Prediction Models of Shear Parameters and Dynamic Creep Instability for Asphalt Mixture under Different High Temperatures

This study mainly investigates the prediction models of shear parameters and dynamic creep instability for asphalt mixture under different high temperatures to reveal the instability mechanism of the rutting for asphalt pavement. Cohesive force c and internal friction angle φ in the shear strength parameters for asphalt mixture were obtained by the triaxial compressive strength test. Then, through analyzing the influence of different temperatures on parameters c and φ, the prediction models of shear strength parameters related to temperature were developed. Meanwhile, the corresponding forecast model related to confining pressure and shear strength parameters was obtained by simplifying the calculation method of shear stress level on the failure surface under cyclic loading. Thus, the relationship of shear stress level with temperature was established. Furthermore, the cyclic time FN of dynamic creep instability at 60 °C was obtained by the triaxial dynamic creep test, and the effects of confining pressure and shear stress level were considered. Results showed that FN decreases exponentially with the increase in stress levels under the same confining pressure and increases with the increase in confining pressure. The ratio between shear stress level and corresponding shear strength under the same confining pressure was introduced; thus, the relationship curve of FN with shear stress level can eliminate the effect of different confining pressures. The instability prediction model of FN for asphalt mixture was established using exponential model fitting analysis, and the rationality of the model was verified. Finally, the change rule of the parameters in the instability prediction model was investigated by further changing the temperature, and the instability forecast model in the range of high temperature for the same gradation mixture was established by the interpolation calculation.

Mechanical creep instability of nanocrystalline methane hydrates

Fundamental creep mechanisms of nanocrystalline methane hydrates are revealed, which are of importance to evaluate the mechanical stability of gas hydrate-bearing sediments in both terrestrial and planetological environments.

The Micro-Mechanism Analysis on Rock Creep Damage

Rock creep is a hot topic and key point in engineering geology. Based on renormalization group theory, the rock creep instability of micro-mechanism can be described as follow: with the expanding and interaction of micro-cracks and the main fault forms, which leads to failure. Comparing with the similarity of rock creep and rock compression process, and combining results of rock creep process, it is shown that there a distinct pertinence between different stages of rock creep. The relationship characteristics of different evolutionary stages can be used to provide scientific foundation for rock creep.

Temperature-dependent shear flow and the absence of thermal runaway in valley glaciers

We propose a two-dimensional model of a valley glacier in order to reconsider the question of whether thermal runaway could be a viable mechanism for the onset of creep instability in surging glaciers. We do this by providing an approximate solution for the temperature field based on the idea that shear is concentrated at the glacier bed. With this assumption, we show that a closed-form evolution equation for the glacier profile exists. While this is well known for isoviscous flows, it has not been previously derived for variable viscosity flows. During the process of deriving this equation, we show that thermal runaway does not occur. We provide numerical solutions of the model, and are led to infer that enhanced basal heating owing to refreezing of surface meltwater is an essential constituent in raising the bed temperature to the melting point.

Buckling Analysis in Creep Conditions: Review and Comparison

In the case of structures operating at high temperature in normal or accidental conditions, the influence of creep has to be considered at the design stage because this phenomenon may reduce the lifetime significantly. This is true in particular for buckling analysis : in creep conditions, the buckling sometimes occurs after a long period under a compressive load which is lower than the critical load assessed when considering an instantaneous buckling. The main reason is that creep deformations induce an amplification of the initial geometrical imperfections and consequently a reduction of the buckling load. Some Design Codes incorporate special rules and/or methods to take creep buckling into account. Creep buckling analysis methods aim at evaluating critical loading for a given hold period with creep or alternatively critical creep time for a given loading. The Codes where creep buckling is considered also define margins with respect to critical loading : it shall be demonstrated that creep instability will not occur during the whole lifetime when multiplying the specified loading by a coefficient (design factor) depending on the situation level. For the design of NPP, specific creep buckling rules exist in the US, France and Russia. In the US, ASME, Section III, Subsection NH, which is dedicated to high temperature components design, provides limits which are applicable to general geometrical configurations and loading conditions that may cause buckling due to creep behaviour of the material. For load-controlled time-dependent creep buckling, the design factors to apply to the specified loadings are 1.5 for levels A, B or C service loadings and 1.25 for level D service loadings. A design factor is not required in the case of purely strain-controlled buckling. No specific method is provided to obtain critical loading or critical time for creep instability. In France, creep buckling rules included in RCC-MR, Chapter RB or RC 3200 are similar to those of ASME, Subsection NH. In addition, a new simplified method has been developed recently to assess critical creep loading/time for a shell under mechanical loading. Diagrams, presently valid for 316 austenitic steel, have been established from a ring model with perfect plasticity. Creep buckling load is determined applying a reduction factor to Euler instantaneous buckling load, depending on temperature, hold time, thinness of the structure and geometrical imperfection amplitude. This method has been validated by experimental tests and finite element results. It will be included in Appendix A7 of RCC-MR, Edition 2000. In Russia, the document PNAE G-7-002-86 applicable to NPP equipment and pipeline strength analysis, presents stability check analytical calculations to be performed to determine the allowable loading or allowable operation lifetime for typical geometries (cylindrical shells, dished ends) and loadings (external pressure, axial force). In the case of stability analysis under creep, creep deformation is assessed using a Norton law. In Germany, a KTA project including an analytical method for creep buckling analysis had also been proposed at the beginning of 90th to be used in HTR development. Finally, in India, a creep buckling analysis method has been proposed in the framework of PFBR project. As per this approach, elastic-plastic analysis should be performed replacing the instantaneous stress-strain curve at the design temperature by the isochronous curve for the time corresponding to the lifetime of the component and the same temperature. These methods are applied in the case of cylindrical shells under external pressure and comparative results are provided. The RCC-MR method appears to be reasonably conservative and applicable with several creep law types.