Near-threshold fatigue crack growth behavior in copper
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I,
INTRODUCTION
THE fracture
mechanics approach to the evaluation of fatigue crack growth behavior provides an extremely valuable tool for the development of rational design criteria useful in assessing structural integrity and preventing failure. The use of the preexisting defect tolerant approach in fatigue life prediction can reduce or eliminate the crack initiation phase of a fatigue crack which may account for a very large percentage of the cyclic life. However, at very low stress intensity ranges, preexisting cracks either grow at very low rates or do not grow at all under cyclic loading. Since fatigue crack growth behavior near the threshold is extremely important in applications involving very long life considerations, it is necessary to develop these data in structural materials. This paper presents the results of an evaluation of the near-threshold fatigue crack growth rate behavior in commercially pure copper. The effects of material condition (annealed, quarter-hard, and full-hard) and load ratio were investigated in detail. The influences of waveform and specimen geometry on slow crack propagation rates were also considered. Auger analysis was used to examine the effects of oxide deposits on threshold crack growth behavior. In particular, the role of crack closure during nearthreshold fatigue crack propagation was emphasized.
II.
MATERIAL
Near-threshold fatigue crack growth tests in air were performed in ETP (Electrolytic Tough Pitch) copper with 99.95 pet purity. Three material conditions were characterized: annealed, quarter-hard, and full-hard. Quarter-hard and full-hard specimens were cold-rolled to 11 pet and 31 pet reduction in thickness, respectively. To produce annealed specimens, quarter-hard materials were heat-treated at 550 ~ for one hour. The mechanical properties are shown in Table I. While annealed copper shows a considerable amount of cyclic hardening, quarter-hard and full-hard copper show
PETER K. LIAW, Senior Engineer, and M.G. PECK, Research Technician, are both with Westinghouse Research and Development Center, Materials Engineering Department. T.R. LEAX is Advanced Engineer, Westinghouse Steam Turbine Generator Division, and R. S. WILLIAMS is Senior Engineer, Westinghouse Research and Development Center, Physical Metallurgical Department, Pittsburgh, PA 15235. Manuscript submitted October 13, 1981. METALLURGICALTRANSACTIONS A
cyclic softening. Even though the annealed and quarter-hard (full-hard) materials have significantly different monotonic yield strengths, both have approximately the same order of cyclic yield strength. This appears to be consistent with Laird and Feltner's I finding that a material of high stacking fault energy possesses a unique cyclic stress and strain curve regardless of the previous deformation history. Interestingly, the three materials have nearly the same ultimate tensile strength; see Table I.
III.
EXPERIMENTAL TECHNIQUE
Near-threshold fatigue crack growth tests in annealed and full-hard copper were performed using WOL (wedge-openloading)
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