Volume 08 - Mechanical Testing and Evaluation Part 10 pdf

1 Comparison of static KIc, dynamic KId, and dynamic-instrumented KIdi impact fracture toughness of precracked specimens of ASTM A 533 grade B steel, as a function of test temperature..

39 K B Yoon, A Saxena, and D L McDowell, Influence of Crack-Tip Cyclic Plasticity on Creep-Fatigue

Crack Growth, Fracture Mechanics: Twenty Second Symposium, STP 1131, ASTM, 1992, p 367

40 A Saxena and B Gieseke, Transients in Elevated Temperature Crack Growth, International Seminar on High Temperature Fracture Mechanics and Mechanics, EGF-6, Elsevier Publications, 1990, p iii–19

41 N Adefris, A Saxena, and D.L McDowell, Creep-Fatigue Crack Growth Behavior in 1Cr-1Mo-0.25V

Steels I: Estimation of Crack Tip Parameters, J Fatigue Mater Struct., 1993

42 A Saxena, Limits of Linear Elastic Fracture Mechanics in the Characterization of High-Temperature

Fatigue Crack Growth, Basic Questions in Fatigue, Vol 2, STP 924, R Wei and R Gangloff, Ed.,

ASTM, 1989, p 27–40

43 “Practices of Load Verification of Testing Machines,” E 4 94, Annual Book of Standards, Vol 3.01,

ASTM, 1994

44 A Saxena, R.S Williams, and T.T Shih, Fracture Mechanics—13, STP 743, ASTM, 1981, p 86

45 “Test Method for Plane-Strain Fracture Toughness of Metallic Materials,” E 399, Annual Book of ASTM Standards, Vol 3.01, ASTM, 1994, p 680–714

46 A Saxena and J Han, “Evaluation of Crack Tip Parameters for Characterizing Crack Growth Behavior

in Creeping Materials,” ASTM Task Group E24-04-08/E24.08.07, American Society for Testing and Materials, 1986

47 H.H Johnson, Mater Res Stand., Vol 5 (No 9), 1965, p 442–445

48 K.H Schwalbe and D.J Hellman, Test Evaluation, Vol 9 (No 3), 1981, p 218–221

49 P.F Browning, “Time Dependent Crack Tip Phenomena in Gas Turbine Disk Alloys,” doctoral thesis, Rensselaer Polytechnic Institute, Troy, NY, 1998

50 W.R Caitlin, D.C Lord, T.A Prater, and L.F Coffin, The Reversing D-C Electrical Potential Method,

Automated Test Methods for Fracture and Fatigue Crack Growth, STP 877, W.H Cullen, R.W

Landgraf, L.R Kaisand, and J.H Underwood, Ed., ASTM, 1985, p 67–85

51 P.K Liaw, A Saxena, and J Schaefer, Eng Fract Mech., Vol 32, 1989, p 675, 709

52 P.K Liaw and A Saxena, “Remaining-Life Estimation of Boiler Pressure Parts—Crack Growth Studies,” Electric Power Research Institute, EPRI CS-4688, Project 2253-7, final report, July 1986

53 P.K Liaw, M.G Burke, A Saxena, and J.D Landes, Met Trans A, Vol 22, 1991, p 455

54 P.K Liaw, G.V Rao, and M.G Burke, Mater Sci Eng A, Vol 131, 1991, p 187

55 P.K Liaw, M.G Burke, A Saxena, and J.D Landes, Fracture Toughness Behavior in Ex-Service

Cr-Mo Steels, 22nd ASTM National Symposium on Fracture Mechanics, STP 1131, ASTM, 1992, p 762–

58 A Saxena, P.K Liaw, W.A Logsdon, and V.E Hulina, Eng Fract Mech., Vol 25, 1986, p 289

59 V.P Swaminathan, N.S Cheruvu, A Saxerna, and P.K Liaw, “An Initiation and Propagation Approach

for the Life Assessment of an HP-IP Rotor,” paper presented at the EPRI Conference on Life Extension and Assessment of Fossil Plants, 2–4 June 1986 (Washington, D.C.)

60 N.S Cheruvu, Met Trans A, Vol 20, 1989, p 87

61 R Viswanathan, Damage Mechanisms and Life Assessment of High-Temperature Components, ASM

International, 1989

62 C.E Jaske, Chem Eng Prog., April 1987, p 37

63 P.K Liaw, A Saxena, and J Schaefer, Creep Crack Growth Behavior of Steam Pipe Steels: Effects of

Inclusion Content and Primary Creep, Eng Fract Mech., Vol 57, 1997, p 105–130

Impact Toughness Testing

Introduction

DYNAMIC FRACTURE occurs under a rapidly applied load, such as that produced by impact or by explosive detonation In contrast to quasi-static loading, dynamic conditions involve loading rates that are greater than those encountered in conventional tensile tests or fracture mechanics tests Dynamic fracture includes the case

of a stationary crack subjected to a rapidly applied load, as well as the case of a rapidly propagating crack under

a quasi-stationary load In both cases the material at the crack tip is strained rapidly and, if rate sensitive, may offer less resistance to fracture than at quasi-static strain rates For example, values for dynamic fracture

toughness are lower than those for static toughness (KIc) in the comparison shown in Fig 1

Fig 1 Comparison of static (KIc), dynamic (KId), and dynamic-instrumented (KIdi ) impact fracture toughness of precracked specimens of ASTM A 533 grade B steel, as a function of test temperature The stress-intensity rate was about 1.098 × 10 4 MPa · s -1 (10 4 ksi · s -1 ) for the dynamic tests and about 1.098 × 10 6 MPa · s -1 (10 6 ksi · s -1 ) for the dynamic-instrumented tests Source: Ref 1

Because many structural components are subjected to high loading rates in service, or must survive high loading rates during accident conditions, high strain rate fracture testing is of interest and components must be designed against crack initiation under high loading rates or designed to arrest a rapidly running crack

Furthermore, because dynamic fracture toughness is generally lower than static toughness, more conservative analysis may require consideration of dynamic toughness

Measurement and analysis of fracture behavior under high loading rates is more complex than under static conditions There are also many different test methods used in the evaluation of dynamic fracture resistance Test methods based on fracture mechanics, as discussed extensively in other articles of this Section, produce quantitative values of fracture toughness parameters that are useful in design However, many qualitative methods have also been used in the evaluation of impact energy to break a notched bar, percent of cleavage area on fracture surfaces, or the temperature for nil ductility or crack arrest These qualitative tests include methods such as the Charpy impact test, the Izod impact test, and the drop-weight test Other less common tests are the explosive bulge test, the Robertson test, the Esso test, and the Navy tear test (described in the 8th Edition Metals Handbook, Volume 10, p 38–40)

quasi-This article focuses exclusively on notch-toughness tests with emphasis on the Charpy impact test The Charpy impact test has been used extensively to test a wide variety of materials Because of the simplicity of the Charpy test and the existence of a large database, attempts also have been made to modify the specimen, loading arrangement, and instrumentation to extract quantitative fracture mechanics information from the Charpy test Other miscellaneous notch-toughness test methods are also discussed in this article

Reference cited in this section

1 Use of Precracked Charpy Specimens, Fracture Control and Prevention, American Society for Metals,

1974, p 255–282

Impact Toughness Testing

History of Impact Testing

Before fracture mechanics became a scientific discipline, notched-bar impact tests were performed on laboratory specimens to simulate structural failures, eliminating the need to destructively test large engineering components The simulation of structural component failure by notched-bar impact tests is based on severe conditions of high loading rate, stress concentration, and triaxial stress state These tests have been extensively used in the evaluation of ductile-to-brittle transition temperature of low- and medium-strength ferritic steels used in structural applications such as ships, pressure vessels, tanks, pipelines, and bridges

The initial development of impact testing began around 1904 when Considére discovered and noted in a published document that increasing strain rate raises the temperature at which brittle fracture occurs In 1905 another Frenchman, George Charpy, developed a pendulum-type impact testing machine based on an idea by S.B Russell This machine continues to be the most widely used machine for impact testing In 1908 an Englishman by the name of Izod developed a similar machine that gained considerable popularity for a period

of time but then waned in popularity because of inherent difficulties in testing at temperatures other than room temperature

Impact testing was not widely used, and its significance not fully understood, until World War II when many all-welded ships were first built (approximately 3000 of them) Of these 3000 ships, approximately 1200 suffered hull fractures, 250 of which were considered hazardous In fact, 19 or 20 of them broke completely in two These failures did not necessarily occur under unusual conditions; several occurred while the ships were at anchor in calm waters In addition to ship failures, other large, rigid structures, such as pipelines and storage tanks, failed in a similar manner All failures had similar characteristics They were sudden, had a brittle appearance, and occurred at stresses well below the yield strength of the material It was noted that they originated at notches or other areas of stress concentration, such as sharp corners and weld defects These failures were often of considerable magnitude: in one case a pipeline rupture ran for 20 miles

The Naval Research Laboratory, along with others, launched a study of the cause of these fractures It was noted that often, but not always, failures occurred at low temperatures More detailed historical research

revealed that similar failures had been recorded since the 1800s but had been largely ignored The results of this study renewed interest, and further investigation revealed that materials undergo a transition from ductile behavior to brittle behavior as the temperature is lowered In the presence of a stress concentrator such as a notch, it takes little loading to initiate a fracture below this transition temperature, and even less to cause such a fracture to propagate These transitions were not predictable by such tests as hardness testing, tensile testing, or, for the most part, chemical analysis, which were common tests of the times It was then discovered that a ductile-to-brittle transition temperature could be determined by impact testing using test specimens of uniform configuration and standardized notches Such specimens were tested at a series of decreasing temperatures, and the energy absorbed in producing the fracture was noted The Charpy pendulum impact testing machine was used At first, test results were difficult to reproduce The problem was partly resolved by producing more uniformly accurate test equipment The notch most often used was of a keyhole type created by drilling a small hole and then cutting through the test bar to the hole by sawing or abrasive cutting It was soon found that by using specimens with sharper notches, better-defined transition temperatures that were more reproducible could

be determined A well-defined notch with a V configuration became the standard Steels in particular could then

be tested and the ductile-to-brittle transition temperature obtained

Two problems remained First, testing machines had to be standardized very carefully or the results were not reproducible from one machine to another The other problem was that the transition temperature found by testing small bars was not necessarily the same as that for full-size parts

