Coupled Fatigue Crack Initiation And Propagation Model Utilizing A Single Blunt Notch Compact Tension Specimen

Many power generation facilities equipped with turbomachinery are designed to provide electric energy on an as-needed basis and, as a consequence, impart a mixture of fatigue and creep damage to high-value components at elevated temperatures. Cracks are often initiated on free surfaces of these parts near stress-raising features and propagate under thermal-mechanical cycling until the component is removed from service. Whether the emphasis is on creep or fatigue failure, most conventional structural life prediction approaches decouple crack initiation from crack propagation. Turbine designers are in need of approaches that span the full life cycle of components in which both initiation and propagation are the consequences of a variety of mechanical failure modes. Although recent fracture mechanics methods have been developed to account for fatigue- and creep-crack growth, a tacit assumption is that a precrack exists. Another main limitation of these approaches is the small scale yielding assumption, associated with linear elastic fracture mechanics, in which extensive plasticity invalidates such analyses. In this study, utilizing a blunt notch compact tensile specimen, experimental routines involving crack initiation and propagation within a single specimen at elevated temperatures with plastic-inducing loads and hold periods were conducted. Founded on existing elastic and elastic-plastic fracture mechanics (EPFM), a coupled crack initiation and propagation model is presented. Through the use of the EPFM parameter J, the proposed models are observed to accurately predict crack initiation and replicate crack propagation rates based on the imposed experimental conditions. The model is demonstrated on an austenitic stainless steel, type 304, subjected to moderate temperatures in air. Mechanical testing, metallurgical analysis, and analytical modeling allow for a simplified phenomenological life prediction model capable of predicting crack initiation and propagation at elevated temperatures. Consequently, structural analysis of critical locations of components can span the gamut of crack initiation, crack growth, and instability (i.e., total life assessment)