The influence of tempering temperature on small fatigue crack behavior monitored with surface acoustic waves in quenched
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INTRODUCTION
T H E unusual behavior of small surface fatigue cracks compared to large through-cracks was documented more than ten years ago by Pearson. tll Since then, this "small crack effect" (SCE), where small cracks grow rapidly at AK levels below the long crack threshold and then often decelerate as they continue to extend, has been widely investigated in a range of materials and microstructures, t2-sl Perhaps the major cause for the SCE is the breakdown in similitude between the two crack types, tg,~~ It may be improper to apply K, a parameter based on linear elastic fracture mechanics (LEFM), to small fatigue cracks. I1~,123 As evidence of this, several studies have shown the dependence of the small crack growth rate on AK and mo'. [4'8'13-16] If LEFM were valid, AK should uniquely describe small crack growth. Crack closure may be another possible cause of the SCE. There is evidence that closure begins at a low value when a crack is initiated, increases with crack growth, and saturates at the long crack closure value when the crack is sufficiently large, t5,~~ Thus, small cracks experience a reduced crack closure level compared to long cracks, allowing them to grow below the long crack threshold. Small crack growth rate deceleration could be caused by the increase in closure with growth. Finally, microstructural interactions may also explain the SCE. Substantial crack growth variations from grainto-grain could explain the unusual behavior of small cracks. I15'23] Crack growth within a certain grain could
B. LONDON, formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA, is Research Scientist, McDonnell Douglas Research Laboratories, St. Louis, MO, 63166. D.V. NELSON, Associate Professor, is with the Department of Mechanical Engineering, Stanford University, Stanford, CA 94305. J.C. SHYNE, Emeritus Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA, is with Failure Analysis Associates, Palo Alto, CA 94303. Manuscript submitted September 15, 1988. METALLURGICAL TRANSACTIONS A
be rapid or slow, depending on its crystallographic orientation or the overall grain size. Small cracks could begin to grow rapidly after initiation, but with increasing crack length, the crack front would encompass more grains; the overall growth rate is the average of the propagation rate through each grain. [8] The difference between large and small cracks, therefore, may be that growth rate averaging does not occur for small cracks. Grain boundaries also play a major role in small crack growth. Several studies have found crack propagation to be temporarily halted or completely arrested at grain boundaries, depending on the crystallographic misorientation of adjacent grains, t2,3,8-1~ This could also explain the deceleration in crack growth rate with increasing AK for small cracks. A key area in small crack research is to define the small crack regime. The ao parameter first introduced by Kitagawa et a l . [4'331 and later re
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