ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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be addressed when using optical pyrometry. Commercial units are widely available for this type of
measurement and usually come with detailed instructions.
Crack-Length Measurement. Electric potential drop is a common method of crack-length determination, both at
room temperature and at elevated temperatures. The major consideration for elevated-temperature testing is to
ensure that temperature fields are uniform and constant between the potential drop leads. Changes in
temperature can result in a false indication of crack-length changes. The error is due to the resistivity change in
the material caused by a change in temperature, and this change, in turn, is dependent on the resistivity
characteristics of the material. For the direct current potential drop (DCPD) method, a 3 °C (5.5 °F) change in
temperature in an aluminum alloy can result in a 1% change in the DCPD signal, whereas for the same change
in resistivity, Inconel 718 requires a 100 °C (180 °F) change in temperature (Ref 53). The electric potential
drop method of crack-length measurement, used commonly in both room- and elevated-temperature isothermal
crack growth testing, has also been applied to TMF testing (Ref 54). As with any other technique, modifications
and improvements are continually being developed and documented in the literature, as in the work of Shin et
al. (Ref 55), where modification to the dc potential drop system is reported.
Compliance methods are equally valid at elevated temperatures as at room temperature, provided that some
simple considerations are addressed. Nonconstant thermal gradients must be avoided, because any change in the
dimensions of any part of a mechanical extensometer can result in a change in electrical output. The gage
portion, which is normally attached to the specimen through special high-temperature extension arms or feed
rods, is usually shielded from the specimen (Fig. 20) and cooled with either blowing air or circulating water.
Whatever the setup, the gage should reach thermal equilibrium and stability before measurements are made, in
order to avoid false indications due to thermal transients.
Fig. 20 Feed-rod attachment for high-temperature tests. Feed rods are made of quartz for tests up to
1000 °C or ceramics for tests at higher temperatures.
Optical methods are less widely used today than previously because of the availability of automated methods
for crack-length measurement. Optical methods, such as use of a traveling microscope, are useful for reference
measurements and are equally valid at room or elevated temperature. At elevated temperature, however, the
resolution of crack length may be reduced because of deterioration of the specimen surface through oxidation.
When specialized optical methods such as laser interferometry are used, the deterioration of the specimen
surface over time should be considered so that the optical information is still available with sufficient
resolution.
Special Methods. Other specialized techniques see occasional application in high-temperature crack growth
testing. For example, crack growth over prolonged periods of time has been measured using real-time
holographic interferometry (Ref 56). Compact-tension specimens were tested under sustained load at 120 °C
(250 °F) for 860 h, and crack-opening displacement (COD) was monitored. The COD measurements had to be
converted to crack lengths using an elastic-plastic finite-element analysis code and clip gage COD
measurements on a reference sample.
Validity of K Solutions. The reduction of crack length versus N data to a da/dN-ΔK curve depends on having a
K solution for the specimen geometry being used. Stress-intensity solutions are widely available, or can be
easily generated, for almost any specimen geometry used in laboratory testing. These solutions, however, are
based on the assumption of a homogeneous material and do not take into account any thermal gradients or
thermal stresses that may arise in elevated-temperature crack growth testing. For situations where the specimen
and grips are at a uniformly high temperature, such as within a furnace, there is no problem. For situations
where the temperature is nonuniform, such as with inductance or quartz lamp heating with cooled grips,
temperature gradients are developed along the axis of the specimen. These gradients, in turn, may result in
thermal stresses that produce thermal K-values that alter the isothermal K solution for a given specimen. These
thermal K-values, therefore, must be taken into account in the K analysis of the particular crack geometry.
Ohta et al. (Ref 57) found that crack growth rate data at 300 °C (570 °F) in a MT specimen only matched room-
temperature data in a low-alloy ferritic steel (SB46) when the parabolic temperature gradient that existed along
the specimen was taken into account using finite element analysis, which was confirmed with experimental
measurements. Coker et al. (Ref 58) conducted a finite element analysis of a single-edge-crack specimen with
thermal gradients between the plane of the crack and the room-temperature grips. For the specific geometry and
temperature profile, stress-intensity factors due to thermal stresses were below 2 MPa for a/W ≤ 0.8.
