Volume 08 - Mechanical Testing and Evaluation Part 10 pdf

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 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 (No 9), 1965, p 442–445 48 K.H Schwalbe and D.J Hellman, Test Evaluation, Vol (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 CrMo Steels, 22nd ASTM National Symposium on Fracture Mechanics, STP 1131, ASTM, 1992, p 762– 789 56 P.K Liaw and A Saxena, “Crack Propagation Behavior under Creep Conditions,” Int J Fract., Vol 54, 1992, p 329–343 57 W.A Logsdon, P.K Liaw, A Saxena, and V.E Hulina, Eng Fract Mech., Vol 25, 1986, p 259 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 Fig 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 × 104 MPa about 1.098 × 106 MPa · s-1 (106 ksi · s-1 (104 ksi · s-1) for the dynamic tests and · s-1) for the dynamic-instrumented tests Source: Ref 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 quasistatic 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) 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 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 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 dropweight 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 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 °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 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 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: Title Designation 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 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 General configuration of anvils and specimen in Charpy test Fig 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 There are also three basic types of standard Charpy specimens (Fig 5): the Charpy V-notch, the Charpy UNotch, and the Charpy keyhole specimen These dimensions are based on specifications in ASTM E 23, ISO 148, and ISO 83 The primary specimen and test procedure involves the Charpy V-notch test Other Charpytype specimens are not used as extensively because their degree of constraint and triaxiality is considerably less than the V-notch specimen Fig 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) Unotch 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 and 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 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 keyholenotch 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 notchbroaching 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 Effect of specimen 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materials show some form of inelastic behavior (Ref 1, 2) The elastomers show hysteresis, and the glasses show some form of yielding The inelastic behavior is not restricted to the tip of a crack, but is present in some form or another throughout the material The inelasticity is a direct result of the time dependence of the motions of the polymer chains With the exceptions of certain untoughened epoxy resins and related thermosets, inelasticity is the norm Hence, the expectation of many theories of fracture mechanics that Hookian behavior can be assumed is not to be realized Even theories that assume elastic-plastic criteria are inadequate because they assume plastic behavior at the crack tip and elastic behavior throughout the remainder of the specimen, whereas in the real materials, there is viscoelastic deformation of some form or other occurring in the bulk of the specimen The presence of inelasticity in the entire specimen, as well as at the crack tip, results in additional energy being required for crack propagation Hence, in any mechanical test the energy measured to propagate a crack consists of the surface energy of the crack, energy of plastic deformation at the crack tip, and energy of inelastic deformation of the entire specimen (Ref 3) Because the latter two forms of energy absorption are a direct result of the time-dependent behavior of the polymer chains, the energy absorbed displays a strong dependence on the rate at which stress is applied The crack opening displacements in polymeric materials can be quite large and, hence, the microstrain at a crack tip will be similarly large In polymeric materials displaying minimal levels of plasticity and/or inelasticity, such as untoughened epoxies, the crack opening displacement is quite small At the other extreme is the elastomer, or rubber, where the crack opening displacement is so large that the process is usually referred to as tearing The crack opening displacement can reflect