Abstract:Focusing on the mixed mode fracture(Ⅰ-Ⅱ),the cracked straight through Brazilian disc(CSTBD) of gypsum was chosen for investigating the mechanical characteristics of mixed mode fracture.Adopting the displacement servo,the applied load and the displacement were monitored.Based on the peak loading and classical theory of mixed mode fracture,the mixed mode fracture toughness of rocks were calculated.The PFC3D code was chosen to simulate the evolution of cracks and adopt the point loading to replace the traditional loading manner.On the basis of the experimental results of cracked straight through Brazilian disk test under different angle β,by testing whether it was consistent for the outcomes of laboratory experiments and numerical simulations，microcosmic parameters were calibrated.By analyzing the mixed-mode Brazilian disk under microcosmic and macrocosmic perspectives,the microcosmic characteristics of simulations were coincident with the Brazilian disk destructive processes.The results showed that when the value of β was between 0 and 75°，there was a linear relation between K_Ⅰ-K_Ⅱ and β；according to the experimental results，a fitting equation between peak load values and β could be established to predict the peak load values with different β;according to three-dimensional simulations,the critical values of tensile and press transition angle on crack tips were greater in the three dimensional space than that in the two dimensional space，the error range was between 4% and 7%, in addition that the critical angle had significant spatial effect and gradually increases from surface to mid-span section.As β augments，the maximum tensile stress point moved from the crack tip towards the center of Brazilian disk and the rate monotonously increased with the increase of the loading angle.The three-dimensional analytical results denotes that the spatial effect of three fracture modes were different,the mode was minimal，and yet the Brazilian split was maximal.