Experimental Study On Failure Behavior Of 3D Printed Rock-like Materials With Double-cross Fractures

Double cross-fracture is the basic unit of intermittent cross-joints in engineering rock mass,which directly affects the mechanical behavior of rock mass with intermittent crossjoints.Due to the complexity of geometry between fracture groups and within fracture groups,the failure modes,peak strength,elastic modulus,deformation behavior and other failure behaviors are also extremely complex.In this paper,rock-like materials with double crossfractures are selected as the research object.Four characteristic factors,namely,the length of rock bridge,the inclination angle of rock bridge,the length of minor fracture and the angle between main and minor fracture,are selected to study the influence of each characteristic parameter on the failure behavior of rock-like materials with double cross-fractures.The main work is as follows :(1)The geometric parameters of double cross-fractures are complex and cumbersome,and they are interrelated and interactive.In this paper,the rock bridge morphology of double cross-fractures is taken as the research core,and experiments are designed and carried out.The geometric size of the specimen was 70 mm in length,70 mm in width and 8 mm in thickness.The length of rock bridge is 3mm,6mm,9mm,12 mm,15mm and 18 mm.The dip angles of rock bridge are set as 0°,30°,45°,60°,75°,90°,105°,120°,135° and 150°,with a total of 10 levels;the length of secondary fissure is 2mm,4mm,5mm,6mm,8mm and 10 mm.The angles of main and minor fractures are 15°,30°,45°,60°,75° and 90°,which are totally six levels.(2)Based on the photosensitive resin 3D printing technology,four types of specimens with different rock bridge length,rock bridge inclination,minor fracture length and main and minor fracture angle were prepared.Then,uniaxial compression test was carried out to obtain the total stress-strain curve of the specimen during loading process,and the fracture images of different stages were obtained by high-speed camera.At the same time,the main stress field and displacement field of the test block were obtained by digital image correlation(DIC)technology.From the full stress-strain curve of the specimen,it can be seen that the deformation process of the specimen obtained by the material and its specimen preparation method in this experiment under uniaxial loading conditions presents four stages : fracture pressure sealing,elastic deformation to micro-elasticity,unstable fracture,and deformation after fracture.The brittleness is obvious,and it has obvious rock-like properties.(3)Based on the test results,the influence of each characteristic parameter on the failure mode,peak strength,elastic modulus and deformation behavior of the double-cross fracture specimen was analyzed,and the deformation and failure mechanism of the double-cross fracture of 3D printing rock material was revealed.With the increase of the length of the rock bridge,the hindering effect on the fracture at the fracture tip in the fracture crack increases.The peak strength of the specimen decreases first and then increases,and the stress concentration in the middle section of the rock bridge decreases.With the increase of rock bridge inclination,the rock bridge area presents four different load states,and the sample presents three failure modes.The peak strength of the specimen decreases first and then increases.With the increase in the length of minor fracture,the dominant rock bridge of double-cross fracture changes from the main fracture rock bridge to the main and minor fracture rock bridge,and then to the minor fracture rock bridge.The specimen is divided into two failure modes at about 10 mm of minor fracture,and the peak strength increases first and then decreases.With the increase of the angle between the main and minor fractures,the rock bridge dominated by double-cross fractures changes.The specimen is divided into three failure modes at the angle of 30° and 75° between the main and minor fractures,and the peak strength increases first and then decreases.(4)Based on the test results,the failure mode of the specimen with the minor fracture length is about b = 10 mm,and the specimen with the main and minor fracture angles is aboutβ = 30° and β = 75°.In order to determine the subcritical parameters,numerical simulation was carried out by RFPA software.Firstly,a fracture calculation model with different minor fracture lengths and different cross-fracture angles is established to simulate the whole process of fracture initiation,propagation and failure of prefabricated fracture specimens under load.The results are compared with those of physical specimens,and the simulation parameters are adjusted and optimized.Then,the critical parameters of failure mode transformation of double cross-fracture specimens are identified,and the failure mode and mechanism of double cross-fractures of rock-like materials are further revealed.