Mechanical properties and crack evolution of double-layer composite rock-like specimens with two parallel fissures under uniaxial compression

A large number of two-layer rock-like specimens composed of different combinations of materials and containing two fissures with 30°, 45° or 60° fissure angles were manufactured and carried out a series of uniaxial compression tests. A special parameter R related to the upper rock layer strength and lower rock layer strength was defined, and it was regarded to characterize the behavior of the different double-layer composite rock-like specimens. The axial strength of the double-layer composite rock-like specimens containing two parallel fissures was greatly weakened by the presence of the fissures and the degree of weakening was related to the fissure angle and R, while the elastic and deformation modulus (Es and E50) were mainly affected by R. The axial strain ε1c fluctuates between 9‰ and 12‰ and gradually stabilizes and was not directly related to the fissure angle or R. The digital image correlation method was used to analyze crack evolution behavior and it showed that the rock failure characteristics of the double-layer composite rock-like specimens were obviously different from those of ordinary single-layer rock specimens. The ultimate failure modes could be divided into two major categories of coalescence mode and no-coalescence mode, and each mode could be subdivided into three types, which were mainly influenced by R and secondly influenced by the fissure angle. The results also indicate that the interface between the upper rock layer and the lower rock layer can play a role in linking cracks.