In-depth residual stress analysis considering manufacturing process and cyclic loading of bolts

Abstract

Although bolts are considered to be a well-established structural element, only general statements on the influence of the different manufacturing processes on the fatigue life are available. Detailed investigations of the in-depth residual stress path and the residual stress stability under cyclic loading have not yet been carried out. A challenge in determining the residual stresses is the complex geometry of the thread and the expected residual stress state with steep gradients close to the surface.This paper examines the influence of the manufacturing process of the main two process types on the residual stress state. The focus lies on the residual stress and material state of high tension bolts (HT) rolled before heat-treatment (RBHT) and rolled after heat-treatment (RAHT) both with hot-dip galvanization (tZn). To get detailed information about in-depth residual stresses, neutron diffraction is used. In addition, the residual stress stability under cyclic loading is examined.For the first time, in-depth residual stress distributions are determined on unloaded and loaded RAHT M 36 and RBHT M 24 bolts, tZn, grade 10.9. The results show that RAHT bolts in the unloaded state have compressive residual stresses below the surface in the thread root in the magnitude of −225 MPa. In the case of RBHT bolts, low residual stresses are present at the surface in the magnitude of −160 MPa. The different progression of the residual stresses is associated with the manufacturing process.Loaded RAHT and RBHT bolts exhibit different stress characteristics. In the loaded state, RAHT bolts have residual compressive stresses on the surface, while RBHT bolts have residual tensile stresses. This may be one reason for the significantly longer fatigue life of RAHT bolts compared to RBHT bolts.