Residual stress tensor induced by machining

Residual stresses are the self-equilibrium stress state that remains within a component after all external loads have been removed. The residual stresses are inevitable, and can be induced from plastic deformation due to mechanical loading, temperature gradient due to thermal loading or phase transformation in manufacturing or assembly processes. It is beneficial for the component with compressive residual stresses at the surface to resist corrosion crack acceleration. Therefore, control of machined surface residual stresses is important for industry.; In this research, an analytical model is built upon the machining forces, tool geometry and work material properties. It is believed that mechanical loads---chip formation forces from shearing process and plowing force from tool tip---workpiece contact are the primary external loads to generate the residual stress in machining. Considering a moving tool, the machined surface is subjected to two stress-loading cycles generated from the chip formation and plowing forces. The plastic strains obtained after the cyclic loading, then, were used to derive residual stresses.; A face center cube (FCC) experiment design was performed for the research to study the impacts of selected variables and validate the proposed analytical model for predicting residual stress generation. Three levels of tool tip radius, depth of cut and rake angle were chosen to vary the chip formation force and plowing force in the experiments to verify residual stress generation. Eight specimens of the FCC cubic corners were electro-polished to reveal the residual stress distribution underneath the machined surface. X-ray diffraction method was used to measure the resultant surface strain and stress tensors.; As the measurement results indicated, the residual stresses in the cutting direction on the machined surface are generally tensile, when the tool tip radius is small or the tool is sharp. When the tool tip radius increases, the residual stresses tend to decrease or move from tensile to compressive. Additionally, not only does the result reveal the impact of the variables on residual stress generation, but also proves that the sequential chip formation force and the plowing force are the two primary forces to determine the residual stresses on the machined surface. The residual stress distributions predicted corresponded well to the measured distributions. Consequently, the mechanism of residual stresses was revealed, and the analytical model developed along with its assumptions was validated.