Experimental study on time-dependent stress and strain of in-plane shear (Mode II) fracture process of rock

Abstract

Shear-box test with strain measurement was used to study time-dependent stress and strain of in-plane shear (Mode II) fracture process of rock and to reveal the mechanism of Mode II fracture. Numerical results show that the maximum shear stress τ_max at the crack tip is much larger than the maximum tensile stress σ₁ and the ratio of τ_max/σ₁ is about 5, which favors Mode II fracture of rock. Test results indicate that the strain-time curve comprises three stages: the linear deformation stage, the micro-cracking stage and the macroscopic crack propagation. The strain in the direction of the original notch plane is negative, due to restraining effect of compressive loading applied to the original notch plane. Both σ₁ and τ_max are increased as the load increases, but the slope of τ_max is larger than that of σ₁ and the value of τ_max is always larger than that of σ₁. Therefore, τ_max reaches its limited value at peak load before σ₁ and results in Mode II fracture of rock. Shear-box (i.e. compression-shear) test becomes a potential standard method for achieving the true Mode II fracture and determining Mode II fracture toughness of rock.

Keywords

time-dependent stress and strain / Mode II fracture / strain measurement / fracture mechanism / finite element method / rock

References

[1] HuangJ., WangS.. An experimental investigation concerning the comprehensive fracture toughness of some brittle rocks [J]. Int J Rock Mech Min Sci & Geomech Abstr, 1985, 22(2): 99-104

[2] SchlangenE., van MierJ. G. M.. Crack propagation in sandstone: Combined experimental and numerical approach [J]. Rock Mech. & Rock Eng, 1995, 28(2): 93-110

[3] PetitJ. P.. Normal stress dependent rupture morphology in direct shear test on sandstone with applications to some natural fault surface features [J]. Int J Rock Mech Min Sci & Geomech Abstr, 1988, 25(6): 411-419

[4] WatkinsJ.. Fracture toughness test for soil-cement samples in mode II [J]. Int J Fract, 1983, 23: 135-138

[5] JONES D L, CHISHOLM D B. Further parametric and boundary condition studies of the compact shear specimen [C]// LUXMOORE A R, OWEN D R J, eds. Pro 3rd Inter Confer on Num Methods in Fract Mech. Swansea, Pineridge, 1984: 697–709.

[6] AnandrajahA., VardyA. E.. Mode I and Mode II fracture of adhesive joints [J]. J Strain Analysis, 1984, 19(3): 173-183

[7] RaoQ.-hua.Pure shear fracture of brittle rock-a theoretical and laboratory study [D], 1999, Lulea, Division of Rock Mechanics, Lulea University of Technology: 99-103

[8] RaoQ. H., SunZ. Q., StephanssonO., LiC. L., StillborgB.. Shear fracture (Mode II) of brittle rock [J]. International Journal of Rock Mechanics and Mining Sciences, 2003, 40(3): 355-375