two extremes in deformation behavior: shear yielding or crazing (Ref 3) Both reflect large amounts of plastic deformation at the crack tip In the case of some polymers, for example, polycarbonate, a large yield zone is observed In others, the phenomenon is referred to as crazing, where the apparent crack is really a zone of fibrous material produced by the stress field ahead of the crack This phenomenon can be present in glassy materials as well as semicrystalline materials, and it corresponds to microyielding to levels of several hundred percent strain A similar phenomenon can also be observed in unnotched specimens where regions in the bulk of the specimen display what is usually described as stress whitening In addition to the behavior described above, polymers are also sensitive to the environment, both gaseous and liquid (Ref 3) An example of the effects of gaseous environments is the effect of atmospheric ozone on crack propagation rates in natural rubber (Ref 4) In the case of a liquid the behavior can be caused by several different effects (Ref 5, 6) First, there is always the possibility that the liquid may be a solvent and be absorbed by the polymer; the absorption process may occur more rapidly at the tip of a crack In this case the liquid will plasticize the polymer, lowering its glass transition temperature and thereby altering all of its fundamental properties Second, the liquid may react chemically with the polymer, changing its fundamental structure and properties on a microscopic or macroscopic scale Third, the liquid may simply wet the polymer, lowering the surface energy and making crack or craze propagation much easier A well-known example of such behavior is the effect of carbon tetrachloride on polycarbonate References cited in this section N.G McCrum, B.E Read, and G Williams, Anelastic and Dielectric Effects in Polymeric Solids, Wiley, 1967 I.M Ward, Mechanical Properties of Solid Polymers, Wiley, 1983, p 15 E.H Andrews, Cracking and Crazing in Polymeric Glasses, The Physics of Glassy Polymers, R.N Haward, Ed., Wiley, 1973, p 394 R Natarajan and P.E Reed, J Polym Sci A, Polym Chem., Vol (No 10), 1972, p 585 G.A Bernier and R.P Kambour, Macromolecules, Vol 1, 1968, p 393 E.H Andrews, G.M Levy and J Willis, J Mater Sci., Vol 8, 1973, p 1000 Fracture Resistance Testing of Plastics Kevin M Kit and Paul J Phillips, University of Tennessee, Knoxville Historical Development Fracture in polymers was first studied intensively for rubber, and tests were developed logistically in the early 1900s (Ref 7, 8) Standard test methods included tensile testing with “dog-bone” specimens where the breaking strength was obtained By the 1920s, standard tests for tear strength, using “trouser-type” specimens, were in use Such methods are still in common use, the tensile test to failure using a dog-bone specimen being one of the most popular for the characterization of all kinds of polymers (Ref 8) As new polymers are developed and testing is needed, tensile testing on sheets or thin films as a method of characterization still tends to be preferred over the standardized ASTM tests for fracture strength This may occur sometimes due to the amount of specimen available and at other times due to the simplicity of specimen preparation and characterization Fracture testing using standardized linear fracture mechanics approaches, such as KIc/GIc methods, has been used for decades as a means of carrying out fracture testing (Ref 3, 9) However, because of the previously mentioned inelasticity problems, polymers have stress distributions at the tip of a crack that cannot be calculated or described adequately by the assumptions of classical elasticity theory Such approaches clearly cannot describe adequately the behavior of even the most well-behaved systems Early attempts at describing the fracture phenomenon in a more realistic manner recognized that the most important parameter describing the phenomenon was the energy absorbed by the fracture process (Ref 10) The energy balance approach was suggested very early by Griffith (Ref 11) but was used for rubber by Rivlin and Thomas (Ref 12) who used a to describe the total work needed to create a unit area of surface (or the tearing energy in the case of rubber) Attempts at applying this approach were made successfully by Andrews and coworkers (Ref 3), Kambour (Ref 13), and Berry (Ref 14) The beginning of a generalized theory of fracture mechanics, not requiring linear fracture assumptions, was developed by Andrews (Ref 15) Study of fracture then concentrated for several years on the development and understanding of the mechanisms of craze formation because, clearly, the formation of crazes ahead of the crack is the major contributor to the energy absorbed in fracture in most polymers (Ref 15) Indeed, because crazing is the precursor to fracture itself, it justifies attention on that ground alone A concurrent development, which now is used in the testing of polymers, is the J-integral method; it is essentially the equivalent of GI for a nonlinear system Discovered by Rice (Ref 16, 17), and developed independently by Begley and Landes (Ref 18, 19), the J-integral method has been applied successfully to polymers by the Williams group and others (Ref 20, 21, and 22) The disadvantage of the method is that it requires multiple specimens in its strict form, discouraging widespread use A single specimen method was developed and used successfully on polypropylene by Ouederni and Phillips (Ref 23), but it has not yet been converted into a standard ASTM method References cited in this section E.H Andrews, Cracking and Crazing in Polymeric Glasses, The Physics of Glassy Polymers, R.N Haward, Ed., Wiley, 1973, p 394 L.E Weber, The Chemistry of Rubber Manufacture, Griffin, London, 1926, p 336 K Memmler, The Science of Rubber, R.F Dunbrook and V.N Morris, Ed., Reinhold, 1934, p 523 G.R Irwin, in Encyclopaedia of Physics, Vol 6, Springer Verlag, 1958 10 N.G McCrum, C.P Buckley, and C.B Bucknall, Principles of Polymer Engineering, Oxford University Press, 1997, p 201 11 A.A Griffith, Phil Trans R Soc (London) A, Vol 221, 1921, p 163 12 R.S Rivlin and A.G Thomas, J Polym Sci., Vol 10, 1953, p 291 13 R.P Kambour, Appl Polym Symp., Vol 7, John Wiley & Sons, 1968, p 215 14 J.P Berry, J Polym Sci A, Polym Chem., Vol 2, 1964, p 4069 15 E.H Andrews, J Mater Sci., Vol 9, 1974, p 887 16 J.R Rice, J Appl Mech (Trans ASME), Vol 35, 1968, p 379 17 J.R Rice, Fracture, Vol 2, 1968, p 191 18 J.A Begley and J.D Landes, in Fracture Toughness, ASTM STP 514, 1972, p 19 J.D Landes and J.E Begley, in Fracture Toughness, ASTM STP 514, 1972, p 24 20 J.M Hodgkinson and J.G Williams, J Mater Sci., Vol 16, 1981, p 50 21 S Hashemi and J.D Williams, Polym Eng Sci., Vol 26, 1986, p 760 22 Y.W Mai and P Powell, J Polym Sci B, Polym Phys., Vol 29, 1991, p 785 23 M Ouederni and P.J Phillips, J Polym Sci B., Polym Phys., Vol 33, 1995, p 1313 Fracture Resistance Testing of Plastics Kevin M Kit and Paul J Phillips, University of Tennessee, Knoxville Fracture Test Methods for Polymers Several methods have been developed specifically for determining the fracture toughness of polymeric materials ASTM D 5045 (Ref 24) describes a method for determining the linear elastic fracture toughness (KIc and GIc) of polymers This methodology is appropriate for highly crosslinked thermosets (e.g., epoxy) or glassy thermoplastics incapable of significant plastic deformation (e.g., polystyrene) ASTM D 6068 (Ref 25) describes a method for measuring J-R curves (a measure of elastic-plastic fracture toughness) for polymer specimens that are not large enough to experience conditions of plane strain during loading However, methods originally developed to characterize the elastic-plastic fracture of ductile metallic materials are most commonly used (with slight modifications) to characterize ductile polymers These methods are based on the concept of the J-integral to determine plane strain fracture toughness values To date, the most commonly used method is that of ASTM E 813 (Ref 26) This method was discontinued in 1989 and replaced by ASTM E 1737 (Ref 27) The differences between the two are minor, but the methods for data analysis and reporting described in ASTM E 1737 should now be followed J-Integral Testing ASTM E 1737 is more general than ASTM E 813 and describes the method for determining either JIc or Jc under plane stress conditions JIc is the critical value of the J-integral at which onset of stable crack growth occurs If stable crack growth is not observed, then Jc is defined as the value of the J-integral at which unstable crack growth (i.e., failure) occurs The J-integral is a measure of the amount of energy absorbed (due to both elastic and plastic responses) during the growth of a crack through the material of interest Experimentally, J is determined as a function of crack extension, Δa, in a notched specimen loaded in tension J is calculated according to (Ref 28): (Eq 1) where U is the area under the load-displacement curve