MODELLING OF RESIDUAL STRESS RELAXATION OF SHOTPEENED ASTM A516 GRADE 70 STEEL

Abstract

Residual stress is defined as the remaining stress present in an object with the absence of an external load. It can be divided into tensile and compressive residual stress. Compressive residual stress is beneficial to prolong the fatigue life of the product especially for products made of metallic material. It was demonstrated that the fatigue life of metallic materials can be extended by the near-surface macroscopic compressive residual stress which can be introduced by shot peening process whereby fatigue crack initiation and crack growth can be reduced. However, the initial residual stress field inherent or induced in the finished product may not remain stable during the operation due to the relaxation of the residual stress. The previous empirical model to predict the residual stress relaxation did not incorporate the surface hardness parameter. The main objective of this research is to determine the relaxation of the residual stress of ASTM A516 Grade 70 carbon steel which is widely used in the automotive and oil industries. Empirical and numerical model were particularly generated for this material at the end of this research. This study involved simulation and experimental methods. The simulation part was performed by developing a CAD model with the same dimension of the actual sample. The simulation method consists of shot peening simulation to induce the initial residual stress and simulation was the residual stress relaxation. On the other hand, the experimental part began with the preparation of the sample material according to standard dimension, followed by the introduction of residual stress in the material through shot peening process at low and high intensities. The cyclic load was applied to both variables with low load (20% of Yield Strength) and high load (80% of Yield Strength). The load was varied by the number of cycles. Finally, the residual stress was measured using X-Ray diffraction on the samples to study the relaxation trend. Based on the results, the residual stress relaxed during the first cycle. The experiment results of residual stress relaxation was validated numerically and showed good agreement. Hence, the experimental result was validated by the simulation result. Finally, two sets of equations (numerical model) were developed for the residual stress relaxation of this material. Of the two, the FE model developed can be used to predict the value of residual stress in any cycle.