AN INVESTIGATION OF RESIDUAL STRESSES INDUCED BY FORMING PROCESSES ON THE FATIGUE RESISTANCE OF AUTOMOTIVE WHEELS

Abstract

The nature and scope of this thesis relates to obtaining an understanding of the way in which sheet metal forming processes affect the residual stresses (plastic deformation) and the fatigue perfomiance of wheel centre discs made of a dual phase steel. Automotive wheels are mainly manufactured from sheet metal plate by forming processes using tools and dies. This manufacturing process is directly concemed with deforming the plate under predominantly applied tensile forces where the component acquires a sfress state which is multi-axiial. This stress state is both complex and difficult to assess from an analytical point of view and thus the work reported in this thesis is of an applied nature, and seeks to understand the results of this stress state on residual stresses, hardness and, hence fatigue behaviour. Components from each stage of the centre disc manufacturing process and of the assembled and painted wheel will be assessed and characterised. The first part of the work involved a literature review of the factors influencing sheet metal forming, i.e. elongation, anisofropy, grain size, springback, residual sfresses and fatigue. This review gave the researcher a background from which he could confidently approach the various investigations that were required for successfiil completion of this project. The residual stress analysis led to an investigation that provided a new technique for residual stress measurement using the high speed centre hole drilling method. This new measurement technique gave improved results when assessing the residual stress behaviour of textured sheet materials subjected to plastic deformation. It is felt that this technique will also minimise the inherent scatter that is perceived by many an analyst to be a fundamental part of residual stress assessment by the hole drilling method. The concept of this measuring technique has since been patented and is subject to a South African Patent Application No.2000/2042. The thesis then deals with the process analysis of centre disc manufactiure as well as the analysis of the fatigue properties of specimens removed from each stage of the manufacturing process and of the complete wheel. Here the complexity of the cenfre disc manufacturing process is discussed and assessed in terms of the process effects on microstructure, surface hardness, induced residual stresses and fatigue performance. The aim of this part of the work is to seek relationships with respect to surface hardness, induced residual stress and fatigue life in terms of the various production stage components including the centre disc of the complete wheel. The results showed that relationships do exist and that fatigue life prediction can be based on surface hardness as well as residual sfress assessments for centre discs manufactured from dual phase steel plate. The final part considers modelling the fatigue life of automotive wheels based on the mean stress models of Soderberg, Goodman and Gerber and incorporating the induced residual stresses into the equations. The Gerber model was found to give the better correlation with regard to fatigue life prediction when compared with the actual fatigue data. hi conclusion, this research has led to the development of a model showing the relation and impact of the centre disc manufacturing process on induced residual stress levels and fatigue properties of dual phase steel plate.