and B and b are the dimensions of the specimen in the plane of the crack Testing is most commonly performed on single-edge notched bend or on compact tension specimens containing machined notches (see Fig in the article “Fracture Toughness Testing” in this Volume) ASTM E 1737 specifies that the specimen be fatigued so that a sharp “precrack” is formed at the base of the notch However, this is not a viable technique for most thermoplastic polymers The accepted method for creating a precrack in polymer samples is to tap a fresh, unused razor blade into the notch immediately preceding the test, as specified in ASTM D 6068 and D 5045 To ensure the existence of plane strain conditions at the crack tip, specimen thickness, B, and the original uncracked ligament, bo (i.e., the distance the crack would have to extend to separate the specimen into two pieces), must be greater than 25JIc/σy where JIc is the elastic-plastic fracture toughness and σy is the yield strength Because JIc is generally not known a priori, specimen dimensions must be based on an estimated value of JIc and then verified after testing It has been shown (Ref 29) that the specimen size requirements specified by ASTM E 1737 can be relaxed for some polymers, such as low-density polyethylene and a polypropylene copolymer, to B, bo > 17 JIc/σy In order to arrive at a value of JIc, J-integral values are plotted as a function of crack extension, Δa, to form a so-called R-curve This data may be collected using single specimen or multiple specimen techniques The multiple specimen technique is widely accepted as a valid measure of the elastic-plastic fracture toughness of polymers and is commonly employed However, results from the much simpler single specimen technique have also been shown to be valid, and the implementation of this technique is increasing These techniques differ only in the determination of the R-curve; specimen requirements and data analysis to determine JIc are identical Both are summarized in the following sections Multiple Specimen Technique In both techniques, it is desirable to determine J at a minimum of ten equally spaced Δa points In the multiple specimen technique, each J-Δa point on the R-curve is generated with a different specimen Each specimen is loaded to a level judged to produce a desired, stable crack growth extension, Δa, and is then unloaded Polymer specimens are then removed from the test frame and fractured in liquid nitrogen (This last step deviates from ASTM E 1737, which specifies that the specimens be fatigued first.) The precrack, stable crack growth and freeze-fracture regions of the fracture surface are usually easily identifiable (Ref 25), and an optical microscope is used to measure Δa (the length of the stable crack growth region) at nine points equally spaced across the thickness of the specimen These nine values are averaged as described by ASTM E 1737 J is then calculated according to: J = Jcl + Jpl (Eq 2) where Jel and Jpl are the elastic and plastic components of J, calculated as: (Eq 3) (Eq 4) K is a function of maximum load and specimen geometry, ν is Poisson's ratio, E is Young's modulus, Apl is the area under the load-displacement curve for the entire loading-unloading cycle, and BN is specimen thickness For single-edge notch and compact tension specimens, η = 2, while for the disk-shape compact tension specimen, η is a function of geometry Equations for K for each specimen type are given in Annex of ASTM E 1737 Single Specimen Technique The single specimen technique relies on the ability to determine the extent of crack growth, Δa, while the specimen is loaded in the test frame If this can be done, then many J-Δa data pairs can be collected from one specimen Crack growth is usually determined by an elastic compliance method or by an electrical resistance method In the elastic compliance method, the specimen is unloaded periodically during the test At each unloading point, Δa is calculated as a function of the slope of the unload line, Young's modulus, and specimen geometry However, due to the viscoelastic behavior of polymers, accurate determination of crack lengths by this method is suspect (Ref 30, 31) Another method determines the crack length by measuring the voltage drop across the uncracked ligament through which a constant direct current is passed This method is also not generally applicable to polymers because most are poor conductors However, Ouederni and Phillips (Ref 23) have developed a method that involves measuring crack extension