Surface zone hardening during the bending fatigue of nickel
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IT has been suggested by Kramer ~3 that a very hard surface layer, 50 to 200/~m deep approximately, of dislocation debris develops in materials during fatigue. He has postulated that a fatigue crack initiates when a "surface layer stress" reaches a critical value, independent of applied stress amplitude or environment, the influence of both being on the number of cycles needed to reach the necessary level. He investigated the increase in the fatigue life obtained by periodically removing this outer layer, under conditions where persistent slip bands or cracks were not present? The existence of such a zone has been contentious, for Block and Johnson 4 detected no difference between the surface and subsurface dislocation density in deformed copper, and Fourie 5 reported a "soft surface effect." Lukas and Klesnil 6 studied substructures developed during fatigue and likewise found no change in the substructure adjacent to the surface. The respective viewpoints were summarized by Kramer and Kumar 7 and Fourie. 8 Some of Kramer's tests were repeated by Basinski, 9 who arrived at a different conclusion, supporting Fourie's analysis, and the consensus now seems to be that a hard surface layer does not develop. 9-~ Some volume hardening or softening is, however, usual during fatigue, and cracks are considered to initiate when a limiting or saturation hardness is attained? T M Therefore, points of interest remaining from Kramer's postulate are: 1) Is the same saturation hardness, albeit not excessively high, attained in the surface zone during fatigue independent of strain amplitude? 2) Is the magnitude of the saturation hardness influenced by the environment? 3) Is there a surface zone effect, in that the hardening varies with depth beneath the surface? The investigation reported here addressed these questions. The extent of the surface zone hardening was determined by microhardness measurements. Such measurements have been used to assess fatigue damage in steels/5 but when a low strain hardening exponent material gives broad data spread, the usefulness of the technique is limited. More rewarding has been the use C. E. PRICE is Professor, School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078. J. A. FILA, formerly at Oklahoma State University, is now with LearAvia Corp., Reno, NV 89506. Manuscript submitted May 21, 1980. METALLURGICAL TRANSACTIONS A
of microhardness measurements to evaluate the plastic zone size around crack tips. 16'~vThe microhardness measurements were used in the present case both to measure the degree of hardening after fatigue, and to determine whether the hardening varied with depth beneath the surface. The latter was accomplished by using progressively increasing indenter weights in a particular vicinity. The analogy is that microhardness can be used to determine both the magnitude and depth of case carburisation in steels, with small indenter weights giving the case hardness and larger weights the core hardness. PROCEDURE The material selected was nickel, In
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