MODELING THERMOMECHANICAL FATIGUE-CRACK GROWTH-RATES IN TI-24AL-11NB

A model is developed for predicting the crack growth rate in a titanium aluminide alloy under thermomechanical cycling. The model utilizes a linear summation of cycle-dependent and time-dependent terms, where the cycle-dependent behaviour is dependent on temperture as well as DELTAK and R. The time-dependent term is obtained through an integration of da/dt over any cycle, where da/dt depends on K and temperature. A unique feature of this model is the use of a retardation coefficient with the crack growth terms to account for creep or stress relaxation under load at high temperatures. Evolution equations are derived for the retardation term, which can vary during an individual load cycle. The model is calibrated using solely isothermal data and applied to several thermomechanical fatigue conditions, where good correlation is obtained between experimental data and model predictions.