directly with a video camera A thin copper grid deposited on the surface of the specimen serves as a scale reference Another J-integral technique that has been successfully applied to polymers is the normalization method (Ref 31) This method does not require specimen unloading or in situ measurements of crack growth The crack length is calculated by separating total displacement into elastic and plastic components, each of which is a function of crack length After a fitting procedure is used to establish a relationship between plastic displacement and crack length, the actual crack length can be calculated at any point on the load-displacement curve Zhou et al (Ref 31) used this technique to determine JIc for two rubbertoughened nylons and found their results very close to values obtained by the standard multiple specimen method Determination of JIc Before the data can be analyzed, it must be checked to verify that it spans a sufficiently large range of Δa This procedure to determine qualifying data is detailed in ASTM E 1737 Qualifying J data must also be less than the smaller of boσy/20 and Bσy/20 to ensure that all data points are measured under plane strain conditions Qualified data are fit by the method of least squares to the curve described by: (Eq 5) where C1 and C2 are fitting parameters and k = mm (0.04 in.) A linear blunting line must also be constructed along the line defined by: J = 2σyΔa (Eq 6) where σy is the average of the 0.2% offset yield strength and the ultimate tensile strength The blunting line accounts for deflection that occurs due to plastic deformation near the crack tip prior to the onset of stable crack growth ASTM E 1737 specifies that the J value at the intersection of the fit data and a line offset 0.2 mm (0.008 in.) from the blunting line defines an interim value, JQ, which is used to verify the existence of plane strain conditions If both B and bo are indeed greater than 25JIc/σy, and some additional data qualifications are met, then the value of JQ is taken to be equal to JIc Experimental and fit R-curves for an acrylonitrilebutadiene-styrene (ABS) copolymer are shown in Fig along with the blunting and 0.2 mm offset lines The intersection of the fit R-curve and the 0.2 mm offset line indicates a JIc of 5.31 kJ/m2 Fig Experimental R-curve for an ABS copolymer showing power-law fit, blunting line, and 0.2 mm offset line Source: Ref 32 Modifications for Polymeric Materials Due to the unique properties of polymers, several modifications to the J-integral method have been proposed and used Some of these modifications that affect the collection of J-Δa data have already been mentioned, and these are quite widely accepted as standard In some cases, crack tip blunting may not occur before or during stable crack growth in polymers Crack tip blunting can be verified by direct microscopic observation or if J data follows the blunting line (J = 2σyΔa) for small amounts of crack growth Some of the data in Fig lie on the blunting line, indicating that blunting does occur (Ref 32) If blunting is not known to occur, JIc should be determined by extrapolating a linear fit to the JΔa data to zero crack growth (Δa = 0) It has been argued in Ref 33 that J-Δa data should, under conditions of plane strain, follow: (Eq 7) For small crack growth, J should vary linearly with Δa, and the value of JIc should be determined as previously explained Optical microscopy (Ref 34, 35) has shown that crack blunting does not occur in certain grades of high-density polyethylene, toughened nylon 6/6, ABS, and toughened polycarbonate As further evidence, the J data collected from the high-density polyethylene (Ref 35) does not follow the blunting line for small Δa, as shown in Fig Fig Experimental R-curve for a high-density polyethylene showing the dashed blunting line and the absence of blunting behavior Source: Ref 35 If crack tip blunting does occur, the procedure described will yield conservative values of JIc If blunting is known to occur, then JIc should be determined by the methods of ASTM E 1737 or ASTM E 813 The determination of JIc by ASTM E 813 differs in that JIc is taken at the intersection of a linearly fit R-curve and the blunting line This construction is shown in Fig for the same data used in Fig The intersection of the linear R fit and the blunting line indicates a JIc of 3.95 kJ/m2 (compare to the ASTM E 1737 value of 5.31 kJ/m2) The method in ASTM E 813 usually gives more conservative values than that in ASTM E 1737 Chang et al (Ref 29, 