While these values are small, near-threshold tests involving very small values of ΔK could be substantially
influenced by thermal stresses. It is important, therefore, to have calculations or estimates of thermal-stress-
induced values that have nonuniform temperature profiles. Unfortunately, there are very few papers in the
literature where this particular problem is addressed. Nonetheless, this consideration can be important in high-
temperature testing, particularly when testing in the near-threshold regime or when thermal gradients are
severe.
Thermal gradients and effective K-values resulting from direct resistance heating are discussed by Cunningham
and Griffin (Ref 59). The analysis shows when direct resistance heating becomes important during the cyclic
conditions of thermal fatigue. The results indicate that stress intensities resulting from thermal gradients due to
direct resistance heating are generally small except when thermal cycling frequencies are high. The thermal
gradients and K-values resulting from thermal cycling are reported in a subsequent publication (Ref 60). There,
for high thermal frequencies, significant magnitudes of K can be developed at the crack tip due to the thermal
transients. For those involved in TMF crack growth testing, this information should be considered when
determining the stress intensity at the crack tip.
References cited in this section
40. Y. Asada, M. Kitagawa, N. Shimakawa, T. Kodaira, T. Asayama, and Y. Wada, Standardization of
Procedures for the High-Temperature Crack Growth Testing for FBR Materials in Japan, Nucl. Eng.
Des., Vol 133, 1992, p 465–473
41. S. Mall and T. Nicholas, Ed., Elevated Temperature Crack Growth, MD-Vol 18, ASME, 1990
42. H. Sehitoglu, Ed., Thermomechanical Fatigue Behavior of Materials, STP 1186, ASTM, 1993
43. M.J. Verrilli and M.G. Castelli, Ed., Thermomechanical Fatigue Behavior of Materials, STP 1263,
ASTM, 1995
44. W.J. Jones, Ed., Thermomechanical Behavior of Advanced Structural Materials, AD-Vol 34/AMD-Vol
173, ASME, 1993
45. H. Kitagawa and T. Tanaka, Ed., Fatigue 90, Materials and Components Engineering Publications, Ltd.,
Birmingham, UK, 1990
46. J.-P. Bailon and J.I. Dickson, Fatigue 93, Engineering Materials Advisory Services, Ltd., West
Midlands, UK, 1993
47. T. Nicholas, M.L. Heil, and G.K. Haritos, Predicting Crack Growth under Thermo-Mechanical Cycling,
Int. J. Fract., Vol 41, 1989, p 157–176
48. G.A. Hartman, A Thermal Control System for Thermal/Mechanical Cycling, J. Test. Eval., Vol 13,
1985, p 363–366
49. W.P. Wood and J.M. Cork, Pyrometry, McGraw-Hill, 1941
50. W.E. Forsythe, Optical Pyrometry: Temperature—Its Measurement and Control in Science and
Industry, Reinhold Publishing, 1941
51. H.J. Kostkowski and R.D. Lee, “Theory and Methods of Optical Pyrometry,” Monograph 41, National
Bureau of Standards, 1962
52. J.C. Richmond and D.P. DeWitt, Ed., Applications of Radiation Thermometry, STP 895, ASTM, 1985
