Effects of ligament length and normal stress on the U-shaped pattern of acoustic emission energy in quasi-brittle material shear cracking

Shear cracking in quasi-brittle materials is a fundamental issue in engineering applications, with acoustic emission (AE) patterns during their cracking process offering potential fracture precursors. These patterns are influenced by geometry and environmental conditions of structures. As in-situ structures exceed specimen dimensions and endure normal stress, assessing their influence on AE patterns is essential for bridging lab-field gaps and for identifying AE precursor patterns with real predictive capability. However, yet characteristic AE patterns under realistic in-situ dimensions and stresses remains elusive. Direct shear experiments on jointed mortar specimens with varying ligament lengths and normal stresses revealed two AE energy distribution patterns: a single-burst pattern featuring one major fracture-related energy burst, and a U-shaped patterns characterized by two energy bursts separated by quiet periods. The former burst of U-shaped pattern arises from localized shear cracking at the ligament's accelerated cracking onset. Ligament length dominates the AE pattern: longer ligaments enhance crack accommodation capacity, facilitating localized shear cracking at accelerated cracking onset, thus favoring distinct U-shaped patterns. Elevated normal stress promotes compression-induced shear cracking, transitioning single-burst to U-shaped patterns in short ligaments but reducing U-shaped identifiability in medium-length ligaments. A statistical damage mechanical model quantifies the AE energy relationship between the fracture point and accelerated cracking onset, showing a magnitude difference ΔM between these two points ≤0.6 for U-shaped patterns. These results indicate the U-shaped pattern probably emerges consistently in large-scale structures, with ΔM confined within a narrow range. This provides important physical foundations for reliable warning for quasi-brittle material fracture.