ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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Fig. 16 Scanning electron micrographs depicting (a) the ductile mechanisms observed in pristine
UHMWPE and (b) the brittle mechanisms found in acrylic bone cement
References cited in this section
5. R.W. Hertzberg and J.A. Manson, Fatigue of Engineering Plastics, Academic Press, 1980
59. L. Engel, H. Klingele, G.W. Ehrenstein, and H. Schaper, An Atlas of Polymer Damage, Wolfe Science
Books, Vienna, 1981
Fatigue Testing and Behavior of Plastics
Lisa A. Pruitt, University of California at Berkeley
References
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
9. J.D. Ferry, Viscoelastic Properties of Polymers, Wiley, 1961
10. G.C. Sih, Handbook of Stress Intensity Factors, Lehigh University Press, 1973
11. P.C. Paris, M.P. Gomez, and W.P. Anderson, Trends Eng., Vol 13, 1961, p 9
12. R.O. Ritchie, Proc. of the 5th Int. Conf., M.G. Yan, S.H. Zhang, and Z.M. Zheng, Ed., Pergamon Press,
1988, p 5
13. S. Suresh, Fatigue of Materials, 2nd ed., Cambridge University Press, 1998
14. A.G. Evans, Z.B. Ahmad, D.G. Gilbert, and P.W.R. Beaumont, Acta Metall., Vol 34, 1986, p 79
15. C.B. Bucknall and W.W. Stevens, in Toughening of Plastics, Plastics and Rubber Institute, London,
1978, p 24
16. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th ed., Wiley, 1996
17. T.L. Anderson, Fracture Mechanics: Fundamentals and Applications, CRC Press, 1995
18. A.S. Argon, Proc. of 7th Int. Conf. on Advances in Fracture Research, K. Samala, K. Ravi-Chander,
D.M.R. Taplin, and P. Rama Rao, Ed., Pergamon Press, 1989
19. R.W. Lang, J.A. Manson, R.W. Hertzberg, and R. Schirrer, Polym. Eng. Sci., Vol 24, 1984, p 833
20. A.J. Kinloch and F.J. Guild, in Advances in Chemistry Series 252: Toughened Plastics II: Novel
Approaches in Science and Engineering, American Chemical Society, Washington, D.C., 1996, p 1
21. T.R. Clark, R.W. Hertzberg, and N. Nohammadi, in 8th Int. Conf. Deformation, Yield and Fracture of
Polymers, Plastics and Rubber Institute, London, 1991, p 31/1
22. E.J. Kramer and L.L. Berger, Advances in Polymer Science 91/92, H.H. Kausch, Ed., Springer-Verlag,
1990, p 1
23. H.R. Azimi, R.A. Pearson, and R.W. Hertzberg, J. Mater. Sci., Vol 31, 1996, p 3777
24. S.A. Sutton, Eng. Fract. Mech., Vol 6, 1974, p 587
25. B. Mukherjee and D.J. Burns, Mech. Eng. Sci., Vol 11, 1971, p 433
26. Y.-Q. Zhou and N. Brown, J. Poly. Sci. B, Polym. Phys., Vol 30, 1992, p 477
27. E.J. Moskala, in 8th Int. Conf. Deformation, Yield and Fracture of Polymers, Plastics and Rubber
Institute, London, 1991, p 51/1
28. S. Arad, J.C. Radon, and L.E. Culver, J. Mech. Eng. Sci., Vol 13, 1971, p 75
29. A.S. Argon and R.E. Cohen, Advances in Polymer Science 91/92, H.H. Kausch, Ed., Springer-Verlag,
1990, p 300
30. J.A. Manson, R.W. Hertzberg, and P.E. Bretz, Advances in Fracture Research, D. Francois, Ed.,
Pergamon Press, 1981, p 443
31. L. Pruitt and S. Suresh, Polymer, Vol 35, 1994, p 3221
32. L. Pruitt and D. Rondinone, Polym. Eng. Sci., Vol 36, 1996, p 1300
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50. G.M. Connelly, C.M. Rimnac, T.M. Wright, R.W. Hertzberg, and J.A. Manson, J. Orth. Res., Vol 2,
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54. C.M. Rimnac, T.M. Wright, and R.W. Klein, Polym. Eng. Sci., Vol 28, 1988, p 1586
55. M. Goldman, R. Ranganathan, R. Gronsky, and L. Pruitt, Polymer., Vol 37 (No. 14), 1996, p 2909–
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56. D. Baker, R. Hastings, and L. Pruitt, Polymer, 1999, in press
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59. L. Engel, H. Klingele, G.W. Ehrenstein, and H. Schaper, An Atlas of Polymer Damage, Wolfe Science
Books, Vienna, 1981
Fatigue Testing of Brittle Solids
J.A. Salem, Glenn Research Center at Lewis Field; M.G. Jenkins, University of Washington
Introduction
CERAMICS AND GLASSES subjected to static or cyclic loads exhibit time-dependent failure due to the
growth in inherent and induced flaws to a critical size (Ref 1, 2, 3, 4, 5, 6, and 7). Cyclic loading is not required
to generate crack extension. Hence, the phenomenon is referred to as “static fatigue.” The phenomenon is also
referred to as subcritical crack growth, stress corrosion, delayed failure, slow crack growth, or environmentally
induced fatigue.
