manufacturing Processes for Engineering Materials, Addison-Wesley, 2nd ed., 1991

Introduction to Manufacturing and Design

Henry W Stoll, Northwestern University

Production Systems

Manufacturing processes convert starting material into finished parts and products Production systems link, coordinate, and integrate the various manufacturing processes used in production Production systems are often designed and optimized to reduce manufacturing lead time, and as a result they may involve manual operations, mechanized operations, automated operations, or a mix of all three

A distinction is generally made between mechanization and automation According to Schey (Ref 3), mechanization means that something is done or operated by machinery and not by hand whereas automation means a system in which many or all of the processes involved in the production, movement, and inspection of parts and material are automatically performed or controlled by self-operating devices Automation implies sensing, closed-loop control, and some degree of decision making in addition to mechanization Flexible automation includes the added capability of being easily reprogrammed or adapted to meet varying or new production needs

The kind of production systems used in modern factories depend greatly on the variety of products being produced and on production quantities If production quantities are low and product variety is extensive, then the production systems are likely to be manual with an emphasis on accommodating variety A glass blower who makes a wide variety of different ornamental decorations is an example In this case, the glass blower probably does his or her work at one workbench with all needed tools readily at hand He or she is ideally set up to make individual pieces, one at a time, and in any order necessary to meet customer demand

At the other extreme, if production quantities are large and only one product is made, it is likely that dedicated mechanization or automation would be used For example, a company that makes millions of a certain type of lightbulb each year is likely to manufacture the lightbulb using a production machine that is fully automated but capable of producing only that one particular type of lightbulb Figures 4 and 5 depict the relationship among product complexity, production volume, and level of automation

Fig 4 Production volume versus product complexity Source: Ref 6

Fig 5 Level of automation versus product complexity Source: Ref 6

Although production systems are not of direct concern in material selection, they do impact part and product design in a variety of ways For example, if a part is to be bowl fed, it must be properly designed for that environment This implies that the material must hold up under bowl feeding conditions Similarly, if a part is to be handled by a robot, it must have the appropriate design features that allow gripping and it must be capable of withstanding the gripping forces involved Production systems therefore often superimpose additional constraints on design and material selection Leading-edge companies that manufacture products in large quantities often utilize a "design for automation" approach in which the needs of the production system are considered very early in the product design and the systems integrator or supplier of the automation equipment is an important member of the concurrent engineering team

References cited in this section

3 M.P Groover, Fundamentals of Modern Manufacturing, Prentice-Hall, 1996

6 M Andreasen, S Kahler, and T Lund, Design for Assembly, IFS Publishing, 1983

Introduction to Manufacturing and Design

Henry W Stoll, Northwestern University

Interaction between Design and Manufacturing

The design of a product and its method of manufacture are intimately connected This interdependence cuts across all aspects of the manufacturing system and exists at the part, assembly, and system levels of product design and manufacturing The interaction between design and manufacturing is easiest to see at the part level Suppose, for example, that a part is to be designed as a casting If the part is to be manufactured using a casting process, then a material suitable for the particular casting process being considered must be selected This material must also have suitable functional properties such as strength, corrosion resistance, weight, and so forth Given a specific material and casting process, the part must then be properly designed to satisfy both functional and processing requirements This means that not only must the finished part have a certain desired shape and set of material properties, it must also have an acceptable parting line, sufficient draft, and a geometry that facilitates proper solidification of the material to achieve the desired material properties without porosity and other flaws

The interaction between design and manufacturing at the assembly level should be one of ease and simplicity How a product is assembled is determined by its design If assembly is considered as part of design, then many potential difficulties and quality risks can usually be avoided If assembly is not considered, manufacturing will still find a way to assemble the product, but the cost of assembly as well as indirect cost is likely to be higher and product quality may suffer

Design and manufacturing interactions are perhaps hardest to delineate, quantify, and control at the systems level, and yet

it is at this level that these interactions have the most long-term impact and far-reaching consequences Consider, for example, the sheet metal used in making an automobile From the design point of view a wide selection of different sheet metal thicknesses, roll widths, coatings, and material properties is desirable so that the most optimal design can be achieved with respect to structural rigidity, weight, and cost From the manufacturing point of view, however, the ability

to process a wide selection of different thicknesses and types of sheet metal means added cost and complexity in purchasing and supplier relations, material handling and storage, processing equipment and tooling, setups and operations, and so forth What is needed is the right balance that gives design the flexibility it needs while still allowing manufacturing the ability to simplify and standardize its operations How a company manages interactions such as this can greatly affect its long-term economic viability and competitiveness

