Analysis of anisotropy and plastic spin effects on localization phenomena

Summary The main objective of the paper is the investigation of the influence of the anisotrophy and plastic spin effects on criteria for adiabatic shear band localization of plastic deformation. A theory of thermoplasticity is formulated within a framework of the rate-type covariance material structure with a finite set of internal state variables. The theory takes into consideration such effects as plastic non-normality, plastic-induced anisotropy (kinematic hardening), micro-damage mechanism, thermomechanical coupling and plastic spin. The next objective of the paper is to focus attention on cooperative phenomena in presence of the plastic spin, and the discussion on the influence of synergetic effects on localization criteria. A particular constitutive law for the plastic spin is assumed. The necessary condition for a localized plastic deformation region to be formed is obtained. This condition is accomplished by the assumption that some eigenvalues of the instantaneous adiabatic acoustic tensor vanish. A procedure has been developed which allows us to discuss two separate groups of effects on the localization phenomenon along a shear band. Plastic spin, spatial covariance and kinematic hardening effects are investigated at an isothermal process in an undamaged solid. In the second case, an adiabatic process in a damaged solid is discussed when the spatial covariance terms and the plastic spin are neglected. Here the thermomechanical coupling, micro-damage mechanism and kinematic hardening effects are examined. For both cases, the criteria for adiabatic shear band localization are obtained in an exact analytical form. Particular attention is focused on the analysis of the following effects: (i) plastic non-normality; (ii) plastic spin; (iii) covariant terms; (iv) plastic strain-induced anisotropy; (v) micro-damage mechanism; (vi) thermomechanical couplings. Cooperative phenomena are considered, and synergetic effects are investigated. A discussion of the influence of the plastic spin, kinematic hardening and covariant terms on the shear band localization conditions is presented. A numerical estimation of the effects discussed is given. Received 24 April 1997; accepted for publication 23 December 1997     

纸页受力微损伤演化过程的声发射及多元统计特征

纸页受载后的损伤断裂行为涉及其使用性能,对其损伤断裂过程的研究不仅有助于揭示纸页破坏行为的物理本质,而且有助于对纸页质量及其生产、加工工艺作出更加科学合理的评定。本文基于声发射(Acoustic Emission,AE)实验研究纸页内部微损伤演化过程,给出了评估纸页宏观损伤断裂性能的定量描述方法。首先,根据AE信号与微损伤事件之间存在本质上的关联性,通过采集和观测纸页试样受载过程中的AE信号来研究纸页损伤断裂的基本特征;其次,针对所采集AE信号的数据量大、蕴含信息丰富导致数据分析和处理困难的特点,本文采用多元随机变量的分析方法。利用概率熵和Andrews曲线法等统计分析手段对AE数据展开研究分析,并最终利用一条损伤状态轨迹曲线实现了对纸页微损伤演化过程的完整宏观描述。结果表明,纸页微损伤演化过程呈现明显阶段性变化特征;Andrews曲线的聚类结果有效地区分了不同损伤阶段的相似性和差异性,同时,不同损伤阶段中纸样的微观形貌观测提供了实验性的证据。本文以一种典型的牛皮箱板纸为试件,验证了上述方法的有效性。     

激光诱导击穿光谱(LIBS)法微损快速检测党参中Pb

中药材重金属元素快速检测对污染监控及人们健康具有重要意义。激光诱导击穿光谱技术(Laser Induced Breakdown Spectroscopy, LIBS)属于一种快速检测方法,研磨压片等预处理方法相对样品消解已有所简化,但破坏了样品的物理性质,且不能满足中药材大宗品种、大批量检测需求。若进一步简化样品预处理,将更加凸显LIBS快速检测的优势。本文建立了激光诱导击穿光谱技术(LIBS)快速微损检测中药材样品重金属元素定标方法。线性相关系数R2为0.7764,建立的微损定标曲线线性可用于切片党参LIBS快速检测,对待测党参切片样品检测平均相对误差为3.74%,与电感耦合等离子体质谱法(ICP-MS)对比,相关系数R2为0.7957,验证了LIBS技术微损检测的可行性。制备的党参参考定标样品可多次重复用于待测样定标和仪器标定等。实验对待测党参样品仅进行切片处理,避免了研磨、压片等预处理,更加充分地体现LIBS快速检测的优势,为LIBS技术应用于中药材重金属元素快检等领域提供了一种新方法。     

