The Fractal Characteristics of Coal Sample’s Fragments Subjected to Cyclic Loading

Abstract

A series of test were performed to study the fractal characteristics of fatigue failure of coal samples. The research revealed the difference of fractal characteristics in conventional compression test and cyclic loading test, and obtained the relationship among the fractal dimension, size, mass and quantity of fragments. In cyclic loading test, three levels of confining pressure (0, 5 and 10 MPa) were applied during axial cyclic loading at 0.25 and 0.5 Hz frequency. These tests showed that: (1) The size and mass distribution of the debris are affected by the confining pressure, frequency, and loading mode. Compared to conventional compression experiments, fatigue failure from coal samples produces more debris and smaller dimensions. (2) The size of the pieces is less affected by the experimental method. However, as the size range of the fragments decreases, the shape of the fragments changes from a long thin strip to a rectangular parallelepiped. (3) The fractal dimension of the cyclic load test is larger than the traditional compression experiment. In the cyclic load test, the fractal characteristics are more obvious as the confining pressure increases.