ASM Metals HandBook Volume 12 - Fractography
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of AISI type 304 stainless steel) with ultimate tensile strengths of 648 MPa (94 ksi) and 356 MPa (52 ksi) at room temperature and 600 °C (1110 °F), respectively, was tested to determine the effect of the type of cyclic load on the fatigue crack growth rate (Ref 255). Through-thickness cracks in thin-wall cylindrical test specimens were grown to half crack lengths ranging from 0.55 to 1.23 mm (0.02 to 0.048 in.) using fast-fast strain cycling. At the predetermined half length, one slow-fast cycle was introduced, and the fast-fast strain cycling was resumed. The fatigue test wave form is illustrated in Fig. 96. The wave form was triangular, fully reversed, with a total strain range of 0.8 to 1.5%. The fast-fast cycles were applied at a strain rate of ±50 × 10-5 s-1, and the slow-fast cycle consisted of a 1.4 × 10-5 s-1 rising strain ramp and 28 × 10-1 s-1 declining strain ramp. All test were conducted in air at a temperature of 600 °C (1110 °F). Fig. 96 Fatigue test strain wave from. A, strain, rate = 50 × 10-5 s-1 (fast-fast cycles); B, strain rate = 1.4 × 10-5 s-1 (the slow-rising strain ramp of the slow-fast cycle); C, strain rate = 28 × 10-5 s-1 (the fast-declining strain ramp of the slow-fast cycle). Source: Ref 255 The introduction of one slow-fast cycle always increased the fatigue crack growth rate, sometimes by up to a factor of two. When the fast-fast cycling was resumed after the slow-fast cycle, the crack propagated by a fatigue striation forming mechanism, in which case there was a local increase in the striation spacing, or by intergranular fracture (Fig. 97). Where the crack propagated by a striation-forming mechanism or by intergranular fracture was determined by the ratio \u2206Jc/\u2206Jf, where \u2206Jc = range of the creep J-integral and \u2206Jf = range of the cycle J-integral. The values of \u2206Jc and \u2206Jf can be determined from plots of load versus crack-opening displacement at half crack length for the slow-fast cycle, \u2206Jc, and the fast-fast cycle, \u2206Jf (Ref 255). When the value of \u2206Jc/\u2206Jf is less than 0.1, the crack propagates by a striation-forming mechanism; if the value exceeds 0.1, the crack propagates by intergranular fracture when fast-fast cycling is resumed . In general, fatigue striations were formed when half crack lengths were less than about 0.85 mm (0.033 in.) and the total strain range was less than 1%. Intergranular fractures occurred at the longer half crack lengths and the higher total strain ranges. Fig. 97 Effect of one slow-fast cycle on the fracture appearance of an SUS 304 stainless steel that was fatigue tested in air at 600 °C (1110 °F). (a) Increase in fatigue striation spacing after the introduction of the slow-fast cycle at a half crack length = 0.97 mm (0.038 in.) (b) Intergranular fracture after the application of one slow- fast cycle at a half crack length of 1.0 mm (0.039 in.). Crack growth direction in both figures is right to left. Source: Ref 255 From a material standpoint, the factors that determined whether the cracking occurred by striation fatigue or intergranular fracture were the degree of creep damage and the size of the damaged zone produced during the slow-fast cycle. If the creep-damaged zone ahead of the crack tip was smaller than the stain-deformed region resulting from the fast-fast cycling before the slow-fast cycle, the crack propagated by striation-forming fatigue. However, if the creep-damaged zone was larger than that of the fast-fast cycling and if wedge cracks were produced at the grain boundaries adjacent to the crack tip, intergranular fracture resulted. Regardless of the cracking mechanism, the fatigue crack growth rate is accelerated until the fatigue crack escapes the creep-damaged zone (Ref 255). Examples 27. A fully heat treated IN-738 nickel-base superalloy was tested to determine the effect of wave form on fatigue