Stress-relaxation constitutive relations for modeling residual stresses from quenching

There is increasing pressure to predict the residual stresses that invariably result from quenching components of any appreciable size. Thermal gradients in cooling lead to differential thermal contraction strains, building stresses that only partially relax by plastic flow upon complete cooling. The thermomechanical response is most commonly modeled using the monotonic plastic stress-strain relations (flow stress). However, the thermal strains are small, no more than about 0.01, and change sign during cooling, whereas the flow stress data is typically available at the much larger strains of hot-working conditions, lacks fidelity in the small-strain range, and is measured under isothermal conditions. The talk will describe how the stress-relaxation (creep) constitutive behavior can be used to directly link strain and stress for the modeling of thermal stress development on cooling/quenching. Although the approach is general, the initial application will focus on quenching high-strength Al alloys from solution treatment. A method for measuring the necessary properties data will also be described and initial results on AA7050 over the range of cooling from solution treatment will be presented. Other projects in the recently started Purdue Heat Treating Consortium (PHTC), an industry-focused research center in the School of Materials Engineering at Purdue, will be highlighted.