53. J.K. Donald and J. Ruschau, Direct Current Potential Difference Fatigue Crack Measurement
Techniques, Fatigue Crack Measurement: Techniques and Applications, K.J. Marsh, R.A. Smith, and
R.O. Ritchie, Ed., EMAS, West Midlands, UK, 1991, p 11–37
54. G.A. Hartman and D.A. Johnson, DC Electric Potential Method Applied to Thermal/Mechanical Fatigue
Crack Growth Testing, Exp. Mech., Vol 11, 1987, p 106–112
55. C.-S. Shin, W.H. Huang, and H.-Y. Chen, An Improved DC Potential Drop System for Crack Length
Measurement, J. Chinese Inst. Eng., Vol 16, 1993, p 29–40
56. T.R. Hsu, R. Lewak, and B.J.S. Wilkins, Measurements of Crack Growth in a Solid at Elevated
Temperature by Holographic Interferometry, Exp. Mech., Vol 18, 1978, p 297–302
57. A. Ohta, M. Kosuge, S. Matsuoka, E. Takeuchi, Y. Muramatsu, and S. Nishijima, Significant Effect of
Thermal Stresses on Fatigue Crack Propagation Properties, Int. J. Fract., Vol 38, 1988, p 207–216
58. D. Coker, B.K. Parida, and N.E. Ashbaugh, “Thermal Stresses due to a Temperature Gradient on a
Single Edge Notch Specimen,” Report UDR-TM-93-07, University of Dayton Research Institute, 1993
59. S.E. Cunningham and J.H. Griffin, On the Importance of Direct Resistance Heating in Thermo-
Mechanical Fatigue, Int. J. Fract., Vol 46, 1990, p 257–270
60. S.E. Cunningham and J.H. Griffin, Estimating the Importance of Cyclic Thermal Loads in Thermo-
Mechanical Fatigue, Int. J. Fract., Vol 47, 1991, p 161–180
Fatigue Crack Growth Testing
Ashok Saxena, Georgia Institute of Technology; Christopher L. Muhlstein, University of California, Berkeley
References
1. P.C. Paris and F. Erdogan, J. Basic Eng. (Trans. ASME), Series D, Vol 85, 1963, p 528–534
2. P.C. Paris, Proc. 10th Sagamore Conf., Syracuse University Press, 1965, p 107–132
3. J.R. Griffiths and C.E. Richards, Mater. Sci. Eng., Vol 11, 1973, p 305–315
4. J.C. Grosskruetz, Strengthening in Fracture and Fatigue, Metall. Trans., Vol 3, 1972, p 1255–1262
5. J.R. Rice, in Fatigue Crack Propagation, STP 415, ASTM, 1967, p 247–311
6. N.E. Dowling, in Flaw Growth and Fracture, STP 631, ASTM, 1977, p 139–158
7. Standard Test Method for Measurement of Fatigue Crack Growth Rates, E 647-91, Annual Book of
ASTM Standards, Vol 03.01, 1992, ASTM, p 674–701
8. N.E. Dowling and J.A. Begley, in Mechanics of Crack Growth, STP 590, ASTM, 1976, p 82–103
9. N.E. Dowling, in Cracks and Fracture, STP 601, ASTM, 1977, p 131–158
10. H.S. Lamba, The J-Integral Applied to Cyclic Loading, Eng. Fract. Mech., Vol 7, 1975, p 693–696
11. J.R. Rice, J. Appl. Mech. (Trans. ASME), Vol 35, 1968, p 379–386
12. J.W. Hutchinson, J. Mech. Phys. Solids, Vol 16, 1968, p 337–347
13. J.R. Rice and G.F. Rosengren, J. Mech. Phys. Solids, Vol 16, 1968, p 1–12
14. W.R. Brose and N.E. Dowling, in Elastic-Plastic Fracture, STP 668, ASTM, 1979, p 720–735
15. W. Elber, Fatigue Crack Closure under Cyclic Tension, Eng. Fract. Mech., Vol 2, 1970, p 37–45
