Analysis of Fatigue-Crack Growth in a High-Strength Steel—Part I: Stress Level and Stress Ratio Effects at Constant Amplitude

Abstract

High-strength pressure vessel steel surface flaw or part-through crack (PTC) specimens were selected for studies of fatigue crack growth rate (da/dN) under constant amplitude cycling to assess the effects of varied stress ratio R (minimum nominal stress/maximum nominal stress, σmin/σmax) and stress level (maximum nominal stress/yield stress, σmax/σys). Analyzed within the framework of linear elastic fracture mechanics, these studies warrant the following conclusions regarding fatigue-crack growth in this material: • Crack growth does not appear to be influenced by stress level, per se, even for stress levels approaching net section yield. • It is moderately influenced by both positive (tension-tension) and negative (tension-compression) stress ratios. • It is principally related to the tensile range of cyclic stress as expressed by the fracture mechanics stress-intensity range parameter, ΔK. Utilizing the results of this investigation, a normalizing relationship expressing da/dN as a function of both ΔK and R, which is applicable to both positive and negative values, is discussed. It is concluded that the stress-intensity range ΔK provides a viable analytical approach to fatigue crack-growth analyses relevant to high-strength pressure vessels.