Prediction of limit strains in superplastic materials

A simple model has been developed to study the effects of static and dynamic grain growth and strain rate on the ductility of superplastic materials. One-half of a tensile specimen was modeled numerically using a power-law creep constitutive equation, with either a constant strain rate or constant extension rate. Static grain growth was shown to reduce material ductility at all rates of growth, except when the imposed strain rate was very low. Dynamic grain growth was shown to enhance ductility at lower growth rates and for intermediate imposed strain rates, while decreasing ductility at higher growth rates. Comparison of simulated and experimental results reveals the relationship between strain and grain size, thus justifying the treatment of dynamic grain growth as a function of accumulated strain.