Abstract
The friction coefficient, tip curvature, and different-width crack state influence the stress intensity factor (SIF). The maximum circumferential tensile stress (MTS) and minimum strain energy density criterion (S) face challenges in explaining the mode-II fracture propagation of cracks. The maximum radial shear stress (MSS) and modified twin shear stress factor (ITS) criteria are proposed as the brittle mode-II fracture criteria. The experiments and numerical analysis are also performed. The results indicate that the fracture angles of the MSS and ITS were similar and different from the results of MTS and S. The equivalent stress intensity factors (ESIFs) from the mixed mode I-II are proposed to determine the fracture mode. There are different fracture models for different cracks under tensile and compressive stresses. The ratio of the tensile strength to uniaxial compressive strength influenced the fracture angle of ITS. The lateral pressure coefficient (k) had a significant effect on the mode-II fracture angle when the angle between the crack and the vertical direction is less than 40° and the lateral pressure coefficient is more than 0. Because the same fracture mode k (k > 0) can inhibit mode-I fracturing, conversely, it can also promote mode-I fracturing. Experimental results and numerical simulations of fracture propagation under uniaxial compression confirmed that the theoretical results were correct.
Highlights
Rock develops in complex geological environments and includes various defects or flaws. ese flaws weaken the mechanical properties of the rock mass and modify the stress distributions
When the shear stress acting on the main crack exceeds the friction stress between the cracks, the stress will concentrate at the crack tip. e cracks continue to grow and curve toward the direction of the maximum principal stress, when the stress strength factor meets or exceeds KIe equals the mode-I fracture toughness (KIC)
2 mm, the crack length was 2a 10 mm, the curvature radius was ρ 1 mm at the crack tip, and the crack was nonclosed during loading. e relationship between the crack angles and the fracture angles is shown in Figure 3, where the mixed mode I-II nonclosed crack was analyzed according to maximum circumferential tensile stress (MTS), S, maximum radial shear stress (MSS), and ITS. e fracture angles based on MTS and S were similar. e fracture angles based on S were influenced by v; the results were quite different when β < 40° and k < 0
Summary
Rock develops in complex geological environments and includes various defects or flaws. ese flaws weaken the mechanical properties of the rock mass and modify the stress distributions. E crack initiation and growth on a rock specimen subjected to compressive stress has been investigated experimentally [1, 3, 7, 14,15,16,17,18,19,20,21,22,23,24,25,26]. By using the finite element fracture software called Franc2D, the energy release rate (G), crack propagation, fracturing time, and static tensile and normaldistributed stresses were calculated to represent the crack initiation and growth in a rock specimen [11, 27]. E maximum tangential stress [31], maximum energy release rate [32], and minimum energy density criterion [33, 34] have typically been considered as the fracture initiation criteria to identify the crack growth mechanism of brittle rocks. Mixed mode I-II ESIFs are proposed to determine the fracture mode. e relationships between the fracture angle and model of crack propagation with a crack angle and thickness, lateral pressure coefficient, and ratio of tensile strength to compressive strength are discussed. e consistency between the theoretical and numerical results was verified
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