ASM Metals Handbook - Desk Edition (ASM_ 1998) Episode 12 pot

The fracture surface contains vestigial marks that indicate the amounts of high-energy ductile, or tough and low-energy brittle crack extension that produced the fracture.. Fatigue Stria

Stereo Views

Stereo-pair photographs of fracture surfaces provide a means of viewing the fracture contours in simulated three dimensions The basic technique for preparing such photographs entails taking two pictures of a subject area, the second from an angle slightly different from the first The photographs are then examined under a visual condition in which, for example, the viewer's left eye focuses on the first picture and his right eye focuses on the second Stereo viewers are available, and in most cases are necessary, to ease the task of viewing stereo pairs The effect is to convince the brain that the eyes are indeed seeing a three-dimensional scene If the angular displacement between the two pictures is appropriate (an included angle of 12° or 14° is desirable), the illusion is very vivid

Stereo images by light microscopy have been used only to a limited extent because of the restricted depth of field They provide a useful means of studying fractures at magnifications generally not greater than 200×

Stereo-pair photographs can be taken using a single-lens camera, if it is provided with a mount that will pivot about a horizontal axis through the subject After the subject is properly aligned beneath the camera with the mount vertical, the camera is swung to an angle of 6° to 7° from vertical and an exposure is made The second exposure is made with the camera at the same angle as for the first exposure, but swung to the other side of the vertical, or zero, position

With the stereomicroscope shown in Fig 3, it is possible to observe the voids that are a part of ductile dimpled fracture, and, in brittle fractures, cleavage facets and some of the river marks can be discerned It is most useful for preliminary examination of fracture surfaces, leaving final documentation of fine details for scanning electron microscopy

Fig 3 Optical stereomicroscope with camera box partly removed for direct viewing The light source is two 40

watt incandescent tubes above the specimen

An example of a light-microscope stereo pair is shown in Fig 4 The subject is a fracture surface in a low-carbon steel casting that cracked along prior austenite grain boundaries during a straightening operation The bold relief contours of the columnar fracture are evident when viewed in stereo

Fig 4 Stereo view of a fracture surface of cast experimental low-carbon steel The smooth columnar contours

in the lower portions of the photographs were the result of cracking along prior austenite grain boundaries during straightening The upper portions of the photographs also show intergranular fracture, but along ferrite grain junctions, produced by impact in the laboratory to expose the original crack surface Light fractograph (stereo pair) 3×

If a tool such as a parallax bar is used, quantitative measurements of topographic depths and elevations can be obtained

Fractographic Features Revealed by Light Microscopy

THE MACROSCOPIC APPEARANCE of a fracture surface has often been used to appraise the degree of ductility and

of toughness present in a metal According to the concepts of fracture mechanics, toughness is the critical material property associated with overload or rapid fracture The fracture surface contains vestigial marks that indicate the amounts of high-energy (ductile, or tough) and low-energy (brittle) crack extension that produced the fracture

Tensile-Fracture Surface Marks in Unnotched Specimens

Tensile-fracture surface marks have been classified into three zones by configuration: the fibrous zone, the radial zone, and the shear-lip zone This shear-lip zone is the highest-energy portion of the fracture The relative amount of shear lip provides an indicator of the toughness of the material The three zones are shown in Fig 5 Fractures consisting solely of one zone occur only under conditions of extreme ductility or brittleness

Fig 5 Tensile-fracture surface marks (a) Schematic representation of zones within a typical tensile fracture of

an unnotched cylindrical specimen The surfaces of the fibrous and radial zones are usually normal to the tensile axis, as shown The shear-lip surface is always at about 45° to the tensile axis (b) Fracture surface of a

4340 steel tensile specimen showing fibrous zone (A), radial zone (B), and shear lip (C) 3×

Fatigue Marks

The formation of cracks under conditions of repeated or cyclic stress has been denoted as fatigue cracking Zones of crack propagation on fatigue fractures exhibit several types of surface marks, such as beach marks, striations, and ratchet marks

Beach Marks The term "beach mark" describes the macroscopic features present on the fracture surface as illustrated in

Fig 6 Beach marks indicate a local region of variation in crack-growth rate Such beach marks may, but do not always, indicate fatigue as the mode of cracking Stress-corrosion fractures may also show beach marks

Fig 6 Fatigue-fracture surface of an AISI 1050 shaft (35 HRC) subjected to rotating bending Numerous

ratchet marks (small shiny areas at surface) indicate that fatigue cracks were initiated at many locations along

a sharp snap-ring groove The eccentric pattern of oval beachmarks indicates that the load on the shaft was not balanced The final rupture area is near the left (low stress) side, where there may have been no fatigue action

The presence of beach marks is fortuitous, at least for the investigator, because beach marks permit the origin to be easily determined and provide the analyst with other information concerning the manner of loading, the relative magnitude of the stresses, and the importance of stress concentration (see Fig 6)

Striations The term "striation" refers to a "line" on the fracture surface indicating the position of a crack front after an

increment of crack propagation has occurred Each increment of propagation is due to a cycle of stress, i.e., a cyclic load The distance between striations indicates the advance of the crack front during each succeeding cycle Fatigue striations are not readily resolvable with the light microscope and are best viewed using a scanning electron microscope (see the section "Interpretation of Scanning-Electron Microscope Fractographs" )

Ratchet marks are macroscopic features that may be seen on fatigue fractures in shafts and flat leaf springs, and they

may also occur in ductile fractures in overtorqued fasteners In fatigue fractures, ratchet marks are the result of multiple fatigue-crack origins, each producing a separate fatigue-crack zone (Fig 6) As two approaching cracks meet, a small step

is formed The small steps are the ratchet marks

Although ratchet marks are most apparent on the peripheries of fractures in shafts, the stepped appearance is characteristic whenever fatigue cracks emanate from several origins and subsequently meet to form one principal crack front

Discontinuities

Fractures originate from a broad variety of discontinuities within the metal structure, such as laps and seams from primary metal forming, shrinkage and gas cavities in cast structures, hot tears, inclusions, segregation of impurities, and imperfections in welds Many of these features are illustrated in the section "Discontinuities Leading to Fracture" in the article "Use of Fractography for Failure Analysis."

Interpretation of Optical Fractographs

A RUDIMENTARY KNOWLEDGE of how to "read" a fracture surface must be gained so that meaningful fractographs can be taken from which to describe the fracture process Fractography can provide information about the conditions of stress, the effects of temperature and chemical environment, and the origin of the fracture and how the crack progressed to final rupture

States of Stress

Information regarding the stresses that caused a fracture can be learned from a casual examination of the fractured part

In many types of fracture, the general plane of fracture is perpendicular to the maximum principal tensile stress These types, called group I fractures in the following discussion, include cleavage and other brittle fractures, ductile fractures (also called microvoid coalescence and dimpled rupture) under plane-strain conditions (in thick sections), fatigue fractures (at least in the intermediate stages), and stress-corrosion cracks

Other types of fracture propagate along planes of maximum shear stress These types, called group II fractures here, include ductile fractures under plane stress (that is, in thin sections or near free surfaces), shear fractures, and the very early stages of fatigue fractures in pure or relatively impurity-free metals In a ductile material, the shear stresses cause considerable deformation prior to fracture, although the deformation is not always obvious because the shape of the part is not changed except for flow on the surface Figure 7(a) is a photograph showing deformation in a fractured shaft Torsional single-overload fractures (group II) of a ductile material usually occur on the transverse shear plane, straight across the cylinder, and exhibit a telltale swirled appearance (Fig 7b) The final-fracture area will be at the center of the bar A brittle material in pure torsion will fracture in a plane perpendicular to the tensile-stress component, which is 45° to

the specimen axis (group I fracture) A spiral-type fracture is one characteristic of this type of loading and material and can be demonstrated by twisting a piece of chalk to fracture (Fig 8) The elastic-stress distribution in pure torsion is maximum at the surface and zero at the center Thus, fracture normally originates at the highest-stressed region (the surface) in pure torsion Longitudinal torsional fractures are sometimes observed (for example, Fig 9) because longitudinal planes have the same magnitude of shear stress as transverse planes, and longitudinal planes usually have lower toughness, due to the shape and distribution of inclusions

Fig 7 Splined shaft of 6118 steel that fractured from a single torsional overload (a) Photograph ( 2×) of the

shaft showing the deformation of the splines in the region of fracture, which would not occur if the fracture were caused by fatigue The shaft, 28 mm (1 in.) in diameter, was made of 6118 steel and had a hardness

of 23 HRC Being made of a ductile metal, it was twisted in pure torsion with a single overload, yielding the fracture on the transverse shear plane shown in (b) (b) Fracture surface of the shaft, showing the rotary deformation characteristic of a single-torsion-overload fracture in a ductile metal If there is a combined bending component, the region of final, fast fracture will be offset from the center of the section This type of fracture should not be confused with one resulting from rotating-bending fatigue, which does not have the gross distortion seen here Light fractograph 2×

Fig 8 Torsional brittle fracture of chalk Fracture follows the 45° direction of maximum tensile stress

Fig 9 A torsional-fatigue fracture in an induction-hardened 1037 steel shaft 25 mm (1 in.) in diameter that

finally fractured in longitudinal shear No clear point of origin is visible because the surfaces rubbed as the crack propagated Light fractograph 0.95×

Crack Origins

An interest in the exact location of the point of origin of a fracture derives from the importance of determining what initiated the fracture The initial examination of a fracture is concerned with the recognition of all features that may point

to the crack origin

Gross Aspects of Fractures Some indications of crack-propagation direction can be seen by examination of the

gross aspects of a broken part They relate to the order in which events occured, sometimes called "fracture sequencing." The fragments of a fractured structure can be reassembled in approximate juxtaposition, without allowing the fracture surfaces to touch, and then the telltale indications should be sought

First, a fast-running crack in sheet or plate will frequently branch as it propagates but will almost never join another crack

to continue as a single crack Second, if a running crack joins a pre-existing fracture, it will usually meet it at approximately a 90° angle, not at a shallow angle Third, it is almost impossible for an intersecting crack to cross and propagate beyond a pre-existing fracture These considerations lead to the following useful guidelines concerning crack origins:

• The direction to the crack origin is always opposite to that of crack branching, as shown in Fig 10

• If a crack meets another at about 90°, it occurred later and the origin should not be sought in it but in the earlier crack This is known as the T-junction method of crack-origin location (Fig 11)

