INFLUENCE OF A NEGATIVE R RATIO ON THE CREEP-FATIGUE BEHAVIOR OF THE N18 NICKEL-BASE SUPERALLOY

Crack growth rate measurements, performed on smooth specimens during c reep-fatigue testing of the N18 nickel base superalloy at 650 degrees C, reveal a strong detrimental effect of the compressive part of the l oading cycle. This effect is attributed to the residual stresses in th e wake of the crack, which promote early crack opening. A finite eleme nt analysis is conducted in order to predict the evolution of the open ing stress level as a function of the stress ratio. Good agreement is found between numerical and experimental results. The influence of the constitutive equation of the material on the crack closure level is t ested and the effects of kinematic hardening and viscosity are more sp ecifically examined. An engineering simplified method is proposed to d etermine the opening stress intensity factor as a function of crack le ngth, stress ratio and maximum stress. This method, developed for crac ks in smooth specimens, is then applied to small cracks growing from a notch root. Good agreement is found between the results given by the engineering simplified method and the result of the finite element ana lysis.