Observations on the stability of the residual stresses after cold forming and unidirectional loading

The present paper addresses the investigation of the evolution of the residual stresses (RS) under unidirectional cycling loading in cold formed parts. Through a basic three point bending process, the bare phenomena of relaxation were studied by excluding the disturbance of external factors, like friction or temperature gradients. Initially, samples of high strength fine grained steel S960QL (1.8933) with different geometries were manufactured, followed by a stress relief heat treatment. The samples were then bended with attentive process control to obtain a homogeneous stress state. X-Ray diffractometry (XRD) was employed for the experimental measurement of the axial residual stresses along a path in the center line of the specimens. The RS were also predicted through a finite element (FE) model with ABAQUS© using non-linear kinematic hardening. The bended samples were then subjected to further cyclic loading in the bending direction with two different amplitudes, i.e. complete elastic and partially plastic regime. The relaxation of the residual stresses was stepwise analyzed through X-Ray measurements and FE simulations. It was observed, that bending can be an efficient way to study the residual stresses stability during cyclic loading, as it allows the simultaneous examination of different loading situations.