Introduction to Manufacturing and Design

Henry W Stoll, Northwestern University

Design for Manufacture Practices

The interaction between design and manufacture has resulted in a variety of design and manufacturing practices This section provides a brief overview of various practices that are of general importance to design and manufacturing Some

of these practices, such as standardization and group technology, span several organizations within a company and have their greatest effect at the systems level Others, such as design for assembly, focus on the interaction between design and specific manufacturing processes Still others, such as failure mode and effects analysis and value engineering, can be used at all levels and in all contexts to improve both product design and the method of manufacture Related coverage is provided in the Sections "The Design Process" and "Criteria and Concepts in Design" in this Volume

Design for life-cycle manufacturing is a philosophy of product design that includes the full range of policies, techniques, practices, and attitudes that cause a product to be designed for the optimal manufacturing cost, the optimal

achievement of manufactured quality, and the optimal achievement of life-cycle support (serviceability, reliability, and maintainability) This philosophy is often implemented by companies that perceive product design to be a competitive advantage The concepts of design for assembly, concurrent engineering (see the article "Concurrent Engineering" in this Volume), and other systematic design approaches that cause the design team, from the outset, to consider all elements of the product life cycle from conception through disposal are all considered to be part of this general, overarching philosophy The philosophy is implemented both through the product realization process utilized within the company and through the general culture and attitudes embodied by the company employees

The design for assembly (DFA) method was developed by Geoffrey Boothroyd while at the University of Massachusetts (Amherst) Details of the methodology are presented in Ref 7 Design for assembly is also discussed in detail in the article "Design for Manufacture and Assembly" in this Volume Based largely on industrial engineering time-study methods, the DFA method seeks to minimize the cost of a product within constraints imposed by other design requirements This is done by first reducing the number of parts and then ensuring that the remaining parts are easy to assemble

Design for "X" (DFX) Methods. A variety of "design for" methods and approaches are being developed and used to improve the designs of parts and products with respect to specific manufacturing processes and activities Examples of methods aimed at manufacturing activities include design for service, design for testing, design for disassembly, and so forth Design for casting, design for plastic injection molding, and design for machining are examples of DFX methods directed toward improving the design of parts that are to be manufactured using a specific manufacturing process Many companies have specialized manufacturing facilities that can only process parts or assemblies that have acceptable characteristics such as part spacing or size Guidelines, developed by manufacturing engineers who are very familiar with the capability of the specific facility for use by design engineers, is another example of a DFX methodology Design for

"X" methods seek to provide guidance to the designer that helps ensure that parts and products are correctly designed to

be compatible with a given process or activity Benefits include lower tooling costs, reduced process cycle times, and improved process yields

Design for Quality. Variability is the enemy of manufacturing It is a major cause of poor quality resulting in unnecessary manufacturing cost, product unreliability, and ultimately, customer dissatisfaction and loss of market share Variability reduction and robustness against variation of hard-to-control factors are therefore recognized as being of paramount importance in the quest for high-quality products In a design for quality approach, the design team seeks to design the product and process in such a way that variation in hard-to-control manufacturing and operational parameters

is minimal The ideas behind this approach are largely attributable to the efforts of G Taguchi and the cost-saving approaches to quality control pioneered in Japan An important element of this approach is the extensive and innovative use of statistically designed experiments The Taguchi method and other design for quality approaches are described in the articles "Robust Design" and "Design for Quality" in this Volume, and they are treated extensively in the literature (see, for example, Ref 8)

Process-Driven Design. In process-driven design, a method of assembly or other manufacturing process plan is developed prior to developing the product design This plan is then used to guide the product design thereby ensuring a coordinated product and process that results in an optimization of the overall manufacturing system Process-driven design is based on the recognition that product design decisions often inadvertently limit the manufacturing options available for use in production of the product Process-driven design methods have been successfully applied in many different industries For example, many of the modern innovative manufacturing and assembly methods now commonly used in the automotive, airplane, and farm machinery industries can be traced to process-driven design practices

