Plasticity and stress heterogeneity influence on mechanical stress relaxation residual stress measurements

Two common problems of mechanical strain relaxation (MSR) residual stress measurement methods are investigated in this work: (1) assumption of stress uniformity and (2) the effect of plasticity at relaxation. A new MSR technique, designed specifically for highly non-uniform in-plane residual stress fields, is applied in this work to measure the residual stress field resulted from pure bending of an Al7075 alloy. The method involves introducing a straight cut across the whole part in a single increment, and collecting full field displacement fields from the side surface. Application of a 2D high resolution digital image correlation (DIC) method proved successful in this work. The reconstructed residual stress agrees well with that predicted by FE modelling. It is shown that the direction of the propagation of the slit has a major influence on plastic flow during relaxation. The major conclusion from this work is that it is possible to substantially reduce, or completely eliminate, plastic flow on relaxation by careful planning of the slit orientation and the cutting schedule.