Geometry Effects on Mode I Brittle Fracture in VO-Notched PMMA Specimens

This paper gathers experimental and theoretical investigations about both the geometry-dependent fracture initiation angle and the fracture strength in VO-notched polymethyl methacrylate (PMMA) specimens under mode I loading conditions. The numerical analyses revealed that despite the application of pure mode I loading on the geometrically symmetric VO-notched samples, the maximum tangential stress occurs at two points symmetrically placed on either side of the notch bisector line. The experimental tests performed on some specimens showed that a crack does not necessarily propagate along the notch bisector line. Stress-based theoretical studies were then carried out to justify the experimental findings. The conventional maximum tangential stress (MTS) criterion gave weak predictions of the fracture. Therefore, the predictions were checked with the generalized MTS (GMTS) criterion by taking into consideration the higher-order stress terms. It was demonstrated that the GMTS criterion predictions have satisfactory consistency with the experimental results of the crack initiation angle and the fracture strength.

A New Unified Solution for Deep Tunnels in Water-Rich Areas considering Pore Water Pressure

Pore water pressure has an important influence on the stresses and deformation of the surrounding rock of deep tunnels in water-rich areas. In this study, a mechanical model for deep tunnels subjected to a nonuniform stress field in water-rich areas is developed. Considering the pore water pressure, a new unified solution for the stresses, postpeak zone radii, and surface displacement is derived based on a strain-softening model and the Mogi-Coulomb criterion. Through a case study, the effects of pore water pressure, intermediate principal stress, and residual cohesion on the stress distribution, postpeak zone radii, and surface displacement are also discussed. Results show that the tangential stresses are always larger than the radial stress. The radial stress presents a gradually increasing trend, while the tangential stress presents a trend of first increasing and then decreasing, and the maximum tangential stress appears at the interface between the elastic and plastic zones. As the pore water pressure increases, the postpeak zone radii and surface displacement increase. Because of the neglect of the intermediate principal stress in the Mohr-Coulomb criterion, the postpeak zone radii, surface displacement, and maximum tangential stress solved by the Mohr-Coulomb criterion are all larger than those solved by the Mogi-Coulomb criterion. Tunnels surrounded by rock masses with a higher residual cohesion experience lower postpeak zone radii and surface displacement. Data presented in this study provide an important theoretical basis for supporting the tunnels in water-rich areas.

Field investigation and analysis of rockburst and spalling in a deep hard-rock mine

In order to investigate the ground pressure disasters in deep hard-rock mines, field investigation and theoretical analysis were carried out in a deep hard-rock mine. It is found that the degree and number of ground pressure disasters in the mine have increased significantly with depth. When the maximum tangential stress between 0.4-0.6 times the uniaxial compressive strength of surrounding rock, surrounding rock is prone to local spalling. When maximum tangential stress is greater than 0.6 times uniaxial compressive strength, serious failure is easy to occur, such as rockbursts and large-area collapses. After excavation, the rebound strain and displacement of surrounding rock increases linearly with buried depth, and the strain energy released of surrounding rock increases rapidly with the second power of buried depth. The rapidly increasing strain energy is main reason why deep ground pressure disasters in the mine are becoming more and more serious. In terms of surrounding rock support, energy-absorbing materials such as energy-absorbing bolts can well absorb strain energy released by surrounding rock. The energy-absorbing bolts are used for design of roadway support in the mine.

Research on the Size Effect of Unstable Fracture Toughness by the Modified Maximum Tangential Stress (MMTS) Criterion

This paper investigates the unstable fracture toughness of specimens of different heights using the double-K model for three-point bending tests on notched concrete beams. It is shown that unstable fracture toughness exhibits a significant size effect. The modified maximum tangential stress (MMTS) criterion is used to explain the size effect of unstable fracture toughness. The MMTS criterion considers the higher order terms of the Williams series expansion of the stress field. The results show that the MMTS criterion can reasonably estimate unstable fracture toughness. It is recommended that the minimum height of the specimen be 200 mm when three-point bending tests on notched beams are used to determine unstable fracture toughness.

A Comparison of the Fracture Behaviour of Various Concrete Grades under Mixed Mode I/II Loading

The Brazilian disc test with central notch is widely used to evaluate mixed mode I/II fracture resistance of brittle materials such as rocks and concrete. An analytical evaluation is used based on the maximum tangential stress (MTS) criterion and the generalised maximum tangential stress (GMTS) criterion. In this contribution two concrete types, a C 50/60 and a high strength concrete, were compared using the GMTS criterion. Also, the influence of critical distance rC on the fracture resistance under the mixed mode I/II was studied.