The fracture behavior of anisotropic shale plays a critical role in hydraulic fracturing and the stability of underground engineering. However, most previous study has focused on the pure mode I fracture, while the mixed-mode I–II fracture behavior remains relatively unexplored. Therefore, this study investigates the fracture behavior of layered shale under mixed-mode I–II loading. Firstly, the asymmetric notched semicircular bending (ANSCB) tests were conducted on Longmaxi shale, considering various bedding dip angles (α) and crack inclination angles (β), and were combined with acoustic emission (AE) and digital image correlation (DIC) monitoring. Subsequently, a systematic analysis was conducted on the mechanical load–displacement curves, AE characteristic parameters, DIC-based displacement and strain fields, and failure modes. Finally, the anisotropy of mixed-mode I–II fracture toughness and its influence on failure mode, fracture angle, and fracture process zone (FPZ) were examined. Additionally, correlation between DIC and AE results and the application of mixed fracture models in hydraulic fracturing operations were discussed. The results indicated that the Longmaxi shale exhibits significant anisotropy in mode I/II fracture toughness and the effective fracture toughness (Keff). The isotropic model underestimates both KI and Keff while overestimating KII. Variations in KI, KII, and Keff exhibit pronounced anisotropic characteristics, primarily governed by angles α and β, with elastic anisotropy playing a comparatively minor role. Anisotropy ratios AR−α = 2.47 and AR−β = 2.21 indicate that α exerts a stronger influence on Keff than β. DIC analysis identified five typical failure modes: tensile failure along or through bedding planes, shear failure along or through bedding planes, and mixed tensile–shear failure. Integrated DIC and AE results suggest that nearly all specimens were dominated by tensile failure, except at the lower and upper limits of angles α and β, i.e., α = 0°@β = 15° and α = 90°@β = 45°. The FPZ was notably influenced by anisotropy, leading to significant variations in fracture angles between the front and back surfaces of some specimens. Both the fracture angle and FPZ dimensions (width and length) demonstrated a clear increasing trend with angle β, while angle α exhibited no significant influence.

Experimental Investigation on the Mixed-Mode I–II Fracture Behavior in Layered Shale

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发表日期：

2026年4月9日

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Tianshou Ma,
Zexin He,
Haonan Wang,
Yang Liu,
Ran Xu,
Yi Wang

摘要

The fracture behavior of anisotropic shale plays a critical role in hydraulic fracturing and the stability of underground engineering. However, most previous study has focused on the pure mode I fracture, while the mixed-mode I–II fracture behavior remains relatively unexplored. Therefore, this study investigates the fracture behavior of layered shale under mixed-mode I–II loading. Firstly, the asymmetric notched semicircular bending (ANSCB) tests were conducted on Longmaxi shale, considering various bedding dip angles (α) and crack inclination angles (β), and were combined with acoustic emission (AE) and digital image correlation (DIC) monitoring. Subsequently, a systematic analysis was conducted on the mechanical load–displacement curves, AE characteristic parameters, DIC-based displacement and strain fields, and failure modes. Finally, the anisotropy of mixed-mode I–II fracture toughness and its influence on failure mode, fracture angle, and fracture process zone (FPZ) were examined. Additionally, correlation between DIC and AE results and the application of mixed fracture models in hydraulic fracturing operations were discussed. The results indicated that the Longmaxi shale exhibits significant anisotropy in mode I/II fracture toughness and the effective fracture toughness (K_eff). The isotropic model underestimates both K_I and K_eff while overestimating K_II. Variations in K_I, K_II, and K_eff exhibit pronounced anisotropic characteristics, primarily governed by angles α and β, with elastic anisotropy playing a comparatively minor role. Anisotropy ratios AR−α = 2.47 and AR−β = 2.21 indicate that α exerts a stronger influence on K_eff than β. DIC analysis identified five typical failure modes: tensile failure along or through bedding planes, shear failure along or through bedding planes, and mixed tensile–shear failure. Integrated DIC and AE results suggest that nearly all specimens were dominated by tensile failure, except at the lower and upper limits of angles α and β, i.e., α = 0°@β = 15° and α = 90°@β = 45°. The FPZ was notably influenced by anisotropy, leading to significant variations in fracture angles between the front and back surfaces of some specimens. Both the fracture angle and FPZ dimensions (width and length) demonstrated a clear increasing trend with angle β, while angle α exhibited no significant influence.