含裂纹结构可靠度的多尺度求解

针对环境作用下金属结构从微观损伤到宏观失效的复杂过程,选用具有局部细化特征的CAS小波,先对疲劳裂纹扩展方程进行离散处理,将微分方程转化为代数方程组,然后用完全积分可靠性分析模型,对一般环境中和引入环境系数的含裂纹结构的可靠度进行求解.最后,验证了利用CAS小波多尺度方法求解复杂环境中结构疲劳破坏过程是可行的,并能很好地解决短裂纹与长裂纹扩展的光滑过渡.     

高纯铜初始层裂的微损伤特性研究

本文对平面冲击加载下高纯铜初始层裂的微损伤特性进行了研究. 利用准三维的表面轮廓测试技术, 对冲击加载“软回收”的样品截面进行测试. 通过对测试数据的重构、量化和统计分析, 结果表明: 拉伸应力持续时间和加载应力幅值的增加, 都会加剧样品内部损伤局域化程度. 样品内损伤区域宽度是亚微米尺度的损伤演化的结果, 并且亚微米尺度的演化速率随着拉伸应变率的增加而单调递增. 通过统计获得了样品内微损伤的尺寸分布特征, 并分析了其与损伤演化进程的关联.     

THEORETICAL ANALYSIS ON THE NUCLEATION OF MICRO-DAMAGE IN CEMENT MORTAR UNDER COMPRESSIVE LOADING AND SULFATE ATTACK

Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated.Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techniques.The formation of delayed ettringite crystal is analyzed using scanning electron microscope and energy disperse spectrum methods.Deformation of micro-voids and the distribution of stress at the surface of a micro-void are calculated.It is found that the nucleation of micro-cracks is caused by the tensile stress at the voids’ surface,and such damage nucleation will be speeded up by the remote uniaxial compressive load.     

一种新型微孔洞损伤模型

 在前人的基础上引入单位体积微损伤个数概念,从而将宏观损伤参量与微观孔洞体积结合起来,并且在孔洞长大扩展时计入惯性效应的影响,得到了一个新的二次型微孔洞损伤演化方程,指出表面扩张效应和惯性效应分别在孔洞长大的不同阶段起主导作用,最后以921钢板撞击实验为参考,通过比较自由面速度时程曲线,得出的结果与实验相符,验证了本模型的正确性。     

PBX炸药的拉伸断裂损伤分析

采用巴西实验作为间接拉伸加载手段,研究了某PBX(polymer-bonded explosive)炸药试样拉伸作用下的断裂损伤特性.发展了PBX炸药的光学制备技术,获得了试样在光学显微镜下的细观表面形貌和断裂损伤形貌,结合高速摄影和数字相关分析技术获得了试样的形变和破坏过程.实验结果表明该PBX炸药的静态拉伸强度低于...     

Quantitative characterization of toughening mechanisms of rubber modified polymers

The volume changes of rubber modified polymers under creep at room temperature were successfully used to characterize the toughening mechanisms of blends with brittle polymer matrices such as high impact polystyrene.This approach cannot be applied to pseudo-ductile polymers such as polypropylene and polyamide,because they are ductile when stretched at low speed at room temperature.Based on the time-temperature equivalence princi ple,the volume change at low temperature is proposed to characterize quantitatively the toughening mechanisms of polymer blends with ductile matrices,which is illustrated by applying this approach to rubber modified polypropylene     

延性材料动态断裂的实验研究和数值分析

本文对延性材料纯铜和ＬＹ１２铝合金进行了层裂实验，对软回收试件进行了仔细的定量和显微金相观测，观察到了两种材料中不同微损伤的形核、增长和聚合机制，为理论分析提供了真实的实验结果。将文〔４〕提出的延性动态损伤模型编入了一维Ｌａｇｒａｎｇｅ有限差分动力学程序，对两组层裂实验进行了数值模拟，计算结果与实验吻合较好。