Fig 10 Schematic representation of the information conveyed by crack branching with regard to the location of

the crack origin

Fig 11 Schematic representation of the T-junction method of determining which fracture surface to search to

locate the crack origin Because B does not cross A but meets it at about 90°, B occurred later and cannot contain the crack origin

The initial section of fracture (containing the crack origin) transfers its original load to adjoining sections, in all probability overstressing them If these sections do not contain imperfections, succeeding fractures (assuming a normally ductile material) will be preceded by a certain amount of plastic deformation

Fibrous marks, tear ridges, and beach marks can also indicate the location of a crack origin Chevron markings also can be used Where the curvature of such marks is slight, the origin is generally on the concave side of the crack-front curve

In general, the region of crack initiation will be flat and will lack any free-surface shear-lip zone The shear-lip zone appears only at some distance from the origin and becomes larger as the distance increases (see Fig 12)

Fig 12 Fracture in a welded pressure vessel of 4340 steel displaying a flat origin at the top with a shear lip

beginning on either side of it The shear lip increases in width with increasing distance from the origin The radial marks below the origin and the chevron patterns to the left and right also indicate the directions of fracture Light fractograph 6×

Location of Origins in Impact-Overload Fractures Figure 13 shows the fracture of a 12% Cr steel bar that was

notched and then struck with a hammer Two blows were necessary to complete the fracture The fracture marks are radial They may be traced downward to a common intersection Also present are crack arrests, one at A and a second at

B, where fracture progress came to a complete halt before the second blow was struck The arrest marks are parallel to the crack front, and lines drawn normal to them should intersect at or near the origin The contours of the final fracture marks,

at C and D, also point to the general location of the beginning of fracture

Fig 13 Locating the origin in an impact fracture, produced by two hammer blows, in a notched bar of 12% Cr

steel Fracture origin can be found in three ways: by tracing the radial marks in the lower portion of the fracture to their point of convergence (the arrows on the curved lines indicate the direction of crack propagation); by drawing normals to the crack-arrest fronts labeled A and B; and by projecting the tangents to the final radial marks at C and D toward the bottom The crack came to a full stop at B with the first hammer blow and resumed motion at the second hammer blow Light fractograph 3×

Fracture Progress

Many types of fractures, including most service fractures, occur by a sequence involving crack initiation, subcritical crack propagation (due to ductile crack extension, fatigue, corrosion fatigue, stress-corrosion cracking or hydrogen embrittlement), and fast fracture, which occurs when the remaining cross section can no longer support the applied load The fracture processes leave telltale marks on the fracture surfaces, which enable a trained investigator to locate the initiation sites, to discern the propagation direction and crack-front shape, and to distinguish the fast-fracture zone This information can lead to an understanding of the stress levels and conditions leading to fracture

Fracture Changes During Crack Propagation

Several influences may affect the growth of a crack, causing it to progress thereafter by a mechanism of fracture different from that in effect when cracking started These influences include local differences in microstructure; changes in stress-

intensity factor, K; changes in chemical or thermal environment: differences in stress state

Changes Caused by Local Differences in Structure Microstructure exerts a pronounced influence on local

fracture appearance The presence of two or more types of microstructure may result in different fracture mechanisms being involved and a different fracture appearance A simple example is a fracture in a chilled white iron part Fracture is

by cleavage through the chilled zone and is fibrous in the pearlitic zone

Another structure difference is that of case and core in carburized, flame-hardened, and induction-hardened parts The difference in properties between such structures can cause a crack to proceed by quite different fracture mechanisms in adjacent regions

Changes Caused by Altered Environments A fracture-mechanism change as a result of different chemical and

stress environmental conditions is shown in Fig 14 A corrodent generated small pits below a layer of chromium plate and provided the environment for the growth of stress-corrosion cracks, which originated at the pits The stress may have been residual or applied As the stress-corrosion cracks grew, the stress intensity at the crack tip increased for the applied cyclic loads At some critical level of environment and cyclic-stress intensity, the fracture mechanism changed to one of fatigue The fatigue cracks propagated until the critical crack-tip stress-intensity values were reached, and then unstable fracture occurred in an essentially ductile manner

Fig 14 Changes in fracture mechanism and appearance that were caused by changes in chemical and stress

environment for a chromium-plated aluminum alloy 7079-T6 forging Small corrosion pits formed beneath the layer of chromium plate, as at A, and generated stress-corrosion cracks B Growth of these cracks altered the stress intensity at the crack tips, leading to propagation of fatigue cracks C Final, fast fracture D occurred when the critical crack-tip stress-intensity value was reached Light fractograph 5.7×

Interpretation of Scanning-Electron Microscope Fractographs

AT LOW MAGNIFICATIONS, the features in scanning electron microscope (SEM) fractographs strongly resemble the aspects of the fracture apparent to the naked eye; but at high magnifications, more detail is visible which needs to be categorized and interpreted if the fractograph is to be related to the micromechanisms of fracture that were active

It is important to realize that microscopic features of fractures ordinarily differ widely within a small area The principal categories of fracture features (or fracture modes) are as follows:

• Cleavage features (tongues, microtwins, and location of cleavage-crack origins)

• Quasicleavage features

• Dimples from microvoid coalescence

• Tear ridges

• Fatigue striations

• Separated-grain facets (intergranular fracture)

• Mixed fracture features, including binary combinations of cleavage features, dimples, tears, fatigue striations, and intergranular-fracture features

• Features of fractures resulting from chemical and thermal environments

Transgranular Cleavage Features

In cleavage fracture, the fracture path follows a transgranular plane that is usually a well-defined crystallographic plane This plane of fracture is one of the {100} planes in most body-centered-cubic metals Cleavage fracture is produced, usually at low temperature, under a condition of high triaxial stress that is, at the root of a notch or at a high deformation rate, as, for example, by impact loading, or as a result of environmental factors

Figure 15 provides three views, at increasing magnification, of an area in an impact fracture exhibiting features that are typical of cleavage It is apparent that the fracture plane changes orientation from grain to grain As a result, the average grain size can be measured on the fractograph and related to grain-size measurements on a metallographic section The change of orientation from grain to grain leads to a branching of the crack along different planes and to a very chaotic overall appearance of the fracture surface At higher magnification, many features typical of cleavage can be identified In Fig 15(b), the evidence of change in orientation between grain A and grain B is particularly clear because of the river patterns that begin in grain B at the interface The river patterns, which represent steps between different local cleavage facets of the same general cleavage plane, are well defined

Fig 15 Cleavage fracture in a notched impact specimen of hot-rolled 1040 steel broken at -196 °C (-321 °F),

shown at three magnifications The specimen was tilted in the scanning electron microscope at an angle of 40°

to the electron beam The cleavage planes followed by the crack show various alignments, as influenced by the orientations of the individual grains Grain A, at the center in fractograph (a), shows two sets of tongues (see arrowheads in fractograph b) as the result of local cleavage along the {112} planes of microtwins created by plastic deformation at the tip of the main crack on {100} planes Grain B and many other facets show the cleavage steps of river patterns The junctions of the steps point in the direction of crack propagation from grain A through grain B, at an angle of about 22° to the horizontal plane The details of these forks are clear in fractograph (c)

Quasicleavage Features

In steels that have been quenched to form martensite and then tempered to precipitate a fine network of carbide particles, the size and orientation of the available cleavage planes within a grain of prior austenite may be poorly defined True cleavage planes have been replaced by smaller, ill-defined cleavage facets, which usually are initiated at carbide particles

or large inclusions The small cleavage facets have been referred to as quasicleavage planes, because, although they look like cleavage planes with river patterns radiating from the initiation sites, until recently they have not been clearly identified as crystallographic planes Quasicleavage features tend to be more rounded, indicating a somewhat higher energy absorption than that of true cleavage

Quasicleavage facets on a fracture surface of a quenched and tempered 4340 steel specimen broken by impact at -196 °C (-321 °F) are shown in Fig 16 The poorly defined cleavage facets are connected by tear ridges and shallow dimples

Fig 16 Quasicleavage in the surface of an impact fracture in a specimen of 4340 steel The same area is shown

in both fractographs, but at different magnifications The small cleavage facets in martensite platelets contain river patterns and are separated by tear ridges Shallow dimples, marked by arrowheads, are also visible Direction of crack propagation is from bottom to top in each fractograph The specimen was heat treated for 1 h

at 843 °C (1550 °F), oil quenched, and tempered for 1 h at 427 °C (800 °F) Fracture was by Charpy impact at -196 °C (-321 °F)

Quasicleavage, or cleavage in complex microstructures, is more difficult to identify than the cleavage found in carbon steel made up of ferrite and pearlite When identification is uncertain, it is essential to relate the fracture features

low-to the microstructure, including the prior austenite grain size, the martensite plate size, and the distribution, size, spacing, and volume fraction of fine carbide particles precipitated during tempering

Dimples Formed by Microvoid Coalescence

If the temperature of fracture is raised from very low (for example, liquid-nitrogen temperature) to higher levels, the fracture mechanism changes from brittle or cleavage fracture to ductile fracture by microvoid initiation, growth, and coalescence At low magnification, the fracture surface may exhibit both fibrous and cleavage regions Fibrous regions are usually observed near the free surface in a shear-lip zone or at the origin of fracture at the center of a smooth (unnotched) tensile specimen Cleavage regions are typically observed in the surfaces of flat, plane-strain fractures and in regions of high crack velocity However, at high magnification, the fibrous-fracture region, and also the cleavage region, may show successive fracture mechanisms that is, the fracture features may include both cleavage facets and dimples Both dimples and cleavage facets are visible in the Charpy impact fracture shown in Fig 17 An overall view of the region of transition between ductile and cleavage fracture is shown in Fig 17(a) The upper portion of the fracture surface

in this view is a result of cleavage, and the lower portion is a result of fracture by microvoid coalescence The cleavage facets in Fig 17(a) and 17(b) are at a considerable distance from the notch, which is below the region shown in the fractograph In the intermediate-magnification view of the region of microvoid coalescence (Fig 17c), facet A cannot be positively identified as a cleavage-fracture feature; it could be the result of a fracture along a grain boundary, or even possibly a microvoid surface that has stretched Around this facet is a region of ductile fracture, which originated at the interface between the matrix and a carbide particle (in the deep dimple)

Fig 17 Dimples and cleavage facets exhibited in three aspects of a Charpy impact fracture at room

temperature in a specimen of hot-rolled 1040 steel, tilted in the scanning electron microscope at an angle of

