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.

Low energy shock response of a melt cast simulant material

To manufacture its insensitive munitions (MURAT MUnitions à Risque ATténué), NEXTER Munitions uses a melt cast explosives as an EIDS (Extremely Insensitive Detonating Substance). Unlike commonly used and well-documented EIDSs such as PBX, melt cast have a high volumetric matrix percentage. Moreover, in its life cycle, the ammunition can undergo severe loads, such as cannon firing, accidental shocks and terminal ballistics events. The objective of this paper is therefore to analyse, how these dynamic loads induce changes in the material (damage, cracking, de-cohesion), and then, to evaluate how these alterations influence the pyrotechnic properties of the melt cast explosive. To address the first point, we delimited the scope of the study in pressure and strain rate ranges which corresponds to the context of the ammunition. To safely explore this area, we have created an inert material that is morphologically and mechanically representative of the melt case explosive. It is used to setup and validate the experimental technic that will be applied to damage the melt cast explosive in the future. In this article the mechanical behaviour of the inert material is investigated under simple compression and passive confinement. This was done under the quasi-static and dynamic regimes thanks to a compression press and a Split Hopkinson Pressure Bars setup. Firstly, the results obtained show the emergence of a damage which increases with the loss of cohesion of the material during the test. This seems to be related to the extension strain. Then, for all tests, a strain rate dependent mechanical response is observed. Finally, the end of the test shows the material behaviour without cohesion. Then, a rate dependent ultimate shear criterion is deduced. To complete the interpretation these results, a model is proposed. It intends to be simple as it tries to describe the whole degradation of the material with a unique scalar parameter.

A cadaveric analysis of cervical fixation: the effect of intermediate fixation points and dynamization in multilevel cervical fusions

Object The authors conducted a study to compare biomechanical effects on the cervical spine of bridging fixation and intermediate fixation techniques, in both fixed and dynamic modes. Methods A biaxial, servohydraulic machine biomechanically tested 23 human cervical spines for stiffness and strain in compression, extension, flexion, and lateral bending through 3 specimen states: 1) intact, 2) defect (corpectomy and discectomy), and 3) grafting with plate application in 1 of 4 constructs: C3–7 dynamized long strut (DLS), C3–7 fixed long strut (FLS), C3–5–7 dynamized multisegment (DMS), and C3–5–7 fixed multisegment (FMS). Results Compared with FMS, FLS had significantly greater strain in extension (at C-3 and at the rostral and caudal parts of the graft) and in lateral bending (at C-3 and at the caudal part of the graft). Fixed (FLS and FMS) constructs had greater flexion stiffness than did dynamized (DLS and DMS) constructs and showed a trend toward greater lateral bending stiffness. Instrumentation revealed greater extension strain with the long fixed (FLS and DLS) constructs than with the multifixed (FMS and FMS) constructs at the rostral and caudal parts of the graft but no significant differences between the dynamized (DLS and DMS) and fixed (FLS and FMS) constructs. Conclusions Multisegmental fixation provided greater stabilizing forces than did bridging constructs for both dynamized and fixed plates. Use of multisegmental fixation can potentially decrease strain at the screw-plate interface and reduce the rate of hardware failure.

Twinning Behavior in AZ31-B Polycrystal Texture Subjected to In-Situ Bending Test

A commercial Mg alloy, AZ31B, has been used widely. In the texture of AZ31B sheet, each grain has its c-axis almost parallel to the sheet normal. Therefore, at the bending process of the sheet, basal slip system can not accommodate an in-plane plastic strain which is perpendicular to the c-axis of each grain. It is known that {10―,12} twin can be formed by applying an extension strain parallel to the c-axis, which is equivalent to the a-axis compression strain. So in the bending deformation of the AZ31B sheet with a texture microstructure, it is expected that {10―,12} twinning occurs. In this study, an in-situ bending test of AZ31B sheet with a texture was conducted under a confocal scanning laser microscope to observe twinning by applying compression stress along a direction almost perpendicular to the c-axis of grains. In addition, EBSD techniques were used for the analysis of crystal orientations. The process of twin development observed by the in-situ bending test can be summarized as follows; with the increase of the deformation strain, the total area of twins increases. However, it is noted that the growth of twins is apparent while the number of twins is almost constant during plastic bending deformmation. EBSD analysis suggested that twinning behavior obey Schmid’s law even in the polycrystal.