Mixed Hardening Characteristics of the Anisotropic Coal under Cyclic Loading

• Published in: IOP conference series. Earth and environmental science 2020-10, Vol.570 (4), p.42033
• Main Authors: Huang, Bingxiang, Xing, Yuekun, Jin, Feng, Zhao, Long
• Format: Article
• Language: English
• Subjects: Acoustic emission; anisotropic; Anisotropy; Coal; Compression; Compressive strength; cyclic loading; Cyclic loads; Fracture surfaces; Fracturing; Hardening; Isotropy; Kinematics; Load distribution; Loading rate; mixed hardening; Permeability
• ISSN: 1755-1307; 1755-1315
• DOI: 10.1088/1755-1315/570/4/042033

Pulse fracturing has been used to increase permeability and weaken the strength of the coal seam, making the coal fracture under cyclic loading. During the cyclic loading, the rock-like materials tend to present mixed hardening (mixed mode of isotropic and kinematic hardening) from initial yielding to failure (critical yielding). At present, understanding of the mixed hardening characteristics in anisotropic coal involving massive cleats remains challenging and crucial. In this paper, the cylinder specimens (diameter: 50 mm; length: 100 mm) of the coal were tested under cyclic uniaxial loading (loading rate: 900 N/s), and acoustic emission (AE) was employed to characterize the hardening process. The samples were drilled at the angle (α) of 0°, 45° and 90° with the coal surface cleat respectively. The upper-stress limit (Pmax) increases by 2 kN at each loading cycle, and the lower-stress limit (Pmin) was kept at 1 kN. Several findings were obtained based on the experimental results. (1) Uniaxial compression strength and the cyclic number increase with α, presenting isotropy. (2) The remarkable accumulation of AE energy is the feature of identifying plastic hardening. Both the isotropic and kinematic hardening processes are significant for the specimen with α dip angle (α) of 90°, validated by the dramatically increased isotropic and kinematic hardening indexes. However, the coal presents a slight isotropic and kinematic hardening, with a ranging from 0° to 45°. (3) The remarkably mixed hardening (α=90°) corresponding to a complex fragmentation of the coal, which is supposed to be caused by the continuous weakening of coal matrix (including the butt cleat). In contrast, at α of 0° and 45°, the mixed hardening characteristics are slight. Accordingly, the final fracture surfaces of coal tend to be relatively single, roughly along with the surface cleats. Thus, we can infer that the slightly mixed hardening is due to the weakening of surface cleats. Based on the mixed hardening characteristics of anisotropic coals, conducting pulse fracturing to apply cyclic loading perpendicular to the cleat surface is supposed to be beneficial for generating more complex fractures, to improve coal permeability.