Abstract:

To reveal the deformation and damage characteristics of coal under true three-dimensional cyclic excavation disturbances during coal mining, a series of raw coal mechanical tests was conducted under graded cyclic true triaxial stress conditions. The stress-strain behavior, strain sensitivity coefficients, and loading-unloading response ratios were analyzed. The experimental results indicate that during the true triaxial cyclic loading process, the principal strain corresponding to the direction of the cyclic principal stress dominates the deformation, with relatively larger deformation. When s₂ is large under cycle s₁, and when s₁ is large under cycle s₃, in both of the above conditions, e₂ will exhibit a strain reversal effect near the failure stage. Cyclic loading or unloading of s₂ may lead to instability and failure of coal, reflecting that the true triaxial ultimate strength curve is arc-shaped. The principal strain sensitivity coefficient corresponding to the cyclic principal stress is the highest and exhibits a synchronous trend with the volumetric strain sensitivity coefficient. When the graded cyclic loading of s₁ is applied, the unloading modulus is always greater than the loading modulus. As the number of cycles increases, the loading modulus first increases and then decreases, while the unloading modulus continuously decreases. The load-unload response ratio first drops sharply, then declines slowly, gradually approaching a value of 1, with the curve showing a noticeable upward trend near failure. The pattern is entirely opposite when graded cyclic unloading of s₃ is applied. As the number of graded cycles s₂ increases, the loading modulus and unloading modulus gradually approach each other. The experiments demonstrate that the strain reversal effect in e₂ under true triaxial cyclic loading and the subsequent deviation from 1 in the loading-unloading response ratio can serve as reference criteria for predicting coal failure.

Key words: true triaxial, cyclic load, coal, strain sensitivity, load-unload response ratio