Fortunately, the problem with standardization was resolved by the Army They learned that impact testing was

a necessity for producing successful armor plate and gun tubes Research at the Watertown Arsenal resulted in the development of standard test specimens of various impact levels The Army made these available to their various vendors so that the vendors could standardize their own testing machines This program was so successful that such specimens were made available to the public, at a nominal charge, starting in the 1960s Next, the manufacturers of testing equipment were pressured into making equipment available that would meet these exacting standards

The problem of differing transition temperatures for full-size parts and test specimens was discovered when a series of full-size parts was tested using a giant pendulum-type impact machine and these results were compared with those determined using small standard test bars made from the same material A partial solution

to this problem was the development of the drop-weight test (DWT) and the drop-weight tear test (DWTT) These tests produced transition temperatures similar to those found when testing full-size parts Unfortunately, such tests are adaptable only for plate specimens of limited sizes and have not become widely used

The Charpy V-notch test continues to be the most used and accepted impact test in use in the industry However, the restricted applicability of the Charpy V-notch impact test has been recognized for many years (Ref 2) Charpy test results are not directly applicable for designs, and the observed ductile-to-brittle transition depends on specimen size Nonetheless, the Charpy V-notch test is useful in determining the temperature range

of ductile-to-brittle transition

Reference cited in this section

2 C.E Turner, Impact Testing of Metals, STP 466, ASTM, 1970, p 93

Impact Toughness Testing

Types of Notch-Toughness Tests

In general, notch toughness is measured in terms of the absorbed impact energy needed to cause fracturing of the specimen The change in potential energy of the impacting head (from before impact to after fracture) is determined with a calibrated dial that measures the total energy absorbed in breaking the specimen Other quantitative parameters, such as fracture appearance (percent fibrous fracture) and degree of ductility/deformation (lateral expansion or notch root contraction), are also often measured in addition to the

fracture energy Impact tests may also be instrumented to obtain load data as a function of time during the fracture event In its simplest form, instrumented impact testing involves the placement of a strain gage on the tup (the striker)

Many types of impact tests have been used to evaluate the notch toughness of metals, plastics, and ceramics In general, the categories of impact tests can be classified in terms of loading method (pendulum stroke or drop-weight loading) and the type of notched specimen (e.g., Charpy V-notch, Charpy U-notch, or Izod) The following descriptions briefly describe the key types of impact tests that are used commonly in the evaluation

of steels or structural alloys

The Charpy and Izod impact tests are both pendulum-type, single-blow impact tests The principal difference, aside from specimen and notch dimensions, is in the configuration of the test setup (Fig 2) The Charpy test involves three-point loading, where the test piece is supported at both ends as a simple beam In contrast, the Izod specimen is set up as a cantilever beam with the falling pendulum striking the specimen above the notch (Fig 2b)

Fig 2 Specimen types and test configurations for pendulum impact toughness tests (a) Charpy method (b) Izod method

The Charpy V-notch test continues to be the most utilized and accepted impact test in use in the industry It is written into many specifications While this test may not reveal exact ductile-to-brittle transition temperatures for large full-size parts, it is easily adaptable as an acceptability standard on whether or not parts are apt to behave in a brittle manner in the temperature range in which they are likely to be used

The drop-weight test is conducted by subjecting a series (generally four to eight) of specimens to a single impact load at a sequence of selected temperatures to determine the maximum temperature at which a specimen breaks The impact load is provided by a guided, free-falling weight with an energy of 340 to 1630 J (250 to

1200 ft · lbf) depending on the yield strength of the steel to be tested The specimens are prevented by a stop from deflecting more than a few tenths of an inch

This is a “go, no-go” test in that the specimen will either break or fail to break It is surprisingly reproducible For example, Pellini made 82 tests of specimens from one plate of semikilled low-carbon steel At -1 °C (30 °F) and 4 °C (40 °F), all specimens remained unbroken At -7 °C (20 °F), only one of 14 specimens broke; however, at -12 °C (10 °F), 13 of the 14 specimens broke At temperatures below -12 °C (10 °F), all specimens broke

The drop-weight tear test (DWTT) uses a test specimen that resembles a large Charpy test specimen The test specimen is 76 mm (3 in.) wide by 305 mm (12 in.) long, supported on a 254 mm (10 in.) span The thickness

of the specimen is the full thickness of the material being examined The specimens are broken by either a falling weight or a pendulum machine The notch in the specimen is pressed to a depth of 5 mm (0.20 in.) with

a sharp tool-steel chisel having an angle of 45° The resulting notch root radius is approximately 0.025 mm (0.001 in.) One result of the test is the determination of the fracture appearance transition curve The “average” percent shear area of the broken specimens is determined for the fracture area neglecting a region “one thickness” in length from the root of the notch and “one thickness” from the opposite side of the specimen These regions are ignored because it is believed that the pressing of the notch introduces a region of plastically deformed material which is not representative of the base material Similarly the opposite side of the specimen

is plastically deformed by the hammer tup during impact The fracture appearance plotted versus temperature defines an abrupt transition in fracture appearance This transition has been shown to correlate with the transition in fracture propagation behavior in cylindrical pressure vessels and piping

Impact Toughness Testing

Charpy Impact Testing

As previously noted, the specimen in the Charpy test is supported on both ends and is broken by a single blow from a pendulum that strikes the middle of the specimen on the unnotched side The specimen breaks at the notch, the two halves fly away, and the pendulum passes between the two parts of the anvil The height of fall minus the height of rise gives the amount of energy absorption involved in deforming and breaking the specimen To this is added frictional and other losses amounting to 1.5 or 3J (1 or 2 ft · lbf) The instrument is calibrated to record directly the energy absorbed by the test specimen

Methods for Charpy testing of steels are specified in several standards including:

Designation Title

ASTM E 23 Standard Test Methods for Notched Bar Impact Testing of Metallic Materials

BS 131-2 The Charpy V-Notch Impact Test on Metals

BS 131-3 The Charpy U-Notch Impact Test on Metals

BS 131-6 Method for Precision Determinations of Charpy V-Notch Impact Energies for Metals

ISO 148 Steel—Charpy Impact Test (V-Notch)

ISO 83 Steel—Charpy Impact Test (U-Notch)

DIN-EN 10045 Charpy Impact Test of Metallic Materials

These standards provide requirements of test specimens, anvil supports and striker dimensions and tolerances, the pendulum action of the test machine, the actual testing procedure and machine verification, and the determination of fracture appearance and lateral expansion

The general configuration of the Charpy test, as shown in Fig 3 for a V-notch specimen, is common to the requirements of most standards for the Charpy test Differences between ASTM E 23 and other standards include differences in machining tolerances, dimensions of the striker tip (Fig 4), and the ASTM E 23 requirements for testing of reference specimens The most pronounced difference between standards is the different geometry for the tip of the striker, or tup The tup in the ASTM specification (Fig 4a) is slightly flatter than in many other specifications (Fig 4b) From a comparison of results from Charpy tests with the two different tup geometries, differences appeared more pronounced for several steels at impact energies above 100

J (74 ft · lbf) (Ref 3) From this evaluation, a recommendation was also made to use the sharper and smoother tup (Fig 4b) if the national standards are unified further

Fig 3 General configuration of anvils and specimen in Charpy test

Fig 4 Comparison of striker profiles for Charpy testing (a) ASTM E 23 (b) Other national and international codes: AS1544, Part 2; BS 131, Part 2; DIN 51222; DS10 230; GOST 9454; ISO R148; JIS B7722; NF A03-161; NS 1998; UNI 4713-79 Source: Ref 3

There are also three basic types of standard Charpy specimens (Fig 5): the Charpy V-notch, the Charpy Notch, and the Charpy keyhole specimen These dimensions are based on specifications in ASTM E 23, ISO

U-148, and ISO 83 The primary specimen and test procedure involves the Charpy V-notch test Other type specimens are not used as extensively because their degree of constraint and triaxiality is considerably less than the V-notch specimen

Charpy-Fig 5 Dimensional details of Charpy test specimens most commonly used for evaluation of notch toughness (a) V-notch specimen (ASTM E 23 and ISO 148) (b) Keyhole specimen (ASTM E 23) (c) U- notch specimen (ASTM E 23 and ISO 83)

The Charpy V-notch impact test has limitations due to its blunt notch, small size, and total energy measurement (i.e., no separation of initiation and propagation components of energy) However, this test is used widely because it is inexpensive and simple to perform Thus, the Charpy V-notch test commonly is used as a screening test in procurement and quality assurance for assessing different heats of the same type of steel Also, correlation with actual fracture toughness data is often devised for a class of steels so that fracture mechanics analyses can be applied directly Historically, extensive correlation with service performance has indicated its usefulness

The keyhole and U-notches were early recognized (1945) as giving inadequate transition temperatures because

of notch bluntness Even the V-notch does not necessarily produce a transition temperature that duplicates that

of a full-size part Under current testing procedures, the Charpy V-notch test is reproducible and produces close approximations of transition temperatures found in full-size parts It is widely used in specifications to ensure that materials are not likely to initiate or propagate fractures at specific temperature levels when subjected to impact loads

Equipment

Charpy testing requires good calibration methods Machine belting should be examined regularly for looseness, and broken specimens should be examined for unusual side markings Anvils should also be examined for wear Testing Machines Charpy impact testing machines are available in a variety of types Some are single-purpose machines for testing Charpy specimens only Others are adaptable to testing Izod and tension impact specimens also They are offered in a range of loading capacities The most common of these capacities are 325 and 160 J (240 and 120 ft · lbf) Some machines have variable load capabilities, but most are of a single-fixed-load type When purchasing or using a machine, be sure that the available loading is such that specimens to be tested will break with a single blow, within 80° of the machine capacity (as shown by the scale on the machine)

While loading capacity depends on the anticipated strength of specimens to be tested, the maximum value of such specimens is the principal consideration Very tough specimens may stop the hammer abruptly without breaking A number of such load applications have been known to cause breakage of the pendulum arm On the other hand, lower-capacity machines may be more accurate and more likely to meet standardization requirements For most ordinary steel testing applications, the machine with a capacity of 160 J (120 ft · lbf) makes a good compromise choice Testing of a large number of very tough specimens may require a machine with a capacity of 325 to 400 J (240–300 ft · lbf)

