Effect of specimen thickness on Mode II fracture toughness of rock

Abstract

Anti-symmetric four-point bending specimens with different thickness, without and with guiding grooves, were used to conduct Mode II fracture test and study the effect of specimen thickness on Mode II fracture toughness of rock. Numerical calculations show that the occurrence of Mode II fracture in the specimens without guiding grooves (when the inner and outer loading points are moved close to the notch plane) and with guiding grooves is attributed to a favorable stress condition created for Mode II fracture, i.e. tensile stress at the notch tip is depressed to be lower than the tensile strength or to be compressive stress, and the ratio of shear stress to tensile stress at notch tip is very high. The measured value of Mode II fracture toughness K_{II C} decreases with the increase of the specimen thickness or the net thickness of specimen. This is because a thick specimen promotes a plane strain state and thus results in a relatively small fracture toughness.

Keywords

Mode II fracture toughness / rock / fracture / stress analysis / specimen thickness

References

[1] CarpinteriA, FerraraG, MelchiorriGShahS P, SwartzS E, BarrB. Single edge notched specimen subjected to four point shear: an experimental investigation[A]. Fracture of Concrete and Rock: Recent Developments[C], 1989, Houston, Elsevier Applied Science Press: 605-614

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

[3] ChongK P, KuruppuM D, KuszmaulJ SShahS P, SwartzS E. Fracture toughness determination of rocks with core-based specimen[A]. Fracture of Concrete and Rock, SEM/RILEM International Conference[C], 1987, Houston, Elsevier Applied Science Press: 13-25

[4] PangH L J. Mixed mode fracture analysis and toughness of adhesive joints[J]. Engineering Fracture Mechanics, 1995, 51(4): 575-538

[5] RichardH A. A new compace shear specimen[J]. Int J Fract, 1981, 17: R105-107

[6] BiolziL. Mixed mode fracture in concrete beams[J]. Engineering Fracture Mechanics, 1990, 35(1/2/3): 187-193

[7] WangGui-yao, SunZong-qi, XuJi-chen. Rock fracture under pure shear loading[J]. Trans Nonferrous Met Soc China, 1996, 6(3): 17-21(in Chinese)

[8] RaoQiu-huaPure shear fracture of brittle rock, a theoretical and laboratory study[D], 1999, Sweden, Lulea University of Technology

[9] WangGui-yaoStudy of Mode II fracture of rock and its engineering application[D], 1996, Changsha, Central South University of Technology

[10] FettT. Stress intensity factors for edge subjected to mixed mode four-point bending[J]. Theoretical and Applied Fracture Mechanics, 1991, 15: 99-104