流变作用下露采岩质边坡和采空区的稳定性分析

针对井下开采转露天开采现状,分析了岩体材料蠕变力学特性对采空区和边坡稳定性的响应。采用改进的西原蠕变模型,对边坡安全系数的求解进行了推导,然后通过工程算例进行了数值计算,研究了采空区安全顶板的厚度和采空区边坡稳定性的影响。计算结果表明:蠕变效应下,采空区安全顶板的厚高比要比工程经验规定的值偏大,且采空区边坡垂直位移的变化对其稳定性有显著的影响,拉剪塑性区主要分布在边坡的中上部和深部采空区,并不是在矿体中,算例验证了该模型的计算是可靠的,为采空区边坡的稳定性分析提供了参考。     

盐岩蠕变模型的建立及其在地下天然气储气库中的应用

本文以地下储气库盐岩单轴蠕变试验为例,对盐岩的蠕变特性进行了分析,建立了盐岩的蠕变模型;通过最小二乘法原理回归得出了盐岩蠕变模型中的各个参数。利用该模型对地下天然气储气库进行了数值计算分析,结果表明模型可以很好的反映盐岩的蠕变规律且可用来预测盐岩天然气储气库的长期稳定性。     

玻纤增强阻燃PBT长期弯曲蠕变行为预测

采用万能电子拉力机测试了不同应力下玻纤增强阻燃PBT(PBT-RG301)的短期蠕变数据,并采用时间应力等效原理、Burgers模型以及Findley指数定律预测了长期蠕变行为.结果发现:依据时间应力等效原理可预测10000h后体系的蠕变数值,在4000s实验时间内Burgers模型和Findley指数定律均可很好的拟合实验结果,但Burgers模型预测的长期蠕变数据高于Findley指数定律和时间应力等效原理预测数值,实验时间14h的跟踪数据也证实了该结果.     

桂林市南洲大桥组合梁徐变效应分析

钢—混凝土组合梁是由两种性质不同的材料结合所组成的一个整体,由于混凝土的徐变受到钢梁的约束,必然会使组合梁截面产生应力重分布及内力重分布,这种重分布会对组合梁的长期变形产生影响。本文采用有限元软件MIDAS对桂林市南洲大桥组合梁进行了徐变计算。徐变对组合梁应力的影响随时间的增长逐渐递增,但徐变影响速度逐渐减慢;钢梁上缘...     

Experimental investigation and material modelling of fresh and UV aged Japanese lacquer (Urushi)

Urushi is a complex natural polymer that has been used to protect and decorate objects for many hundreds of years. It is an important material as decorated objects can obtain great value and historical worth. These objects are often exposed to environments that are detrimental to both their aesthetic appeal and structural performance and restoration and conservation procedures are needed to preserve these objects over long periods of time. The conservation work requires a detailed understanding of the material properties of the Urushi lacquer film. However, Urushi exhibits complex viscoelastic behaviour under load that has not been fully characterised to date. This paper presents the sample preparation technique and experimental data from a comprehensive mechanical testing programme for Urushi film. The viscoelastic response was investigated by tests at various loading speeds and creep and recovery tests. A number of constitutive models were fitted to the creep and recovery data and a good fit was seen with a number of these, most notably a modified generalized Kelvin fluid (MGKF) model. Some samples of Urushi were artificially aged by laboratory exposure to ultraviolet (UV) irradiation before mechanical testing. The ageing increased strength and reduced the ductility of the Urushi and it was shown that the MGKF model was capable of modelling the ageing behaviour by using ageing-time dependent material parameters. The models were implemented in the commercial FEA software ABAQUS, offering the potential to accurately model the Urushi behaviour in a complex structure.     

Influence of grain boundary sliding on diffusion in yttria doped zirconia

Grain boundary sliding during high temperature deformation can lead to stress concentrations and an enhancement of diffusion in mobile boundaries. Experiments were conducted on a fine grained 3 mol% yttria stabilized tetragonal zirconia, under conditions associated with superplastic flow involving grain boundary sliding. Tracer diffusion studies under creep conditions and without load indicate that there is no enhancement in either the lattice or grain boundary diffusivities. The experimental creep data are consistent with an interface controlled diffusion creep mechanism.     

