Characteristics of residual stresses produced by finish machining processes and challenges in their assessment

Joseph Francis Zawadzki, Purdue University

Abstract

A study has been made of residual stresses induced by machining processes in the context of their relevance to the performance of manufactured components. Based on consideration of the nature of the residual stresses arising in machining and component geometries typical of applications, the x-ray diffraction measurement technique was selected for this residual stress assessment. The accuracy and uncertainty in stress measurement by x-ray diffraction was analyzed using a round-robin study involving turned and heat treated components. It was found that while the variability and uncertainty intrinsic to the measurement technique were relatively small for the heat treated component, there was substantial intrinsic uncertainty in the machined surface stress measurements. This was found to arise from the phenomenon called Ψ-splitting which is a consequence of large shear stresses, stress gradients, and bending of lattice planes in the volume sampled by the x-ray beam. These effects contributing to Ψ-splitting are undoubtedly a consequence of the intense (anisotropic) deformation of the surface due to the material removal action of the tool. The use of the measurement technique was studied using three examples wherein prior stress data is not extensively available. First, the technique was found to measure well residual stresses generated by a superfinishing process widely used in many bearing and diesel fuel system applications. The presence, or lack thereof, of anisotropy in the stress distribution and stress gradients near the surface was examined. Second, the technique was used to map surface and subsurface residual stresses in a component of complex geometry containing multiple functional features - a diesel engine crankshaft - created by grinding and lapping processes. A methodology for achieving this difficult measurement was formulated and proved. Third, an attempt was made to study the interactive and individual effects of residual stress and surface finish on the fatigue life of components using a standard tension/compression fatigue test. This analysis requires careful characterization of residual stresses. Based on the three case studies and the round-robin test results, conclusions are presented on the challenges arising in accurate residual stress measurement on machined surfaces, characteristics of the stress distribution arising from different types of machining processes, and critical locations in components where residual stress assessment has to be carried out. These observations offer a rational basis for reducing the complexity of residual stress measurements in manufacturing processes.

Degree

Ph.D.

Advisors

Chandrasekar, Purdue University.

Subject Area

Industrial engineering

COinS