The state of residual stress in the near surface region of homogeneous and heterogeneous materials after grinding
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INTRODUCTION
T H E R E have been numerous investigations of residual stresses caused by shot peening and grinding in particular (see References 1 and 2). It is known that reflection angles v s sin 2 0 (~ is the angle between the normal to the surface of the specimen and the normal to the diffracting plane) measured by the X-ray method are subject to ~b-splitting, 3 that is, they form different curves for ---0 due to the principal strain components being inclined to the surface. 4'5'6 This has provided a new impetus to the X-ray measuring techniques, particularly to the analysis 7-11 of the data in terms of the entire stress tensor. The literature mentioned is concerned with details on grinding residual stresses I~ and with the superposition of elastic ~2 and plastic ~3 strains. The aim of this investigation is to contribute to the explanation of how residual stresses develop after grinding and how they are compensated in homogeneous and heterogeneous materials. Therefore, the residual stress tensor as a function of depth below the original surface had to be determined. This made it necessary to measure the distribution of both lattice strains in the grinding- and transverse-directions by means of X-rays of different wavelengths and also with X-rays of one wavelength after controlled layer removal. When formally calculating the stress tensor from the measured strain tensor, as is done in this study, Hooke's law is involved and it is assumed that the principal axial system of the stresses and strains is identical. In order to calculate the residual stresses, the "XEC" (X-ray Elastic Constants), measured in the elastic range of the material, as well as the lattice constant in the unstressed condition d*~lOO) (often measured in the nondeformed condition) are employed. The justification of this procedure must be considered, particularly when measuring residual stresses by X-rays on heavily deformed and thermally stressed surfaces, and on textured materials.
V. M. HAUK, Professor, R. W. M. OUDELHOVEN, Research Associate, and G. J. H. VAESSEN, Research Associate, are all with Institut fiir Werkstoffkunde, RWTH Aachen, D-5100 Aachen, West Germany. Manuscript submitted August 31, 1981.
METALLURGICAL TRANSACTIONS A
M A T E R I A L S AND E X P E R I M E N T A L PROCEDURE
The materials used are listed in Table I and were at our disposal from previous investigations. The last three materials are heavily heterogeneous, and thus measurements could be made in both phases. The values of the yield stress are given to characterize the strength of the material. The silver brass was heavily deformed and had severe preferred orientations. The widths of the samples were from 5 mm to 30 mm (0.196 up to 1.18 inch), and the samples were 0.2 mm up to 5.5 mm (0.008 up to 0.217 inch) thick. The grinding was adjusted to the material so that the surface was as smooth as possible. The arithmetical average which was used to characterize the surface roughness, was measured according to DIN 4768 in the transverse direction. The values
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