30° to the electron beam The machined notch of the specimen was below the region shown in (a) The overall direction of crack propagation was upward Although equiaxed dimples pre-dominate, certain grain orientations near the top of (a) were unfavorable for ductile fracture by microvoid coalescence and local cleavage occurred,

as shown in detail in (b), which is a higher-magnification view of the outlined area in (a) Fractograph (c), a higher-magnification view of the region at the center of (a), shows a deep dimple, which initiated the local ductile fracture immediately surrounding it The smooth surface at A shows no river patterns and should not be identified as a cleavage facet; it could be a grain-boundary surface, or perhaps a region of stretching

Tearing

Tearing designates a mechanism of local fracture that is often found at a discontinuity in the crack advance by another fracture mechanism It occurs when small regions or ligaments fracture by plastic flow or necking Tearing is frequently observed when small unbroken areas remain behind the main crack front The occurrence of tearing is accompanied by the formation of tear ridges, which are typically sharp and thus produce bright contrast in the SEM image Tearing may also produce flat-topped, featureless areas having some of the characteristics of local glide-plane decohesion, similar to facet A in Fig 17(c)

Fatigue Striations

The advantages offered by the use of the SEM include:

• Easier identification and evaluation of the origin of the fatigue fracture, whether near a free surface, at

an edge, or at the bottom of a notch or a groove

• Better differentiation between stages I and II of fatigue-fracture progress by viewing the overall fracture

at low magnification not obtainable with a transmission electron microscope

• Estimates of crack-growth rates, which are used in fracture mechanics evaluation of loads or for estimation of total number of cycles to failure

• Simpler quantitative analysis of fracture surfaces to determine which portions of fracture surfaces resulted from microvoid coalescence, from intergranular separation, and from cleavage fracture

The main disadvantage of scanning electron microscopy for the investigation of fatigue fractures is that fatigue striations are not as sharply defined as with transmission electron microscopy This lack of resolution occurs because a striation represents only a small surface displacement, which often fades out in the electron image By shadowing the fracture surface with a gold-palladium film 10 nm (0.4 in.) thick, it is possible to enhance striation contrast markedly Striation spacings as small as 25 nm (1.0 in.) have been measured in aluminum alloys with the SEM With scanning electron microscopes that have resolutions better than 10 nm, spacings as small as 10 nm can be resolved

It is desirable to reveal as much as possible of the surface details within all secondary cracks associated with fatigue-crack branching This can best be achieved with the specimen oriented with the crack-propagation direction pointing toward the secondary-electron collector to gain maximum penetration of the primary electron beam into the depth of the secondary cracks

The SEM fractographs in Fig 18 and 19 show some of the characteristic features of fatigue-fracture surfaces

Fig 18 Transition from stage I to stage II of a fatigue fracture in a coarse-grain specimen of aluminum alloy

2024-T3 The transition from stage I (upper left) to stage II is well defined The presence of (Fe.Si)-rich inclusions did not affect the fracture path markedly The inclusions, which were fractured, range from 5 to 25

m (200 to 1000 in.) in diameter The stage II area shows a large number of approximately parallel fatigue patches containing very fine fatigue striations that are not resolved at this magnification

Fig 19 Fatigue fracture in type 304 stainless steel tested at room temperature The vertical secondary cracks

in (a) are grain-boundary separations The well-defined striations in (b) resulted from the planar slip characteristic of stainless steels

Intergranular Fracture

Intergranular fracture (or decohesive rupture) is simply described as grain-boundary separation It can occur by catastrophic brittle separation, or by separation plus microvoid coalescence on the interfaces of grains Such fractures are regarded as the result of a severe reduction in grain-boundary energy by a Gibbsian (thermodynamic) adsorption mechanism In its simplest form, segregation of metallic or gas-metal impurities can alter the grain-boundary free energy Furthermore, grain-boundary energy can vary over a temperature range, leading to brittle-ductile fracture transitions and thermally induced brittle or ductile fractures Variations in segregation at grain boundaries can also lead to mixed mechanisms of fracture, characterized by the appearance of dimpled areas in an intergranular fracture Intergranular brittle fracture in the absence of an aggressive environment may be the result of segregation of a thermally activated impurity, which allows the grains to separate along smooth interfacial planes Typical examples of fractures involving grain-boundary segregation are shown for tungsten, iridium, and a tungsten-rhenium alloy in Fig 20

Fig 20 Intergranular brittle fractures in tungsten, iridium, and a tungsten-3 wt% rhenium alloy (a) Sintered

tungsten rod drawn to 1.5 mm (0.060 in.) diam, recrystallized for 100 h at 10 -6 torr and 2600 °C (4712 °F), and fractured in tension (b) Iridium sheet annealed for 50 h in purified helium at 1700 °C (3092 °F) and broken by bending (c) Tungsten-3 wt% rhenium alloy that was prepared in the same manner as the sintered tungsten rod in fractograph (a) Microvoids ("bubbles") at grain boundaries resulted from segregation of potassium (an impurity)

Features Indicative of Mixed Mechanisms of Fracture

A fracture that occurs by operation of two or more intermingled mechanisms of fracture is generally labeled a mode" fracture This is not to be confused with the successive operation of different fracture mechanisms, which can be analyzed sequentially and therefore require no special discussion The occurrence of fracture by mixed mechanisms often indicates that the usual factors that determine the operative mechanism, such as state of stress, loading history, microstructure, and environment, favor both mechanisms; and that the local fracture mechanism is determined by a combination of deviations in these factors and the influence of secondary variables, such as local grain orientation

"mixed-The reasons for mixed mechanisms of fracture are as diverse as the fractures in which they have been observed However,

it is useful to identify the individual fracture mechanisms that contribute to such mixtures and to establish the circumstances that can lead to their individual occurrence These circumstances establish limits The occurrence of mixed mechanisms of fracture usually indicates that interacting influences have caused the fracture to depart from either limiting mechanism

Mixed fracture mechanisms can result from quite different causes, and caution must be exercised in inferring the cause of fracture from the fracture features alone

Features of Fractures Resulting from Chemical and Thermal Environments

For a specific metal or alloy, certain fracture mechanisms, such as cleavage, microvoid coalescence, fatigue, and intergranular separation, are often associated with particular environmental and stress states The accumulation of experience with such fractures arising from known states of environment and stress makes it possible to ascertain, with the aid of fractography, the causes of fractures occurring under unknown service conditions Some instances in which environment caused a specific fracture response that can be characterized through electron-microscope fractography are illustrated in Fig 21 and 22

Fig 21 Intermingled cleavage facets and dimples in two views of a stress-corrosion fracture in a step-cooled

two-phase Ti-6Al-2Sn-4Zr-6Mo alloy exposed to a 3 % NaCl aqueous solution Cleavage facets formed in the alpha phase, and poorly developed dimples, such as at the sites marked A, formed in the beta phase

Fig 22 Intergranular fracture in copper alloy C71500 (copper nickel, 30%) that became embrittled by

grain-boundary oxidation during extended exposure to high-temperature steam in a heat exchanger Crack penetration (which was cyclic, as intergranular layers of oxide formed, broke and reformed) produced fine striations that could be mistaken for fatigue striations

Guide to Nondestructive Testing and Inspection Methods

Introduction

NONDESTRUCTIVE TESTING (NDT) and inspection techniques commonly used to detect and evaluate flaws (irregularities or discontinuities) or leaks in engineering systems are reviewed in this Section Of the many different NDT techniques used in industry, liquid penetrant and magnetic particle testing account for about one-half of all NDT, ultrasonics and x-ray methods about another third, eddy current testing about 10%, and all other methods for only about 2% (Ref 1) It should be noted that the techniques reviewed in this Section are by no means all of the NDT techniques utilized They do represent, however, the most commonly employed methods Table 1 is a simplified breakdown of the complexity and relative requirements of the five most frequently used NDT techniques Table 2 compares common NDT methods Additional information on the various types of NDT methods can be obtained from the American Society for

Nondestructive Testing (Columbus, OH), Nondestructive Testing, Volume 03.03, published annually by ASTM (Philadelphia, PA), and in Nondestructive Evaluation and Quality Control, Volume 17, ASM Handbook

Table 1 The relative uses and merits of various nondestructive testing methods

Test method

Ultrasonics X-ray Eddy current Magnetic

particle

Liquid penetrant Capital cost Medium to high High Low to medium Medium Low

Time of results Immediate Delayed Immediate Short delay Short delay

Effect of geometry Important Important Important Not too

important

Not too important

Access problems Important Important Important Important Important

breaking

Formal record Expensive Standard Expensive Unusual Unusual

Operator training Important Important Important Important

Portability of equipment High Low High to medium High to

medium

High

Dependent on material

composition

Very Quite Very Magnetic only Little

Capabilities Thickness gaging: some

composition testing

Thickness gaging

Thickness gaging;

grade sorting

Defects only Defects only

Source: Ref 1

Table 2 Comparison of some nondestructive testing methods

Method Characteristics detected Advantages Limitations Example of use

Ultrasonics Changes in acoustic

impedance caused by cracks, nonbonds, inclusions, or interfaces

Can penetrate thick materials; excellent for crack detection; can be automated

Normally requires coupling to material either by contact to surface or immersion in a fluid such as water Surface needs to

be smooth

Adhesive assemblies for bond integrity; laminations;

hydrogen cracking

Radiography Changes in density from

voids, inclusions, material variations; placement of internal parts

Can be used to inspect wide range of materials and thicknesses; versatile;

film provides record of inspection

Radiation safety requires precautions; expensive; detection

of cracks can be difficult unless perpendicular to x-ray film

Pipeline welds for penetration,

inclusions, and voids; internal defects in castings

Visual optical Surface characteristics such

as finish, scratches, cracks,

or color; strain in transparent materials; corrosion

Often convenient; can be automated

Can be applied only to surfaces, through surface openings, or to transparent material

Paper, wood, or metal for surface finish and uniformity

Eddy current Changes in electrical

conductivity caused by material variations, cracks, voids, or inclusions

Readily automated;

moderate cost

Limited to electrically conducting materials; limited penetration depth

Heat exchanger tubes for wall thinning and cracks

Liquid

penetrant

Surface openings due to cracks, porosity, seams, or folds

Inexpensive, easy to use, readily portable, sensitive

to small surface flaws

Flaw must be open to surface

Not useful on porous materials or rough surfaces

Turbine blades for surface cracks or porosity; grinding cracks

Magnetic

particles

Leakage magnetic flux caused by surface or near- surface cracks, voids, inclusions, or material or geometry changes

Inexpensive or moderate cost, sensitive both to surface and near- surface flaws

Limited to ferromagnetic material; surface preparation and post-inspection demagnetization may be required