This phenomenon depends on the chemical makeup and temperature of the environment and the applied stress-
intensity factor. For relatively low stress-intensity factors, a fatigue limit below which crack growth arrests has
been measured for static loading (Ref 8, 9, 10, and 11). However, the threshold occurs at low velocities, and the
threshold is generally ignored in engineering applications.
Although slow crack growth is more severe for glass and glassy polycrystalline ceramics, single crystals such
as sapphire and magnesium fluoride also exhibit time-dependent failure under static loads (Ref 12, 13, and 14).
Thus, the structural application of glasses and ceramics requires consideration of static fatigue or slow crack
growth behavior, and testing standards (Ref 15, 16, 17, 18, and 19) have been developed to generate data and
determine fatigue parameters for materials screening and life prediction. Two types of flaws are generally used
in the determination of the slow crack properties of ceramics: short, inherent flaws or long, induced cracks.
Although machining processes such as surface grinding can introduce flaws, such flaws are generally short
enough to be treated as inherent flaws. In some cases, the material can be heat treated (Ref 20) or etched (Ref
21) to minimize the presence of such populations. The testing standards focus on preexisting flaws within the
material and allow those flaws to grow until catastrophic failure occurs. The fatigue parameters are inferred
from strength data instead of by direct measurement of the crack growth.
In addition to the crack growth due to an environmentally induced stress corrosion mechanism, many ceramics
materials exhibit enhanced fatigue crack growth during cyclic loading, and cyclically induced fatigue crack
growth has been measured in vacuum (Ref 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32). This behavior is
somewhat akin to that exhibited by metals, and measurements with short cracks can be considerably different
from those measured with long cracks. This sensitivity to crack length is a consequence of the mechanisms used
to toughen ceramics (e.g., grain bridging in the crack wake), which are extrinsic in nature. Fatigue crack growth
of long cracks in toughened ceramics results as the toughening mechanism breaks down during load reversals,
whereas for short cracks or ceramics without the presence of a toughening mechanism, crack growth is
dominated by environmental factors and intrinsic fatigue mechanisms. Factors such as grain size and grain
boundary phase, which generally relate to the toughening or environmental sensitivity, influence the sensitivity
to cyclic loading (Ref 29, 30, 31, 32, and 33).
The distinction between environmentally induced fatigue and cyclically induced fatigue is not always clear, and
some ceramics will exhibit a combination of both mechanisms.
Structural components made from ceramics materials that are expected to have long lives will fail from small
cracks developed over long times. Thus, the development of standardized cyclic fatigue test methods has also
revolved around the use of small, inherent flaws.
References cited in this section
1. C. Gurney and S. Pearson, Fatigue of Mineral Glass under Static and Cyclic Loading, Proc. R. Soc. A.,
Vol 192, 1948, p 537–543
2. E.B. Shand, Experimental Study of Fracture of Glass, Part I: The Fracture Process, J. Ceram. Soc., Vol
37, 1954, p 52–60
3. S. Pearson, Delayed Fracture of Sintered Alumina, Proc. Physicl Soc., Vol 69, Part 12, Section B, Dec
1956, p 1293–1296
4. R.E. Mould and R.D. Southwick, Strength and Static Fatigue of Abraded Glass under Controlled
Ambient Conditions, Part II: Effect of Various Abrasions and the Universal Fatigue Curve, J. Am.
