An Energy Model of a Failure Mechanism and its Application

The energy expression is presented for a failure mechanism, and it is appliedin an Accelerated Life Test (ALT) and an Accelerated Reliability Growth Test (ARGT). The conditionsof the common failure mechanism are obtained. The essential relationship between the conditions andthe Accelerated Factor (A_f) is proposed by using the energy model.

Describing failure in geomaterials using second-order work approach

Geomaterials are known to be non-associated materials. Granular soils therefore exhibit a variety of failure modes, with diffuse or localized kinematical patterns. In fact, the notion of failure itself can be confusing with regard to granular soils, because it is not associated with an obvious phenomenology. In this study, we built a proper framework, using the second-order work theory, to describe some failure modes in geomaterials based on energy conservation. The occurrence of failure is defined by an abrupt increase in kinetic energy. The increase in kinetic energy from an equilibrium state, under incremental loading, is shown to be equal to the difference between the external second-order work,involving the external loading parameters, and the internal second-order work, involving the constitutive properties of the material. When a stress limit state is reached, a certain stress component passes through a maximum value and then may decrease. Under such a condition, if a certain additional external loading is applied, the system fails, sharply increasing the strain rate. The internal stress is no longer able to balance the external stress, leading to a dynamic response of the specimen. As an illustration, the theoretical framework was applied to the well-known undrained triaxial test for loose soils. The influence of the loading control mode was clearly highlighted. It is shown that the plastic limit theory appears to be a particular case of this more general second-order work theory. When the plastic limit condition is met, the internal second-order work is nil. A class of incremental external loadings causes the kinetic energy to increase dramatically, leading to the sudden collapse of the specimen, as observed in laboratory.

The temperature-dependent fracture strength model for ultra-high temperature ceramics

Breaking down the entire structure of a material implies severing all the bonds between its atoms either by applying work or by heat transfer. Because bond-breaking is indifferent to either means, there is a kind of equivalence between heat energy and strain energy. Based on this equivalence, we assume the existence of a constant maximum storage of energy that includes both the strain energy and the corresponding equivalent heat energy. A temperaturedependent fracture strength model is then developed for ultrahigh temperature ceramics （UHTCs）. Model predictions for UHTCs, HfB2, TiC and ZrB2, are presented and compared with the experimental results. These predictions are found to be largely consistent with experimental results.

Triaxial shear behavior of a cement-treated sand——gravel mixture

A number of parameters,e.g.cement content,cement type,relative density,and grain size distribution,can influence the mechanical behaviors of cemented soils.In the present study,a series of conventional triaxial compression tests were conducted on a cemented poorly graded sandegravel mixture containing 30% gravel and 70% sand in both consolidated drained and undrained conditions.Portland cement used as the cementing agent was added to the soil at 0%,1%,2%,and 3%(dry weight) of sandegravel mixture.Samples were prepared at 70% relative density and tested at confining pressures of 50 kPa,100 kPa,and150 kPa.Comparison of the results with other studies on well graded gravely sands indicated more dilation or negative pore pressure in poorly graded samples.Undrained failure envelopes determined using zero Skempton’s pore pressure coefficient (= 0) criterion were consistent with the drained ones.Energy absorption potential was higher in drained condition than undrained condition,suggesting that more energy was required to induce deformation in cemented soil under drained state.Energy absorption increased with increase in cement content under both drained and undrained conditions.