Railroad wheels for cracks; large castings

Source: Ref 1

References cited in this section

2 V.E Panhuise et al., Quantitative Nondestructive Evaluation, Nondestructive Evaluation and Quality

Control, Vol 17, ASM Handbook, ASM International, 1989, p 661-715

3 M.P Kaplan et al, Damage Tolerance of Aircraft Systems, Fatigue and Fracture, Vol 19, ASM Handbook,

ASM International, 1996, p 557-588

Uses of NDT

Although flaw detection is usually considered the most important aspect of NDT, there are also other important application areas for these methods These include leak detection, metrology, structure or microstructure characterization, stress/strain response determination, and rapid identification of metals and alloys Each of these is briefly described in the following paragraphs; references to direct the reader to more detailed information are also supplied

Leak Detection and Evaluation Because many objects must withstand pressure, the nondestructive determination of

leakage is extremely important The NDT area known as leak detection utilizes many techniques, as described in the article "Leak Testing" in this Section Each technique has a specific range of applications, and a particular leak detection technique should be selected only after careful consideration

Metrology The measurement of dimensions, referred to as "metrology," is one of the most widely used NDT activities,

although it is not considered with other conventional NDT activities, such as flaw detection In recent years, conventional tools for metrology, such as micrometer calipers, vernier calipers, and dial gages, have been supplemented with modern high-technology metrology tools, such as laser inspection, coordinate measuring machines, and machine vision and robotic inspection systems (Ref 4, 5, 6)

The selection of a metrology system is highly dependent on the specific requirements of a given application Standard reference works on the topic should be consulted for conventional metrology In addition, other NDE methods, such as eddy current, ultrasonic, optical holography, and speckle metrology, often find application in the field of metrology

Structure or Microstructure Characterization Another interesting area of NDT is microstructural

characterization, which can be done in situ without damaging the object by using replication microscopy This is widely used to assess the condition of power plant and petrochemical metallic components on a large scale (Ref 7), or by using conventional optical microscopy techniques with portable equipment, including polishing, etching, and microscopic equipment In addition, it is possible to characterize the microstructure through the correlation with some type of NDT information For example, the transmission of ultrasonic energy has been correlated with the microstructure of gray cast iron

Microstructure can often be characterized by determining physical or mechanical properties with NDT techniques because there is usually a correlation among microstructure, properties, and NDT response Characterizing microstructure from NDT responses is a relatively recent area of NDT application, and new developments are occurring frequently

Stress/Strain and Dynamic Response Determination The local strain at a specific location in an object under a

specific set of loading conditions can be determined by using strain sensing methods such as photoelastic coatings, brittle coatings, or strain gages (Ref 8) If the stress-strain behavior of the material is known, these strain values can be converted into stress values

Residual stresses in materials can be nondestructively measured by a variety of methods, including x-ray diffraction (Ref 9), ultrasonics (Ref 10), and electromagnetics (Ref 11, 12) With the x-ray diffraction technique, the interatomic planar distance is measured, and the corresponding stress is calculated The penetration depth of x-rays is of the order of only 10

m (400 in.) in metals Therefore, the technique is limited to measurements of surface stresses Its use has been generally limited to the laboratory because of the lack of field-usable equipment and concern with radiation safety

With ultrasonic techniques, the velocity of the ultrasonic waves in materials is measured and related to stress These techniques rely on a small velocity change caused by the presence of stress, which is known as the acoustoelastic effect

In principle, ultrasonic techniques can be used to measure bulk as well as surface stresses Because of the difficulty in differentiating stress effects from the effect of material texture, practical ultrasonic applications have not yet materialized

With electromagnetic techniques, one or more of the magnetic properties of a material (such as permeability, magnetostriction, hysteresis, coercive force, or magnetic domain wall motion during magnetization) are sensed and correlated to stress These techniques rely on the change in magnetic properties of the material caused by stress; this is known as the magnetoelastic effect These techniques, therefore, apply only to ferromagnetic materials, such as steel

Rapid Identification of Metals and Alloys Quality assurance during the fabrication of hardware, subassemblies,

and assemblies sometimes requires a reliable system of rapid identification of metals and alloys Because various metals may become mixed during storage or use, and because lengths of strip, sheet, plate, billets, bars, wire, and fabricated products may have lost their identifying marks, some means of sorting mixed lots is necessary The best method of identifying such items is by quantitative chemical analysis However, chemical or spectrographic analyses require extensive, time-consuming procedures and expensive equipment that may not be fully utilized Also, a complete chemical analysis often may be unnecessary

Common methods of rapid identification of metals include techniques involving the magnetic properties and weight of the metal, spark testing (of ferrous alloys), and chemical spot testing Each of these methods is discussed in Ref 13 Spark testing is also described in the Section "Recycling and Life-Cycle Analysis" in this Handbook

References cited in this section

4 C Bixby, Laser Inspection, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM

International, 1989, p 12-17

5 D.H Genest, Coordinate Measuring Machines, Nondestructive Evaluation and Quality Control, Vol 17,

ASM Handbook, ASM International, 1989, p 18-28

6 J.D Meyer, Machine Vision and Robotic Inspection Systems, Nondestructive Evaluation and Quality

Control, Vol 17, ASM Handbook, ASM International, 1989, p 29-45

7 A.R Marder, Replication Microscopy Techniques for NDE, Nondestructive Evaluation and Quality

Control, Vol 17, ASM Handbook, ASM International, 1989, p 52-56

8 L.D Lineback, Strain Measurement for Stress Analysis, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p 448-453

9 P.S Prevey, X-Ray Diffraction Residual Stress Techniques, Materials Characterization, Vol 10, ASM

Handbook, ASM International, 1986, p 380-392

10 Y.H Pao, W Sachse, and H Fukuoka, Acoustoelasticity and Ultrasonic Measurement of Residual Stresses,

Physical Acoustics: Principles and Methods, Vol XVII, W.P Mason and R.M Thruston, Ed., Academic

Press, 1984, p 61-143

11 H Kwun and G.L Burkhardt, Electromagnetic Techniques for Residual Stress Measurements,

Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, ASM International, 1989, p

159-163

12 W.L Rollwitz, Magabsorption NDE, Nondestructive Evaluation and Quality Control, Vol 17, ASM

Handbook, ASM International, 1989, p 143-158

13 R Mason and R.L Lessard, Rapid Identification of Metals and Alloys, Metals Handbook: Desk Edition,

Types of Leaks Two basic types of leaks are real leaks and virtual leaks A real leak essentially is a localized leak; that

is, a discrete passage through which fluid can flow (crudely, a hole) Such a leak can take the form of a tube, a crack, an orifice, and so on A system also can leak through permeation of a somewhat extended barrier; this type of real leak is called a distributed leak A gas can flow through a solid having no holes large enough to permit more than a small fraction

of the gas to flow through any one hole This process involves diffusion through the solid and also can involve various surface phenomena such as absorption, dissociation, migration, and desorption of gas molecules Virtual leaks are leaks that involve the gradual desorption of gases from surfaces and components within a vacuum system It is not uncommon for a vacuum system to have both real leaks and virtual leaks at the same time

Leak Testing of Fluid Systems at Pressure

Leak-testing methods are classified according to the pressure and fluid (gas or liquid) in the system Table 1 lists the common fluid-system leak-testing methods and the methods used in leak testing of vacuum systems

Table 1 Pressure and vacuum system leak-testing methods

Gas systems at pressure

• Direct sensing by:

o Infrared gas analyzers

Liquid systems at pressure

• Unaided visual methods

• Aided visual methods

Acoustic Methods

Turbulent flow of a pressurized gas through a leak produces sound of both sonic and ultrasonic frequencies If the leak is large, it probably can be detected with the ear This is an economical and fast method to find gross leaks Sonic emissions also are detected using such instruments as stethoscopes and microphones that have a limited ability to locate and determine the approximate size of a leak Electronic transducers enhance detection sensitivity

Bubble Testing

A simple method used to leak test small vessels pressurized with any gas is to submerge them in a liquid and observe for bubbles If the test vessel is sealed at atmospheric pressure, a pressure differential can be obtained by pumping a partial vacuum over the liquid and by heating the liquid The sensitivity of this test is increased by reducing (a) the pressure above the liquid, (b) the liquid density, (c) the depth of immersion in the liquid, and (d) the surface tension of the liquid

Specific-Gas Detectors

Many available types of leak detectors react to either a specific gas or a group of gases that have some specific physical and/or chemical property in common

Sulfur hexafluoride detectors operate on the principle of electron-capture detectors, used widely in the field of gas

chromatography The sensing chamber of a sulfur hexafluoride detector consists of a cylindrical cell, which has a centrally mounted insulated probe The inner wall of the cell is coated with a radioactive element (300 millicuries of tritium) Low-energy electrons emitted by the tritium are collected on the central probe by means of a polarizing voltage maintained between the probe and the cell wall The resulting electric current is amplified and displayed on a conventional meter The display meter is a taut band-suspension microammeter reading 0 to 50, and response time of the instruments one second Leak-rate sensitivity is 10-8 mL/s

Mass-Spectrometer Testing A mass spectrometer basically is a device to sort charged particles The sample gas

enters the analyzer where its molecules are bombarded by a stream of electrons emitted by a filament The bombarded molecules lose an electron and become positively charged ions, which are electrostatically accelerated to a high velocity Because the analyzer lies in a magnetic field perpendicular to the ion path, the ions travel distinct, curved paths according

to their mass The radii of these paths are determined by ion mass, the magnitude of initial acceleration, and the strength

of the magnetic field With a constant magnetic field, any group of ions having the same mass can be made to travel the specific radius necessary to strike the ion collector The positive charge of the ion is imparted to the target, or collector, and the resulting current flow is proportional to the quantity of the ions of that particular mass

Application of Specific-Gas Detectors

The proper method for using a specific-gas detector is based on the function of the leak detector, the fluid that is leaking, and the type of vessel being tested One of the best methods of using a leak detector is discussed below

Probing Probes, which will react to a number of gases, can be used in either of two ways: the probe mode and the

monitoring of an enclosure placed around the pressurized item

In the probe mode, the external surface of the pressurized vessel is scanned either with a portable detector having a short probe attached, or with a long probe connected to a stationary leak detector by flexible tubing (Fig 1) In general, the connection of a long probe to a stationary detector reduces sensitivity because of the slow release of absorbed gases in the probe tubing, which results in a high background reading Because a correspondingly longer time is required for the gas to flow up the tube to the sensing element, it is difficult to pinpoint the location of the leak

