In order to predict the initiation angles of cracks,a new yield criterion was established for geotechnical materials with small plastic deformation.According to this yield criterion,the radii of the plastic region around crack tips for mode Ⅰ crack,mode Ⅱ crack and Ⅰ-Ⅱ mixed mode crack were obtained,and the plastic regions under uniaxial tension and pure shear were analyzed.The results showed that loading conditions,crack inclination angles and material internal friction angles affect the plastic core regions significantly.Based on the assumption that cracks propagate along the shortest radius of the plastic core region boundary,a fracture criterion which can predict crack initiation angles was obtained.The pre-existing test results agrees well with this fracture criterion,and its prediction result is more precise than those by other criteria.
