The formation of fine relaxation cracks along the compressive maximum shear crack

The formation of fine short relaxation cracks frequently found along the cyclically propagating 45° compressive maximum shear crack were analyzed. Comparison was made of the results computed from two different fracture criteria, the minimum strain energy density (MSED) and the maximum tensile (tangential) stress (MTS). The measured directions θ₀ of the relaxation cracks were in excellent accord with both the theoretical MSED directions and the MTS directions. The computed MSED crack directions were predicated on two conditions: 1) the strain energy density factor was minimum, S _min, and 2) the volume change component S _v of S _min was greater than the distortion component S _d. Such requirements correspond to crack directions with locally maximum tensile tangential stresses.