An Extension Strain Type Mohr–Coulomb Criterion

Extension fractures are typical for the deformation under low or no confining pressure. They can be explained by a phenomenological extension strain failure criterion. In the past, a simple empirical criterion for fracture initiation in brittle rock has been developed. In this article, it is shown that the simple extension strain criterion makes unrealistic strength predictions in biaxial compression and tension. To overcome this major limitation, a new extension strain criterion is proposed by adding a weighted principal shear component to the simple criterion. The shear weight is chosen, such that the enriched extension strain criterion represents the same failure surface as the Mohr–Coulomb (MC) criterion. Thus, the MC criterion has been derived as an extension strain criterion predicting extension failure modes, which are unexpected in the classical understanding of the failure of cohesive-frictional materials. In progressive damage of rock, the most likely fracture direction is orthogonal to the maximum extension strain leading to dilatancy. The enriched extension strain criterion is proposed as a threshold surface for crack initiation CI and crack damage CD and as a failure surface at peak stress CP. Different from compressive loading, tensile loading requires only a limited number of critical cracks to cause failure. Therefore, for tensile stresses, the failure criteria must be modified somehow, possibly by a cut-off corresponding to the CI stress. Examples show that the enriched extension strain criterion predicts much lower volumes of damaged rock mass compared to the simple extension strain criterion.

Fatigue Life of Aluminum Alloys Based on Shear and Hydrostatic Strain

The main purpose of this paper is to propose, based on the literature review, a new multiaxial fatigue strain criterion, analogous to the Dang Van stress criterion, considering the maximum amplitude of the shear strain and volumetric strain. The proposed strain criterion was successfully verified by fatigue tests in cyclic bending with torsion of specimens made of 2017A-T4 and 6082-T6 aluminum alloy. The scatter of test results for cyclic bending and the combination of cyclic bending and torsion is included in the scatter of tests for the cyclic torsion of the analyzed materials. Fracture surfaces for respective bending and torsion in the 6082-T6 aluminum test with strain control showed that, in the case of bending, cracks can be observed that develop from the surface of the specimen towards the bending plane. They are inclined from the fatigue crack at an angle of 45° in relation to the crack surface and the remaining cracks come from the static fracture. In the case of torsion, however, a conical fracture at 45° and a static torsion zone can be observed.

The Maximum Stress Failure Criterion and the Maximum Strain Failure Criterion: Their Unification and Rationalization

The maximum strain failure criterion is unified with the maximum stress failure criterion, after exploring the implications of two considerations responsible for this: (1) the failure strains for the direct strain components employed in the maximum strain criterion are all defined under uniaxial stress states, not uniaxial strain states, and (2) the contributions to the strain in a direction as a result of the Poisson effect do not contribute to the failure of the material in that direction. Incorporating these considerations into the maximum strain criterion, the maximum stress criterion is reproduced. For 3D stress/strain state applications primarily, the unified maximum stress/strain criterion is then subjected to further rationalization in the context of transversely isotropic materials by eliminating the treatments that undermine the objectivity of the failure criterion. The criterion is then applied based on the maximum and minimum direct stresses, the maximum transverse shear stress and the maximum longitudinal shear stress as the invariants of the stress state, instead of the conventional stress components directly.

Research on Design Method for Thermal Insulation of Oil Pipeline in Permafrost Regions

The most important problems encountered in the design and operation of oil pipelines in permafrost regions are frost heaving and thawing settlement. In this paper, we conducted an investigation and analysis on the status of freezing damage of the operating oil pipelines. Based on the investigation and research, we calculated the temperature field of the oil pipeline and analysed the thawing ability under the conditions of the strain criterion. Finally, we proposed a design method for pipeline insulation and the maximum amount of thawing displacement that the pipeline can withstand. It has certain guiding significance for the thermal insulation design of oil pipelines in permafrost regions.