Ceram. Soc., Vol 42, 1959, p 582–592
5. S.M. Weiderhorn, Influence of Water Vapor on Crack Propagation in Soda-Lime Glass, J. Am. Ceram.
Soc., Vol 50, 1967, p 407–414
6. A.G. Evans and F.F. Lange, Crack Propagation and Fracture in Silicon Carbide, J. Mater. Sci., Vol 10
(No. 10), 1975, p 1659–1664
7. J.E. Ritter and J.N. Humenik, Static and Dynamic Fatigue of Polycrystalline Alumina, J. Mater. Sci.,
Vol 14 (No. 3), 1979, P 626–632
8. S.M. Weiderhorn and L.H. Boltz, Stress Corrosion and Static Fatigue of Glass, J. Am. Ceram. Soc., Vol
62 (No. 7–8), 1970, p 547–548
9. B.J.S. Wilkins and E. Dutton, Static Fatigue Limit with Particular Reference to Glass, J. Am. Ceram.
Soc., Vol 59 (No. 3–4), 1976, p 108–112
10. T.A. Michalske, The Stress Corrosion Limit: Its Measurement and Implications, Fracture Mechanics of
Ceramics, Vol 5, R.C. Bradt, A.G. Evans, D.P.H. Hasselman, and F.F. Lange, Ed., Plenum Publishing
Corp., 1983
11. T. Fett, K. Germerdonk, A. Grossmuller, K. Keller, and D. Munz, Subcritical Crack Growth and
Threshold in Borosilicate Glass, J. Mater. Sci., Vol 26, 1991, p 253–257
12. S.M. Wiederhorn, Moisture Assisted Crack Growth in Ceramics, Int. J. Fract. Mech., Vol 4 (No. 2),
1968, p 171–177
13. S.M. Wiederhorn, B.J. Hockey, and D.E. Roberts, Effect of Temperature on the Fracture of Sapphire,
Philos. Mag., Vol 28 (No. 4), 1973, p 783–796
14. T.A. Michalske, B.C. Bunker, and S.W. Freiman, Stress Corrosion of Ionic and Mixed Ionic/Covalent
Solids, J. Am. Ceram. Soc., Vol 69 (No. 10), 1986, p 721–724
15. DD ENV 843-3 Advanced Technical Ceramics—Monolithic Ceramics—Mechanical Properties at
Room Temperature, Part 3: Determination of Subcritical Crack Growth Parameters from Constant
Stressing Rate Flexural Strength Tests, British Standards Institution, London, 1997
16. “Testing Method for Bending Fatigue of Fine Ceramics,” JIS R 1621, Japanese Standards Association,
Tokyo, Japan, 1996
17. “Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by
Constant Stress-Rate Flexural Testing at Ambient Temperature,” C 1368, Annual Book of ASTM
Standards, ASTM, Vol 15.01, 1999, p 706–714
18. “Test Methods for Static Bending Fatigue of Fine Ceramics,” JIS R 1632, Japanese Standards
Association, Tokyo, Japan, Dec 1998
19. Standard Practice for Constant-Amplitude, Axial Tension-Tension Cyclic Fatigue of Advanced
Ceramics at Ambient Temperatures,” C1361, Annual Book of ASTM Standards, ASTM
20. J.A. Salem, N.N. Nemeth, L.M. Powers, and S.R. Choi, Reliability Analysis of Uniaxially Ground
Brittle Materials, J. Eng. Gas Turbines Power (Trans. ASME), Vol 118, 1996, p 863–871
21. D.H. Roach and A.R. Cooper, “Etching of Soda Lime Glass-Another Look,” in the third proceedings of
a symposium sponsored by the Etel Institute for Silicate Research/The University of Toledo and the
Northwestern Ohio Section of the Am. Ceram. Soc., W. Kneller, Ed., March 1983, p 1–7