Fig 1 Schematic showing connection of a long probe by a flexible hose to a stationary leak detector to scan the

external surface of a pressurized vessel

In the monitoring of an enclosure placed around the pressurized item, the item is enclosed in a plastic bag and the accumulation of tracer gas in the bag is monitored by a leak detector connected to the bag by a short probe

Back pressuring is a method of pressurized testing that typically is used with small, hermetically sealed electronic

components such as integrated circuits, relays, and transistors In this method, the test unit is placed in a pressurized container filled with a tracer gas and is kept there for a time to allow tracer gas to flow into the unit through any leaks that exist

In using back pressuring, care must be taken to ensure that the leaking components contain tracer gas Therefore, it is important that the time between back pressuring and leak testing be suitably controlled For example, in a test specification for transistors, the transistors were subjected to helium pressure of 100 psi for 16 h, air washed for 4 min, and then leak tested within 3 h If more than 3 h elapsed before leak testing, the transistors were pressurized again

Because leaks generally are expected to be quite small, very sensitive tracer-detector combinations must be used The tracer gas typically used is helium or krypton 85, and detection is by use of a helium mass spectrometer or a nuclear-radiation detector, respectively Absolute values of the size of the leak sometimes are difficult to determine with back pressuring, because of (a) adsorption and absorption of the tracer gases and (b) different detector-response times for different leak directions

Visual Inspection

Introduction

VISUAL INSPECTION is a nondestructive testing technique that provides a means to detect and examine a variety of surface flaws, such as corrosion, contamination, surface finish, and surface discontinuities Visual inspection is the most widely used method for detecting and examining surface cracks

Visual inspection methods range from examination with the naked eye to the use of interference microscopes to measure the depth of scratches in the finish of finely polished and lapped surfaces Equipment used to aid visual inspection includes:

• Flexible and rigid borescopes to illuminate and observe internal, closed, and otherwise inaccessible areas

• Image sensors for remote sensing and to develop permanent visual records in the form of photographs, videotapes, and computer-enhanced images

• Magnifying systems to evaluate surface finish, surface shapes (profile and contour gaging), and surface microstructures

• Dye and fluorescent penetrants and magnetic particles to enhance the observation of surface cracks (and sometimes near-surface conditions in the case of magnetic-particle inspection)

Borescopes

A borescope (Fig 1) is a long, tubular optical device that illuminates and allows the inspection of surfaces inside narrow tubes and difficult-to-reach chambers The tube, which can be rigid or flexible with a wide variety of lengths and diameters, provides the necessary optical connection between the viewing end and an objective lens at the distant, or distal, tip of the borescope Three ways to achieve the optical connection are:

• A rigid tube with a series of relay lenses

• A tube (normally flexible, but also rigid) with a bundle of optical fibers

• A tube (normally flexible) with wiring that carries the image signal from a charge-coupled device (CCD) imaging sensor at the distal tip

Fig 1 Three typical designs of borescopes (a) Rigid borescope with a lamp at the distal end (b) Flexible

fiberscope with a light source (c) Rigid borescope with a light guide bundle in the shaft

These three basic tube designs can have either fixed or adjustable focusing of the objective lens at the distal tip The distal tip also has prisms and mirrors that define the direction and field of view (see Fig 2) Generally, a fiber optic light guide and a lamp producing white light is used in the illumination system, although ultraviolet light can be used to inspect surfaces treated with liquid fluorescent penetrants Light-emitting diodes at the distal tip are sometimes used for illumination in videoscopes with working lengths greater than 15 m (50 ft)

Fig 2 Typical directions and field of view with rigid borescopes

Rigid Borescopes

Rigid borescopes generally are limited to use in applications with a straight-line path between the observer and the area to

be observed The sizes range from 0.15 to 30 m (0.5 to 100 ft) long and 0.9 to 70 mm (0.035 to 2.75 in.) in diameter Magnification usually is 3 to 4×, but magnifying power up to 50× is available The illumination system is either an incandescent lamp located at the distal end (Fig 1a) or a light guide bundle made of optical fibers (Fig 1c), which conduct light from an external source

Rigid borescopes typically have a series of achromatic relay lenses in the optical tube The borescopes generally have a 55° field of view, although the field of view can range from 10 to 90° Some rigid borescopes have orbital scan (Fig 1c), which involves the rotation of the optical shaft for scanning purposes The amount of rotation can vary from 120 to 370° Some rigid borescopes also have movable prisms at the tip for scanning The illumination system can be either a distal lamp or a light guide bundle, and the various features may include orbital scan, various viewing heads, and adjustable focusing of the objective lens

Miniborescopes Instead of conventional relay lenses, miniborescopes have either a single image-relaying rod or quartz

fiber in the optical tube Miniborescopes are 110 and 170 mm (4.3 and 6.7 in.) long, and range from 0.9 to 2.7 mm (0.035

to 0.105 in.) in diameter High magnification (up to 30×) can be achieved at minimal focal lengths, and an adjustable focus is not required because the scope has an infinite depth of field Miniborescopes use an integral light guide bundle

Hybrid borescopes use rod lenses combined with convex lenses to relay the image A larger light guide bundle is used,

which provides higher illumination and a larger image with a higher degree of contrast

Hybrid borescopes are up to 990 mm (39 in.) long, and 5.5 to 12 mm (0.216 to 0.47 in.) in diameter All hybrid borescopes have adjustable focusing of the objective lens and a 370° rotation for orbital scan

Extendable borescopes allow the user to construct a longer borescopic tube by joining extension tubes The

borescopes are available with either a fiber-optic light guide or an incandescent lamp at the distal end Extendable borescopes with an integral lamp have a maximum length of approximately 30 m (100 ft) Maximum length of scopes with a light guide bundle is about 8 m (26 ft), which allows smaller tube diameters (as small as 8 mm, or 0.3 in.) Interchangeable viewing heads are also available Extendable borescopes do not have adjustable focusing of the objective lens

Rigid chamberscopes allow more rapid inspection of larger chambers Chamberscopes have variable magnification

(zoom), a lamp at the distal tip, and a scanning mirror, which allows the user to observe in different directions The higher illumination and greater magnification of chamberscopes allow the inspection of surfaces as far as 910 mm (36 in.) away from the distal tip of the scope

Mirror sheaths convert a direct-viewing borescope into a side-viewing scope A mirror sheath is designed to fit over

the tip of the scope, and, therefore, reflects an image from the side of the scope Side, forward-oblique, and retrospective viewing heads provide better resolution and a higher degree of image contrast A mirror sheath also produces an inverse image and can produce unwanted reflections from the shaft

Scanning In addition to the orbital scan feature described previously, some rigid borescopes have the ability to scan

longitudinally along the axis of the shaft This is accomplished using a movable prism with a control at the handle Typically, the prism can shift the direction of view through an arc of 120°

Flexible Borescopes

Flexible borescopes are used primarily in applications that do not have a straight passageway to the point of observation Two types of flexible borescopes are flexible fiberscopes and videoscopes with a CCD image sensor at the distal tip

Flexible Fiberscopes A typical fiberscope (Fig 1b) consists of a light guide bundle, an image guide bundle, an

objective lens, interchangeable viewing heads, and remote controls for articulation of the distal tip Fiberscopes are available in diameters from 1.4 to 13 mm (0.055 to 0.5 in.) and ranging up to 12 m (40 ft) long Special quartz fiberscopes are available in lengths up to 90 m (300 ft)

Fibers in the image guide must be precisely aligned so they are in an identical relative position to each other at their terminations for proper image resolution Fiber diameter also is a factor in obtaining good image resolution With smaller diameter fibers, a brighter image with better resolution can be obtained because more fibers can be packed in the image guide Higher resolution makes it possible to use an objective lens with a wider field of view and also to magnify the image at the eyepiece This allows better viewing of objects at the periphery of the image

The interchangeable distal tips provide various directions and fields of view on a single fiberscope However, because the tip can be articulated for scanning purposes, distal tips with either a forward or side viewing direction usually are sufficient Fields of view are typically 40 to 60°, although they can range from 10 to 120° Most fiberscopes provide adjustable focusing of the objective lens

Videoscopes with CCD probes involve the electronic transmission of color or black and white images to a video

monitor The distal end of electronic videoscopes contains a CCD chip The objective lens focuses the image of an object

on the surface of the CCD chip, converting light to electrons, which are stored in each picture element, or pixel, of the CCD device Thus, the image of the object is stored on the CCD device

Videoscopes with CCD probes produce images having spatial resolutions of the order of those described in Fig 3 Resolution depends on the object-to-lens distance and the fields of view, factors that affect the degree of magnification Generally, videoscopes produce higher resolution than fiberscopes

Fig 3 Typical resolution of charge-coupled device videoscopes with a (a) 90° field of view, (b) 60° field of

view, and (c) 30° field of view

Another advantage of videoscopes is their longer working length With a given amount of illumination at the distal tip, videoscopes can return an image over a greater length than fiberscopes Videoscopes help reduce eye fatigue Also, there

is no honeycomb pattern or irregular picture distortion as with some fiberscopes, the electronic form of the image signal allows digital image enhancement and the potential for integration with automatic inspection systems, and the display allows the generation of reticles on the viewing screen for point-to-point measurements

Special Features

Measuring borescopes and fiberscopes contain a movable cursor that allows measurements during viewing When

the object under measurement is in focus, the movable cursor provides a reference for dimensional measurements in the optical plane of the object This capability eliminates the need to know the object-to-lens distance when determining magnification factors

Working channels are used in borescopes and fiberscopes to pass working devices, such as measuring instruments,

retrieval devices, and hooks to help insert thin, flexible fiberscopes, to the distal tip

Selection

Factors that influence the choice of a flexible or rigid borescope for use in a specific application include focusing, illumination, magnification, working length, direction of view, and environment

Focusing and Resolution In general, the optical quality of a rigid borescope improves as the size of the lens

increases Therefore, a borescope with the largest possible diameter should be used For fiberscopes, the resolution is dependent on the alignment accuracy and fiber diameter in the image bundle Smaller-diameter fibers provide more resolution and edge contrast when combined with good geometrical alignment of the fibers

Illumination The required intensity of the light source is determined by the reflectivity of the surface, the area of

surface to be illuminated, and the transmission losses over the length of the scope At working lengths greater than 6 m (20 ft), rigid borescopes with a lamp at the distal end provide the greatest amount of illumination over the widest area Fiber-optic illumination in scopes with working lengths less than 6 m (20 ft) is always brighter and is suitable for heat-sensitive applications because filters can remove infrared frequencies