Failure mode and effects analysis (FMEA) is an important design and manufacturing engineering tool intended to help prevent failures and defects from occurring and reaching the customer It provides the design team with a methodical way of studying the causes and effects of failures before the design is finalized Similarly, it helps manufacturing engineers identify and correct potential manufacturing and/or process failures In performing a FMEA, the product and/or production system is examined for all the ways in which failure can occur For each failure, an estimate is made of its effect on the total system, its seriousness, and its frequency of occurrence Corrective actions are then identified to prevent failures from occurring or reaching the customer The great value of FMEA lies in its systematic approach in analyzing product and/or process performance in critical detail Also, interaction between manufacturing and design engineering required while conducting a FMEA can be very effective in helping to ensure product and process conformance at an early stage of design A more complete discussion of the FMEA approach is provided in several other articles in this Volume and in Ref 9

Value engineering provides a systematic approach to evaluating design alternatives that is often very useful and may even point the way to innovative new design approaches or ideas Also called value analysis, value control, or value management, value engineering uses a multidiscipline team to analyze the functions provided by the product and the cost

of each function Based on results of the value analysis, where value is the ratio of function and cost, creative ways are sought to eliminate waste and undesired functions and to achieve required functions at the lowest possible cost while optimizing manufacturability, quality, and delivery Value engineering is very broad in scope A disadvantage of the value engineering approach is its reliance on cost data that often cannot be accurately determined until after design decisions have been made This can detract from its usefulness as a tool for enhancing the quality of early design decisions On the other hand, when used properly, value analysis can be a very useful tool for helping to make material and process selection decisions Value engineering is widely discussed in the literature (see Ref 10, for example) See also the article "Value Analysis in Materials Selection and Design" in this Volume

Standardization. This practice seeks to limit and/or reduce manufacturing information content of products by eliminating specials and standardizing wherever possible Information content of the product and quality risks are reduced when part variations (e.g., in the types of screws used) are kept to a minimum It is seldom justifiable, for example, to use several screw sizes or drive styles in one assembly Minimizing part variations also simplifies manufacturing by reducing the information content of the production system required to produce the part Substitution of standard (off-the-shelf) components in place of special-purpose designs helps reduce part variations as well as total information content of the manufacturing system A stock item is always less expensive than a custom-made item Standard components require little or no lead time and are more reliable because characteristics and weaknesses are well known They can be ordered in any quantity at any time They are usually easier to repair and replacements are easier to find Use of standardized components puts the burden on the supplier and makes the supplier do more

Standardization and rationalization (S&R) is a combined design and business approach or philosophy that specifically targets reduction of part proliferation companywide In essence, S&R seeks to further leverage the benefits of

standardization by minimizing the number of standard parts used In the S&R approach, standardization is defined as the reduction in number of different parts used in current and former designs Rationalization is the identification of the

fewest number of parts required for use in future designs Such an approach is ideal for material selection Consider, for example, a company that buys a large variety of different thermoplastic materials In the S&R approach, each of these different plastic materials would be evaluated with respect to volume purchased per year, required properties, and other relevant considerations Based on this evaluation, a "rationalized" list is developed that is agreed upon by design and manufacturing personnel as being sufficient Thermoplastic materials used in all new designs are then selected from this short list Over time, the number of different materials used by the company decreases, saving time and money Less time

is needed to select materials, and product and process development time is reduced because the materials are well understood within the company Money is saved because fewer materials are bought in higher volume from established suppliers

Group Technology (GT) is an approach to design and manufacturing that seeks to reduce manufacturing system information content by identifying and exploiting the sameness or similarity of parts based on their geometrical shape and/or similarities in their production process Group technology is implemented by using classification and coding systems to identify and understand part similarities and to establish parameters for action Manufacturing engineers use

GT to decide on more efficient ways to increase system flexibility by streamlining information flow, reducing setup time and floor space requirements, and standardizing procedures for batch-type production Design engineers use GT to reduce design time and effort as well as part and tooling proliferation With increasing emphasis on flexible and integrated manufacturing, GT is also an effective first step in structuring and building an integrated database Standardized process planning, accurate cost estimation, efficient purchasing, and assessment of the impact of material costs are benefits that are often realized A more complete discussion of GT, especially as it applies to manufacturing, is given in Ref 11

