Study on the influence of hole defects with different shapes on the mechanical behavior and damage law of coal and rock

Abstract

To study the effect of different shapes of hole defects in coal and rocks on their mechanical behavior and macro damage law, the microscopic mechanical parameters required for particle flow code (PFC) simulation were calibrated with laboratory test data, and then the evolution process of crack and stress field in coal and rocks with circle, square, triangular and trapezoidal holes under uniaxial compression were researched. The findings indicate that: the existence of hole defects lowers the elastic modulus, peak stress, peak strain and other mechanical parameters of coal and rock, and the reduction degree is influenced by the shape of defect. Meanwhile, the existence of hole defects promotes the generation and evolution of meso-cracks in coal and rock. For coal and rock with hole defects, the crack initiation stress and expansion stress are less than those of intact coal and rocks. The crack initiation stress and expansion stress of coal and rocks with trapezoidal hole defects are the smallest, and the coal and rocks with circular hole defects are the largest. The existence of hole defects weakens the damage degree of coal and rocks to some extent. With the increase of axial strain, the evolution curve of the number of meso-cracks shows stage characteristics, which consists of the calm period before the crack initiation point, the stable growth stage between the crack initiation point and the dilatation point, and the accelerated growth stage after the dilatation point. Before the initiation of crack, the concentration zone of compressive stress is located on the left and right sides of the hole defect, and the concentration zone of tensile stress is located on the upper and lower sides of the hole defect. The concentration of tensile stress is the main reason for the initiation and propagation of cracks, while the existence of compressive stress chain among macroscopic cracks is the cause of the residual strength of coal and rocks after failure.

A review of FMLs performance test methods and index evaluation

“…However, the mechanical properties of FMLs are their unique anisotropy, and there is a strong correlation between their failure form and its anisotropy, especially the carbon fiber reinforced composite material, which is composed of fiber and matrix, and the accumulation of microscopic defects and cracks determines the characteristics of macroscopic failure [140][141][142]. In the process of finite element simulation, the researchers use the single cell model under the fine scale, and build the internal fiber bundle of the material and woven or woven geometry parameters, pretreatment through CAD modeling and the finite element meshing software, and adopt appropriate failure criteria based on different failure modes to calculate the stress and strain distribution after damage on the mesoscale [143][144][145][146].…”