31, 35, and 36) have analyzed J data of high-impact polystyrene (HIPS), ABS, a polycarbonate (PC)/ABS blend, and a polycarbonate/polybutylene terephthalate (PBT) blend by three methods (ASTM E 1737, ASTM E 813, and the no-blunting method described previously) As can be seen in Table 1, the noblunting method is the most conservative, while ASTM E 1737 is the least conservative If no direct evidence of crack tip blunting exists, the most conservative method for calculating JIc should be used Table Comparison of JIc data for several polymers determined by different methods JIc, kJ/m2 HIPS ABS No blunting 3.24 3.57 ASTM E 813 3.60 3.95 ASTM E 1737 4.30 5.31 Method PC/ABS 3.00 3.55 7.85 PC/PBT 5.47 7.17 13.41 Fig Experimental R-curve for an ABS copolymer showing linear fit and blunting line Source: Ref 32 Several workers have shown that the plane strain thickness requirements specified by ASTM E 813 and ASTM E 1737 are too conservative in certain cases, while not conservative enough in others Rimnac et al (Ref 38) and Huang (Ref 39) have shown that the requirement is too conservative for tough thermoplastics, ultrahighmolecular-weight polyethylene (JIc = 95 kJ/m2), and rubber-toughened nylon 6/6 (JIc = 30 kJ/m2) Both studies found that size-independent values of JIc were obtained for specimen thicknesses greater than 6JIc/σy, which is approximately 25% of the recommended minimum thickness Conversely, Lu et al (Ref 40) found that sizeindependent values of JIc for a relatively brittle PC/ABS blend (JIc = kJ/m2) were not obtained until the thickness was greater than 64JIc/σy, which is more than twice the recommended minimum thickness In light of these results, it is recommended that JIc be determined for various thicknesses to ensure that the true plane strain value is obtained Linear Elastic Fracture Toughness Other methods also exist to determine the plane strain fracture toughness of polymers ASTM D 5045 specifies a procedure for determining the critical strain energy release rate, GIc, of polymers This parameter is equivalent to JIc for materials that exhibit linear (or nearly linear) elastic behavior (Ref 41) ASTM D 5045 specifies the use of single-edge notch bend or compact tension specimens Precracks are created by tapping a fresh, unused razor blade into the machined notch immediately preceding the test The samples are then loaded to a level that causes a 2.5% apparent crack extension However, significant deviation from linear elastic behavior must not occur at this load level The procedure for testing this requirement is detailed in ASTM D 5045 An interim value of the critical strain energy release rate, GQ is determined by: (Eq 8) where φ is a function of b and the original crack length, a This interim value can be qualified as the plane strain critical strain energy release rate if plane strain conditions are verified The standard specifies that B, b, and a must be greater than 2.5 (KIc/σy)2 where KIc is the plane strain fracture toughness and is related to GIc by: (Eq 9) Using this relation, the size requirement for plane strain conditions can be written as: (Eq 10) Using typical values for E (1 GPa, or 145 ksi), σy (60 MPa, or 8.7 ksi), and ν (0.4), the size requirement is B, b, a > 50GIc/σy, which is twice the size requirement for determining plane strain JIc Due to the viscoelastic properties of polymers, test temperature and strain rate should be well controlled and reported The standard recommends 23 °C (73 °F) and a crosshead speed of 10 mm/min (0.4 in./min) The orientation of the specimen with respect to processing direction (e.g., extrusion direction and mold flow direction) should also be reported because of the strong dependence of mechanical properties on molecular orientation that often develops during processing Testing of Thin Sheets and Films In order to ensure the existence of plane strain state, the dimensions of the sample normal to the applied stress are usually required to be greater than 25JIc/σy, where JIc is the elastic-plastic fracture toughness and σy is the yield strength Both JIc and σy are generally considerably lower than the corresponding values for metallic materials, but the ratio JIc/σy is usually much larger for polymeric materials Therefore, the plane strain size requirements for polymeric fracture specimens are often unrealistic (on the order of cm or in.) In many applications, the properties of polymeric materials are strongly dependent on the level of molecular orientation and crystallinity These levels, in