16. W. Elber, The Significance of Fatigue Crack Closure, STP 486, ASTM, 1971, p 230–242
17. Standard Method for Plane-Strain Fracture Toughness of Metallic Materials, E 399-90, Annual Book of
ASTM Standards, Vol 3.01, 1992, ASTM, p 569–596
18. K.T. Venkateswara Rao, W. Yu, and R.O. Ritchie, Metall. Trans. A, Vol 19 (No. 3), March 1988, p
549–561
19. A.W. Thompson and R.J. Bucci, Metall. Trans., Vol 4, April 1973, p 1173–1175
20. G.R. Yoder and D. Eylon, Metall. Trans. A, Vol 10, Nov 1979, p 1808–1810
21. A. Saxena and S.J. Hudak, Review and Extension of Compliance Information for Common Crack
Growth Specimens, Int. J. Fract., Vol 14 (No. 5), 1978, p 453–468
22. J.W. Dally and W.F. Riley, Experimental Stress Analysis, 3rd ed., McGraw-Hill, 1991
23. R.O. Ritchie, G.C. Garrett, and J.F. Knott, Int. J. Fract. Mech., Vol 7, 1971, p 462–467
24. C.Y. Li and R.P. Wei, Mater. Res. Stand., Vol 6, 1966, p 392–445
25. R.O. Ritchie and J.F. Knott, Acta Metall., Vol 21, 1973, p 639–648
26. R.O. Ritchie and K.J. Bathe, Int. J. Fract., Vol 15, 1979, p 47–55
27. G. Clark and J.F. Knott, J. Mech. Phys. Solids, Vol 23, 1975, p 265–276
28. H.H. Johnson, Mater. Res. Stand., Vol 5, 1965, p 442–445
29. G.H. Aronson and R.O. Ritchie, J. Test. Eval., Vol 7, 1979, p 208–215
30. M.A. Ritter and R.O. Ritchie, Fat. Eng. Mater. Struct., Vol 5, 1982, p 91–99
31. G.M. Wilkowski and W.A. Maxey, Fracture Mechanics: 14th Symposium—Vol II: Testing and
Applications, STP 791, J.C. Lewis and G. Sines, Ed., ASTM, 1983, p II-266 to II-294
32. R.J. Selines and R.M. Pelloux, Effect of Cyclic Stress Waveform on Corrosion Fatigue Crack
Propagation in Al-Zn-Mn Alloys, Metall. Trans., Vol 13, 1972, p 2525–2531
33. A. Saxena, S.J. Hudak, Jr., J.K. Donald, and D.W. Schmidt, J. Test. Eval., Vol 6 (No. 3), May 1978, p
167–174
34. R.J. Bucci, Development of a Proposed ASTM Standard Test Method for Near-Threshold Fatigue Crack
Growth Rate Measurement, Fatigue Crack Growth Measurement and Data Analysis, S.J. Hudak, Jr. and
R.J. Bucci, Ed., STP 738, ASTM, 1981, p 5–28
35. K.T. Venkateswara Rao, Y.W. Kim, C.L. Muhlstein, and R.O. Ritchie, Mater. Sci. Eng. A, Vol 192/193,
1995, p 474–487
36. R.J. Dauskardt, W. Yu, and R.O. Ritchie, J. Am. Ceram. Soc., Vol 70 (No. 10), Oct 1987, p C248–C252
37. R.W. Hertzberg, W.A. Herman, and R.O. Ritchie, Scr. Metall., Vol 21, 1987, p 1541
38. W.G. Clark and S.J. Hudak, The Analysis of Fatigue Crack Rate Data, Application of Fracture
Mechanics to Design, J.J. Burke and V. Weiss, Ed., Vol 22, Plenum, 1979, p 67–81
39. E.P. Phillips, “Results of the Second Round Robin on Opening Load Measurement Conducted by
ASTM Task Group E24.04.04 on Crack Closure Measurement and Analysis,” Technical Memorandum
109032, National Aeronautics and Space Administration, Nov 1993
40. Y. Asada, M. Kitagawa, N. Shimakawa, T. Kodaira, T. Asayama, and Y. Wada, Standardization of
Procedures for the High-Temperature Crack Growth Testing for FBR Materials in Japan, Nucl. Eng.