Charpy impact machines are of a pendulum type They must be very rigid in construction to withstand the repeated hammering effect of breaking specimens without affecting the operation of the pendulum mechanism The machine must be rigidly mounted Special concrete foundations are sometimes used, but at least the machine must be bolted down to an existing concrete foundation, which should be a minimum of 150 mm (6 in.) thick The pendulum should swing freely with a minimum of friction Any restriction in movement of the pendulum will increase the energy required to fracture the specimen This produces a test value that is higher than normal There will always be small effects of this type, and they are usually compensated for, along with windage friction effects, by scale-reading adjustments built into the equipment

While the pendulum must be loose enough to swing freely with little friction, it must not be loose enough to produce inaccuracies, such as nonuniform striking of the specimen The components must be sturdy enough to resist deformation at impact This is particularly true of the anvil and pendulum It is important that the instrument be level Some machines have a built-in bubble-type level Others have machined surfaces where a level can be used In operation, the pendulum is raised to the proper height and held by a cocking mechanism that can be instantly released

ASTM E 23 specifies that tests should be made at velocities between 3 and 6 m/s (10 and 20 ft/s) and that this

is defined as “the maximum tangential velocity of the striking member at the center of strike.” When hanging freely, the striking tup of the pendulum should be within 2.5 mm (0.10 in.) of touching the area of the specimen where first contact will be made The anvil that retains the test specimen must be made such that the specimen can be squarely seated The notch must be centered so that the pendulum tup hits directly behind it

Most impact testing machines have scales that read directly in foot-pounds (scales also may read in degrees)

As noted, the scale can be adjusted to compensate for windage, pendulum friction, and other variations The scale should read zero when the pendulum is released without a specimen being present Pendulum and anvil design, configuration, and dimensions are important It is also important that the broken specimens be able to fly freely without being trapped in the anvil by the pendulum Proper anvil design, such as that shown in Fig 6, can minimize jamming

Fig 6 Typical anvil arrangement with modification that reduces the possibility of jamming

Specimens As previously noted, there are three commonly used standard Charpy impact test specimens, which are similar except for the notch (Fig 5) The V-notch bar is the most frequently used specimen, although some specific industries still use the other types of test bars The steel casting industry, for instance, uses the keyhole-notch specimen more frequently There are also many varieties of subsize specimens that should be used only when insufficient material is available for a full-size specimen, or when the shape of the material will not allow removal of a standard specimen

It is important that specimens be machined carefully and that all dimensional tolerances be followed Care must

be exercised to ensure that specimens are square It is easy to grind opposite sides parallel, but this does not ensure squareness The machining of the notch is the most critical factor The designated shape and size of the notch must be strictly followed, and the notch must have a smooth (not polished) finish Special notch-broaching machines are available for V-notching A milling machine with a fly cutter can also be used

In preparing keyhole-notch specimens, the hole should be drilled at a low speed to avoid heat generation and work hardening Use of a jig with a drill bushing ensures accuracy After the hole has been drilled, slotting can

be done by almost any method that meets specifications, but care should be exerted to prevent the slotting tool from striking the back of the hole In all cases it is desirable to examine the notch at some magnification A stereoscopic microscope or optical comparator is suitable for this examination In fact, a V-notch template for use with the optical comparator can be used to ensure proper dimensions

Specimens must generally be provided with identification markings This is best done on the ends of the specimen In preparing specimens where structural orientation is a factor (e.g., rolling direction of wrought materials), such orientation should be taken into consideration and noted, because orientation can cause wide variations in test results If not otherwise noted, the specimen should be oriented in the rolling direction of the plate (forming direction of any formed part) and the notch should be perpendicular to that surface (orientation

A in Fig 7) This produces maximum impact values All notching must be done after any heat treatment that might be performed

Fig 7 Effect of specimen orientation on impact test results

While correlation exists between full-size specimens and subsize specimens, such correlation is not direct Many specifications (ASTM and ASME, for example) specify differing acceptable values for various specimen sizes (Table 1)

Table 1 Conversion table for subsize Charpy impact-test specimens

Minimum average impact strength for three specimens

Minimum impact strength for one specimens or for set of three specimens

Calibration ASTM E 23 goes into considerable detail to ensure proper calibration of testing machines Other relevant standards for qualification or calibration of the test machines are:

ASTM E

1236

Standard Practice for Qualifying Charpy Impact Machines as Reference Machines

BS 131-7 Verification of the Test Machine Used for Precision Determination of Charpy V-Notch

Test Method

Once the equipment has been properly set up and calibrated and the specimens have been correctly prepared, testing can be done Prior to each testing session, the pendulum should be allowed at least one free fall with no test specimen present, to confirm that zero energy is indicated Specimen identification and measurements are then recorded along with test temperature The pendulum is cocked, and the specimen is carefully positioned in the anvil using special tongs (Fig 8) that ensure centering of the notch The quick-release mechanism is actuated, and the pendulum falls and strikes the specimen, generally causing it to break The amount of energy absorbed is recorded (normally in foot-pounds), and this data is noted adjacent to the specimen identification on the data sheet The broken specimens are retained for additional evaluation of the fracture appearance and for measurement of lateral expansion where required The broken halves are often placed side by side, taped together, and labeled for identification

Fig 8 Use of tongs to place a specimen in a Charpy impact testing machine for testing

The release mechanism must be consistent and smooth Test specimens must leave the impact machine freely, without jamming or rebounding into the pendulum; requirements on clearances and containment shrouds are specific to individual machine types The test specimen must be accurately positioned on the anvil support within 5 s of removal from the heating (or cooling) medium; requirements for heating time depend on the heating medium Identification marks on test specimens must not interfere with the test; also, any heat treatment

of specimens should be performed prior to final machining

A daily check procedure of the apparatus must be conducted to ensure proper performance Verification of the testing system is required using Army Materials and Mechanics Research Center (AMMRC) standardized specimens; verification should be completed at least once a year or after any parts are replaced or any repairs or adjustments are made to the machine An operational testing sequence is recommended, as well as specifics on dial energy reading, lateral expansion measurement (technique and measuring fixture), and fracture appearance estimation

Test Temperature Specimen temperature can drastically affect the results of impact testing If not otherwise stated, testing should be done at temperatures from 21 to 32 °C (70–90 °F) Much Charpy impact testing is done

at temperatures lower than those commonly designated as room temperature Of these low-temperature tests, the majority are made between room temperature and -46 °C (-50 °F), because it is within this range that most ductile-to-brittle transition temperatures occur A certain amount of testing is also done down to -196 °C (-320

°F) for those materials that may be used in cryogenic service Some additional testing (mainly research) is done

at the liquid helium and liquid hydrogen temperatures (-269 and -251 °C, or -452 and -420 °F) Such testing requires special techniques and will not be discussed here For testing at temperatures down to or slightly below -59 °C (-75 °F), ethyl alcohol and dry ice are most commonly used This combination solidifies at around -68

°C (-90 °F) A suitable insulated container should be used to cool the test specimens (a container insulated with

a layer of styrofoam works fine) A screen-type grid raised at least 25 mm (1 in.) above the bottom of the container allows cooling liquid to circulate beneath the specimens A calibrated temperature-measuring device,

such as a low-temperature glass or metal thermometer or a thermocouple device, should be placed so as to read the temperature near the center of a group of specimens being cooled The solution should be agitated sufficiently to ensure uniformity of bath temperature The cooling liquid should cover the specimen by at least

25 mm (1 in.) The specimen-handling tongs should be placed in the same cooling bath as the specimens When the specimens have been placed in the alcohol bath along with the tongs, chips of solid CO2 (dry ice) can be added and the solution agitated Experience will dictate the amount of dry ice required to reach a certain temperature Once the temperature is reached, it seems to hold steady with only an occasional addition of a small chip of dry ice The specimens in a liquid bath should be held within +0 and -1.5 °C (+0 and -3 °F) of test temperature for at least 5 min prior to testing The specimens should then be removed one at a time with the cooled tongs and tested within 5 s of removal from the bath Watch the temperature between tests because the tongs can raise the bath temperature if left out of the bath too long The commercial cooling baths that are available range from insulated stainless steel containers to containers with self-contained refrigeration units Also available are thermocouple devices that can be placed in the cooling bath and will give a digital temperature readout Dry ice cannot be stored for any length of time, but there is a device that produces

“instant” dry ice from a CO2 compressed gas bottle Testing between -59 and -196 °C (-75 and -320 °F) requires a liquid medium that will not solidify at these temperatures Various liquids are available One that has been successfully used is isohexane (adequate ventilation should be provided and care exercised to avoid inhalation of the volatile organic fumes) Liquid nitrogen replaces the dry ice as a coolant material, and the procedure is then similar to that for dry ice and alcohol It is wise to keep handy a large, easy-to-handle piece of metal to serve as a temperature moderator in case the temperature becomes lower than desired It can be plunged into the bath and, acting as a heat sink, can cause the temperature to rise quickly

High-Temperature Testing Occasionally, high-temperature impact testing is performed This can be done using

an agitated, high-flashpoint oil (heat treating quenching oils may work) or other liquid medium that is stable at the desired test temperature The bath and specimens are then held at temperature in a furnace or oven for at least 10 min prior to testing

Test Results

Results of impact testing are determined in three ways In the first method, already discussed, they can be read directly from the testing machine (in joules or foot-pounds) This is the most commonly specified test result It

is desirable to test three specimens at each test temperature; the average value of the three is the test result used

If a minimum test value is specified for material acceptance, not more than one test result of the three should fall below that value If the value of one of the three specimens is about 6 J (5 ft · lbf) lower than the average, or lower than the average value by greater than of the specified acceptance value, the material should be either rejected or retested In retesting, three additional specimens must be tested, and all must equal or exceed the specified acceptance value Since it is often required or important to determine the ductile-to-brittle transition temperature, impact test results are plotted against test temperature Somewhere in that transition zone between the high-energy and low-energy values is an energy value that can be defined as the transition temperature When the transition is very pronounced, this value is easily determined However, because the more common case is a less sharply defined transition, an energy value may be specified below which the material is considered to be brittle (below the ductile-to-brittle transition temperature) Such a value may vary with material type and requirements, but the value of 20 J (15 ft · lbf) is often used as a specified value

