Fractal Characteristics of Coal Specimens’ Surface Cracks in Triaxial Conventional Compression and Cyclic Loading Tests

This paper presents an experimental investigation of the fractal characteristics about surface cracks of failed coal specimens in triaxial conventional compression and cyclic loading tests. The fractal characteristics under different confining pressures, test types, and cyclic loading frequencies are summarized and analyzed. The main achievements are as follows: (1) Under the same confining pressure, the average fractal value of failed coal specimen surface cracks in conventional compression test is obviously smaller than that in cyclic loading test for obvious differences in crack propagation modes between the two tests. Conventional compression test is beneficial to the development of large cracks in coal specimen, while cyclic loading test is helpful to the growth of small cracks. (2) The average fractal value of failed coal specimen surface cracks in cyclic loading test with the frequency of 0.25 Hz is slightly larger than that in cyclic loading test with the frequency of 0.5 Hz. The reason is that the lower the frequency of cyclic loading, the more surface cracks generated in coal specimen under the same cyclic number. (3) Whether in triaxial conventional compression test or cyclic loading test, the average fractal value of failed coal specimen surface cracks under the confining pressure of 5 MPa is larger than that under the confining pressure of 10 MPa because of the higher the confining pressure, the stronger the restriction of crack growth.