Predicting Creep and Creep/Fatigue Crack Initiation and Growth for Virtual Testing and Life Assessment of Components

Abstract

Predicting creep and creep/fatigue crack initiation and growth, under static and cyclic loading, in engineering materials at high temperatures is an important aspect for improved life assessment in components and development of virtual test methods. In this context, a short overview of the present standards and codes of practice as well as experimental methods and models to predict failure are presented. Following a brief description of engineering creep parameters and basic elastic–plastic fracture mechanics methods, high-temperature fracture mechanics parameters are derived by analogy with plasticity concepts. Techniques are shown for determining the creep and fatigue fracture mechanics parameters K and C * to predict experimental crack growth using uniaxial creep data. An analytical ‘failure strain/constraint’ based ductility exhaustion model called the NSW model is presented. The model uses a ductility exhaustion argument and constraint at the crack tip is able to predict, within a range of as much as a factor of ∼30 crack initiation and growth at high temperatures, over the plane stress to plane strain regimes. By taking into account angular damage distribution around the crack tip NSW model is further refined as the NSW-MOD model. This model is able predict a much improved upper/lower bound of a factor of between ∼0.5–7, depending on the creep index n, in steady-state crack initiation and growth over the plane stress/strain region in components containing defects. The presence of cyclic load is assumed to introduce a cycle-dependent fatigue component with a linearly summed cumulative damage effect with the creep response. The prediction for the fatigue component can be handled using either the Paris law or the method proposed by Farahmand in this book. Hence, this chapter does not get into the details of the fatigue response. In conclusion, the creep/fatigue modeling presented can be used as a tool in component design of metallic parts, as well as in life assessment of cracked components at elevated temperatures and in predicting virtual cracking behavior in fracture mechanics specimens.