Fracture Appearance Method Other methods of specifying ductile-to-brittle transition temperature are

sometimes presented along with the energy values obtained The first of these auxiliary tests is the appearance method The fractured impact bars are examined and the fractures compared with a series of

fracture-standard fractures or overlays of such fractures By this method the percentage of shear fracture is determined The amount of shear fracture can also be determined in another way This is done by carefully measuring the dimensions of the brittle cleavage exhibited on the specimen fracture surface (Fig 9), and then referring to Table 2 These methods are described in detail in ASTM A 370 The percentage of shear can be plotted against test temperature and the transition temperature can be ascertained using the shear percentage value specified

Table 2 Tables of percent shear for measurements made in both inches and millimeters for impact-test specimens

Because these tables are set up for finite measurements or dimensions A and B (see Fig 9), 100% shear is to be reported when either A or B is zero

Fig 9 Sketch of a fractured impact test bar The method used in calculating percent shear involves measuring average dimensions A and B to the nearest 0.5 mm (0.02 in.) and then consulting a chart (Table 2) to determine the percent shear fracture (Courtesy of ASTM)

Unlike Charpy energy, fracture appearance is indicative of how a specimen failed It is therefore useful when attempting to correlate results of Charpy testing with other toughness test methods that use different specimen geometries and loading rates However, the fracture-appearance method can also be subjective In one round-robin test survey of 20 specimens (Ref 6), results showed that agreement was best when operators are experienced, samples are close to the fracture-appearance transition, and when simple, two-dimensional figures are used for assessment

Lateral-Expansion Method The other auxiliary method of determining transition temperature is the expansion method This procedure is based on the fact that protruding shear lips are produced (perpendicular to

lateral-the notch) on both sides of each broken specimen The greater lateral-the ductility, lateral-the larger lateral-the protrusions This lateral expansion can be expressed as a measure of acceptable ductility at a given test temperature The broken halves from each end of each specimen are measured The higher values from each side are added together, and this total is the lateral-expansion value A minimum value of lateral expansion must be specified as a transition value These test results are then plotted against test temperature and a curve interpolated The impact energy (in joules or foot-pounds) is also reported These methods are described in detail in ASTM A 370 and E 23

Applications

Test criteria for Charpy V-notch impact testing usually involve:

• A minimum impact energy value

• Shear appearance of fractured test bars expressed in percent

• Lateral expansion

For steels, the minimum acceptable values most commonly specified for these three evaluation methods are, respectively: 20 J (15 ft · lbf), 50% shear, and 1.3 mm (50 mil) As a general rule of thumb, Charpy V-notch impact strengths of 14 J (10 ft · lbf) and lower are likely to initiate fractures An impact strength of 27 J (20 ft · lbf) is likely to propagate brittle fracture once initiated, and values well above 27 J (20 ft · lbf) are necessary to arrest fracturing once it has been initiated

Charpy impact testing does not produce numbers that can be used for design purposes, but is widely used in specifications such as ASTM A 593, “Specification for Charpy V-Notch Testing Requirements for Steel Plates for Pressure Vessel.” Other applications are briefly described below

Nuclear Pressure Vessel Design Code For nuclear pressure vessels, the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Ref 7) and the Code of Federal Regulations (Ref 8) currently use fracture mechanics principles that dictate toughness requirements for pressure vessel steels and weldments The specified toughness requirements are obtained using Charpy V-notch test specimens coupled with the nil-ductility transition temperature (NDTT) per ASTM E 208 The actual approach involves a

reference temperature, designated RTNDT, and the reference fracture toughness curve, KIR The reference fracture toughness curve defined in Appendix G, Section III, of the ASME Code uses an experimentally

determined relationship between toughness and temperature that is adjusted along the temperature axis according to an index reference temperature

The reference toughness curve, KIR, is assumed to describe the minimum (lower bound) fracture toughness for all ferritic materials approved for nuclear pressure boundary applications having a minimum specified yield

strength of 345 MPa (50 ksi) or less The value of RTNDT is obtained by measuring the drop-weight nil-ductility transition temperature and performing standard Charpy V-notch tests The nil-ductility transition temperature is determined initially, and then a set of three Charpy V-notch specimens is tested at a temperature that is 33 °C

(60 °F) higher than the nil-ductility transition temperature to measure the temperature, TCV, which ensures an increase in toughness with temperature Charpy energies of 68 J (50 ft · lbf) and lateral expansion of 0.89 mm (35 mil) are used to ensure this condition

The nil-ductility transition temperature becomes the RTNDT temperature if the Charpy results equal or exceed

the above limits If the Charpy values at TCV or the nil-ductility transition temperature plus 33 °C (60 °F) are lower than required, additional Charpy tests should be performed at higher test temperatures, usually in

increments of 5.6 °C (10 °F), until the requirements are satisfied and TCV is measured The RTNDT temperature

then becomes the temperature (TCV) at which the criteria are met minus 33 °C (60 °F) Thus, the reference temperature is always either greater than or equal to the nil-ductility transition temperature

Steel Bridge Toughness Criteria The American Association of State Highway and Transportation Officials (AASHTO) has adopted Charpy impact toughness requirements for primary tension members in bridge steels based on section thickness, yield strength, and expected service temperature They are based on the fracture toughness corresponding to the maximum loading rate expected in service (Ref 9)

Correlations with Fracture Toughness Empirical attempts have been made to correlate the Charpy impact

energy with KIc to allow a quantitative assessment of critical flaw size and permissible stress levels Most of these correlations are dimensionally incompatible, ignore differences between the two measures of toughness (in particular, loading rate and notch acuity), and are valid only for limited types of materials and ranges of data Additionally, these correlations can be widely scattered However, some correlations can provide a useful guide to estimating fracture toughness; in fact, the preceding design criteria for nuclear pressure vessel and bridge steels are partially based on such correlative procedures

Some of the more common correlations are listed in Table 3 (Ref 9, 10, 11, 12, 13, 14, 15, and 16) with appropriate units Note that some of the correlations attempt to eliminate the effects of loading rate; the

dynamic fracture toughness, KId, is correlated with Charpy energy Other attempts have been made to improve and explain some of the correlations (see, for example, Ref 17) A study has also been conducted using a portion of the Charpy energy to separate initiation and propagation components in the Charpy test (Ref 18) The

results from this study for an upper-shelf JIc correlation for pressure vessel steels were not significantly better than the Rolfe-Novak correlation listed in Table 3 A statistically based correlation for lower-bound toughness has also been developed for pressure vessel steels (Ref 19, 20) Thus, simple and empirical correlations can be

used as general guidelines for estimating KIc or KId within the limits of the specific correlation

Table 3 Typical Charpy/KIc correlation for steels

Begley-Logsdon—three points (Ref 12)

(KIc)1 = 0.45 σy at 0% shear fracture temperature

(KIc)2 From Rolfe-Novak Correlation at 100% shear fracture

temperature

(KIc)3 = [(KIc)1 + (KIc)2] at 50% shear fracture temperature

KIc = ksi

σ y = ksi

Marandet-Sanz—three steps (Ref 13)

T100 = 9 + 1.37 T28J

KIc = 19 (CVN) 1/2

Shift KIc curve through T100 point

T100 = °C, for which KIc = 100 MPa

T28 = °C, for which CVN = 28J

KIc = MPa CVN = J

Wullaert-Server (Ref 14)

Ic,d = ksi CVN = ft · lbf

σ y = ksi corresponding to approximate loading rate

Upper-shelf region

Rolfe-Novak—σy> 100 ksi (Ref 15)

Ic = ksi CVN = ft · lbf

σ y = ksi

Ault-Wald-Bertolo—ultrahigh-strength steels (Ref 16)

(KIc /σ y ) 2 = 1.37 (CVN/σ y ) - 0.045 K

Ic = ksi CVN = ft · lbf

σ y = ksi

1.0 ksi = 6.8948 MPa; 1.0 ksi = 1.099 MPa ; 1.0 ft · lbf = 1.356 J; CVN is the designation for Charpy impact energy; σy is the yield stress; and E is the Young's modulus

As previously described, a lower-bound KIR toughness curve is shifted relative to a reference temperature,

RTNDT, and used to define the ductile-to-brittle transition The RTNDT is a critical value and is defined very conservatively in terms of Charpy and dynamic tear specimen results Continued application of these requirements is now a principal limitation to continued operation of several commercial nuclear power plants (Ref 21) Recent work by ASTM Committee E-8 has proposed a method to obtain a new reference temperature and a method to define, using a probabilistic approach, a median ductile-to-brittle transition curve from a set of six properly tested small samples that would, in many cases, be precracked Charpy specimens Statistical confidence bounds would then be available for this median transition “master curve,” which would be specific

to the particular nuclear plant of interest and could be used to assure that the pressure vessel had adequate toughness for continued operation

A generalized prediction method to predict KIc transition curves has also been developed with data from various steels including 2.25Cr-1Mo, 1.25Cr-0.50Mo, 1Cr and 0.50Mo chemical pressure vessel steels, and ASTM A

508 C1.1, A 508 C1.2, A 508 C1.3 and A 533 Gr.B C1.1 nuclear pressure vessel steels (Ref 22) This method

consists of a master curve of KIc and a temperature shift, ΔT, between fracture toughness and Charpy V-notch impact transition curves versus yield strength relationship for T0, where T0 is the temperature showing 50% of

the upper-shelf KIc value The KIc transition curves predicted using both methods showed a good agreement

with the lower bound of measured KIc values obtained from elastic-plastic, Jc, tests

References cited in this section

3 O.L Towers, Effects of Striker Geometry on Charpy Results, Met Constr., Nov 1983, p 682–685

4 J.M Holt, Ed., Charpy Impact Test: Factors and Variables, STP 1072, ASTM, 1990

5 T.A Siewert and A.K Schmieder, Ed., Pendulum Impact Machines: Procedures and Specimens for Verification, STP 1248, ASTM, 1995

6 B.F Dixon, Reliability of Fracture Appearance Measurement in the Charpy Test, Weld J., Vol 73 (No

8), Aug 1994, p 39–46

7 “Rules for Construction of Nuclear Power Plant Components,” ASME Boiler and Pressure Vessel Code, Section III, Division 1 , Appendices, Nonmandatory Appendix G, American Society for Mechanical Engineers, 1983

