Measurement of elastic follow-up for combined applied and residual stresses

Tensile residual stress can reduce the load carrying capability of a structure. However, residual stresses may be redistributed during the life of a component by, for example, permanent deformation. This paper explains an experiment carried out to understand how applied and residual stresses interact and to seek a method of measuring elastic follow-up during the interaction. A friction stir welded aluminium alloy plate was subjected to a series of incrementally increasing load and unload cycles, whilst simultaneously measuring residual stresses and deformation. In-situ loading of the specimen during the residual stress measurements allowed the relaxation of the residual stress to be quantified. The elastic follow-up has been estimated and measured by considering both the structural stiffnesses of the specimen and the relaxation of the residual stress. It was found that global yielding, which can result in no net change of incompatibility, has to be considered when calculating elastic follow-up. An estimation of the elastic follow-up factor based on the structural stiffnesses of the specimen was found to be non-conservative and an elastic follow-up factor of 2.9 was measured. That is three times as much plastic strain is required to relax the residual stress when compared to the fixed-displacement case.