Comprehensive study on the crack tip parameters of two types of disc specimens under combined confining pressure and diametric concentrated forces

•Fracture parameters of HCCD and CCCD specimens were comprehensively calibrated.•Change of crack tip fracture parameters was analyzed.•Fitting equation between mode II loading angle and confining pressure was given.•Advantages and disadvantages of CCCD and HCCD specimens were compared. Stress intensity factors and T-stress of centrally cracked circular disc (CCCD) and hollow central cracked disc (HCCD) specimens with different geometric parameters under the combined diametric concentrated forces and confining pressure were calculated using Finite Element Method. The influence of crack length, crack inclination angle, aperture ratios and confining pressure on the mixed mode SIFs, T-stress and biaxiality ratio of CCCD and HCCD specimens is investigated in this paper. Compared with the HCCD specimens, CCCD specimens can more easily transform pure mode I loading to pure mode II loading. The results obtained in this paper are helpful to understand the mechanical mechanism of rock fracture under combined confining pressure and diametric concentrated forces.