Mechanisms associated with transient fatigue crack growth under variable-amplitude loading : An experimental and numerical study

An experimental and numerical study has been made of the mechanisms of fatigue crack growth and crack-closure behavior in an αβ titanium alloy Ti-4A1-4Mo-2Sn-0.5Si (IMI 550), following both single and block tensile overloads. Closure immediately behind the crack front (near-tip closure) was found to be the main factor controlling load-interaction effects. Single tensile overloads were found to remove near-tip closure, and slightly reduce far-field closure along the crack length, resulting in an initial acceleration in fatigue crack growth rates. Subsequent delayed retardation of crack growth rates was accompanied by an increase in the near-tip closure load, due to the enlarged compressive residual stress in the overload plastic zone. High/low block overloads caused greater retardation than single overloads of the same magnitude, and this was attributed to changes in the degree of closure in the wake of the crack. Numerical predictions of such transient behavior, based on a modified Dugdale model, are found to be in close agreement with experimental results, both in terms of observed crack growth rates and crack opening displacements. Load-interaction effects were found to be most severe when the baseline stress intensity range (ΔK) was close to the fatigue threshold (ΔK_TH ), or, when the overload maximum stress intensity (K_max ) approached the fracture toughness of the material. At low ΔK levels, the magnitude of the delay was sensitive to microstructure and found to be enhanced in coarse-grained β-heat-treated microstructures compared to standard fine-grained αβ microstructures. Based on these results, mechanistic sequences are suggested to explain the transient fatigue crack growth behavior following single and block tensile overload cycles.