Quasibritle fracture of smooth shafts in torsion

The initiation of an edge annular longitudinal shear crack (III fracture mode) during torsion of a circular cross-section bar made of an elastoplastic material with ultimate strain is considered. The fracture process of such materials is described using a modified Leonov–Panasyuk–Dugdale model, which uses an additional parameter – the diameter of the plastic zone (the width of the prefracture zone). To obtain the critical parameters of quasibrittle fracture, a double (sufficient) strength criterion for mode III cracks in an elastoplastic material is proposed. Diagrams of quasibrittle fracture of a circular shaft with an edge crack are plotted. For quasiductile and ductile types of fracture, the ultimate loads are found numerically by the finite element method. The difference between the numerical and analytical models is that the characteristics of the investigated materials correspond to the deformation modes in full-scale yielding. It is found that the results of numerical experiments are in good agreement with the results of calculations using the analytical model of fracture of specimens during torsion in the mode of small-scale plastic flow.