Effect of Stress Triaxiality on Ductile Fracture Initiation of Low Strength Steels

Abstract

Tensile Tests on circumferentially notched round bar tensile specimens with notches of various acuity have been made on SS41 and SM50C in order to clarify the effect of stress triaxiality on ductile fracture initiation. Tests on tensile and bend specimens with a slit or fatigue cracks have also been made in order to obtain the relation between the fracture toughness and the condition of ductile fracture initiation.
The plastic strain at the ductile crack initiation decreased sharply with increase of stress triaxiality in the range where σ_m/σ (σ_m: mean stress, σ: effective stress) is smaller than about 1.2, but it became almost constant for larger σ_m/σ than 1.2. A similar dependence on stress triaxiality σ_m/σ was observed in the dimple size distribution on the fracture surface.
The fact that the crack tip opening displacement at fibrous crack initiation is almost independent of various factors, such as specimen thickness, crack depth, loading mode and so on, may be explained by this stress triaxiality dependence of ductile fracture initiation, because the value of the stress triaxiality σ_m/σ around the crack tip is larger than about 1.2 regardless of the factors above mentioned.