A fracture mechanics criterion for thermal-mechanical fatigue crack growth of gas turbine materials

Abstract

The application of several fracture mechanics data correlation parameters to predicting the crack propagation life of turbine engine hot section materials was evaluated. A survey was conducted to determine the conditions where conventional fracture mechanics approaches may not be adequate to characterize cracking behaviour. Both conventional linear and non-linear fracture mechanics analyses were considered. Isothermal and thermal-mechanical (variable temperature) crack propagation tests were performed in Hastelloy-X, and B-1900 + Hf materials. The crack growth data were reduced using the stress intensity factor, the strain intensity factor and the J- integral . None of these three parameters successfully correlated the crack growth data. All three parameters showed strain range effects and significant scatter between the various testing conditions (in-phase, out-of-phase, isothermal). The parameter which showed the most effectiveness in correlating high temperature and variable temperature crack growth was a modified stress intensity factor ( ΔK σ ) computed using the measured load, the closing bending moment caused by the increase compliance with crack length and with the effective opening stress. The ΔK σ was shown to rationalize the effect of mode of testing (stress vs strain controlled) and the mode of fracture. Furthermore, the isothermal data at T min and T max where shown to provide an upper and lower bounds for the thermal-mechanical fatigue data if plotted in terms of ΔK σ .