Des., Vol 133, 1992, p 465–473
41. S. Mall and T. Nicholas, Ed., Elevated Temperature Crack Growth, MD-Vol 18, ASME, 1990
42. H. Sehitoglu, Ed., Thermomechanical Fatigue Behavior of Materials, STP 1186, ASTM, 1993
43. M.J. Verrilli and M.G. Castelli, Ed., Thermomechanical Fatigue Behavior of Materials, STP 1263,
ASTM, 1995
44. W.J. Jones, Ed., Thermomechanical Behavior of Advanced Structural Materials, AD-Vol 34/AMD-Vol
173, ASME, 1993
45. H. Kitagawa and T. Tanaka, Ed., Fatigue 90, Materials and Components Engineering Publications, Ltd.,
Birmingham, UK, 1990
46. J.-P. Bailon and J.I. Dickson, Fatigue 93, Engineering Materials Advisory Services, Ltd., West
Midlands, UK, 1993
47. T. Nicholas, M.L. Heil, and G.K. Haritos, Predicting Crack Growth under Thermo-Mechanical Cycling,
Int. J. Fract., Vol 41, 1989, p 157–176
48. G.A. Hartman, A Thermal Control System for Thermal/Mechanical Cycling, J. Test. Eval., Vol 13,
1985, p 363–366
49. W.P. Wood and J.M. Cork, Pyrometry, McGraw-Hill, 1941
50. W.E. Forsythe, Optical Pyrometry: Temperature—Its Measurement and Control in Science and
Industry, Reinhold Publishing, 1941
51. H.J. Kostkowski and R.D. Lee, “Theory and Methods of Optical Pyrometry,” Monograph 41, National
Bureau of Standards, 1962
52. J.C. Richmond and D.P. DeWitt, Ed., Applications of Radiation Thermometry, STP 895, ASTM, 1985
53. J.K. Donald and J. Ruschau, Direct Current Potential Difference Fatigue Crack Measurement
Techniques, Fatigue Crack Measurement: Techniques and Applications, K.J. Marsh, R.A. Smith, and
R.O. Ritchie, Ed., EMAS, West Midlands, UK, 1991, p 11–37
54. G.A. Hartman and D.A. Johnson, DC Electric Potential Method Applied to Thermal/Mechanical Fatigue
Crack Growth Testing, Exp. Mech., Vol 11, 1987, p 106–112
55. C.-S. Shin, W.H. Huang, and H.-Y. Chen, An Improved DC Potential Drop System for Crack Length
Measurement, J. Chinese Inst. Eng., Vol 16, 1993, p 29–40
56. T.R. Hsu, R. Lewak, and B.J.S. Wilkins, Measurements of Crack Growth in a Solid at Elevated
Temperature by Holographic Interferometry, Exp. Mech., Vol 18, 1978, p 297–302
57. A. Ohta, M. Kosuge, S. Matsuoka, E. Takeuchi, Y. Muramatsu, and S. Nishijima, Significant Effect of
Thermal Stresses on Fatigue Crack Propagation Properties, Int. J. Fract., Vol 38, 1988, p 207–216
58. D. Coker, B.K. Parida, and N.E. Ashbaugh, “Thermal Stresses due to a Temperature Gradient on a
Single Edge Notch Specimen,” Report UDR-TM-93-07, University of Dayton Research Institute, 1993
59. S.E. Cunningham and J.H. Griffin, On the Importance of Direct Resistance Heating in Thermo-
Mechanical Fatigue, Int. J. Fract., Vol 46, 1990, p 257–270
60. S.E. Cunningham and J.H. Griffin, Estimating the Importance of Cyclic Thermal Loads in Thermo-
Mechanical Fatigue, Int. J. Fract., Vol 47, 1991, p 161–180
Fatigue Crack Growth Testing
Ashok Saxena, Georgia Institute of Technology; Christopher L. Muhlstein, University of California, Berkeley
Selected References
Fatigue Crack Growth Rate Testing
• D. Blatt, R. John, and D. Coker, Single Edge Notched Specimens with Clamped Ends for Automated
Crack Growth Testing, Proc. SEM Spring Conf. on Experimental Mechanics, Society for Experimental
Mechanics, 1993
• D. Blatt, R. John, and D. Coker, Stress Intensity Factor and Compliance Solutions for a Single Edge
Notched Specimen with Clamped Ends, Eng. Fract. Mech., Vol 47 (No. 4), March 1994
• D.C. Freeman and M.J. Strum, Near Threshold Fatigue Testing, Report DE93009069/XAB, Gov. Res.
