Utility of Surface-Flawed Tensile Bars in Cyclic Life Studies

Abstract

Surface-flawed tensile bar specimens were developed and tested to investigate the applicability of fracture mechanics to relatively small crack sizes under varying stress-time-temperature conditions and to derive ths stress intensity relationships for such surface cracks. The geometric influences on continuous cycling, as well as time-dependent, creep/fatigue, crack propagation were observed and characterized for Inconel 718. The influence of stress ratio on cyclic crack growth rate curves was examined, and the equivalent stress intensity concept was substantiated. A residual life calculation method was developed, and verification test results were obtained wherein equal accuracy in calculated lives (following precracking) was observed for flat tensile specimens and simple cylindrical rotating disks. The verification tests established the viability of the elastic fracture mechanics method for predicting crack growth under conditions near the elastic/plastic limit, provided that pertinent material data are used and careful attention is paid to stress redistribution.