Magnification and field of view are interrelated; field of view decreases as magnification increases The precise

relationship between magnification and field of view is specified by the manufacturer

The degree of magnification in a particular application is determined by the field of view and the distance from the objective lens to the object The magnification increases when either the field of view or the lens-to-object distance decreases

Working length can sometimes dictate the use of a particular type of scope For example, a rigid borescope with a long

working length might be limited by the need for additional supports In general, videoscopes allow a longer working length than fiberscopes

Direction of View Flexible fiberscopes and videoscopes, because of their articulating tip, are often adequate with

either a side or forward viewing tip

Circumferential and panoramic heads are designed to inspect tubing and other cylindrical structures A centrally located mirror permits right-angle viewing of an area just scanned by the panoramic view

The forward viewing head permits the inspection of the area directly ahead of the viewing head It is commonly used when examining facing walls or the bottoms of blind holes and cavities

Forward-oblique heads bend the viewing direction at an angle to the borescope axis, permitting the inspection of corners

at the end of a bored hole The retrospective viewing head bends the cone of view at a retrospective angle to the borescope axis, providing a view of the area just passed by the advancing borescope It is especially suited to inspecting the inside neck of cylinders and bottles

Environment Flexible and rigid borescopes can be manufactured to withstand a variety of environments Although

most flexible and rigid borescopes can operate at temperatures ranging from -34 to 66 °C (-30 to 150 °F), specially designed scopes can be used at temperatures to 1925 °C (3500 °F) Scopes can also be manufactured for use in liquid media

Special scopes are required for use at pressures above ambient and in atmospheres exposed to radiation Scopes used in a gaseous environment should be made explosion proof

Applications

The principal use of borescopes is in equipment maintenance programs, where they can help reduce or eliminate the need for costly teardowns Some types of equipment, such as turbines, have access ports that are specifically designed for borescopes Borescopes provide a means to check in-service defects in various equipment, such as automotive components (Fig 4), turbines, and process piping

Fig 4 In-service defects as seen through a borescope designed for automotive servicing (a) Carbon on valves

(b) Broken transmission gear tooth (c) Differential gear wear

Optical Sensors

Visible light, which can be detected by the human eye and with optical sensors, has some advantages over inspection methods based on nuclear, microwave, and ultrasound radiation For example, one of the advantages of visible light is the capability of tightly focusing the probing beam on the inspected surface High spatial resolution can result from this sharp focusing, which is useful in gaging and profiling applications

Different types of image sensors used in visual inspection include:

• Vidicon or plumbicon television tubes

• Secondary electron-coupled (SEC) vidicons

• Image orthicons and image isocons

• Charge-coupled device sensors

• Holographic plates (see the article "Holography" in this Section)

Television cameras with vidicon tubes are useful at higher light levels (approximately 0.2 lm/m2, or 10-2 ftc), while orthicons, isocons, and SEC vidicons are useful at lower light levels

Charge-coupled devices are suitable for use in many different information-processing applications, including image sensing in television-camera technology Charge-coupled devices offer an advantage over vacuum-tube image sensors because of the reliability of their solid-state technology, their operation at low voltage and low power dissipation, extensive dynamic range, visible and near-infrared response, and geometric reproducibility of image location Image enhancement (or visual feedback into robotic systems) typically involves the use of CCDs as the optical sensor or the use

of television signals that are converted into digital form

Optical sensors are also used in inspection applications that do not involve imaging However, in some applications, incoherent light sources are very effective in nonimaging inspection applications using optical sensors

Magnifying Systems

Magnifying systems are used in visual reference gaging When tolerances are too tight to judge by eye alone, optical comparators or toolmakers' microscopes are used to achieve magnifications ranging from 5 to 500×

A toolmakers' microscope consists of a microscope mounted on a base that carries an adjustable stage, a stage

transport mechanism, and supplementary lighting Various objective lenses provide magnifications ranging from 10 to 200×

Optical comparators are magnifying devices that project the silhouette of small parts onto a large projection screen

The magnified silhouette is then compared against an optical comparator chart, which is a magnified outline drawing of the workpiece being gaged Optical comparators are available with magnifications ranging from 5 to 500×

Parts with recessed contours can also be successfully gaged on optical comparators using a pantograph One arm of the pantograph is a stylus that traces the recessed contour of the part, and the other arm carries a follower that is visible in the light path As the stylus moves, the follower projects a contour on the screen

Liquid-Penetrant Inspection

Introduction

LIQUID-PENETRANT INSPECTION is a nondestructive method used to find discontinuities that are open to the surface

of solid, essentially nonporous materials Indications of flaws can be found regardless of the size, configuration, internal structure, and chemical composition of the workpiece being inspected, as well as flaw orientation Liquid penetrants can seep into (and be drawn into) various types of minute surface openings (as fine as 0.1 m, or 4 in., in width) by capillary action Therefore, the process is well suited to detect all types of surface cracks, laps, porosity, shrinkage areas, laminations, and similar discontinuities It is used extensively to inspect ferrous and nonferrous metal wrought and cast products, powder metallurgy parts, ceramics, plastics, and glass objects

The liquid-penetrant inspection method is relatively simple to perform, there are few limitations due to specimen material and geometry, and it is inexpensive The equipment is very simple, and the inspection can be performed at many stages in the production of the part, as well as after the part is placed in service Relatively little specialized training is required to perform the inspection In some instances, liquid-penetrant sensitivity is greater for ferromagnetic steels than that of magnetic-particle inspection

The major limitation of liquid-penetrant inspection is that it can detect only imperfections that are open to the surface; some other method must be used to detect subsurface defects and discontinuities Another factor that can inhibit the effectiveness of liquid-penetrant inspection is the surface roughness of the object Extremely rough and porous surfaces are likely to produce false indications

Although the liquid-penetrant method often is used to inspect some types of powder metallurgy parts, the process generally is not well suited to inspect low-density powder metallurgy parts and other porous materials because the penetrant enters the pores and registers each pore as a defect

Physical Principles

Liquid-penetrant inspection depends mainly on the ability of liquid penetrant to effectively wet the surface of a solid workpiece or specimen; flow over the surface to form a continuous, reasonably uniform coating; and migrate into cavities that are open to the surface The cavities of interest usually are very small, often invisible to the unaided eye The ability

of a given liquid to flow over a surface and enter surface cavities depends principally on:

• Cleanness of the surface

• Configuration of the cavity

• Size of the cavity

• Surface tension of the liquid

• Ability of the liquid to wet the surface

The cohesive forces between molecules of a liquid cause surface tension An example of the influence of surface tension

is the tendency of free liquid, such as a droplet of water, to contract into a sphere In such a droplet, surface tension is counterbalanced by the internal hydrostatic pressure of the liquid When the liquid comes into contact with a solid surface, the cohesive force responsible for surface tension competes with the adhesive force between the molecules of the liquid and the solid surface These forces jointly determine the contact angle between the liquid and the surface If the angle is less than 90°, the liquid has good wetting ability

Description of the Process

Regardless of the type of penetrant used and other variations in the basic process, liquid-penetrant inspection requires at least five essential steps:

Surface Preparation All surfaces of a work-piece must be thoroughly cleaned and completely dried before

inspection Discontinuities exposed to the surface must be free from oil, water, and other contaminants for at least 25 mm (1 in.) beyond the area being inspected to increase the probability of detection

Penetrant Application Liquid penetrant is applied in a suitable manner to form a film of the penetrant over the

surface for at least 13 mm ( in.) beyond the area being inspected The penetrant is left on the surface for a sufficient time to allow penetration into flaws Times are based on experience Table 1 shows some typical times for different materials and defect types

Table 1 Typical penetration time for a dye penetrant

All

Cracks 10 Cracks 2-5

Ceramics

Porosity 2-5

Aluminum welds Cracks and pores 10-20

Steel welds Cracks and pores 10-20

Metal-permanent mold casting Shrinkage porosity 3-10

Carbide-tipped cutting tools Poor braze 2-10

Cracks in steel 2-10

Cutting tools

Cracks in tip 2-10

Removal of Excess Penetrant Uniform removal of excess penetrant is necessary for effective inspection, but

overcleaning must be avoided Penetrants can be washed off directly using water, treated first with an emulsifier and then rinsed with water, or removed using a solvent

Developer Application Developer can be applied by dusting (dry powdered) and immersion and spray (water

developers) applications Nonaqueous wet developers can only be applied by spraying The developer should be allowed

to dwell on the surface for a sufficient time (usually 10 min minimum) to permit it to draw penetrant out of any surface flaws to form visible indications of such flaws Longer times could be necessary for tight cracks The developer also provides a uniform background to assist visual inspection

Inspection After being sufficiently developed, the surface is visually examined for indications of penetrant bleedback

from surface openings This examination must be performed in a suitable inspection environment Visible-penetrant inspection is performed in good white light When fluorescent penetrant is used, inspection is performed in a suitably darkened area using black (ultraviolet) light, which causes the penetrant to emit visible light The actions of penetrant and developer are shown in Fig 1

Fig 1 Actions of penetrant and developer (a) Penetrant liquid is drawn into an open crack by capillary action

(b) Excess surface penetrant is removed by wiping with a cloth, washing directly with water, treating with an emulsifier and rinsing, or removing with a solvent (c) Developer applied to the surface draws out penetrant liquid that seeps into the developer forming a visible indication of the surface crack A colored dye or a fluorescence compound is usually added to the penetrant liquid Depending on the amount of penetrant that seeps into the developer, the crack width can appear 100 times larger than its actual size

Penetrant Systems

Liquid penetrant inspection applications have been developed to handle the wide variations in three basic penetrant systems They are broadly classified as (a) the water-washable system, (b) the postemulsifiable system, and (c) the solvent-removable system

The water-washable penetrant system is designed so that the penetrant is directly washable from the surface of

the workpiece using water It can be used to process workpieces quickly and efficiently However, it is important that washing is carefully controlled, because water-washable penetrants are susceptible to overwashing The degree and speed

of removal depend on processing conditions such as spray-nozzle characteristics, water pressure and temperature, duration of rinse cycle, surface condition of the workpiece, and inherent removal characteristics of the penetrant employed

The Postemulsifiable System. High-sensitivity penetrants that are not water washable are used to ensure detection

of minute discoveries in some materials Because they are not water washable, the danger of washing the penetrant out of the flaws is reduced These penetrants require an additional operation in the inspection process An emulsifier must be applied after the application of penetrant and proper penetration (dwell) time The emulsifier makes the penetrant soluble

in water so the excess penetrant can be removed by water rinsing Therefore, the emulsification time must be carefully controlled so the surface penetrant becomes water-soluble but penetrant in the flaws does not Postemulsifiable penetrants include lipophilic (oil base) and hydrophilic (water base)