Announc. Index, 1993
• J.C.P. Kam and W.D. Dover, The Recent Development of Advanced Fatigue Crack Growth Testing
Technology for Large Scale Offshore Tubular Joints, Conf. Proc. Engineering Integrity through Testing,
Engineering Materials Advisory Services Ltd., West Midlands, UK, 1990
• R.K. Nanstad et al., A Computer-Controlled Automated Test System for Fatigue and Fracture Testing,
Conf. Proc. Applications of Automation Technology to Fatigue and Fracture Testing, STP 1092,
ASTM, 1990
• S. Nishijima, S. Matsuoka, and E. Takeuchi, Environmentally Affected Fatigue Crack Growth, Fatigue
90, Materials and Component Engineering Publications, Birmingham, UK, 1990
• H.-H. Over and B. Buchmayr, Collection and Evaluation of Fatigue and Fracture Mechanics Data
According to the European High Temperature Materials Databank (Petten) Standard, Conf. Proc.
Applications of Automation Technology to Fatigue and Fracture Testing, STP 1092, ASTM, 1990
• S. Saki, T. Asakawa, and H. Okamura, Automatic Data Acquisition System for Fatigue Crack Growth in
the Range from SSY to LGSY Region, Proc. Asian Pacific Conf. on Fracture and Strength '93, Japan
Society of Mechanical Engineers, 1993
• J. Solin and J. Hayrynen, Simulation of Mechanical and Environmental Conditions in Fatigue Crack
Growth Testing, Conf. Proc. Applications of Automation Technology to Fatigue and Fracture Testing,
STP 1092, ASTM, 1990
Crack Measurement
• C.J. Beevers, Advances in Crack Length Measurement, Engineering Materials Advisory Services Ltd.,
West Midlands, UK, 1983
• C.J. Beevers, The Measurement of Crack Length and Shape during Fracture and Fatigue, Engineering
Materials Advisory Services Ltd., West Midlands, UK, 1980
• M.S. Domack, Evaluation of K
ISCC
and da/dt Measurements for Aluminum Alloys Using Precracked
Specimens, Conf. Proc. Environmentally Assisted Cracking: Science and Engineering, STP 1049,
ASTM, 1990
• R.L. Hewitt, Accuracy and Precision of Crack Length Measurement Using a Compliance Technique, J.
Test. Eval., Vol 11, 1983, p 150–155
• H.H. Johnson, Calibrating the Electrical Potential Method for Studying Slow Crack Growth, Mat. Res.
Stand., Vol 5, 1965, p 442–445
• J.A. Kapp, G.S. Leger, and B. Gross, “Wide Range Displacement Expressions for Standard Fracture
Mechanics Specimens,” Report ARLCB-TR-84025, Army Armament Research and Development
Center, 1984
• C.Y. Li and R.P. Wei, Calibrating the Electrical Potential Method for Studying Slow Crack Growth,
Mat. Res. Stand., Vol 6, 1966, p 392–394
• D.C. Maxwell, J.P. Gallagher, and N.E. Ashbaugh, “Evaluation of COD Compliance Determined Crack
Growth Rates,” Report AFWAL-TR-84-4062, Wright-Patterson Air Force Base
• A.C. Pickard, R.A. Venables, and S.I. Vukelich, Crack Detection and Crack Length Measurement in the
Gas Turbine Industry, Fatigue Crack Measurement: Techniques and Applications, Engineering
Materials Advisory Services Ltd., West Midlands, UK, 1991
Fatigue Testing and Behavior of Plastics
Lisa A. Pruitt, University of California at Berkeley
Introduction
FATIGUE FRACTURE OF ENGINEERING PLASTICS due to cyclic loading conditions is of critical concern
when designing polymeric components for structural employment. Like all engineering materials, failure often
ensues in the plastic as a consequence of accumulated irreversible damage or growth of a fatigue flaw to a
critical dimension. The fatigue life of a polymeric component is controlled by a number of factors; in general,
polymers are more sensitive to the testing environment than metal or ceramic counterparts. These variables
include the stress or strain amplitude of the loading cycle; the mean stress of the cycle; the presence of stress
concentrations or initial defects in the component; the frequency, temperature, and environment of the test; and
the molecular properties of the polymer. These factors are of considerable interest and practicality for the safe
design of structural polymeric components subjected to repetitive loading.