8 Energy (Title 10), Domestic Licensing of Production and Utilization Facilities (Part 50), Code of Federal Regulations, U.S Government Printing Office, 1981

9 J.M Barsom, The Development of AASHTO Fracture Toughness Requirements for Bridge Steels, Eng Fract Mech., Vol 7 (No 3), Sept 1975, p 605–618

10 J.M Barsom and S.T Rolfe, Correlations Between KIc and Charpy V-Notch Test Results in the

Transition Temperature Range, Impact Testing of Materials, STP 466, ASTM, 1979, p 281–302

11 R.H Sailors and H.T Corten, Relationship Between Material Fracture Toughness Using Fracture

Mechanics and Transition Temperature Tests, Fracture Toughness, Proceedings of the 1971 National Symposium on Fracture Mechanics—Part II, STP 514, ASTM, 1972, p 164–191

12 J.A Begley and W.A Logsdon, “Correlation of Fracture Toughness and Charpy Properties for Rotor Steels,” WRL Scientific Paper 71-1E7-MSLRF-P1, Westinghouse Research Laboratory, Pittsburgh, PA, July 1971

13 B Marandet and G Sanz, Evaluation of the Toughness of Thick Medium-Strength Steels by Using

Linear Elastic Fracture Mechanics and Correlations Between KIc and Charpy V-Notch, Flaw Growth and Fracture, STP 631, ASTM, 1977, p 72–95

14 R.A Wullaert, Fracture Toughness Predictions from Charpy V-Notch Data, What Does the Charpy Test Really Tell Us?: Proceedings of the American Institute of Mining, Metallurgical and Petroleum Engineers, American Society for Metals, 1978

15 S.T Rolfe and S.R Novak, Slow-Bend KIc Testing of Medium-Strength High-Toughness Steels, Review

of Developments in Plane-Strain Fracture Toughness Testing, STP 463, ASTM, 1970, p 124–159

16 “Rapid Inexpensive Tests for Determining Fracture Toughness,” National Materials Advisory Board, National Academy of Sciences, Washington, D.C., 1976

17 What Does the Charpy Test Really Tell Us?: Proceedings of the American Institute of Mining, Metallurgical and Petroleum Engineers, American Society for Metals, 1978

18 D.M Norris, J.E Reaugh, and W.L Server, A Fracture-Toughness Correlation Based on Charpy

Initiation Energy, Fracture Mechanics: Thirteenth Conference, STP 743, ASTM, 1981, p 207–217

19 W.L Server et al., “Analysis of Radiation Embrittlement Reference Toughness Curves,” EPRI

NP-1661, Electric Power Research Institute, Palo Alto, CA, Jan 1981

20 Metal Properties Council MPC-24, Reference Fracture Toughness Procedures Applied to Pressure

Vessel Materials, Proceedings of the Winter Annual Meeting of the American Society for Mechanical Engineers, American Society of Mechanical Engineers, New York, 1984

21 J.A Joyce, Predicting the Ductile-to-Brittle Transition in Nuclear Pressure Vessel Steels from Charpy

Surveillance Specimens, Recent Advances in Fracture, Minerals, Metals and Materials Society/AIME,

1997, p 65–75

22 T Iwadate, Y Tanaka, and H Takemata, Prediction of Fracture Toughness KIc Transition Curves of

Pressure Vessel Steels from Charpy V-Notch Impact Test Results, J Pressure Vessel Technol (Trans ASME), Vol 116 (No 4), p 353–358

Impact Toughness Testing

Instrumented Charpy Impact Test

The use of additional instrumentation (typically an instrumented tup) allows a standard Charpy impact machine

to monitor the analog load-time response of Charpy V-notch specimen deformation and fracturing The primary advantage of instrumenting the Charpy test is the additional information obtained while maintaining low cost, small specimens, and simple operation The most commonly used approach is application of strain gages to the striker to sense the load-time behavior of the test specimen In some cases, gages are placed on the specimen as well, such as for the example shown in Fig 10 (Ref 23)

Fig 10 Charpy specimen with additional instrumentation at the supports

General Description

Instrumentation of the tup provides valuable data in terms of the load-time, P-t, history during impact Extensive efforts have been made to help determine the dynamic fracture toughness, KId, over a range of behavior in linear-elastic, elastic-plastic, and fully plastic regimes An overview of these efforts is given in Ref

the instantaneous load on the specimen at any particular time, ti; the actual energy absorbed, ΔEi, simplifies to (Ref 25):

(Eq 1)

where Eo is the total available kinetic energy of the pendulum ( m · ) and:

where Vo is the initial impact velocity, and m is the effective mass of the pendulum The ability to separate the

total absorbed energy into components greatly augments the information gained by instrumentation temperature diagrams can be constructed to illustrate the various fracture process stages indicative of the fracture mode transition from brittle to ductile behavior (Ref 26)

Load-Fig 11 Load-time response for a medium-strength steel PM, maximum load; PGY, general yield load; PF ,

fast fracture load (generally cleavage); PA, arrest load after fast fracture propagation; tM , time to

maximum load; tGY, time to general yield; WM , energy absorbed up to maximum load

One of the primary reasons for the development of the instrumented Charpy test was to apply existing notch bend theories (slow bend) to the dynamic three-point bend Charpy impact test Obtaining load information during the standard Charpy V-notch impact test establishes a relationship between metallurgical fracture parameters and the transition temperature approach for assessing fracture behavior (Ref 27) Initial studies concentrated on the full range of mechanical behavior from fully elastic in the lower Charpy shelf region to elastic-plastic in the transition region to fully plastic in the upper shelf region (see Fig 11)

Most studies have been performed on structural steels, with primary emphasis on the effect of composition, strain rate, and radiation on the notch bend properties Interest in instrumented impact testing has expanded to include testing of different types of specimens (e.g., precracked, large bend), variations in test techniques (e.g., low blow, full-size components), and testing of many different materials (e.g., plastics, composites, aerospace materials, ceramics) The many variations in test methods is a motivation for standardized test methods, although standardization for instrumented Charpy testing has been slow (see the section “Standards and Requirements” in this article)

Instrumentation

Instrumentation for a typical Charpy impact testing system includes an instrumented striker, a dynamic transducer amplifier, a signal-recording and display system, and a velocity-measuring device The instrumented striker is the dynamic load cell, which is securely attached to the falling weight assembly The striker has cemented strain gages to sense the compression loading of the tup while it is in contact with the test specimen The dynamic transducer amplifier provides direct-current power to the strain gages and typically amplifies the strain gage output after passing through a selectable upper-frequency cutoff

The impact signal is recorded and stored either on a storage oscilloscope or through the use of a transient signal

recorder Digital data from a transient recorder can be reconverted back to analog form and plotted on an x-y

recorder, or the digital data can be transferred to a computer for direct analysis

Triggering is best accomplished through an internal trigger that has the ability to capture the signal preceding the trigger; external triggering from the velocity-sensing device is often used instead of an appropriate internal trigger The velocity-measuring system should be a noncontacting, optical system that clocks a flag on the impacting mass immediately before impact so that initial velocity measurements can be made Velocities must

be determined for all impact drop heights used

The impact machine and the instrumentation package must be calibrated to ensure reliable data Calibration of the Charpy pendulum impact machine is performed in accordance with ASTM E 23, as discussed previously in this article in terms of periodic proof testing of AMMRC calibration specimens to ensure reliable dial energy values

Instrumentation calibration consists of a time base and load-cell calibration with a system frequency response measurement The time base calibration consists of passing a known time mark pulse through the system and calibrating accordingly The load-cell calibration is typically accomplished by testing notched specimens of 6061-T651 aluminum that are only slightly loading-rate sensitive over the range used (Ref 28) The load cell is calibrated when the measured dynamic limit load is only slightly higher than the predetermined quasi-static limit load (measured using the same loading arrangement and anvil dimensions) and when the dial energy (or velocity-determined energy measurement) matches the integrated total energy The relationship used for

obtaining total absorbed energy, ΔEo, from the area under the load-time record follows the approach in Eq 1 and 2

The calculated ΔEo value will match the dial energy reading when the system is calibrated (in addition to the limit load check) Because the aluminum limit load is fairly low (around 7.1 kN, or 1600 lbf), a check on load-cell linearity at higher loads is also needed To accomplish this, the integrated energy/dial energy requirement for a quenched and tempered 4340 specimen (52 HRC) that has a higher fracture load (near 27 kN, or 6000 lbf)

is checked

Low-energy AMMRC calibration specimens can be used for this procedure If the energies match for the 4340 test at the same amplifier gain as for the aluminum calibration, the load-cell calibration is usually linear throughout the usable load range Static linearity checks can also be made if the static loading system exactly duplicates the dynamic loading conditions Daily test checks using the aluminum calibration specimens are suggested to verify load-cell calibration

The system frequency response is determined experimentally by superimposing a constant-amplitude sine wave signal on the output of the strain gage bridge circuit (Ref 29) The peak-to-peak amplitude of the signal should

be equivalent to approximately half the full-scale capacity of the load transducer at a frequency low enough to ensure no signal attenuation The frequency of the sine wave is then increased until the amplitude is attenuated

10% (0.915 dB), and the response time, tR, is calculated as:

(Eq 3)

where f0.915 is the frequency at 0.915 dB (10%) attenuation

Standards and Requirements

Instrumented impact tests that generate P-t plots from instrumented tups require careful attention to test

procedures and analytical methods in order to determine dynamic fracture toughness values with the accuracy and reliability required for engineering purposes Extensive efforts have been made to standardize instrumented impact tests, but many inherent difficulties in analysis and interpretation have impeded the formal development

of standard methods Nonetheless, instrumented impact testing is an accepted method in the evaluation of

irradiation embrittlement of nuclear pressure vessel steels (Ref 30) Several instrumented impact tests have also been developed for plastics (Ref 31) with the ISO standard 179-2 on instrumented Charpy testing of plastics (Ref 32) The following discussions focus on requirements for steels, while more information on impact testing

of plastics and ceramics are addressed in the article“Mechanical Testing of Polymers and Ceramics” in this Volume For nonmetallic materials, such as plastics and ceramics, the application of available models involving energy considerations may be necessary for arriving at the true toughness values (Ref 24)