The Solvent-Removable System Occasionally, it is necessary to inspect only a small area of a workpiece or to

inspect a workpiece on site rather than at a regular inspection station In such situations, solvent-removable penetrants are used Normally, the same type of solvent is used both for precleaning and for removal of excess penetrant This penetrant process is convenient and broadens the range of applications of penetrant inspection

The solvent-removable penetrants have an oil base Optimum solvent removal is accomplished by wiping off as much of the excess penetrant as possible with a paper towel or a lint-free cloth, then slightly dampening a clean cloth with solvent and wiping off the remaining penetrant Final wiping with a dry paper towel or clean cloth is required

The penetrant also can be removed by flooding the surface with solvent, in the same manner as for water-washable penetrants The flooding technique is particularly useful for large workpieces, but it must be very carefully used to prevent removal of the penetrant from the flaws

The solvent-removable system is used mainly in special applications; it is not practical for production applications because it is labor intensive

Visible penetrant inspection uses a penetrant that is usually red in color and produces vivid red indications in contrast to the light background of the applied developer under visible light The visible penetrant indications must be viewed under adequate white light The sensitivity of visible penetrants is regarded as Level 1 and adequate for many applications

Penetrant selection and use depend on the criticality of the inspection, the condition of the workpiece surface, the type of processing, and the desired sensitivity

Water-washable penetrants are designed for the removal of excess surface penetrant by water rinsing directly after

a suitable penetration (dwell) time The emulsifier is incorporated into the water-washable penetrant When this type of penetrant is used, it is extremely important that the removal of excess surface penetrant be properly controlled to prevent overwashing, which can cause the penetrant to be washed out of the flaws

Lipophilic and hydrophilic postemulsifiable penetrants are insoluble in water, and, therefore, are not

removable by water rinsing alone They are designed to be selectively removed from the surface of the workpiece using a separate emulsifier The emulsifier, properly applied and left for a suitable emulsification time, combines with the excess surface penetrant to form a water-washable surface mixture, which can be rinsed from the surface of the workpiece The penetrant that remains within the flaw is not subject to overwashing if the emulsifier is confined to the surface and if the discontinuity is tight (no mechanical rinsing)

Solvent-removable penetrants are used primarily where portability is required and to inspect localized areas To

minimize the possibility of removing the penetrant from discontinuities, the use of excessive amounts of solvent must be avoided

Physical and Chemical Characteristics Both fluorescent and visible penetrants, whether water-washable,

postemulsifiable, or solvent-removable, must have certain chemical and physical characteristics to perform their intended functions Principal requirements of penetrants are:

• Chemical stability and uniform physical consistency

• A flash point not lower than 95 °C (200 °F); penetrants that have lower flash points constitute a potential fire hazard

• A high degree of wettability

• Low viscosity to permit better coverage and minimum dragout

• Ability to penetrate discontinuities quickly and completely

• Sufficient brightness and permanence of color

• Chemical inertness with materials being inspected and with containers

• Low toxicity to protect personnel

• Slow drying characteristics

Water-base emulsifiers usually are supplied as liquid concentrates that are diluted in water to concentrations of 5 to 30% for dip-tank applications and of 0.05 to 5% for spray applications Water-base emulsifiers function by displacing excess surface penetrant from the surface of the part by detergent action The force of the water spray or air agitation of open dip tanks provides the scrubbing action while the detergent displaces the excess surface penetrant

Solvent Cleaners

Solvent cleaners differ from emulsifiers in that they remove excess surface penetrant through direct solvent action The penetrant is dissolved by the solvent Solvent cleaners are flammable and nonflammable Flammable cleaners are free of halogens, but are potential fire hazards Nonflammable cleaners usually contain halogenated solvents, which render them unsuitable for some applications usually because of their high toxicity or because they have undesirable effects on some materials

Developers

Because the amount of penetrant that emerges from a small surface opening is minute, the visible evidence of its presence must be enhanced Developers are used to spread the penetrant available at the defect, thus increasing the amount of light emitted, or the amount of constrast, that makes the defect visible to the unaided eye

Developers must have the following properties/ characteristics for optimal performance

• It must be adsorptive to maximize blotting

• It must have fine grain size and a particle shape that will disperse and expose the penetrant at a flaw to produce strong and sharply defined indications of flaws

• It must be capable of providing a contrast background for indications when color-contrast penetrants are used

• It must be easy to apply

• It must form a thin, uniform coating over a surface

• It must be easily wetted by the penetrant at the flaw (the liquid must be allowed to spread over the particle surfaces)

• It must be nonfluorescent if used with fluorescent penetrants

• It must be easy to remove after inspection

• It must not contain ingredients harmful to parts being inspected or to equipment used in the inspection

operation

• It must not contain ingredients harmful or toxic to the operator

Four forms of developers commonly used are dry powder (Form A), water soluble (Form B), water suspendible (Form C), and nonaqueous solvent suspendible (Form D)

Dry Developers

Dry powder developers are widely used with fluorescent penetrants, but should not be used with visible-dye penetrants because they do not produce a satisfactory contrast coating on the surface of the workpiece Ideally, dry powder developers should be light and fluffy to allow easy application and should cling to dry surfaces in a fine film Powder adherence should not be excessive, because the amount of penetrant at fine flaws is insufficient to seep back into a thick coating

For purposes of storage, handling, and application, powders should not be hygroscopic and should remain dry Moisture impairs their ability to flow and dust easily, and they can agglomerate, pack, and lump up in containers and in developer chambers

Application and Removal Hand-processing equipment usually includes a developer station, which usually is an open

tank for dry developers Workpieces are dipped into the powder, or powder is picked up with a scoop or with the hands, and dropped onto the workpiece Excess powder is removed by shaking and tapping the workpiece Some powders are so light and fluffy that parts are dipped into them as easily as into a liquid

Other effective methods of application are rubber spray bulbs and air-operated spray guns An electrostatic-charged powder gun that can apply an extremely even and adherent coating of dry powder on metal parts also is used For simple application especially when only a portion of the surface of a large part is being inspected a very soft bristle brush often

is adequate

Powder can dry the skin and irritate the lining of air passages Operators should use rubber gloves and respirators Modern equipment often includes an exhaust system on the developer spray booth or on the developer dust chamber, which prevents dust from escaping Powder recovery filters often are included in such installations

Wet Developers

Three types of wet developers are: suspensions of developer powder in water (the most widely used), aqueous solutions of suitable salts, and suspensions of powder in volatile solvents

Water-suspendible developers can be used with both visible and fluorescent penetrants With a fluorescent

penetrant, the dried developer coating must not fluoresce or absorb or filter out black light used for inspection suspendible developers permit high-speed application of developer in mass inspection of small to medium-size workpieces using the fluorescent method A basket of small, irregularly shaped workpieces that has gone through the steps of penetrant application, penetrant dwell, and washing can be coated with developer in one quick dip in a water suspension This method not only is quick, but also, it provides thorough, complete coverage of all surfaces of the pieces being inspected No dry-powder application method has all these advantages to the same degree

Water-Wet developer is applied just after excess penetrant is washed away and immediately before drying After drying, surfaces are uniformly coated with a thin film of developer Developing time is decreased because heat from the drier helps to bring penetrant back out of surface openings, and the developing action occurs immediately with the developer film already in place Workpieces are ready for inspection in a shorter period of time, before excessive bleedout from large openings occurs, so better definition of flaw indications often is obtained

Water-suspendible developers are supplied as a dry powder concentrate, which is then dispersed in water in recommended proportions, usually from 0.04 to 0.12 kg/L ( to 1 lb/gal) The amount of powder in suspension must be carefully maintained Too much or too little developer on the surface of a workpiece can seriously affect sensitivity

Water-soluble developers can be used for both fluorescent and visible postemulsifiable and solvent-removable

penetrants Water-soluble developers are not recommended for use with water-washable penetrants, because of the potential to wash the penetrant from within the flaw if the developer is not very carefully controlled Water-soluble developers are supplied as a dry powder concentrate, which is then dispersed in water in recommended proportions, usually from 0.12 to 0.24 kg/L (1 to 2 lb/gal) Advantages of this form of developer are:

• The prepared bath is completely soluble and does not require any agitation

• The developer is applied prior to drying, thus decreasing the developing time

• The dried developer film on the workpiece is completely water soluble and is easily and completely removed following inspection by simple water rinsing

Nonaqueous solvent-suspendible developers are commonly used for both the fluorescent and the visible

penetrant process This form of developer produces a white coating on the surface of the part, which yields the maximum white color contrast with the red visible penetrant indication and extremely brilliant fluorescent indication

Nonaqueous solvent-suspendible developers are supplied in the ready-to-use condition and contain particles of developer suspended in a mixture of volatile solvents The solvents are carefully selected for their compatibility with the penetrants Nonaqueous solvent-suspendible developers also contain surfactants in a dispersant that coat the particles and reduce their tendency to clump or agglomerate

Nonaqueous solvent-suspendible developers are the most sensitive form of developer used with fluorescent penetrants because the solvent action contributes to the absorption and adsorption mechanisms In many cases where tight, small flaws occur, dry powder, water-soluble, and water-suspendible developers do not contact the entrapped penetrant This results in the failure of the developer to create the necessary capillary action and surface tension that serve to pull the penetrant from the flaw The nonaqueous solvent-suspendible developer enters the flaw and dissolves into the penetrant This action increases the volume and reduces the viscosity of the penetrant

The manufacturer must carefully select and compound the solvent mixture There are two types of solvent-based developers: nonflammable (chlorinated solvents) and flammable (nonchlorinated solvents) Both types are widely used Selection is based on the nature of the application and the type of alloy being inspected

Solvent developers are sometimes applied with a paintbrush, but this is likely to result in smeared indications; application

by a pressure spray can is a preferred method

Selection of Developer

Because developers play such an important role in penetrant inspection, it is very important to select the appropriate developer for a given job For example, on very smooth or polished surfaces, dry powder does not adhere satisfactorily, and wet developers do a better job Conversely, on very rough surfaces dry powder is far more effective

Following are some general rules regarding developer selection:

• Use a wet developer instead of a dry developer on very smooth surfaces

• Use a dry developer versus a wet developer on very rough surfaces

• Wet developers are better suited for high-production inspection of small workpieces because of their greater ease and speed of application

