Fatigue-Crack Growth in D6AC Steel Heat-Treated to Different Values of Fracture Toughness,

Abstract : Measurements of fatigue-crack growth in D6AC steel, heat-treated to various fracture-toughness levels, show that, under conditions where the growth mechanism produces striation markings and the crack extension per cycle is a linear function of a power of the stress intensity range, the rate of fatigue-crack growth is independent of fracture-toughness. At peak stress intensity values greater than 0.7 K sub IC, the rate of growth accelerates as the critical stress intensity is approached and tensile modes of crack extension occur. At peak stress intensity values less than 18 MPa (square root of m), fractographic examination indicates a change in mechanism as the crack extension per cycle decreases more rapidly with decreasing peak stress intensity, approaching a threshold value of stress intensity dependent upon heat-treatment. The implications of these results for the choice of materials for service applications are discussed. (Author)