Standard Methods Extensive efforts in the development of instrumented Charpy tests began in the 1960s and 1970s with the advent of fracture mechanics and precracked Charpy V-notch specimens, when a series of seminars and conferences in the 1970s (Ref 33, 34, 35, and 36) examined the role of instrumented impact testing in the evaluation of dynamic fracture toughness (Ref 24) The International Institute of Welding first attempted to standardize the instrumented Charpy test, but concluded that the test was not sufficiently documented, and the effort was discontinued (Ref 37) A few years later, two significant events prompted

serious consideration of standardization The development of the KIR curve by the Pressure Vessel Research Committee and its inclusion in the ASME Code, Section III, created the need for dynamic initiation toughness,

KId, data Simultaneously, two other related groups began formulating procedures and conducting interlaboratory round robins The Pressure Vessel Research Committee/Metals Property Council Task Group on

Fracture Toughness Properties for Nuclear Components developed procedures for measuring KId values from precracked Charpy specimens (Ref 38)

The Electric Power Research Institute (EPRI) funded work to develop procedures known as the “EPRI Procedures” (Ref 28, 39) This procedure is summarized in the following section, “General Test Requirements.” Since that time, important theoretical and technical developments have occurred, as outlined in Ref 24 Efforts have also been made in the development of standards In 1992, the European Structural Integrity Society (ESIS) formed a working party (formed within ESIS Technical Subcommittee 5) devoted to instrumented impact testing on subsize Charpy-V specimens of metallic materials In 1994, ESIS issued a draft of a standard method for the instrumented Charpy V-notch test on metallic materials (Ref 40) This method allows one to estimate an approximate value of the proportion of ductile fracture surface by one of the following formulas:

where PGY, PM, PIU, and PA are characteristic points on the load-time diagram shown in Fig 12

Fig 12 Load vs time record showing the definitions of the various load points used in various models to

estimate the percent shear fracture; PGY, characteristic value for onset of plastic deformation; PM ,

maximum load; PIU, load at the initiation of unstable crack propagation; PA , load at the end of unstable crack propagation

The working group also performed round-robin testing to help develop the state of knowledge on the dynamic behavior of miniaturized impact specimens (Ref 41) In 1992, a formal committee also was formed for development of a possible JIS standard for evaluation of dynamic fracture toughness by the instrumented Charpy impact testing method Problems to be resolved before the standardization of the instrumented Charpy impact test method are pointed out in Ref 42

General Test Requirements Only subtle differences exist between the “EPRI Procedures” (Ref 28) and the

Pressure Vessel Research Committee procedures for measuring KId values from precracked Charpy specimens (Ref 38, 43) The following test requirements are taken from the EPRI procedures

The load signal obtained from an instrumented striker during an impact test oscillates about the actual load required to deform the specimen Therefore, the signal analysis procedure employed should minimize the deviation of the apparent load from the actual specimen deformation load A simplistic view of the impact event allows three major areas for test specification to be identified: initial loading, limited frequency response, and electronic curve fitting

The impact loading of a specimen will create inertial oscillations in the contact load between striker and specimen, and a time interval between 2τ and 3τ is required for the load to be dissipated, where τ is related to the period of the apparent specimen oscillations and can be predicted empirically for a span-to-width ratio of 4 by:

(Eq 4)

where W is the specimen width, B is the specimen thickness, Cs is the specimen compliance, E is the Young's modulus, So is the speed of sound in the specimen, and τ is typically 30 μs for standard Charpy steel specimens

When any time, t, is less than 2τ, it is not possible to use the striker signal to measure the portion of the

specimen load caused by inertial effects An empirical specification for reliable load and time evaluation is:

Control of t is obtained by control of the initial impact velocity The constant 3 in Eq 5 may be as low as 2.3

without adversely affecting the test results, if the curve-fitting technique described below is followed A value

of 3 was chosen for the case of “unlimited” frequency response The original EPRI procedures corresponded to the 2.3 factor and included the selective filtering for curve fitting (Ref 28) Computer simulations of the Charpy test have approximately verified the value of τ and the 3τ criterion (Ref 44)

The potential problem of limited frequency response of the transducer amplifier is avoided by specifying:

where tR is defined as the 0.915 dB response time of the instrumentation, as indicated in Eq 3 Inadequate response results in a distorted signal response It is important to note that the electronic attenuation must be representative of a resistance-capacitance circuit for Eq 6 to apply

The curve fitting of the oscillations is achieved by specifying a minimum tR The amplitude of the observed oscillations is therefore reduced such that the disparity between tup contact load and effective deformation load

is minimal For the best test, it has been empirically found for resistance-capacitance circuit systems that:

is adequate for the electronic curve fitting without altering the overall curve, when t ≥ 2.3τ When t ≥ 3τ, it is

not necessary to electronically curve fit because the disparity between the contact load and the specimen deformation load is less than approximately 5%

The requirements for obtaining acceptable load-time records (in particular, Eq 5) result in the need to control

Vo By controlling the impact velocity, a corresponding control of kinetic energy (Eo) is inherent The reduction

in striker velocity during the impact loading of the specimen should therefore be minimized A conservative requirement is:

where WM is the system energy dissipated to maximum load PM This requirement ensures that the tup velocity

is not reduced by more than 20% up to maximum load This requirement is seldom a problem for full-impact Charpy V-notch tests; Eq 8 may not be met, however, when precracked Charpy tests are conducted for very tough materials The test requirements for reliable load measurement are summarized as follows:

Inertial effects t ≥ 3τ

Limited frequency response t ≥ 1.1tR, required only if 2.3τ ≤ t <3τ

Electronic curve fitting tR ≥ 1.4τ

Energy criterion Eo ≥ 3WM

The time t corresponds to the shortest time required for measurement after the specimen has been impacted; that is, t is the time to maximum load tM for the elastic fracture, and t is the time to general yield, tGY, in the

postgeneral yield fracture (See Fig 11) The specification for electronic curve fitting is only required if 2.3τ ≤ t

< 3τ Because it is often difficult to ensure that t ≥ 3τ and because the filtering has no adverse effect when t ≥ 2.3τ filtering at tR ≥ 1.4τ is always possible, assuming that t ≥ 1.1tR

Limitations on Testing Violation of any of the general test requirements presented above will invalidate the data obtained from instrumented Charpy V-notch tests Limitations of this testing technique are the same as those for standard Charpy testing The effects of small size relative to typical component size, the rounded machine notch, and shallow notch depth restrict general applicability and usefulness of the Charpy test Note that the notch depth for the Charpy V-notch specimen is too shallow to prevent yielding across the gross section

of the specimen

Instrumentation has allowed separation of energy components and measurement of applied loads throughout the fracture event, but direct determination of the initiation component is not directly possible for ductile (microvoid coalescence) initiation from the instrumented test record Some of these limitations have been addressed by fatigue precracking the Charpy specimen, which eliminates the notch effects and makes it a small fracture-mechanics-type specimen

References cited in this section

23 W Schmitt, W Böhme, and D.-Z Sun, New Developments in Fracture Toughness Evaluation,

Structural Integrity: Experiments, Models, Applications—European Conference of Fracture (ECF) 10,

Vol 1, Engineering Materials Advisory Service, 1990, p 159–170

24 P.R Sreenivasan, Instrumented Impact Testing—Accuracy, Reliability, and Predictability of Data,

Trans Indian Inst Met., Vol 49 (No 5), Oct 1996, p 677–696

25 B Augland, Fracture Toughness and the Charpy V-Notch Test, Br Weld J., Vol 9 (No 7), 1962, p 434

26 G.D Fearnehough and C.J Hoy, Mechanism of Deformation and Fracture in the Charpy Test as

Revealed by Dynamic Recording of Impact Loads, J Iron Steel Inst Jpn., Vol 202, 1964, p 912

27 R.A Wullaert, Application of the Instrumented Charpy Impact Test, Impact Testing of Metals, STP 466,

ASTM, 1970, p 148–164

28 D.R Ireland, W.L Server, and R.A Wullaert, “Procedures for Testing and Data Analysis,” ETI Report TR-75-43, Effects Technology, Inc., Santa Barbara, CA, Oct 1975

29 D.R Ireland, Procedures and Problems Associated with Reliable Control of the Instrumented Impact

Test, Instrumented Impact Testing, STP 563, ASTM, 1974, p 3–29

30 L.E Steele, Ed., Radiation Embrittlement of Nuclear Pressure Vessel Steels: An International Review,

STP 1011, ASTM, 1989

31 S Kessler, G.C Adams, S.B Driscoll, and D.R Ireland, Instrumented Impact Testing of Plastics and Composites Materials, STP 936, ASTM, 1986

32 “Plastic—Determination Of Charpy Impact Properties: Part 2—Instrumented Impact Test,” ISO-179-2, International Organization for Standardization

33 Impact Testing of Metals, STP 466, ASTM, 1970

34 Instrumented Impact Testing, STP 563, ASTM, 1974

35 C.E Turner, in Advanced Seminar on Fracture Mechanics, EURA-TOM-ISPRA Courses, 1975

36 Dynamic Fracture Toughness, The Welding Institute and The American Society for Metals, 1976

37 E.C.J Buys and A Cowan, Interpretation of the Instrumented Impact Test, Weld World, Vol 8 (No 1),

1970, p 70–76

38 Pressure Vessel Research Committee/Metal Properties Council Working Group on Instrumented Precracked Charpy Testing, “Instrumented Precracked Charpy Testing: Report I—Recommended Testing Procedure, Report II—Associated Test Program,” Westinghouse Research Laboratory, Pittsburgh, PA, 1974

39 R.A Wullaert, Ed., CSNI Specialist Meeting on Instrumented Precracked Charpy Testing, Nov 1980,

EPRI NP-2102-LD, Electric Power Research Institute, 1981

40 “ESIS Instrumented Charpy V-Notch Standard,” Proposed Standard Method for the Instrumented Charpy-V Impact Tests on Metallic Materials, Draft 10, ESIS, Jan 14, 1994

41 E Lucon, Instrumented Impact Testing of Sub-Size Charpy-V Specimens: The Activity of the ESIS

TC5 Working Party, ECF 11—Mechanisms and Mechanics of Damage and Failure, Vol 1, Elsevier,

1996, p 621

42 T Kobayashi and I Yamamoto, Progress and Development in the Instrumented Charpy Impact Testing

Method, Bull Jpn Inst Met., Vol 32 (No 3), 1993, p 151–159 (in Japanese)