• Wet developers cannot be used reliably where sharp fillets unavoidably accumulate developer, which can mask flaw indications

• Solvent developers are effective for revealing fine, deep cracks, but are not satisfactory for finding wide, shallow flaws

• Cleaning and reinspecting a rough surface is difficult if a wet developer was used for a prior inspection

The developer does not produce indications but simply absorbs the penetrant already present in or at the flaw and makes it more visible

Equipment Requirements

With the exception of a source of ultraviolet (black) light for use with fluorescent penetrants, there is no special equipment that is absolutely essential for liquid-penetrant inspection Reasonably effective inspection can be performed with a minimum of simple and relatively crude equipment However, this approach should be considered only when: (a)

no more than a few workpieces are involved, (b) specific portions of very large workpieces are being inspected, (c) maximum sensitivity is not required, or (d) inspection must be performed in the field Therefore, most liquid-penetrant inspection is done with equipment designed specifically for the purpose

A variety of equipment is available "Package units" that incorporate all the necessary stations and controls are widely used, especially where relatively small workpieces in a variety of sizes and shapes are being inspected A typical package unit for inspection using a water-washable, fluorescent-penetrant system is shown in Fig 2 This system is designed to process a steady flow of workpieces, which move through seven stations: application of penetrant, draining excess penetrant, water rinsing, inspection under ultraviolet light to check thoroughness of rinsing, drying, application of developer, and final ultraviolet-light inspection for flaws The unit does not include stations for preliminary cleaning and postcleaning; these operations often are performed in another area The equipment shown in Fig 2 is available in a wide range of sizes and can be modified in many ways to fit specific needs For example, if a postemulsifiable system is used, workpieces are coated with emulsifier after the penetrant has been allowed to drain and prior to rinsing

Fig 2 Typical seven-station package equipment unit for inspecting workpieces using a water-washable,

fluorescent-penetrant system

Workpiece sizes and shapes, and production quantities, are the major factors that influence the selection of equipment An arrangement used in a foundry to process a variety of workpieces is shown in Fig 3

Fig 3 Equipment arrangement used in a foundry for liquid-penetrant inspection of a large variety of castings to

rigid specifications Castings are moved by crane and roller conveyor

Precleaning Regardless of the penetrant chosen, adequate precleaning of workpieces prior to penetrant inspection is

absolutely necessary for accurate results Inadequate removal of surface contamination can result in missed relevant indications because:

• The penetrant does not enter the flaw

• The penetrant loses its ability to reveal the flaw because it reacts with a substance contained in the flaw

• The surface immediately surrounding the flaw retains too much penetrant, which masks the true appearance of the flaw

Also, nonrelevant (false) indications can be caused by residual materials holding penetrants

Cleaning methods generally are classified as chemical, mechanical, solvent, and combinations of these

Chemical cleaning methods include alkaline or acid cleaning, pickling or chemical etching, and molten salt bath cleaning

Mechanical cleaning methods include tumbling, wet blasting, dry abrasive blasting, wire brushing, and high-pressure water or steam cleaning Mechanical cleaning can mask flaws by smearing adjacent metal over them and by filling them with abrasive material

Solvent cleaning methods include vapor degreasing, solvent spraying, solvent wiping, and ultrasonic immersion using solvents Ultrasonic immersion is by far the most effective means of ensuring clean parts, but it can be a very expensive capital equipment investment

Cleaning methods and their common uses are listed in Table 2 Major factors in the selection of a cleaning method is the type of contaminant to be removed, the type of alloy being cleaned, and knowing the chemical composition of the workpiece being cleaned It is good practice to test the method on known flaws to ensure that it will not mask the flaws

Table 2 Applications of various methods of precleaning for liquid-penetrant inspection

Method Use

Mechanical

Abrasive tumbling Removing light scale, burrs, welding flux, braze stopoff, rust, casting mold, and core material; should not be used

on soft metals such as aluminum, magnesium, and titanium

Dry abrasive grit

Same as dry except, where deposits are light, better surface and better control of dimensions are required

Wire brushing Removing light deposits of scale, flux, and stopoff

High-pressure

water and steam

Typically used with an alkaline cleaner or detergent; removing typical machine-shop soils such as cutting oils, polishing compounds, grease, chips, and deposits from electrical discharge machining; used when surface finish must be maintained; inexpensive

Ultrasonic cleaning Typically used with detergent and water or with a solvent; removing adherent shop soil from large quantities of

small parts

Chemical

Alkaline cleaning Removing braze stopoff, rust, scale, oils, greases, polishing material, and carbon deposits; typically used on large

articles where hand methods are too labor intensive; also used on aluminum for gross metal removal

Acid cleaning Strong solutions for removing heavy scale; solutions for light scale; weak (etching) solutions for removing lightly

Vapor degreasing Removing typical shop soil, oil, and grease; usually uses chlorinated solvents; not suitable for titanium

Solvent wiping Same as for vapor degreasing except a hand operation; can use nonchlorinated solvents; used for localized

low-volume cleaning

The surface finish of the workpiece must always be considered When further processing is scheduled, such as machining

or final polishing, or when a surface finish of 3.20 m (125 in.) or coarser is allowed, an abrasive cleaning method is frequently a good choice Generally, chemical cleaning methods have fewer degrading effects on surface finish than mechanical methods (unless the chemical used is strongly corrosive to the material being cleaned) Steam cleaning and solvent cleaning rarely have any effect on surface finish

Choice of cleaning method can be dictated by Occupational Safety and Health Administration and Environmental Protection Agency health and safety regulations Factors to consider include quantities of materials that will be used, toxicity, filtering, neutralization and disposal techniques, and worker safety

Penetrant Station The principal requirement of a penetrant station is to provide a means to coat workpieces with

penetrant the entire surface for small workpieces, or over small areas of large workpieces when only local inspection is required The station also should provide a means to drain excess penetrant back into the penetrant reservoir, unless the expendable technique is being used Draining racks usually serve the additional purpose of providing a storage place for parts during the time required for penetration (dwell time)

Emulsifier Station Emulsifier liquid is contained in a tank large enough to permit immersion of the workpieces, either

individually or in batches Accessory equipment includes covers to reduce evaporation and drain valves for cleanout when the bath has to be renewed Suitable drain racks are also a part of this station, to permit excess emulsifier to drain back into the tank

Large workpieces must be coated with emulsifier as fast as possible Multiple spraying or copious flowing of emulsifier from troughs or perforated pipes can be used on some types of automatic equipment For local coating of large workpieces, spraying often is satisfactory, using the expendable technique described for application of penetrant

Rinse Station Water rinsing (washing) of small workpieces frequently is done by hand, either individually or in

batches in wire baskets The workpieces are held in the wash tank and cleaned with a hand-held spray using water at tap pressure and temperature (water temperature should not, however, be below 10 °C, or 50 °F)

Drying Station The recirculating hot-air drier is one of the most important equipment components The drier must be

large enough to easily handle the type and number of workpieces being inspected Heat input, air flow, rate of movement

of workpieces through the drier, and temperature control are all factors that must be balanced The drier may be of the

cabinet type illustrated in Fig 2, or it can be designed so that the workpieces pass through on a conveyor (Fig 3) If conveyor operation is used, the speed must be coordinated with the required drying cycle

Developer Station The type and location of developer station depend on whether dry or wet developer is used For

dry developer, the developer station is downstream from the drier, whereas for wet developer, it immediately precedes the drier, following the rinse station

The dry-developer station usually consists of a simple bin containing the powder Dried workpieces are dipped into the powder and the excess powder is shaken off For larger workpieces that are difficult to immerse in the powder, a scoop can be used to throw powder over the surfaces, after which the excess is shaken off The developer bin should be equipped with an easily removable cover to protect the developer from dust and dirt when not in use

Dust-control systems are sometimes needed when dry developer is used (Fig 4)

Fig 4 Dry-developer bin equipped with dust-control and reclaimer system

The inspection station essentially is a worktable on which workpieces can be handled under proper lighting For

fluorescent methods, the table usually is surrounded by a curtain or hood to exclude most of the white light from the area (see Fig 2) For visible penetrants, a hood is not necessary

Black (ultraviolet) lights can consist of batteries of 100 or 400 watt lamps for area lighting, or, in small stations, can be one or two 100 watt spot lamps mounted on brackets from which they can be lifted and moved about by hand Because of the heat given off by black lights, good air circulation is essential in black-light booths

For automatic inspection, workpieces are moved through booths equipped with split curtains, either by hand or by conveyor (see Fig 3)

Postcleaning Station Postinspection cleaning often is necessary to remove all traces of penetrant and developer

Drastic chemical or mechanical methods are seldom required for postcleaning When justified by the volume of work, an emulsion cleaning line is effective and reasonable in cost In special circumstances, ultrasonic cleaning may be the only satisfactory way of cleaning deep crevices or small holes However, solvents or detergent-aided steam or water is almost always sufficient The use of steam with detergent is probably the most effective of all methods

Selection of Penetrant System

Size, shape, and weight of workpieces, as well as number of similar workpieces to be inspected, can influence the selection of a penetrant system

Sensitivity and Cost The required level of sensitivity and cost usually are the most important factors in selecting a

system The methods capable of the greatest sensitivity are also the most costly There are many inspection operations that require the ultimate in sensitivity, but there are also many where extreme sensitivity not only is not required, but also can produce misleading results

On a practical basis, the three major penetrant systems are broken down into six systems or variations of systems The six systems, in order of decreasing sensitivity and decreasing cost are:

Table 3 compares the sensitivities and uses of the six systems

Table 3 A comparison of penetrant systems

Water washable Postemulsifiable Solvent removable

Visible dye penetrants

Lowest in sensitivity Higher sensitivity than water washables Where water rinse is not feasible, or

desirable Suited for large surface areas For spot inspections

Suited for large surface areas

Suited for large quantities of similar objects Recommended for small areas and simple

For spot inspections

Suited for deep, narrow discontinuities Contaminants must be removed prior to

For environmental reasons, water washable penetrant systems are used even though the solvent system would be preferable

Processing Cycles for Water-Washable Systems

After the workpieces have been precleaned, processing for penetrant inspection should begin immediately A processing flow diagram for the water-washable system, from precleaning to postcleaning, is presented in Fig 5 Time in each station, equipment used, and other factors can vary widely, depending on workpiece size and shape, production quantities

of similar workpieces, and required sensitivity

Fig 5 Processing flow diagram for the water-washable, liquid-penetrant system

Processing Cycles for Postemulsifiable Systems