Energy analysis of stability of twin shallow tunnels based on nonlinear failure criterion

Based on nonlinear Mohr-Coulomb failure criterion, the analytical solutions of stability number and supporting force on twin shallow tunnels were derived using upper bound theorem of limit analysis. The optimized solutions were obtained by the technique of sequential quadratic programming. When nonlinear coefficient equals 1 and internal friction angle equals 0, the nonlinear Mohr-Coulomb failure criterion degenerates into linear failure criterion. The calculated results of stability number in this work were compared with previous results, and the agreement verifies the effectiveness of the present method. Under the condition of nonlinear Mohr-Coulomb failure criterion, the results show that the supporting force on twin shallow tunnels obviously increases when the nonlinear coefficient, burial depth, ground load or pore water pressure coefficients increase. When the clear distance is 0.5to 1.0 times the diameter of tunnel, the supporting force of twin shallow tunnels reaches its maximum value, which means that the tunnels are the easiest to collapse. While the clear distance increases to 3.5 times the diameter of tunnel, the calculation for twin shallow tunnels can be carried out by the method for independent single shallow tunnel. Therefore, 3.5 times the diameter of tunnel serves as a critical value to determine whether twin shallow tunnels influence each other. In designing twin shallow tunnels,appropriate clear distance value must be selected according to its change rules and actual topographic conditions, meanwhile, the influences of nonlinear failure criterion of soil materials and pore water must be completely considered. During the excavation process, supporting system should be intensified at the positions of larger burial depth or ground load to avoid collapses.     

连续SiC纤维增韧SiC基体复合材料研究进展

近年来,SiC纤维增韧SiC基体复合材料(SiC<,f>/SiC)由于具有良好的高温力学、抗化学腐蚀、高的韧性与抗中子辐照等优异性能而受到广泛关注.本文主要从纤维、界面层、基体与应用四方面评述了近年来国内外的研究进展.SiC纤维的性能直接影响了界面层材料与基体制备技术的选择.电泳沉积有望成为一种低成本、节能及对环境无污染的界向层及基体制备技术.在应用方面,作为热结构材料,SiC<,f>/SiC复合材料已经得到了实际应用.核反应堆用SiC<,f>/SiC结构材料的研究到了实际考核验证阶段.高性能SiC纤维的大规模生产是SiC<,f>/SiC广泛应用的前提条件.     

影响公路压实因素的一些体会

压实使路基及路面各结构层的材料具有足够的密实度,可以充分发挥路基土和路面材料的强度,可以减少路基、路面在行车荷载下产生的形变,可以增加路基土和路面材料的不透水性和强度稳定性。本文针对对公路施工的压实要求,从影响路基路面的压实因素来分析,在施工过程中如何保证路基路面的密实度是我们值得关注的问题。     

浸渗法制备SiCf/SiO2复合材料的力学性能研究

使用硅溶胶浸渍沉积和未沉积碳界面层的碳化硅纤维2.5D编织体,制备了SiCf/SiO2复合材料。抗弯测试结果显示,随着处理温度的升高,基体缓慢释放出的结合水对纤维造成氧化损伤,使试样的抗弯强度均下降,但始终表现出非脆性断裂行为,这主要是由于基体与纤维的热膨胀系数差别较大,使得基体与纤维之间未形成强结合。另外,由于碳界面层对纤维的保护使得纤维免遭氧化损伤,沉积过碳界面层的试样在各温度处理后的抗弯强度均高于未沉积过碳界面层的试样。     

化学气相渗透法制备复合材料的数值模拟研究进展

化学气相渗透法是一种比较成熟的制备连续纤维增韧陶瓷基复合材料的方法.从预制体结构模型、I-CVI过程模拟、TG-CVI过程模拟3方面总结了近年化学气相渗透(Chemical vapor infiltration,CVI)数值模拟的研究进展,重点评述了致密化过程的几种模型的假设条件、建立过程、与实验的吻合程度.     

5059铝镁合金TIG焊接接头组织性能研究

通过开展5059铝镁合金4 mm自动TIG焊对比试验，当热输入为13.8 kJ/cm时，焊缝组织为尺寸较大的等轴晶组织，晶内呈树支状生长，平均晶粒度约为2.56级，焊缝区域能谱（EDS）发现第二相含有7.78%Mg、9.20%Mn，接头的抗拉强度低于330 MPa。当降低热输入为12.5 kJ/cm时，通过调整焊接压力架工装的工作状态，保证焊接试板有效压紧，提高焊缝冷却速度，焊缝组织为尺寸较小的等轴晶组织，呈条状规则排列，平均晶粒度约为4.91级，焊缝区域能谱（EDS）发现第二相含有11.10%Mg、8.86%Mn，接头中形成含有Mg、Mn的金属化合物强化相，弥散地分布在铝基体中，且Mg、Mn元素含量较高，焊缝区显微硬度可达到基材显微硬度的90%以上，接头的抗拉强度满足规定值（大于330 MPa）。