粉煤灰掺量对应变硬化水泥基复合材料力学性能及损伤特征的影响

通过单轴拉伸、三点弯曲和抗压强度试验，研究粉煤灰掺量对应变硬化水泥基复合材料（SHCC）力学性能和损伤特征的影响，并利用数字图像相关技术对整个拉伸加载过程进行分析，提出利用开裂面积和分形维数量化SHCC试件的损伤程度，得到SHCC在加载过程中的应变演化和损伤特征.结果表明：随着粉煤灰掺量的增加，SHCC的拉伸强度、抗压强度及抗弯强度均逐渐降低，耗能能力逐渐提高，变形性能得到改善，但损伤程度增加；通过数字图像相关技术获得的应变云图可以直观观测SHCC的应变演化；开裂面积比和分形维数能够有效表征SHCC的损伤程度及开裂复杂性，可作为描述其损伤的度量指标.     

应变硬化垃圾堆体抗局部沉陷研究

通过三种不同材料堆体的局部沉陷模型试验发现局部沉陷条件下应变硬化材料可能产生"自支撑"现象。利用模型试验和退化情况下的解析解验证了数值分析模型模拟应变硬化材料堆体局部沉陷问题的有效性。在此基础上比较了采用不同垃圾应力应变模型的分析结果,考虑堆体材料应变硬化的复合指数模型和线弹性模型计算得到的沉陷区土压力明显小于与之对应的摩尔-库伦模型计算结果,采用传统摩尔-库伦模型计算得到的沉陷区衬垫系统挠曲变形和应变值偏大,因而针对砂土材料的Giroud(1990)Trapdoor土拱效应理论应用于垃圾堆体局部沉陷分析时存在一定的局限性。进一步分析了各种参数对衬垫系统表面土压力和变形的影响,发现沉陷区衬垫系统最大挠度和最大应变随垃圾堆体高度的增加而增加,随垃圾堆体模量和加筋体刚度的增加而减小。最后提出应变硬化垃圾堆体在局部沉陷条件下衬垫系统土工膜的应变计算和加筋层的设计方法,对于垃圾填埋场衬垫系统抗局部沉陷设计具有一定的指导意义。     

设计PVA纤维水泥基复合材料的研究进展

本文综述了设计聚乙烯醇纤维水泥基复合材料(Polyvinyl Alcohol—Engineered Cementitious Composite，简称PVA—ECC)Ig前在国际上的研究进展。文中简要介绍了ECC的概念及其基本性能．详细介绍了具有超高韧性和应变-硬化特性的PVA—ECC的理论研究情况．特别是基干性能设计的ECC的微观机理和结构与材料相结合的综合设计方法(ISMD)的概念。并介绍了PVA—ECC和构件的界面特性试验、弯曲试验、拉／压周期、抗剪试验所取得的成果。在此基础上提出了一些需要进一步研究的问题。     

New formula relating the yield stress-strain with the strength coefficient and the strain-hardening exponent

The main purpose of this paper is to search formulas for different metals that relate the yield stress-strain with the strength coefficient and the strain-hardening exponent. For this purpose, the test data of nine alloys were used as basic data and the applicability of Hollomon’s equation at the yield point of the alloy was studied. This paper explores new equations relating the yield stress-strain with the strength coefficient and the strain-hardening exponent. At the same time, the study introduces a new fracture-ductility parameter. The new fracture-ductility parameter not only describes the applicability of the new equations to these alloys, but may also be better at describing the hardening behavior of a metallic material.     