43 W.L Server, Impact Three-Point Bend Testing for Notches and Precracked Specimens, J Test Eval.,

Vol 6 (No 1), 1978, p 29–34

44 D.M Norris, D Quiñones, and B Moran, Computer Simulation of Plastic Deformation in the Charpy

V-Notch Impact Test, What Does the Charpy Test Really Tell Us?: Proceedings of the American Institute of Mining, Metallurgical and Petroleum Engineers, American Society for Metals, 1978, p 22–

32

Impact Toughness Testing

Precracked Charpy Test

By inducing a fatigue precrack in the Charpy specimen, the notch acuity and depth restrictions are eliminated Early work concentrated on correlations with fracture toughness using only the total absorbed energy (i.e.,

uninstrumented testing) These energy values usually are normalized per unit area (A) below the fatigue crack; the normalized energy values are designated as W/A

Most of the correlations of W/A with fracture toughness have been conducted using slow-bend specimens The

basic problem in reaching an impact correlation is the difference in loading rates between the Charpy impact

and the static KIc tests, particularly for loading-rate sensitive materials (Ref 45) A general trend exists for a correlation between /E and W/A, but the limited data and scatter make this difficult to utilize (Ref 16) A better correlation with KId may be possible The reason for using /E as the basis is the approximate

proportionality between /E and W/A, based on a presumed fracture mechanics relationship (Ref 45)

The precracked Charpy W/A values can also be used to estimate the nil-ductility transition temperature The

typical technique defines an inflation point between lower shelf and transition region behavior as the estimated nil-ductility transition temperature (Ref 46) Some exceptions have been noted to this approach (Ref 47)

Instrumented Data

The types of data and test techniques used for instrumented precracked Charpy testing are the same as those discussed earlier for instrumented Charpy impact testing The 3τ criterion, which limits the impact velocity, becomes more important for deeply cracked, brittle materials The greatest advantage of precracking is the transformation of the Charpy V-notch specimen into a dynamic fracture mechanics test piece The direct calculation of fracture toughness (within certain limitations) is now possible using the instrumented load-time information The following discussion presents the calculational aspects of these fracture toughness parameters

If fracture is known to initiate at maximum load (as it usually does for cleavage initiation), the energy value of

WM (see Fig 11) can be considered an initiation energy However, WM includes contributions other than that caused by the deflection of the specimen Therefore, a compliance energy correction is needed to determine the

true specimen energy, EM (Ref 48) When the fracture is linear elastic (fracture before general yield; see the

first two load-time records in Fig 11), the value of EM can be calculated directly:

(Eq 9)

where CND is the nondimensional specimen compliance (Ref 49) For a fracture occurring after general yield

(see Fig 11), EM is obtained by correcting WM:

(Eq 10)

where CT is the total system compliance calculated at general yield and corrected for the decrease in velocity through general yield:

(Eq 11)

This compliance correction is assumed to be linear with load, but the actual correction is not so simple

However, the error in assuming a linear relationship results in a slightly smaller (conversvative) value of EM

(Ref 43)

It is often desirable to partition the total fracture energy into initiation and postinitiation (propagation)

components Assuming initiation occurs at maximum load, the propagation energy, EP, is:

where ET is the total fracture energy, as determined from a dial indicator, kinetic energy change (initial and

final velocity measurements), or ΔEo

Linear Elastic Fracture Toughness When the fracture is elastic (fracture occurs before general yield), the

stress-intensity factor, KIc, can be calculated by applying linear elastic fracture mechanics:

(Eq 13)

where a is the crack length

The ASTM size requirements for a valid KIc are quite limiting, even if a dynamic yield strength is used However, the general specimen size requirements of ASTM E 399 may be too conservative for dynamic testing

of ferritic medium-strength steels (Ref 50) Therefore, if general yielding has not occurred, a linear-elastic value

of fracture toughness, KIc, generally is calculated The stress intensification rate is calculated as:

(Eq 14) This loading rate reflects the dynamic aspect of the loading, because the lowest for impact loading of precracked Charpy specimens is on the order of 11 × 104 MPa · s-1 × (1 × 105 ksi · s-1)

Postgeneral Yield Fracture Toughness When general yielding occurs, an energy-based value of the J-integral can be used to obtain a measure of fracture toughness The calculation of ductile fracture toughness, JIc, is contingent upon knowing the initiation point of fracture on the load-time record For cleavage-initiated fracture, this point generally corresponds to maximum load However, for fibrous (ductile) initiation, maximum load is generally a nonconservative assumption When the initiation point is known or has been determined

experimentally (Ref 51) and when a/W ≥ 0.5 (Ref 52), then:

(Eq 18)

where σf is the flow stress, defined as the average of the yield stress and the ultimate stress For dynamic loading, the yield stress, σy, and flow stress, σf, of standard Charpy V-notch specimens can be estimated for postgeneral yield behavior as:

(Eq 19) and

(Eq 20) The general form of the equation results from slip-line field solutions for blunt-notch specimens, and the constant 2.99 has been obtained from extrapolation of results from a slip-line field solution that included the tup indentation at the center loading point (Ref 53) The constant of 2.99 reduces to 2.85 for sharp-notch specimens with a fatigue precrack The stress values obtained using this approach agree favorably with high rate tensile test results (Ref 54)

Limitations The test requirements and data analysis procedures described in this article were developed for ferritic pressure vessel steels A review of instrumented precracked Charpy testing can be found in Ref 55, which discusses the theory and applicability of instrumented precracked Charpy testing Not all of the relationships and approaches presented in Ref 55 are universally accepted because standards or recommended practices do not currently exist

Related Test Techniques

Several attempts have been made to use the precracked Charpy specimen at loading rates beyond the limits applicable to quasi-static analysis The procedures described above assume a quasi-static situation for times greater than the limiting values near 3τ One such attempt for larger than Charpy-size specimens is described in Ref 56, in which strain gages were mounted near the crack tip to avoid many of the spurious wave effects Other studies have been conducted using Hopkinson bar techniques (Ref 57) and the shadow optic method of caustics (Ref 58) These studies indicate the need for dynamic analysis when using the instrumented Charpy striker approach at high loading rates

References cited in this section

16 “Rapid Inexpensive Tests for Determining Fracture Toughness,” National Materials Advisory Board, National Academy of Sciences, Washington, D.C., 1976

43 W.L Server, Impact Three-Point Bend Testing for Notches and Precracked Specimens, J Test Eval.,

Vol 6 (No 1), 1978, p 29–34

45 T.M.F Ronald, J.A Hall, and C.M Pierce, Usefulness of Precracked Charpy Specimens for Fracture

Toughness Screening Tests of Titanium Alloys, Metall Trans., Vol 3, April 1972, p 813–818

46 G.M Orner and C.E Hartbower, Transition-Temperature Correlations in Construction Alloy Steels,

Weld J., Vol 40 (No 9), Oct 1961, p 459s

47 J.H Gross, The Effect of Strength and Thickness on Notch Ductility, Weld J., Vol 48 (No 10), Oct

52 J.D.G Sumpter and C.E Turner, Method for Laboratory Determination of Jc, Cracks and Fracture, STP

Impact Toughness Testing

Drop Weight Testing

Because Charpy V-notch testing does not necessarily reveal the same transition temperature as that observed for full-size parts, many other tests have been devised Two such tests have achieved some degree of popularity These are the drop-weight test (DWT) and the drop-weight tear test (DWTT) Both of these tests were developed at the United States Naval Research Laboratory Both tests yield a transition temperature that more nearly coincides with that of full-size parts This has been described as the nil-ductility temperature (NDT) Both tests have limited usage because of the required specimen sizes There are three types of DWT specimens,

as shown in Table 4 The smallest of these measures 16 × 51 × 127 mm ( × 2 × 5 in.), and thus, when four to eight specimens are required, a considerable amount of material is expended Often parts are not of sufficient size or are not shaped in such a manner to allow preparation of such specimens A provision is made for remelting and casting material to specimen size Most DWT tests are made on plate that is 9.5 mm (⅜ in.) thick

or thicker The DWTT is also a plate testing specification This test requires a specimen 76 × 305 mm (3 × 12 in.) by full plate size

Table 4 Standard drop-weight test (DWT) conditions

Drop-weight energy for given yield strength level(a)

Deflection stop,

mm (in.)

Yield strength level,

7.6 (0.3)

620–760 (90– 1650 1200

110) 210–410 (30–60) 350 250

The same piece of test equipment is used for both the drop-weight test and the drop-weight tear test The difference lies in the anvil that holds the specimen The principal requirement is that sufficient impact can be generated to produce cracking This is commonly provided by a vertical structure on which weights can be attached using guides The weights are raised to a measured height, quickly released, and guided to strike the specimen properly

The Drop-Weight Test

The DWT specimen and procedure are shown in Fig 13 and are described in ASTM E 208 The crack inducer

is a bead of hard-facing metal approximately 76 mm (3 in.) long The specimen, 89 × 356 × 19 mm (3½ × 14 ×

¾in.), is placed, weld down, on rounded end supports and is struck by a 27 kg (60 lb) falling weight with sufficient energy to bend the specimen about 5° A cleavage crack forms in the bead as soon as incipient yield occurs (at about 3° deflection), thus forming the sharpest possible notch, a cleavage crack in the test specimen

A series of specimens is tested over a range of temperatures to find the nil-ductility transition temperature

Fig 13 Drop-weight test method

The weld bead is deposited on one side of the specimen at the center using a copper template The weld bead is purposely a hard, brittle deposit (the Murex-Hardex N electrode is recommended) A notch is made in the weld bead, but not in the specimen itself The specimen, after being cooled to the desired temperature, is placed in the anvil with the notched weld deposit facing downward The weight is dropped, striking the back side of the specimen (the amounts of weight and height depend on the strength of the material being tested; see Table 4) The specimen is allowed to deflect slightly under the impact load, controlled by deflection stops This initiates a crack at the notch in the weld bead When the crack reaches the specimen material it will be either propagated

or arrested The specimen is then examined to see whether or not it has fractured A specimen is considered to

be broken if the crack extends to one or both sides of the specimen surface with the weld bead If the crack does not propagate to the edge it is considered a “no break.” If the weld notch is not visibly cracked, or if complete deflection does not occur (determined by mark transfer on the deflection stops), it is considered a “no test.” The nil-ductility transition temperature (NDTT) is the maximum temperature at which the specimen breaks When minimum temperatures are set in material specifications, at least two specimens must be tested at the specified temperature All specimens tested shall show a “no-break” performance

