A threshold criterion of stress intensity range in a steel sheet

Abstract

Fatigue fractures often originate and propagate from geometric discontinuities or material defects in equipment or components. The occurrence and propagation of cracks near the fatigue limit were observed in notched specimens of a steel sheet that simulated this situation, and analyzed using fracture mechanics parameters. The notch caused strain concentration, and local plastic strain arose even at low stress amplitudes. Transgranular fracture occurred, and arrested cracks were observed near the threshold stress amplitude Δσth. The threshold stress intensity range ΔKth had a good power function correlation with the effective notch length aef, but it depended on the length and did not reach a constant value. By identifying and applying the critical power exponent, we proposed a prediction model using the function ΔKthc=Yπ1/2Δσthaef1/κ, which is the critical threshold stress intensity range that has a constant value. κ is equal to 5 for the steel sheet under transverse bending. Since ΔKth c obtained using this critical exponent method does not depend on the crack length, it can be used as a material constant and a failure criterion, and can be applied as a new strength prediction model.