Influence of Residual Stress Distribution on Delayed Fracture of Coil Springs

Abstract

Delayed fracture is one of the most important problems for strengthening spring steels and reducing the weight of coil springs. It has already been reported that the compressive residual stress by shot peening lengthens the delayed fracture life. However, whether the compressive residual stress suppresses the crack initiation or the crack propagation has not been clarified. Therefore, we researched the effect of compressive residual stress on the occurrence of crack and its propagation by using the newly developed AE (Acoustic Emission) system. First, it was clarified that the shot peened spring had a long life for delayed fracture, even if increasing the diffusible hydrogen. It was also clarified that the crack occurred as soon as the experiment started, regardless of shot peening. From the observation of delayed fracture surface, it was also clarified that the QC (Quasi-Cleavage) region became deeper as the surface magnitude and the depth of compressive residual stress became larger. The mean growth rate of crack was estimated by dividing the depth of QC region by the total time to delayed fracture. The mean growth rate of crack became smaller for larger surface compressive residual stress and its depth. It can be concluded that the compressive residual stress caused by the shot peening suppresses the propagation of crack, and contributes to enhance the delayed fracture strength.