When designing for the fatigue life of an engineering plastic, one of two distinct philosophies is generally
practiced. The total life approach is utilized with unnotched specimens that are assumed to be defect free, and
this methodology is predicated on the notion that fatigue failure is a consequence of both crack nucleation and
subsequent growth. Conversely, the defect tolerant approach bases the fatigue life of a component on the
number of loading cycles needed to propagate a crack of an initial size to a critical dimension. Over the last few
decades, numerous researchers have provided detailed reviews (Ref 1, 2, 3, 4, 5, 6, 7, and 8) of fatigue behavior
in polymers based on both total life and fracture mechanics approaches.
This article provides a review of fatigue test methodologies and an overview of general fatigue behavior in
engineering plastics. Many factors affect the fatigue performance of engineering materials, including molecular
and mechanical variables as well as the design of the fatigue test. The appropriate test conditions should be
used when evaluating the life of the polymer. If a structural component is likely to be free of defects and stress
concentrations or if the component is likely to spend the majority of its lifetime in the initiation stage of crack
growth, then the total life philosophy is preferred. On the other hand, fracture mechanics should be used for
safety critical fatigue designs and in flawed structural components likely to sustain a high degree of stable crack
growth prior to fracture.
References cited in this section
1. E.H. Andrews, Testing of Polymers, W. Brown, Ed., Wiley, 1969, p 237
2. P. Beardmore and S. Rabinowitz, Treat. Mater. Sci. Technol., Vol 6, 1975, p 267
3. J.A. Sauer and G.C. Richardson, Int. J. Fract., Vol 16, 1980, p 499
4. R.W. Hertzberg, M.D. Skibo, and J.A. Manson, Fatigue Mechanisms, ASTM STP 675, ASTM, 1979, p
471
5. R.W. Hertzberg and J.A. Manson, Fatigue of Engineering Plastics, Academic Press, 1980
6. R.W. Hertzberg and J.A. Manson, in Encyclopedia of Polymer Science and Engineering, Wiley, 1986, p
378
7. H.H. Kausch and J.G. Williams, in Encyclopedia of Polymer Science and Engineering, Wiley, 1986, p
341
8. J.A. Sauer and M. Hara, Advances in Polymer Science 91/92, H.H. Kausch, Ed., Springer-Verlag, 1990,
p 71
Fatigue Testing and Behavior of Plastics
Lisa A. Pruitt, University of California at Berkeley
Fatigue Crack Initiation
Stress-Based Loading. The traditional total life philosophy for fatigue life prediction is based on an endurance
limit established from stress-log cycle plots, also known as S-N curves. In these tests, uncracked specimens are
subjected to a constant amplitude load cycle until failure occurs. Often fatigue tests are performed on closed-
loop servohydraulic universal test machines, which enable fatigue tests to be performed under a variety of
waveforms over a range of test frequencies (including spectrum loadings). The stress amplitude can be applied
through torsion, rotation, beam bending, or axial loads. Specimens (Fig. 1) are chosen according to the loading
method employed. For example, the standard fatigue test for plastics in ASTM D 671 specifies repeat flexural
stress as a standard fatigue test. In this test, a triangulated specimen geometry (Fig. 1d) provides a uniform
flexural stress across the entire gauge section.
Fig. 1 Schematic of specimens used for total life fatigue analysis. Tests can be done (a) in torsion, (b)
with a rotating cantilever, (c) with a rotating beam, (d) with cantilever reverse bending, or (e) under axial
loading
In fatigue testing, the applied stress, σ
a
, is typically described by the stress amplitude of the loading cycle:
(Eq 1)