References cited in this section

7 G Boothroyd and P Dewhurst, Product Design for Assembly, Boothroyd Dewhurst, 1989

8 P.J Ross, Taguchi Techniques for Quality Engineering, McGraw-Hill, 1988

9 J.J Hollenback, Failure Mode and Effects Analysis, Society of Automotive Engineers, 1977

10 A.E Mudge, Value Engineering, A Systematic Approach, McGraw-Hill, 1971

11 I Ham, Group Technology, Chapter 7.8, Handbook of Industrial Engineering, G Salvendy, Ed., John

Wiley & Sons, 1982

Introduction to Manufacturing and Design

Henry W Stoll, Northwestern University

References

1 J.E Ettlie and H.W Stoll, Managing the Design-Manufacturing Process, McGraw-Hill, 1990

2 M.P Groover, Automation, Production Systems, and Computer Integrated Manufacturing, Prentice-Hall,

1987, p 40

3 M.P Groover, Fundamentals of Modern Manufacturing, Prentice-Hall, 1996

4 J.A Schey, Introduction to Manufacturing Processes, McGraw-Hill, 2nd ed., 1987

5 S Kalpakjian, Manufacturing Processes for Engineering Materials, Addison-Wesley, 2nd ed., 1991

6 M Andreasen, S Kahler, and T Lund, Design for Assembly, IFS Publishing, 1983

7 G Boothroyd and P Dewhurst, Product Design for Assembly, Boothroyd Dewhurst, 1989

8 P.J Ross, Taguchi Techniques for Quality Engineering, McGraw-Hill, 1988

9 J.J Hollenback, Failure Mode and Effects Analysis, Society of Automotive Engineers, 1977

10 A.E Mudge, Value Engineering, A Systematic Approach, McGraw-Hill, 1971

11 I Ham, Group Technology, Chapter 7.8, Handbook of Industrial Engineering, G Salvendy, Ed., John

Wiley & Sons, 1982

Design for Manufacture and Assembly

Geoffrey Boothroyd, Boothroyd Dewhurst, Inc

Introduction

DURING THE 1980s AND 1990s, the United States has been losing millions of dollars per day to its foreign competitors Competitiveness has been lost in many areas, but most notably in automobile manufacture, as highlighted by a worldwide study of this industry that was published in 1990 (Ref 1) The study, which showed that Japan had the most productive plants at that time, attempted to explain the wide variations in automotive assembly plant productivity throughout the world It was found that automation could only account for one-third of the total difference in productivity among plants worldwide and that, at any level of automation, the difference between the most and least efficient plants was enormous

The authors of the study concluded that no improvements in operation can make a plant fully competitive if the product design is defective There is now overwhelming evidence to support the view that product design for manufacture and assembly can be the key to high productivity in all manufacturing industries

Design for Manufacture and Assembly

Geoffrey Boothroyd, Boothroyd Dewhurst, Inc

Introduction to Design for Manufacture and Assembly

It has long been advocated that designers should give attention to possible manufacturing problems associated with a design Traditionally, the idea was that a competent designer should be familiar with manufacturing processes to avoid adding unnecessarily to manufacturing costs Until the 1960s, the supposed solution in education was to provide "shop" courses to familiarize engineering students with the ways products are manufactured However, even this approach has now been abandoned by the colleges because the courses lacked academic content Furthermore, with the increasingly complex technology incorporated within many products, the time pressures put on designers to get designs onto the shop floor, the "we design it, you manufacture it" attitude of designers, and the increasing sophistication of manufacturing techniques, designers have become less and less able to avoid unnecessary manufacturing costs

Since the early 1980s, it has become recognized that more effort is required to take manufacturing and assembly into account early in the product design cycle One way of achieving this is for manufacturing engineers to be part of a simultaneous or concurrent engineering design team

Within this teamworking, design for manufacture and assembly (DFMA) software analysis tools help in the evaluation of proposed designs It is important that design teams have access to such tools in order to provide a focal point that helps identify problems from manufacturing and design perspectives

Use of DFMA software allows a systematic procedure that aims to help companies keep the number of component parts

in an assembly to a minimum and make the fullest use of the manufacturing processes that exist It achieves this by enabling the analysis of design ideas It is not a design system, and any innovation must come from the design team However, it does provide quantification to help decision making at the early stages of design