life (Ref 256). Seven strain wave forms (Fig. 98) were used to conduct fatigue life tests at a total strain range of 0.4 to 1.2%. The strain rates for the fast-fast and the slow-slow wave forms were 10-2 and 10-5 s-1, respectively. For the slow-fast and fast-slow sawtooth wave forms, the slow strain rate was 10-5 s-1 and the fast rate was 10-2 s-1. The ramp rate for the truncated wave forms was 10-2 s-1, and the dwell times at maximum tensile or compressive strains varied from 400 to 700 s. All tests were performed in air at a temperature of 850 °C (1560 °F). Fig. 98 Strain wave forms used in fatigue testing. Source: Ref 256 The fatigue life to crack initiation, defined as the number of cycles required to initiate a 0.5-mm (0.02-in.) long crack, was lower for the SS, TW-TC, SF, and TW-T wave forms than for the FF wave form (see Fig. 98 for explanation of abbreviations). The FS and TW-C types were also lower than FF, but generally not by as much as the SS, TW-TC, SF, and TW-T types. Crack initiation and propagation for the FF wave form were transgranular, the SS, TW-TC, SF, and TW-T forms showed intergranular crack initiation but a mixed propagation. For the FS and TW-C wave forms, fatigue crack initiation was either intergranular or transgranular; the determining factor was the total strain range. The higher range favored intergranular initiation and mixed propagation, while the lower ranges promoted transgranular initiation and propagation. In addition to the wave forms influencing the fracture path, the dwell and ramp rates were also shown to affect the mean stress. It was discovered that a dwell on one side of the strain wave developed a mean stress on the opposite side of the wave. For example, a dwell at maximum tensile strain (maximum tensile stress) developed a compressive mean stress in the fatigue load cycle. This shift in the mean stress was apparently due to creep during dwell at the maximum tensile (or compressive) strain so that returning the specimen to the maximum compressive (or tensile) strain resulted in a shift in the initial zero-mean stress to the compressive (or tensile) side of the fatigue curve. Although not as large as in the dwell wave forms, mean stresses sometimes developed in wave forms having unequal ramp rates, with the mean stress occurring on the side of the faster ramp rate. The one clear trend in the fatigue life tests showed that the FF wave forms exhibited the longest fatigue lives. Because of scatter in the data, no clear trends were established for the other wave forms, although a few individual specimens had longer fatigue lives than some of the shorter-life FF specimens. Because FF loading always produced the optimum (low fatigue crack growth rate) transgranular crack path, the occasional longer fatigue lives of individual non-FF wave form specimens were attributed to such factors as the presence of a compressive mean stress, multiple crack propagation, and crack branching, all of which would contribute to an increase in the fatigue life (Ref 256). References cited in this section 15. R.O. Ritchie, in Environment-Sensitive Fracture of Engineering Materials, Z.A. Foroulis, Ed., The Metallurgical Society, 1979, p 538-564 19. R.M.N. Pelloux, Trans. ASM, Vol 62, 1969, p 281-285 25. D.A. Meyn, Trans. ASM, Vol 61 (No. 1), 1968, p 42 31. P.C. Paris and F. Erdogan, J. Basic Eng., (Trans. ASME), D, Vol 85, 1963, p 528 32. H.H. Johnson and P.C. Paris, Eng. Fract. Mech., Vol 1, 1968, p 3 38. C.D. Beachem, Metall. Trans. A, Vol 3A, 1972, p 437 40. C.A. Zapffe and C.E. Sims, Trans. AIME, Vol 145, 1941, p 225 89. D.A. Meyn and E.J. Brooks, in Fractography and Material Science, STP 733, L.N. Gilbertson and R.D. Zipp, Ed., American Society for Testing and Materials, 1981, p 5-31 92. M.F. Stevens and I.M. Bernstein, Metall. Trans. A, Vol 16A, 1985, p 1879 95. M.B. Whiteman and A.R. Troiano, Corrosion, Vol 21, 1965, p 53 96. M.L. Holtzworth, Corrosion, Vol 25, 1969, p 107 97. R. Langenborg, J.