Characteristics and influencing factors for propagation of primary and secondary cracks in sandstone based on CT scan

In order to study the initiation and propagation of the primary and secondary cracks under compressive stress conditions, the CT tests are conducted on the fracture process of sandstone under uniaxial compression, and the morphological characteristics of the primary and secondary cracks in sandstone are analyzed. Meanwhile, the surface area and volume of the primary cracks and the fractal dimension of the secondary cracks are quantitatively discussed at different deformation stages of sandstone. The length and number of the primary and secondary cracks are calculated at different heights of sandstone using the software Image pro plus 6.0 and Image J to study the propagation and influencing factors of the primary and secondary cracks. The results show that the primary cracks initiate on the sandstone surface in two different directions, and the growth rates of the surface area and the volume of the primary cracks vary with the increase of the load level. Many secondary cracks are produced around the primary cracks. The fractal dimension of the secondary cracks increases nonlinearly with the increase of the load level. Under different load levels, the length and number of the primary and secondary cracks at different heights of sandstone vary. Before the failure of sandstone, the main factor affecting the propagation of the primary and secondary cracks is the length of the primary cracks. After the failure of sandstone, the main factor affecting the propagation of the primary and secondary cracks is the length of the secondary cracks. The conclusion lays a theoretical foundation for the prediction and analysis of rock failure and instability in the future.