Figure 1 summarizes the steps taken when using DFMA software during design The design-for-assembly (DFA) analysis

is conducted first, leading to a simplification of the product structure Then, early cost estimates for the parts are obtained for both the original design and the new design in order to make trade-off decisions During this process, the best materials and processes to be used for the various parts are considered For example, would it be better to manufacture a cover from plastic or sheet metal? Once the materials and processes have been finally selected, a more thorough analysis for design for manufacture (DFM) can be carried out for the detail design of the parts

Fig 1 Typical steps taken when using DFMA software in design

It should be remembered that DFMA is the integration of the separate but interrelated design issues of assembly and manufacturing processes Therefore, there are two fundamental aspects to producing efficient designs: DFA to help simplify the product and quantify assembly costs and the early implementation of DFM to quantify parts cost and allow trade-off decisions to be made for design proposals and material and process selection

Design for Assembly

The DFA analysis tool was developed by the author in the mid-1970s The idea was to stress the economic implications of design decisions This is crucial, because while design is usually a minor factor in the total cost of a product, the design process fixes between 70 and 95% of all costs

The author and his colleague, Peter Dewhurst, developed a personal computer program for DFA that was first introduced

in 1982 Since then, the DFA database of time standards has been expanded to include the assembly of large products, cable harnesses, and printed circuit boards

Reducing the number of separate parts, thereby simplifying the product, is the greatest improvement provided by DFA To give guidance in reducing the part count, the DFA software asks the following questions as each part is added to the product during assembly:

• Is the part or subassembly used only for fastening or securing other items?

• Is the part or subassembly used only for connecting other items?

If the answer is "yes" to either question then the part or subassembly is not considered theoretically necessary If the answer is "no" to both questions the following criteria questions are then considered

• During operation of the product, does the part move relative to all other parts already assembled?

• Must the part be of a different material than, or be isolated from, all other parts already assembled? Only fundamental reasons concerned with material properties are acceptable

• Must the part be separate from all other parts already assembled because the necessary assembly or disassembly of other separate parts would otherwise be impossible?

If the answer to all three criteria questions is "no," the part cannot be considered theoretically necessary

When these questions have been answered for all parts, a theoretical minimum part count for the product is obtained It should be emphasized, however, that this theoretical minimum does not take into account practical considerations or cost considerations, but simply provides a basis for an independent measure of the quality of the design from an assembly viewpoint

While answering these minimum part questions, the design team is challenged to justify the existence of each separate part and this is where brainstorming usually results in considerable product simplification In fact, it is found from more than 70 published case studies that the average reduction in the number of parts is 50% as a result of using the DFA software

Estimating Assembly Time. The next step is to estimate the assembly time for the product design and establish a DFA index in terms of difficulty of assembly

To estimate assembly time, each part in the design is examined for two considerations: how the part is to be acquired (fetched if necessary), oriented, and made ready for insertion; and how it is inserted and/or fastened into the product

The difficulty of these operations is rated, and from this rating standard times are determined for all the operations necessary to assemble each part The DFA time standard is a classification of design features that affect part assembly It

is a system for use in product design similar to the standard time systems used by industrial engineers such as MTM, WorkFactor, or MOST and was developed from 12 years of industry and university experiments Usage has proved the data to be quite accurate for the overall times that are generally within 6% of the actual times

For estimation of the handling time the designer must specify the symmetry of the item, its major dimension, and its thickness In addition, the designer must specify whether the item nests or tangles when in bulk, whether it is fragile, flexible, slippery, sticky, and whether it needs two hands, grasping tools, optical magnification, or mechanical assistance

in the form of cranes

A portion of the database for estimation of the time for insertion of small items is shown in Fig 2 Here it is important to know whether the assembly worker's vision or access is restricted, and whether the item is difficult to align or insert Other portions of the database deal with resistance to insertion, and whether the item requires holding down in order to maintain its position for subsequent assembly operations

Fig 2 Portion of database of manual insertion times (in seconds) without fastening for small parts where no

holding down or regrasping is required Copyright Boothroyd Dewhurst, Inc.; used with permission

For fastening operations, further questions may be required For example, the type of tool used for threaded fasteners and the number of revolutions required are important in a determination of the total fastening time A further consideration relates to the location of the items that must be acquired If turning, bending, or walking are needed to acquire the item, a different database for acquisition and handling is used

Thus, for each classification of handling and insertion, an average time is given leading to an estimate of the total manual assembly time for an item

