Single Shear Failure Research Articles

Deformation Failure Characteristics and Loading Rate Effect of Sandstone Under Uniaxial Cyclic Loading and Unloading

In this paper, uniaxial cyclic loading and unloading tests of equal amplitude loading with four loading rates were conducted on sandstone samples using the testing system of MTS 815.03. The evolution characteristics of stress–strain, peak stress and macro-failure characteristics of sandstone under uniaxial cyclic loading and unloading tests with four equal amplitude loading rates were analyzed. The average strength of sandstone under uniaxial cyclic loading and unloading is less than that under uniaxial compression. And with the increase of loading rate, the peak stress of sandstone nonlinearly increases and its strength dispersion decreases gradually. Moreover, the hysteretic loop of stress–strain curve under uniaxial cyclic loading and unloading has a continuous “migration” phenomenon. When close to failure, the lateral strain will suddenly increase and the strain recovery is small when unloading. The failure of sandstone is the single shear failure at a low loading rate and gradually transformed into splitting ejection failure with the increase of loading rate. The “expansion” phenomenon of sandstone represents that it enters the accelerated failure stage. The larger the loading rate is, the smaller the volume compression before critical points is. After post-peak, the greater the loading rate is, the intenser the volume expansion is. According to above achievements, the failure of the rock can be speculated by monitoring the lateral deformation of rock or lateral strain rate in practical engineering.

Influence of Water Saturation on the Mechanical Behaviour of Low-Permeability Reservoir Rocks

The influence of water on the mechanical properties of rocks has been observed by many researchers in rock engineering and laboratory tests, especially for sedimentary rocks. In order to investigate the effect of water saturation on the mechanical properties of low-permeability rocks, uniaxial compression tests were conducted on siltstone with different water contents. The effects of water on the strength, elastic moduli, crack initiation and damage thresholds were observed for different water saturation levels. It was found that 10% water saturation level caused more than half of the reductions in mechanical properties. A new approach is proposed to analyze the stress-strain relations at different stages of compression by dividing the axial and lateral stress-strain curves into five equal stress zones, where stress zones 1–5 refer to 0%–20%, 20%–40%, 40%–60%, 60%–80% and 80%–100% of the peak stress, respectively. Stress zone 2 represents the elastic range better than stress zone 3 which is at half of the peak stress. The normalized crack initiation and crack damage stress thresholds obtained from the stress-strain curves and acoustic emission activities averaged 31.5% and 83% of the peak strength respectively. Pore pressure is inferred to take part in the deformation of low-permeability siltstone samples, especially at full saturation levels. A change of failure pattern from multi-fracturing to single shear failure with the increase of water saturation level was also observed.

Mechanisms-based failure laws for AS4/3502 graphite/epoxy laminates under in-plane biaxial compression

Failure mechanisms and stress–strain behaviors have been investigated for [±30] 12s and [±45] 12s graphite-epoxy (AS4/3502) laminates under in-plane biaxial compression by using a cruciform biaxial test frame and microscopy of load-interrupted samples. The loading confinement ratio R was varied from 0.24 to nearly 1.0 to measure the sensitivity of sample failure mechanisms and stress–strain behavior to different stress states. Failure modes with an increasing loading confinement ratio for both fiber orientations transitioned from the uniaxial failure mode of in-plane shearing to out-of-plane shearing and massive delamination. The shear failures were on one or more planes that traversed the entire sample thickness and thus encompassed alternate plies in which fibers were aligned and misaligned with respect to the shear plane. The local failure was found to be triggered either by the matrix shear in fiber-aligned plies or fiber-shear in the fiber-misaligned plies. A standard classical laminate theory in combination with the standard rule of mixture theory for composites was used to calculate the maximum matrix and fiber shear stresses. The failure data conformed nicely with a Mohr–Coulomb-type shear failure law. Most interestingly, both types of failures correlated rather remarkably to a single shear failure law when the stresses were smeared and expressed on the scale of the laminae. This latter failure law should have widespread use because of its simplicity.