Application of X-ray stress measurement for residual stress analysis by inherent strain method - Comparison of cosα and sin2Ψ method-

Abstract

Most cold forming processes are perceived as simple surface treatments without a heating process and are widely used to improve the fatigue strength of various engineering components. The method for measuring the residual stresses caused by such surface treatment is very important when evaluating the fatigue strength of engineering components. The inherent strain method is one of the most effective measures for predicting the internal residual stress distribution. The residual stresses within a body are caused by internal permanent strains known as inherent strain or eigenstrain. In the case of cold formed components, the inherent strains are induced by plastic deformation. If a component is cut, the residual stress distribution changes, but the inherent strains of the original shape before cutting are preserved. The inherent strains are predicted by the inverse analysis of a finite element model using the measurement results of residual stresses on the slice or the cut surface of a body. On the other hand, a two-dimensional X-ray diffraction system based on a cosα method is useful for measuring the residual stresses because of its compactness and higher measurement speed than the conventional sin2Ψ method. In this paper, we propose an efficient approach that combines the inherent strain method and an X-ray stress measurement along with a new measurement procedure for the fillet portion of an axisymmetric shaft with a flange after the cold forming process. This report compares the results estimated using the inherent strain method by the sin2Ψ and cosα methods, and confirms that the differences in the results were very small. Furthermore, three advantages of the cosα method—wider measurable area, shorter measurement time, and shear stress measured at the same time as normal stress—are examined. Consequently, it is verified that the cosα method is effective for the proposed new approach.

Keywords

• residual stress
• inherent strain
• finite element method
• x-ray stress measurement
• cosα method
• cold forming process