Design Efficiency (Ease of Assembly). When all the items and operations for assembly of a product have been analyzed, the total assembly time for the product can be estimated, and using standard labor rates, so can assembly costs Also, the efficiency of a design from an ease of assembly point of view can be determined; this is the DFA index

Based on the assumption that all of the critical parts could be made easy to assemble requiring only about 3 s each the minimum assembly time is equal to the theoretical minimum number of parts times three The assembly efficiency percentage or DFA index is equal to the minimum assembly time divided by the estimated total assembly time multiplied

by 100

The maximum value of the DFA index is 100% However, excellent designs can have a DFA index much lower For example, good designs of small electromechanical assemblies have values of around 20% Thus, much depends on the type of assembly and, with experience, the values that are achievable in different circumstances will be known

At the DFA stage, part manufacturing costs are not brought into the analysis, but the DFA index and the estimated assembly times provide benchmarks against which further design iterations, previous estimates for an original product design, or a competitor's product can be compared

Other Assembly Analysis Methods

Assemblability Evaluation Method (AEM). The AEM method, described in 1986 by Miyakawa and Ohashi (Ref

2), uses two indices at the earliest possible stage of design, namely the assembly-evaluation score, E, which is used to assess the design quality or the difficulty of assembly, and the assembly-cost ratio, K, which is used to project assembly

costs relative to current assembly costs The method does not distinguish between manual, robot, or automatic assembly, because according to Myakawa and Ohashi there is a strong correlation between the degrees of assembly difficulty using these three methods

In the AEM, approximately 20 symbols represent the various assembly operations Each symbol has an index to assess the ability to assemble the part under consideration Examples of the symbols and penalty scores are given in Table 1, and examples of their application are given in Table 2

Table 1 Examples of AEM symbols and penalty scores

Elemental operation AEM symbol Penalty score

Downward movement(a) 0

Source: Ref 3

(a) As in a pressing operation

Table 2 Examples of assemblability evaluation and improvement

Product structure Assembly

operations

Ability to assemble part evaluation

score

Ability to assemblability evaluation

Fig 3 Example of subassembly evaluation using assembly-oriented product design method FM, functional

content of procedure; AM, assembly expenditure; KM, characteristic value of procedure (KM = FM/AM)

Characteristic value of subassembly (K) is 0.250 Arrows denote weak technical aspect of assembly Source:

Ref 4

Lucas Method. The Lucas method was developed at the University of Hull, United Kingdom, during the late 1980s (Ref 5) In the Lucas method, three steps are followed, as described below

(required by that particular design solution) A target is set for design efficiency, which is A/(A + B) and is expressed as a percentage The objective is to exceed an arbitrary 60% target value by the elimination of category B parts through redesign The Lucas method authors emphasize assembly-cost reduction and parts-count reduction and include the use of the present author's minimum-parts criteria in a "truth" table to assist in part-count reduction

the part, handling difficulties, and the orientation of the part The score is summed to give the total score for the part, and

a handling/feeding ratio is calculated that is given by the total score divided by the number of A parts A target of 2.5 is recommended

Step 3: Fitting Analysis. Based on the proposed assembly sequence, each part is scored on the basis of whether it requires holding in a fixture, the assembly direction, alignment problems, restricted vision, and the required insertion force The total score is divided by the number of A parts to give the fitting ratio Again, it is recommended that this ratio approach 2.5 for an acceptable design

DAC Method. Sony Corporation claims to have developed a unique set of rules for increased productivity involving design-for-assembly cost effectiveness (DAC) Yamigawa (Ref 6) reiterates the view that it is impossible to design for assembly ease unless one starts at the time of conception before the blueprint for the product is drawn up The improvement of a design at its inception is referred to as the concept of feed-forward design, as opposed to making improvements later with feedback from the manufacturing process

In the DAC method, factors for evaluation are classified into 30 key words The evaluations are displayed on a diagram using a 100-point system for each operation, thus making judgment at a glance easy A list of operations is presented on the DAC diagram, and a bar is drawn that represents the score for that particular operation (Fig 4) Operations with low

scores are easily identified Since 1987, DAC has been introduced in various companies in Japan and overseas Emphasis

is given to the ease with which an operation can be carried out automatically, and the method is used to illustrate problems with the efficiency of the assembly system

Fig 4 Example of part improvement using design-for-assembly cost effectiveness (DAC) method Source: Ref 6

Design for Manufacture (DFM)

