Effects of combination mode on mechanical properties of bi-material samples consisting of rock and coal

We performed uniaxial compression tests on coal–rock, rock–coal, and rock–coal–rock bi-material samples. We studied their mechanical behavior using an acoustic emission monitoring and digital speckle strain measurement systems. The average uniaxial compressive strengths of the rock–coal–rock, coal–rock, and rock–coal bi-material samples were 20.54 MPa, 14.28 MPa, and 14.03 MPa, respectively. The strength of the coal determines the overall strength of the bi-material sample. However, the overall strengths of the coal–rock, rock–coal, and rock–coal–rock bi-material samples were greater than that of coal by 11.76%, 10.19%, and 31.84%, respectively. A crack in the coal part of a bi-material sample can develop and spread to the rock part, thus resulting in its splitting failure. The rock–coal–rock bi-material samples exhibited plastic failure, while the coal–rock and rock–coal bi-material samples exhibited brittle failure. Progressive failure of the coal sample induced rebound deformation of the rock sample. The rebound deformation rates of the rock–coal–rock, coal–rock, and rock–coal samples were 0.57%, 0.09%, and 0.12%, respectively. In the pre-peak stage, the average elastic energy of the rock–coal–rock bi-material sample was 92.57%, indicating a strong ability to store external energy. In the post-peak stage, the average post-peak release energy of the rock–coal–rock bi-material sample was the largest, but the average surplus energy was the smallest. Therefore, the rock–coal–rock bi-material exhibited the highest macro-cracking but the dynamic behavior was the least prominent. The coal–rock and rock–coal bi-material samples exhibited less prominent macro-cracking after failure but a more pronounced dynamic behavior.

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