Full-tensor measurement of multiaxial creep stress relaxation in Type 316H stainless steel

Background: Mechanical constitutive models of metals can be difficult to validate without loss of generality. Creep-induced stress relaxation in stainless steels is one such example: stress triaxiality may play a role in the deformation rate but is challenging to measure experimentally. Objectives: We aimed to determine whether the accuracy of constitutive laws for multiaxial deformation, particularly creep deformation, can be verified by measuring the evolution of the complete stress tensor at a point within a structure. Methods: Creep stress relaxation specimens of Type 316H stainless steel were exposed to 550°C for different durations. We used time-of-flight neutron diffraction and finite element analysis to determine the complete stress tensors at points within the specimens, tracking their development as the residual stress field relaxed. Results: Multiaxial stress relaxation of 316H occurs at 550°C due to plasticity and creep, and can follow a non-proportional deformation path. In our measurements, over-determination of the elastic strain tensor using neutron diffraction was found to reduce stress uncertainty. A popular creep constitutive model for 316H, when used with finite element analysis, predicts slightly higher strain rates than were observed experimentally. Conclusions: Sequential neutron diffraction stress measurements can be used to validate material constitutive laws in situations involving multiaxial deformation. This could be used to substantiate models of plasticity, viscoelasticity and creep. The RCC-MR primary/secondary creep law for 316H stainless steel is conservative for cases involving a complex initial stress field.