Charge signals characterizing the influence of bedding angles on shale damage under cyclic loading and unloading

Abstract

The role of bedding angle on the mechanic properties and failure modes of shale under cyclic loading and unloading conditions is unclear. This study conducted uniaxial cyclic loading and unloading tests on shales from the Longmaxi Formation with different bedding angles (θ = 0°, 22.5°, 45°, 67.5° and 90°), and characterized their damage evolution through both AE and charge signals. Results show that the compressive strengths are higher, and the loading cycles are more in specimens with θ = 0° and 90° than those with θ = 22.5°, 45° and 67.5° during cyclic loading tests, resulting in a silent period of signaling presented in former but not in latter θ. Both AE and charge signals can well reflect the major damages in time-domain analysis, while only charge signals can characterize the minor damages at the silent stages by continuous wavelet transforming into time-frequency plots, leading to their advantages in characterizing the damage evolution in specimens with θ = 0° and 90°, but not with θ = 22.5°, 45° and 67.5°. These differences can be attributed to their different signal acquirement mechanisms. These findings highlight the effectiveness of charge signals in characterizing the shale damage evolution under loading and unloading conditions.