Development of a High‐Temperature Deformation and Life Prediction Model for an Advanced Silicon Nitride Ceramic

Abstract

A new deformation model inclusive of life prediction capability is introduced for describing general thermal‐mechanical loading behavior of an advanced structural ceramic at high temperatures. The model is formulated using the state variable approach. Two internal state variables, namely, “hardening” and “damage” variables, are employed to characterize the current state of the material. The model consists of three rate equations: a flow rule describes the creep rate as a function of the hardening state variable, applied stress, and temperature; and two evoluton rules describe the rate changes of the two internal variables. Material history is accounted for through the evolution of the internal variables. The model was characterized and evaluated based on experimental creep and creep rupture data of an advanced silicon nitride ceramic tested under constant and stepwise‐varied loading conditions. A unique strength of the model, not empowered in conventional approaches such as the Norton power‐law creep and Monkman‐Grant creep rupture relations, is demonstrated with the aid of the hardening variable, which enables the effcts of thermal annealing on subsequent creep and creep rupture behavior to be delineated.