Effects of strain hardening on quasi-static fracture in elasto-plastic solid represented by modified yield strip model

Matching the near tip stress and strain fields, resulting from the continuum mechanics models of the crack with the corresponding fields postulated within the small process region adjacent to the leading edge of the crack, allows one to gain a better understanding of the interrelations between the microstructure and the macroscopic response of a material undergoing extensive deformation and fracture process. In this work, attention is focused on the influence of the strain-hardening phenomenon of the Ramberg-Osgood type on the material fracture response. In particular, through detailed analysis of the relations pertinent to the onset of stable cracking process and to the attainment of the steady-state limit which corresponds to a fully developed energy absorption mechanism that a material is capable to provide, it has been found that even minimal amount of strain-hardening has a significant effect on the predicted value of the JM _ss / J_c ratio. This ratio defines the margin of safety when dealing with fracture associated with a nonlinear elasto-plastic deformation process. The end results of this study can be applied to small scale yielding range and moderate amount of strain-hardening. Plane stress is usually implied, but for nonmetallic materials the division between plane stress and plane strain conditions is blurred (as exemplified by the localized craze which usually precedes fracture in polymer specimens regardless their width). This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of High-Performance Steel on the Tension Stiffening and Cracking Behavior of Reinforced Concrete (RC) Tension Ties

In construction industry, the application of high-performance reinforcement bar is required strongly. Unfortunately, not nearly enough research has been conducted on high-performance steel in comparison with high strength concrete. This paper describes the effect of high-performance steel as reinforcement steel bar on the tension response and cracking behavior of concrete and fiber-reinforced strain-hardening cement-based composite (SHCC) tension members. High-performance steel is characterized by higher strength in comparison to ASTM A615-06 Grade 60 steel. The tension stiffening effect on high-performance reinforcing bars embedded in cement-based composite prism is investigated experimentally. The variables in the study are types of cement-based composite (conventional concrete, synthetic fiber-reinforced cement composite), yielding strength of steel bars (400MPa and 600MPa), and types of loading (monotonic and repeated tension loading).     

固废应变硬化机理与超重力模拟适用性

为了探究城市固废应变硬化机理及模型固废在超重力模拟试验中的适用性,选用塑料和草炭、石英砂和高岭土等材料,分别对固废有机纤维和土颗粒的各项特性进行试验研究. 结果表明,有机质纤维对固废的应变硬化特性起主导作用,可以显著地提高固废强度;土颗粒中石英砂质量分数增加可以提高固废在小应变时的强度,高岭土质量分数增大会导致固废破坏应变及偏应力峰值明显减小. 基于各组分作用,提出定量控制有机纤维质量分数的方法配制模型固废,配制出的模型固废与真实固废的各项特性非常吻合. 采用模型固废强度参数计算得到的填埋场失稳临界水位比与离心模拟试验结果一致,验证了所配固废的超重力模拟适用性,为填埋场变形与稳定超重力研究提供了重要依据.     

考虑变形硬化的管道塑性弯曲屈曲临界应变预测

为了正确评估管道弯曲屈曲的临界应变,在考虑变形硬化对管道塑性弯曲的影响的基础上,对基于截面椭圆化模型的管道塑性弯曲屈曲的临界应变评估方法进行改进,提出一个考虑变形硬化的管道塑性弯曲屈曲的临界应变预测方法。其中,管道材料的应变硬化属于Hollomon类型。此外,使用文献中的测试数据对所提出的表达式进行了检验。结果表明,所提出的预测表达式能够合理地揭示应变硬化行为对管道塑性弯曲时临界屈曲应变的影响,为预防管道弯曲屈曲导致的基于强度的管道设计失效提供帮助。     

TiNi合金的耐磨机制及其摩擦学应用研究

TiNi合金是一种新型的性能优异的摩擦学材料,其摩擦学特性主要与相变超弹性有关,快速应变硬化、耐腐蚀及抗疲劳等特性也对合金的耐磨性能有很大帮助.本研究从相变超弹性产生机制、产生条件及影响因素等方面揭示了其优异摩擦学特性的原因,介绍了提高和充分发挥相变超弹性的措施,回顾了超弹TiNi合金在摩擦学方面的应用研究进展,提出了若干发展方向及亟待解决的问题.