22. F. Gigu, Cyclic Fatigue of Polycrystalline Alumina in Direct Push-Pull, J. Mater. Sci. Lett., Vol 13 (No.
6), 1978, p 1357–1361
23. M.V. Swain, Lifetime Prediction of Ceramic Materials, Mater. Forum, Vol 9 (No. 1–2), 1st and 2nd
Quarter 1986, p 34–44
24. H. Kawakubo and K. Komeya, Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room
Temperature, J. Am. Ceram. Soc., Vol 70 (No. 6), 1987, p 400–405
25. R.H. Dauskardt, W. Yu, and R.O. Ritchie, Fatigue Crack Propagation in Transformation Toughened
Zirconia, J. Am. Ceram. Soc., Vol 70 (No. 10), 1987, p C-248–C-252
26. G. Grathwohl, Fatigue of Ceramics under Cyclic Loading, Mater. Sci. Technol., Vol 19 (No. 4), April
1988, p 113–124 (in German)
27. M.J. Reece, F. Guiu, and M.F.R. Sammur, Cyclic Fatigue Crack Propagation in Alumina under Direct
Tension-compression Loading, J. Am. Ceram. Soc., Vol 72 (No. 2), 1989, p 348–352
28. Y. Mutoh, M. Takahashi, T. Oikawa, and H. Okamoto, Fatigue Crack Growth of Long and Short Cracks
in Silicon Nitride, Fatigue of Advanced Materials, R.O. Ritchie, R.H. Dauskardt, and B.N. Cox, Ed.,
Material and Component Engineering Publications, Ltd., Edgbaston, U.K., 1991, p 211–225
29. S. Horibe and H. Hirahara, Cyclic Fatigue of Ceramics Materials: Influence of Crack Path and Fatigue
Mechanisms, Acta Metall. Mater., Vol 39 (No. 6), 1991, p 1309–1317
30. M. Okazaki, A.J. McEvily, and T. Tanaka, On the Mechanism of Fatigue Crack Growth in Silicon
Nitride, Metall. Trans. A, Vol 22A, 1991, p 1425–1434
31. T. Tanaka, N. Okabe, H. Nakayama, and Y. Ishimaru, Fatigue Crack Growth of Silicon Nitride with
Crack Wedging by Fine Fragments, Fatigue Fract. Eng. Mater. Struct., Vol 17 (No. 7), 1992, p 643–
653
32. R.O. Ritchie, C.J. Gilbert, and J.M. McNaney, Mechanics and Mechanisms of Fatigue Damage and
Crack Growth in Advanced Materials, Int. J. Solids Struct., Vol 37, 2000, p 311–329
33. R. Dauskardt, Cyclic Fatigue-Crack Growth in Grain Bridging Ceramics, J. Eng. Mater. Technol.
(Trans. ASME), Vol 115, 1993, p 115–251
Fatigue Testing of Brittle Solids
J.A. Salem, Glenn Research Center at Lewis Field; M.G. Jenkins, University of Washington
Fatigue Mechanism
Fatigue mechanisms in brittle solids can be classified as environmentally induced fatigue or as cyclic fatigue.
Environmentally Induced Fatigue. Environmentally induced, or static, fatigue in ceramics and glasses is a
chemically activated atomic reaction that is enhanced by stress and temperature. Crack growth occurs by
breakage of the bonds at the crack tip. In particular, the introduction of water to the environment can have a
severe effect on the fatigue parameters, and the moisture in air is sufficient to cause crack growth in ceramics
and glasses subjected to stress.
For aqueous solutions, the rate of crack growth is increased with the increasing concentration of OH- ions,
which break the metal-oxygen-metal bonds in silicate glass and oxide ceramics. For nonoxide ceramics that do
not contain a glassy phase (e.g., SiC), the presence of room temperature water vapor may have little effect on
crack growth rates. Acid base pairs have the strongest influence on crack growth (Ref 14, 34). Michalske et al.
(Ref 34) developed an atomistic model addressing the role of mechanical strain in accelerating chemical
reactions between crack tip bonds in silica and environmental molecules. The strain field at the crack tip
deforms the Si-O bond angle, thereby resulting in bond defects that increase the chemical activity of the bonds.
Chemisorption of the attacking species results in bond breakage. The absolute rate of crack extension depends
on the combined rates of active bond formation and chemical attack at strain-induced defects.
Additional studies on the {1012} of sapphire in water, ammonia, hydrazine, and acetonitrile indicated the same
mechanism—dissociative chemisorption—but with different details that make the fatigue parameter more
sensitive to the specific environment (Ref 14).
In contrast to the slow crack growth mechanism in sapphire, that in magnesium fluoride, which is ionically
bonded, appeared to be ionic solvation of strained ions at the crack tip. The actively corrosive environments, in
this case, water, methanol, and acetonitrile, reduced the electrostatic attraction between oppositely charged ion
pairs at the crack tip.
