Dependence of microelastic-plastic nonlinearity of martensitic stainless steel on fatigue damage accumulation

Self-organized substructural arrangements of dislocations formed during cyclic stress-induced fatigue of metals produce substantial changes in the material microelastic-plastic nonlinearity, a quantitative measure of which is the nonlinearity parameter β extracted from acoustic harmonic generation measurements. The contributions to β from the substructural evolution of dislocations and crack growth for fatigued martensitic 410Cb stainless steel are calculated from the Cantrell model [Proc. R. Soc. London, Ser. A 460, 757 (2004)] as a function of percent full fatigue life to fracture. A wave interaction factor fWI is introduced into the model to account experimentally for the relative volume of fatigue damage included in the total volume of material swept out by an interrogating acoustic wave. For cyclic stress-controlled loading at 551MPa and fWI=0.013 the model predicts a monotonic increase in β from dislocation substructures of almost 100% from the virgin state to roughly 95% full life. Negligible contributions from cracks are predicted in this range of fatigue life. However, during the last 5% of fatigue life the model predicts a rapid monotonic increase of β by several thousand percent that is dominated by crack growth. The theoretical predictions are in good agreement with experimental measurements of 410Cb stainless steel samples fatigued in uniaxial, stress-controlled cyclic loading at 551MPa from zero to full tensile load with a measured fWI of 0.013.