Drop-Weight Tear Test Procedure

The drop-weight tear test (ASTM E 436) is similar in some ways to the drop-weight test The transition fracture appearance occurs at the same temperature as for full-size parts It has the same sudden change from shear to cleavage as that observed in full-scale pieces of equipment The test is relatively simple in terms of both specimen preparation and lack of sensitivity to specimen-preparation techniques The results vary with specimen thickness in the same manner as actual parts do The principal short coming, as in the drop-weight test, is that testing is confined to plate material between 3 and 19 mm (0.125 and 0.750 in.) thick The test specimen is even larger than the DWT specimen: it is 76 mm (3 in.) wide by 305 mm (12 in.) long Tests are made with the same apparatus used for the drop-weight test, but the test fixture for holding the specimen is altogether different A large pendulum-type machine can also be used, but the vertical weight-dropping apparatus is more commonly used Up to 2700 J (2000 ft · lbf) of energy may be required

Test-Specimen Anvil The holder for the test specimen must support the specimen on edge (305 mm, or 12 in., long edge) in such a manner that rotation will not occur when the specimen is struck This usually requires adjustable supports for differing specimen thicknesses Hardened supports at each end suspend the specimen, and a centering guide at one end centers the specimen A slot in the center of the anvil allows downward clearance for the breaking specimen (Fig 14)

Fig 14 Drop-weight tear test specimen and support dimensions

Specimen Preparation Test specimens can be removed by sawing, shearing, or flame cutting Specimen dimensions and tolerances are shown in Fig 14 A notch is impressed at the center of one of the 305 mm (12 in.) edges This is accomplished by using a sharp tool-steel chisel that is hardened The sharp edge should have

an angle of 45 ± 1 or 2° The depth of the notch is 0.5 ± 0.05 mm (0.020 ± 0.002 in.) The combination of the sharpness of the notch (radius of about 0.013 mm, or 0.0005 in.) and the cold working that occurs as the result

of impressing it produces cleavage fractures under the notch

Test Procedure Specimens are tested at various temperatures They are brought to the desired temperature by immersing them in a cooled solution and holding for at least 15 min at temperature The bath should be agitated, and if several specimens are cooled simultaneously they should be separated by several specimen thicknesses Specimens should be broken within 10 s after they are removed from the bath The cooled specimen is inserted in the anvil so that the notch is directly beneath the point of load application, and the test load, which must be only of sufficient magnitude to produce a fracture, is suddenly applied If the specimen buckles under the test load, the test is considered to be invalid Otherwise, the specimen fractures and separates

as it moves into the slotted anvil without the two pieces being jammed against one another

Test Results The test is evaluated by examining the broken pieces The idea is to determine the percentages of the fracture surface that exhibit ductile shear and brittle cleavage The two regions are very different in appearance, and the transition from one to the other is abrupt There are two methods of making this evaluation One is for percentages of shear from 45 to 100% (Fig 15), and the other for percentages from 0 to 45% (Fig 16) The acceptance criterion is percentage of shear at a specific temperature The temperature at which 50% shear occurs is sometimes considered the ductile-to-brittle transition temperature Such tests have often been used for evaluation of line-pipe material for natural-gas transmission lines The specimens from the curved pipe may be flattened prior to testing

Fig 15 Method of calculating percent shear for drop-weight tear testing by examination and measurements made of the fractures This method applies when the percent shear is between 45% and

100% %SA is percent shear area; A is the width of the cleavage fracture at the one “t” line beneath the notch, in.; and B is the length of the cleavage fracture in between the two “t” lines, in

Fig 16 Method of shear calculation for drop-weight tear testing when the percent shear is less than 45%

Impact Toughness Testing

Other Impact Tests

Izod impact testing uses a specimen with a V-notch (Fig 17) that is similar to the Charpy V-notch specimen The principal difference is that the specimen is gripped at one end only, allowing the cantilevered end to be struck by the pendulum (Fig 18) An advantage of this method is that several notches can be made in a single specimen and the ends broken off one at a time The disadvantage that has caused it to lose popularity is that the required time for and method of clamping the specimen in an anvil preclude low-temperature testing Izod specimens can also be round Many testing machines can be used for both Charpy and Izod testing

Fig 17 Izod specimen

Fig 18 Cross section depicting clamped specimen and contact point for testing All dimensional tolerances are ±0.05 mm (0.002 in.) unless otherwise specified The clamping surfaces of A and B are flat and parallel within 0.025 mm (0.001 in.) Finish on unmarked parts is 2 μm (63 μin.) Striker width must

be greater than that of the specimen being tested

Impact toughness values from the British Standard Izod test are compared with various other methods of testing

in Fig 19 These graphs, derived from a large number of test results on carbon and low-alloy steels, are only intended to show comparative trends The curves in Fig 19 should not be used for comparing or compiling specifications (Ref 59)

Fig 19 Impact values obtained with the British Standard Izod test and other test methods The inner dotted band represents the area within which 50% of the results may be expected to fall, while the wider full band covers approximately 80% of results Source: Ref 59

The one-point bend test uses a single-edge cracked specimen and the same testing arrangement as a conventional three-point bend test, except that the end supports are removed (Fig 20) The specimen holder used in a Charpy or Izod test is replaced by a simple frame that supports the specimen, while allowing it to move freely in the horizontal plane Depending on the design of the original pendulum and hammer, the impact tester may require retrofitting with a new hammer and striker that will not interfere with the specimen edges or the support frame When the hammer strikes the specimen, the center portion of the specimen is accelerated away from the hammer; the end portions of the specimen lag behind because of inertia This causes the specimen to bend and to load the crack tip

Fig 20 Experimental setup used to perform the one-point bend test

The primary advantage of the one-point bend test is that the measured stress-intensity history incorporates dynamic effects completely Therefore, no limits need to be imposed on the impact velocity and the test duration to fracture Use of the one-point bend test currently is restricted to small-scale yielding conditions (Ref 60)

Dynamic Notched Round Bar Testing The dynamic notched round bar specimen is a long cylindrical bar with a fatigue precrack (Fig 21) During the test, the specimen is loaded in tension at one end by an impact of sufficiently large magnitude that the resulting stress pulse produces a fracture at the notch In principle, therefore, the dynamic notched round bar test is more amenable to analysis than the Charpy test because the fracture process is completed before the stress pulse has sufficient time to be reflected from the farthest end of the bar

Fig 21 Typical apparatus for dynamic fracture initiation experiment Source: Ref 61

The Charpy test is a simple, low-cost test that rapidly detects changes in ductility However, the Charpy test does have certain disadvantages for quantitative assessments Fracture in the Charpy specimen does not occur under plane-strain conditions Furthermore, the state of stress at the fracture site is unknown and quite complex due to multiple pulse reflections from its various surfaces For these reasons, it is difficult to interpret Charpy results in terms of elastic or elastic-plastic fracture toughness parameters, although, as previously described, instrumented impact testing of precracked Charpy V-notch specimens provides useful results for evaluations of dynamic fracture toughness, as described in more detail in Ref 62

Dynamic notched round bar testing yields data from which a reliable value of the dynamic critical

stress-intensity factor KId can be calculated easily Hence, results are immediately related on a quantitative basis to fracture mechanics parameters However, the test setup is rather elaborate, and more material is required for each specimen compared to Charpy testing As a result, the technique is not suitable for routine testing It may

be used, however, when a precise evaluation of the fracture initiation properties of a particular material is required, perhaps as a function of temperature as well as of loading rate

In this test, measurements of the average stress across the fracture plane and of crack-opening displacement, both as functions of time, are easily obtained Various methods can be used to measure crack-opening displacement, but the stress across the fracture plane is most easily determined by using electric resistance strain gages applied to the surface of the bar downstream from the fracture site In this respect, and several others, the dynamic notched round bar test resembles the Kolsky (or split-Hopkinson) bar used in dynamic plasticity Another example of using a notched round bar in evaluation of dynamic fracture toughness is given

in Ref 63, where the KId toughness of A533-B reactor-grade steel was determined over the temperature range from 3 to 50 °C (37–122 °F), by dynamic loading of notched round bar specimens with axial precompression of the notch

The Schnadt specimen, details of which are shown in Fig 22, has been used primarily in Europe for testing ship plate In the Schnadt test, five test pieces are used with different notch radii, ranging from no notch to a severe notch made by pressing a sharp knife into the bottom of a milled groove A hardened steel pin is inserted in a

hole parallel to and behind the notch, replacing the material normally under compression in the Charpy or Izod tests The specimen is broken by impact as a three-point-loaded beam

Fig 22 Details of the Schnadt notched-bar impact-test specimen References cited in this section

59 J Woolman and R.A Mottram, The Mechanical and Physical Properties of the British Standard DN Steels, Vol 1, Appendix UI, The British Iron and Steel Research Association, 1964, p 442

60 J.H.J Giovanola, One-Point Bend Test, Mechanical Testing, Vol 8, ASM Handbook, 1985, ASM

International, p 271–275

61 L.S Costin, J Duffy, and L.B Freund, Fracture Initiation in Metals Under Stress Wave Loading

Conditions, Fast Fracture and Crack Arrest, STP 627, G.T Hahn and M.F Kanninen, Ed., ASTM,

1977, p 301–318

62 R.H Hawley, J Duffy, C.F Shih, Dynamic Notched Round Bar Testing, Mechanical Testing, Vol 8, ASM Handbook, 1985, ASM International, p 275–284

63 G.R Irwin, J.W Dally, X.-J Zhang, and R.J Bonenberger, Lower-Bound Initiation Toughness of

A533-B Reactor-Grade Steel, Rapid Load Fracture Testing, STP 1130, ASTM, 1991, p 9–23

Impact Toughness Testing

Acknowledgments

Contents of this article were adapted from the following:

• Impact Testing, by R.C Anderson, in Inspection of Metals: Destructive Testing, ASM International,

1988, p 121–170

• Charpy Impact Testing, by William Seaver, in Mechanical Testing, Vol 8, ASM Handbook, 1985, p

261–269

Impact Toughness Testing

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