Design for assembly has generated a revolution in design practices, not principally because it usually reduces assembly costs, but because it has a far greater impact on the total manufacturing costs of a product The reason is that DFA simplifies the product structure, reduces the number of parts, and thereby reduces the total cost of the parts However, to judge the effects of DFA at the early design stage, companion methods for the early estimation of part costs must be made available, and accordingly, many of those who have developed DFA methods are now turning their attention to methods

of assessment of part-manufacturing difficulties

For example, the Hitachi researchers (Ref 7) have introduced a machining producibility evaluation method, which combined with their AEM, gives an overall producibility-evaluation method (PEM)

Similarly, Toshiba Corporation (Ref 8) has developed a processability evaluation method, which combined with other methods, including an assemblability-evaluation method, provides an overall producibility-evaluation method Processability is defined as being proportional to the cost of the part The cost of the part is determined by the selection of the part-processing method, and then by the design of the part shape Various processing methods are considered for a particular part The cost of the part is then determined for all combinations of the selected processing methods and suitable materials Then the design of the part is evaluated to see whether it fits a particular processing method, and finally, a processability evaluation is carried out

Since 1985, the author and his colleagues Dewhurst and Knight have developed methods for designers to obtain cost estimates for parts and tooling during the early phases of design Studies have been completed for machined parts (Ref 9), injection-molded parts (Ref 10), sand-cast, investment-cast and die-cast parts (Ref 11, 12, 13), sheet-metal stampings (Ref 14), and powder-metal parts (Ref 15) The objective of these studies was to provide methods with which the designer or

design team can quickly obtain information on costs before detailed design has taken place For example, an analysis (Ref 16) of an injection-molded heater cover gave the results shown in Fig 5 It was evident that certain wall thicknesses were too large, and that, through some fairly minor design changes, the processing cost could be reduced by 33% If these studies had taken place at the early design stage, the designer could also have considered the cost for an equivalent sheet-metal part, for example In fact, the use of these analysis techniques is now allowing designers and purchasing managers

to challenge suppliers' estimates In one example, it has been reported that Polaroid Corporation has saved $16,000 to

$20,000 on the cost of tooling for an injection-molded part (Ref 17)

Number of cavities required 6 2

Cost of production mold $36,383 $22,925

Cost per part (a) $0.251 $0.168

(a) Includes $0.05 for material

Fig 5 Design-for-manufacture analysis of injection-molded heater cover Source: Ref 16

Early Cost Estimating. The problem of estimating part and tooling costs before the part has been fully detailed is discussed using machining as an example because this is one of the most common shape-forming processes Several

conventional cost estimating methods for machining are available both in handbook form, such as the Machining Data Handbook (Ref 18) and the AM Cost Estimator (Ref 19) and in software form However, all of these methods are meant

to be applied after the part has been detailed and its production has been planned, and they are not tailored for use by a designer During the early stages of design, the designer will not wish to specify, for example, all the work-holding devices and tools that might be needed a detailed design will not yet be available Indeed, a final decision even on the work material might not have been made

For early cost estimating an important assumption has to be made The designer should be able to expect that, when the design is finalized, care will have been taken to avoid unnecessary manufacturing expense at the detail-design stage and that manufacturing will take place under efficient conditions

To illustrate how such an assumption can help in providing reasonable estimates, consider the effect of the metal-removal rate on grinding costs, as shown in Fig 6 These cost curves indicate that as the removal rate is increased the cost of grinding-wheel wear increases in proportion At the same time, the cost of grinding decreases because the grinding cycle

is shortened; in fact, the grinding costs are inversely proportional to the removal rate

Fig 6 Effect of metal removal rate on grinding costs

Under these circumstances, it is easy to show that the minimum cost condition occurs when the grinding costs and wheel costs are equal Hence, we can say that if economic grinding conditions are used in the manufacture of a part, wheel costs can be allowed for by doubling the estimated grinding costs Even if economical conditions cannot be used say, because

of power limitations it is still possible to adjust the grinding costs to allow for wheel costs This simple example illustrates the general approach to early cost estimating

In estimating machining costs, perhaps the simplest method would be to specify the shape and size of the original workpiece and the quantity of material to be removed by machining An estimate could then be made of the material cost needed to manufacture the part, and if an approximate figure were available for the average cost of removal of each unit volume of material by machining, an estimate could also be made of the machining cost Even the tool-replacement costs could be allowed for