3004铝合金“反常”锯齿屈服现象的研究

在应变速率为5.56×10^-5s^-1—5.56×10^-3s^-1的范围内,在不同温度下(从223K至773K),对3004铝合金进行系列拉伸试验,探索其锯齿屈服规律;通过激活能的计算、内耗研究、微观组织观察和能谱分析,探讨锯齿屈服的机理与物理本质.结果表明,3004铝合金在形变过程中会出现动态应变时效现象;发现了一种“反常”的锯齿屈服现象：在出现锯齿屈服的温区内,存在锯齿屈服临界应变量转变温度T_t,关键词：动态应变时效锯齿屈服铝合金内耗     

流变岩体中让压支护作用下隧道力学行为研究

高地应力深埋软岩隧道大变形问题已成为隧道工程建设领域的突出难题. 根据高地应力深埋软岩隧道的变形特征, 基于\"围岩能量吸收、变形释放\"的让压支护是解决软岩隧道大变形问题的有效方法. 针对流变岩体中深埋圆形隧道在让压支护作用下的力学响应问题, 通过引入分数阶微积分理论, 采用Abel黏壶元件建立了改进的分数阶Burgers蠕变模型来表征围岩的时效变形. 此外, 通过在让压支护不同变形阶段引入刚度修正系数, 克服了传统支护未能考虑围岩变形释放的问题. 据此, 本文推导了在考虑支护延迟安装影响下, 不同变形阶段围岩与让压支护相互作用的解析解. 为了验证理论研究的正确性, 对一算例进行了不同解答及工程结果的比对, 吻合较好. 最后, 参数研究结果表明: 围岩与让压支护间的相互作用受蠕变本构模型分数阶阶数影响较大. 隧道的位移或支护压力与让压位移、支护刚度修正系数间存在线性比例关系, 但由于刚度修正系数仅保持在较小的变化范围内, 隧道的位移或支护压力变化并不显著.      

一维六方准晶压电材料反平面Ⅲ型裂纹电塑性分析

论文基于一维六方准晶压电材料反平面问题的基本方程,对Ⅲ型裂纹的电塑性区进行分析.采用条带模型并假设在电塑性区的切应力保持为常数,得到了电塑性区大小的表达式.这种假设方式消除了电场和应力在裂纹尖端的奇异性,这与实际情况相符合,也为一维六方准晶压电材料的断裂分析提供了理论基础.     

高阶奇异项及J积分表达式

从４种典型裂纹情况的特征展开式出发,运用其微分性质和伪正交性质得到了含有高阶奇异项的Ｊ积分表达式．它们是通用的显函数表征的．本研究支持了Ｈｕｉ和Ｒｕｉｎａ的杰出研究和重要观点,即高阶奇异项和非奇异项一样在小范围屈服描述中扮演着重要角色．研究表明,常规柔度方法确定的Ｊ积分已隐含了高阶奇异项的贡献．断裂准则有可能由含高阶奇异项和非奇异项的Ｊ积分的临界值来描述     

沉淀对A1-Cu—Mg合金中锯齿形屈服现象影响及其微观实验研究

经固溶处理的A1-Cu—Mg合金在常应变率拉伸实验中具有显著的锯齿形屈服现象,且屈服行为随固溶处理温度的改变而呈现不同的特征。塑性变形特性与合金材料的微细观结构,尤其是位错运动的演化密切相关。本文运用透射电子显微镜,研究在不同温度下固溶处理的Al—Cu—Mg合金的微观结构,尤其是析出颗粒的大小和含量。并结合宏观的拉伸实验结果,分析Al—Cu—Mg合金动态应变时效的机制。     

国产航空有机玻璃(PMMA)的压缩应力-应变行为研究——Ⅱ.力学性能特征参数分析

讨论从加卸载循环的应力-应变表征曲线提取得到的各材料力学性能,尤其是关于屈服和塑性变形的特征参数。包括加卸载弹性模量E_L~T,E_u~T;屈服点的表征参数:屈服应力σ_s~T,屈服应变ε_s~N,屈服应变能U_s;热激活塑性变形的表征参数:活化体积V_τ和活化能ΔH。讨论这些参数对温度和应变率的依赖关系,寻求由这些特征参数作表述,或是存在于这些参数之间的经验关系或规律。     

Evolution of nanometer voids in polycarbonate under mechanical stress and thermal expansion using positron spectroscopy

Positron Annihilation Lifetime Spectroscopy (PALS) measurements were conducted on polycarbonate subjected to either thermal expansion or to tensile and compressive strains. It was found that thermal expansion affected both the nanometer hole size and the hole number density, whereas mechanical stress affected mainly the size of existing holes, and did not generate or eliminate holes in the quasielastic deformation region. The effect of stress on yield and postyield behavior of this glassy material was also investigated. The deduced hole volume fraction of this polymer at 25°C was 6.8 ± 0.5% from the thermal expansion experiment and 7.2 ± 1.2% from the mechanical loading experiment. When the specimen was under compression, the hole volume fraction was found to continuously decrease. This can be considered as evidence of the inability of the free volume concept in explaining the yield behavior of glassy polymers. ©1995 John Wiley & Sons, Inc.     

基于五参数统一强度理论的刚构桥地震损伤分析

混凝土五参数统一强度理论在主应力空间偏平面上的边界线为十二边形,可通过调整中间主应力的影响系数消除角点的奇异性,以单位体积材料当前塑性耗能及其断裂能的比值定义损伤变量,根据相关流动准则推导混凝土的弹塑性损伤本构关系.该模型可同时考虑混凝土材料中间主应力的影响、高应力状态下的静水应力效应、拉压异性和随动强化效应等,不考虑循环荷载下的刚度折减.由图形回归法给出应力更新的数值方法,编写了相应的Fortran语言程序,并将其用于大跨度连续刚构桥地震损伤的模拟,计算结果验证了所用模型的预测能力.     

TOUGHENING OF POLYCARBONATE WITH PBA-PMMA CORE-SHELL PARTICLES

The miscibility, mechanical properties, morphology and toughening mechanism of PC/PBA-PMMA blends were investigated. The dynamic mechanical results show that PC/PBA-PMMA blend has good miscibility and strong interfacial adhesion. The Izod impact strength of blend PC/PBA-PMMA with 4% (volume fraction) PBA-PMMA core-shell modifier is 16 times higher than that of pure PC. The core-shell volume fraction and thickness of the PMMA shell have effect on the toughness of PC/PBA-PMMA blends. As PMMA volume fraction increases, the toughness of PC/PBA-PMMA blend increases, and reaches a maximum value at 30% volume fraction of PMMA or so. The tensile properties of PC/PBA-PMMA blend with a minimum amount of PBA-PMMA modifier show that brittle-tough transition has no significant variance in comparison with that of pure PC. The scanning electron microscopic (SEM) observation indicates that the toughening mechanism of the blend with the pseudo-ductile matrix modified by small core-shell latex polymer particles is the synergetic effect of cavitation and shear yielding of the matrix.