For corrosive environments such as elevated temperature water or pressurized steam, silicon nitrides undergo
severe pitting or grain loss, depending on the oxide additives and grain morphology (Ref 35, 36). For silicon
nitride with amorphous silica at the grain boundaries, intergranular corrosion results in the Si
3
N
4
grains falling
out. However, for silicon nitrides with even small amounts of oxide additives, the leachability of the grain
boundary is substantially changed. Depending on the additives, pits form under either a protective or
nonprotective corrosion layer (Ref 35). The implication for environmentally induced crack growth is that harsh
environments involving elevated temperature steam, such as turbines, will result in substantially accelerated
crack growth.
Cyclic Fatigue. A variety of mechanisms have been proposed to result in cyclically induced fatigue in ceramics
(Ref 26, 29, 30, 31, 32, 33, 37, 38, and 39). Generally, the fatigue mechanism is related to the crack growth
resistance mechanism of the particular material. For example, the breakdown of bridging grains or
transformation zones that produce R-curve effects in silicon nitrides and transformation-toughened zirconia.
Also, mechanisms such as the coalescence of microcrack clusters at grain boundaries have been proposed (Ref
39). As the material is cycled, the toughening mechanism is broken down, thereby increasing the crack tip
stress-intensity factor, allowing crack extension until the crack growth resistance redevelops.
One of the most critical aspects of cyclic fatigue testing is the size of the crack used. Results generated with
long cracks can be substantially different from those generated with short cracks (Ref 28, 40).
References cited in this section
14. T.A. Michalske, B.C. Bunker, and S.W. Freiman, Stress Corrosion of Ionic and Mixed Ionic/Covalent
Solids, J. Am. Ceram. Soc., Vol 69 (No. 10), 1986, p 721–724
26. G. Grathwohl, Fatigue of Ceramics under Cyclic Loading, Mater. Sci. Technol., Vol 19 (No. 4), April
1988, p 113–124 (in German)
28. Y. Mutoh, M. Takahashi, T. Oikawa, and H. Okamoto, Fatigue Crack Growth of Long and Short Cracks
in Silicon Nitride, Fatigue of Advanced Materials, R.O. Ritchie, R.H. Dauskardt, and B.N. Cox, Ed.,
Material and Component Engineering Publications, Ltd., Edgbaston, U.K., 1991, p 211–225
29. S. Horibe and H. Hirahara, Cyclic Fatigue of Ceramics Materials: Influence of Crack Path and Fatigue
Mechanisms, Acta Metall. Mater., Vol 39 (No. 6), 1991, p 1309–1317
30. M. Okazaki, A.J. McEvily, and T. Tanaka, On the Mechanism of Fatigue Crack Growth in Silicon
Nitride, Metall. Trans. A, Vol 22A, 1991, p 1425–1434
31. T. Tanaka, N. Okabe, H. Nakayama, and Y. Ishimaru, Fatigue Crack Growth of Silicon Nitride with
Crack Wedging by Fine Fragments, Fatigue Fract. Eng. Mater. Struct., Vol 17 (No. 7), 1992, p 643–
653
32. R.O. Ritchie, C.J. Gilbert, and J.M. McNaney, Mechanics and Mechanisms of Fatigue Damage and
Crack Growth in Advanced Materials, Int. J. Solids Struct., Vol 37, 2000, p 311–329
33. R. Dauskardt, Cyclic Fatigue-Crack Growth in Grain Bridging Ceramics, J. Eng. Mater. Technol.
(Trans. ASME), Vol 115, 1993, p 115–251
34. T.A. Michalske and B.C. Bunker, Slow Fracture Based on Strained Silicate Structures, J. Appl. Phys.,
Vol 56 (No. 10), 1984, p 2686–2694
35. T. Yishio and K. Oda, Aqueous Corrosion and Pit Formation of Si
3
N
4
under Hydrothermal Conditions,
Ceram. Trans., Vol 10, 1990, p 367–386
36. M.K. Ferber, H.T. Lin, and J. Keiser, Oxidation Behavior of Non-Oxide Ceramics in a High-Pressure,
High-Temperature Steam Environment, Mechanical, Thermal and Environmental Testing and
Performance of Ceramic Composites and Components, STP 1392, M.G. Jenkins, E. Lara-Curzio, and S.
Gonczy, Ed., ASTM, 2000
37. G. Vekinis, M.F. Ashby, and P.W. Beaumont, R-Curve Behaviour of AP
2
O
3
Ceramics, Acta Metall.