Creep behaviour of porous SOFC electrodes: Measurement and application to Ni-8YSZ cermets

A methodology is proposed in this study to investigate the creep properties of porous Ni-8YSZ cermet. Creep experiments have been conducted under reducing atmosphere at the typical SOFC operating temperatures. Specimens have been loaded in a four-point bending test bench. A special attention has been paid in this work to the analytical and numerical modelling of the mechanical test. It has been highlighted that Ni-8YSZ exhibits substantial creep strain rates even at relative low temperatures (700 °C < T < 850 °C). The creep exponent has been found to be just slightly higher than unity (1 < n < 2) while the activation energy has been determined equal to Q = 115 kJ mol⁻¹.High-temperature plastic strains of both Ni and 8YSZ phases have been estimated through the local stress acting on the cermet particles. This analysis indicates that creep behaviour of the Ni-8YSZ composite is not influenced by the metallic phase, but is controlled by the deformation of the 8YSZ matrix. It is also proposed that cermet creep mechanism involves Zr⁴⁺cations diffusion at the surface rather than in the bulk of the 8YSZ material.Impact of the Ni-8YSZ cermet creep on the internal stresses distribution in SOFC is discussed considering the anode supported cell (ASC) design. It is shown that cermet creep strain can induce a substantial stress decrease in the thin electrolyte.     

Creep behavior and related structural defects in Al2O3-Ln2O3 (ZrO2) directionally solidified eutectics (Ln = Gd, Er, Y)

The eutectic architecture of “in situ” composites prepared by solidification from the melt in the Al₂O₃-Ln₂O₃ (ZrO₂) systems gives rise to materials with a high creep resistance. With the objective to elucidate the high temperature deformation micro-mechanisms, microstructural features are investigated on crept specimens. Compressive creep experiments have been carried out between 1400 and 1550 °C for various eutectic compositions. Different deformation regimes depending on considered systems and conditions of stress and temperature are revealed. Transmission electron microscopy studies emphasize the activation of different slip systems in the alumina phase and the deformation by dislocation climb processes controlled by bulk diffusion.     

Refractory concretes uniaxial compression behaviour under high temperature testing conditions

Two refractory concretes with the same matrix composition, one based on andalusite aggregate and the other on bauxite aggregate, have been studied at temperatures ranging from room temperature to 1200 °C maximal temperature. Samples dried at 110 °C during 24 h to create dried standard specimens and others, fired at different temperatures, were subjected to several uniaxial compression test conditions conducted at various temperatures. The evolution of the materials’ global behaviour from quasi-brittle to viscous was evidenced and correlated to their microstructure evolution. Loading/unloading, creep, and strain rate jump tests helped define the damageable and viscoplastic nature of the behaviour. The influence of the firing temperature and duration on the materials’ behaviour is also reported and discussed. In conclusion, potential constitutive equations that are able to fit the material recorded behaviour are suggested.     

Multi-scale modeling of time-dependent response of smart sandwich constructions

Polymer and polymer based composite structures exhibit time-dependent response, leading to their being described as viscoelastic bodies. The rate of creep (or stress relaxation) in viscoelastic bodies increases with increasing the temperature of the bodies. In this study, we are interested in analyzing the time-dependent response of smart sandwich composites comprising of glass fiber reinforced polymer (GFRP) skins, polyurethane foam core, and lead zirconate titanate (PZT) wafers embedded in the GFRP skins. The PZT is used to monitor lifetime performance of sandwich composites. A multi-scale model is developed to integrate different constitutive models of the constituents in the sandwich structures. Quasi-static and creep tests are conducted for bulk epoxy, GFRP, polyurethane foam, and sandwich specimens under uniaxial tension and bending. The tests were done at room temperature and at 80 °C. The experimental data are used for material characterization and model verification. The multi-scale model that is developed can be used to understand the effect of different responses of the constituents on the overall time-dependent behavior of sandwich structures and examine the feasibility of using PZT wafers for monitoring lifetime performance of sandwich structures.