turn, are strongly dependent on the thermal and mechanical histories experienced during processing Specimens that are produced to fulfill the plane strain condition are likely to have quite different thermal and mechanical histories than polymer materials processed into sheet or film Therefore, the thicker test specimens not reflect the actual properties of the polymer for the intended application For these reasons, ASTM D 6068 is often a more desirable method than the plane strain method of ASTM E 813 or E 1737 This method was developed specifically for the determination of R-curves from thin sheets or films However, this is not a valid method for determining JIc, and results should not be reported as such When using this method, specimen size and the values of C1 and C2 (which characterize the power-law fit of the R-curve) should be reported Other Methods Alternative methods for determining the fracture toughness of polymer materials have recently been proposed Most notable are the normalization and hysteresis methods, which are both single specimen techniques The normalization method does not require unloading cycles or online crack measurement and has been used successfully for metallic materials (Ref 31) The method is based on the assumption that the load, P, on the specimen can be represented by: P = G(a)H(νpl) (Eq 11) where G(a) is a known function of crack length and specimen geometry, and H(νpl) is a function of plastic displacement, νpl After the form of H(νpl) is fit to experimental data, values of a (and hence J) can be determined at any point on the load-displacement curve JIc can then determined from the R-curve using the methods described above Zhou et al (Ref 31) found that the results of this method are slightly less conservative than those determined by ASTM E 813 and more conservative than ASTM E 1737 for two rubbertoughened nylons (nylon 6/6 and an amorphous nylon) The hysteresis method requires the application of multiple load-unload cycles to successively larger displacements (Ref 30, 32, and 37), as shown in Fig The area between the loading and unloading lines on the load-displacement curve is defined as the hysteresis energy, and this is plotted against maximum displacement for each loading cycle, as shown in Fig For small displacements, crack growth does not occur, and the hysteresis energy varies linearly with displacement This data is fit with a linear blunting line After crack growth commences, the hysteresis energy varies nonlinearly with displacement and can be fit with a power law The displacement at which the linear blunting line intersects with the power-law curve is taken as the critical displacement to initiate crack growth, and the value of J at this displacement is taken as JIc It has been found that the results of this method are slightly less conservative than those determined by ASTM E 813 and more conservative than ASTM E 1737 for several polymers (ABS, PC/ABS, HIPS, and PC/PBT) (Ref 32, 36, 37, and 42) Fig Hysteresis loops for several loading-unloading cycles for a PC/PBT blend D, specimen displacement; HR, ratio of hysteresis energy to total strain energy Source: Ref 37 Fig J-integral and hysteresis energy vs displacement for a PC/PBT blend Test rate, mm/min (0.08 in./min) JIC-HE and DC-HE are critical values of J and D for initation of crack propagation Source: Ref 37 References cited in this section 23 M Ouederni and P.J Phillips, J Polym Sci B., Polym Phys., Vol 33, 1995, p 1313 24 “Standard Test Methods for Plane Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials,” ASTM D 5045, Annual Book of Standards, Vol 08.03, ASTM, 1996 25 “Standard Test Method for Determining J-R Curves of Plastic Materials,” ASTM D 6068, Annual Book of Standards, Vol 08.03, ASTM, 1996 26 “Standard Test Method for JIc, A Measure of Fracture Toughness,” ASTM E 813, Annual Book of Standards, Vol 03.01, ASTM, 1989 ... temperatures (-2 69 and -2 51 °C, or -4 52 and -4 20 °F) Such testing requires special techniques and will not be discussed here For testing at temperatures down to or slightly below -5 9 °C (-7 5 °F),... stress-intensity rate was about 1.098 × 104 MPa about 1.098 × 106 MPa · s-1 (106 ksi · s-1 (104 ksi · s-1) for the dynamic tests and · s-1) for the dynamic-instrumented tests Source: Ref Because many structural... 19 ì 51 ì 127 (ắ ì × 102 1.5 (0.06) 210? ?? 410 (30–60) 350 250 5) (4.0) 410? ??620 (60–90) 400 300 620–830 (90– 450 350 120) 830? ?103 0 (120– 550 400 150) P-3 102 1.9 (0.075) 210? ?? 410 (30–60) 350 250 15.9