Mater., Vol 38 (No. 6), 1990, p 1151–1162
38. D.A. Krohn and D.P.H. Hasselman, Static and Cyclic Fatigue Behavior of a Polycrystalline Alumina, J.
Am. Ceram. Soc., Vol 55 (No. 4), 1972, p 208–211
39. M. Masuda, T. Soma, M. Matsui, and I. Oda, Fatigue of Ceramics, Part 1: Fatigue Behavior of Sintered
Silicon Nitride under Tension-Compression Cyclic Stress, J. Ceram. Soc. Jpn. Int. Ed., Vol 96, 1988, p
275–280
40. A. Steffen, R.H. Dauskardt, and R.O. Ritchie, Cyclic Fatigue Life and Crack Growth Behavior of
Microstructural Small Cracks in Magnesia-Partially-Stabilized Zirconia Ceramics, J. Am. Ceram. Soc.,
Vol 74 (No. 6), 1991, p 1259–1268
Fatigue Testing of Brittle Solids
J.A. Salem, Glenn Research Center at Lewis Field; M.G. Jenkins, University of Washington
Fatigue Mechanism
Fatigue mechanisms in brittle solids can be classified as environmentally induced fatigue or as cyclic fatigue.
Environmentally Induced Fatigue. Environmentally induced, or static, fatigue in ceramics and glasses is a
chemically activated atomic reaction that is enhanced by stress and temperature. Crack growth occurs by
breakage of the bonds at the crack tip. In particular, the introduction of water to the environment can have a
severe effect on the fatigue parameters, and the moisture in air is sufficient to cause crack growth in ceramics
and glasses subjected to stress.
For aqueous solutions, the rate of crack growth is increased with the increasing concentration of OH- ions,
which break the metal-oxygen-metal bonds in silicate glass and oxide ceramics. For nonoxide ceramics that do
not contain a glassy phase (e.g., SiC), the presence of room temperature water vapor may have little effect on
crack growth rates. Acid base pairs have the strongest influence on crack growth (Ref 14, 34). Michalske et al.
(Ref 34) developed an atomistic model addressing the role of mechanical strain in accelerating chemical
reactions between crack tip bonds in silica and environmental molecules. The strain field at the crack tip
deforms the Si-O bond angle, thereby resulting in bond defects that increase the chemical activity of the bonds.
Chemisorption of the attacking species results in bond breakage. The absolute rate of crack extension depends
on the combined rates of active bond formation and chemical attack at strain-induced defects.
Additional studies on the {1012} of sapphire in water, ammonia, hydrazine, and acetonitrile indicated the same
mechanism—dissociative chemisorption—but with different details that make the fatigue parameter more
sensitive to the specific environment (Ref 14).
In contrast to the slow crack growth mechanism in sapphire, that in magnesium fluoride, which is ionically
bonded, appeared to be ionic solvation of strained ions at the crack tip. The actively corrosive environments, in
this case, water, methanol, and acetonitrile, reduced the electrostatic attraction between oppositely charged ion
pairs at the crack tip.
For corrosive environments such as elevated temperature water or pressurized steam, silicon nitrides undergo
severe pitting or grain loss, depending on the oxide additives and grain morphology (Ref 35, 36). For silicon
nitride with amorphous silica at the grain boundaries, intergranular corrosion results in the Si
3
N
4
grains falling
out. However, for silicon nitrides with even small amounts of oxide additives, the leachability of the grain
boundary is substantially changed. Depending on the additives, pits form under either a protective or
nonprotective corrosion layer (Ref 35). The implication for environmentally induced crack growth is that harsh
environments involving elevated temperature steam, such as turbines, will result in substantially accelerated
crack growth.
Cyclic Fatigue. A variety of mechanisms have been proposed to result in cyclically induced fatigue in ceramics
(Ref 26, 29, 30, 31, 32, 33, 37, 38, and 39). Generally, the fatigue mechanism is related to the crack growth
resistance mechanism of the particular material. For example, the breakdown of bridging grains or
transformation zones that produce R-curve effects in silicon nitrides and transformation-toughened zirconia.
Also, mechanisms such as the coalescence of microcrack clusters at grain boundaries have been proposed (Ref
39). As the material is cycled, the toughening mechanism is broken down, thereby increasing the crack tip
stress-intensity factor, allowing crack extension until the crack growth resistance redevelops.
One of the most critical aspects of cyclic fatigue testing is the size of the crack used. Results generated with
long cracks can be substantially different from those generated with short cracks (Ref 28, 40).
References cited in this section
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