A nonlinear threshold model applied to spallation analysis

Spallation in heterogeneous media is a complex, dynamic process. Generally speaking, the spallation process is relevant to multiple scales and the diversity and coupling of physics at different scales present two fundamental difficulties for spallation modeling and simulation. More importantly, these difficulties can be greatly enhanced by the disordered heterogeneity on multi-scales. In this paper, a driven nonlinear threshold model for damage evolution in heterogeneous materials is presented and a trans-scale formulation of damage evolution is obtained. The damage evolution in spallation is analyzed with the formulation. Scaling of the formulation reveals that some dimensionless numbers govern the whole process of deformation and damage evolution. The effects of heterogeneity in terms of Weibull modulus on damage evolution in spallation process are also investigated.     

Evolving internal length scales in plastic strain localization for granular materials

A numerical model in the Cosserat continuum for strain localization phenomena in granular materials is developed and proposed in this paper. The model assumes a constant internal length scale that is used to describe the shear band thickness. However, it is observed that the internal length scales need to change to accommodate the possible change in the contact surface between the particles, damage of the particles or/and any change in the local void ratio within the domain, which will change the shear band thickness. The mathematical formulations used in the present numerical model were equipped with evolution equations for the length scales through the Micropolar theory, those formulations are proposed and discussed in this paper. The evolution equations of the internal length scales describe any possible change in the contact surface between the particles, damage of the particles if exists and/or any change in the local void ratio within the domain. Hence, the strain localization described by the enhanced model with evolving internal length scales is more accurate and closer to the real solution. The solution for the shear bands thickness shows more accurate correlation with the experimental results and less dependency on the mesh size when such evolution equations are used. Moreover, the shear band thickness and inclination evolve during the deformation process.     

The onset and progression of damage in isotropic paper sheets

An experimental investigation is performed and analyzed in order to examine the onset and evolution of damage processes in thin isotropic paper sheets made of mechanical pulp. A microscopy technique has been used to estimate the relative fraction of bond and fibre breaks. It has been found that the active damage mechanism is bond failure, hence supporting the assumption of an isotropic scalar valued damage variable.All experiments have been performed by simultaneous with the mechanical loading monitoring the acoustic emission activity. Three different experimental setups have been designed offering the possibility to analyze the influence of stress gradients, as well as different levels of the ratios between the in-plane normal stresses, on the onset of damage. It is concluded that stress gradients in the paper specimens have a large influence on the onset of damage. When stress gradients are present a non-local theory has to be used in the analysis. In this way compliance with an isotropic damage criterion is achieved. The characteristic length, determining the gradient sensitivity, has been found to be of the same order of magnitude as some average fibre length.To study the evolution of the damage processes, wide and short specimens have been loaded in tension resulting in stable damage processes. With the assumptions made regarding the mechanical behavior of the paper material after onset of damage, the damage and the cumulative number of acoustic events curve correlates very well. The experimentally obtained data is used to determine material parameters in a proposed damage evolution law. It is found that the assumed damage evolution law can, for isotropic paper materials with bond rupture as the prevalent failure mechanism, be further simplified as only one specific material dependent damage evolution parameter has to be determined in experiments.     

固体的统计细观力学——-连接多个耦合的时空尺度

从固体力学所面临的新的挑战------多物理、多尺度耦合及其现状的描述开始, 以层裂 过程为例, 说明了这些多尺度非平衡问题的基本困难在于, 在固体中不同尺度上有不同的微 结构层次及不同的演化物理和速率. 接下来, 概述了一些针对这一困难的独特的思路及 其成果. 第3部分强调了一些统计平均方法的范式, 以及处理包含多个时间和空间尺度的问 题的新思路, 特别是非平衡损伤演化导致宏观失效的问题. 在第4部分, 简要评述了一些连 接多个空间和时间尺度的细观力学框架, 如位错理论, 物理细观力学, Weibull理论, 随机 理论等, 并且阐述了其中蕴含的跨尺度耦合的机理. 然后, 在第5部分, 回到了描述损 伤演化过程的框架, 也就是统计细观损伤力学以及它的跨尺度封闭近似. 基于这些跨尺度框 架, 在第6部分, 对控制跨尺度耦合的可能机理进行了评述和比较. 由于对失效时灾变 的洞察与跨尺度强耦合紧密相关, 一些非平衡和强相互作用的新概念在第7部分进行了讨 论. 最后, 以一个简短的总结和一些建议结束.     

Comparing Two Damage Models Under Shear Stress

Recently, a new macro-mechanical model for ductile damage was presented by Wierzbicki, Xue et al., trying to address the role assumed by Lode angle and pressure sensitivity. The new model requires several experimental tests to identify all its parameters, but should outperform the “standard” formulations at high triaxiality levels. In this work a comparison between the Lemaitre’s Continuous Damage Mechanics model and the Wierzbicki’s one is proposed. After calibration of both models, the displacement field measured using the Digital Image Correlation technique in a large shear test case is compared with results of Finite Element simulations obtained using the two damage models considered. Results are not conclusive but show that the new damage model is quite accurateas rupture criterion, but damage evolution history deviates to some extent from the experimentally observed behavior.     

Modeling of damage in unidirectional ceramic matrix composites and multi-scale experimental validation on third generation SiC/SiC minicomposites

The purpose of this paper is to experimentally validate a 1D probabilistic model of damage evolution in unidirectional SiC/SiC composites. The key point of this approach lies in the identification and validation at both local and macroscopic scales. Thus, in addition to macroscopic tensile tests, the evolution of microscopic damage mechanisms – in the form of matrix cracks and fiber breaks – is experimentally analyzed and quantified through in-situ scanning electron microscope and computed tomography tensile tests. A complete model, including both matrix cracking and fiber breaking, is proposed on the basis of existing modeling tools separately addressing these mechanisms. It is based on matrix and fiber failure probability laws and a stress redistribution assumption in the vicinity of matrix cracks or fiber breaks. The identification of interfacial parameters is conducted to fit the experimental characterization, and shows that conventional assumptions of 1D probabilistic models can adequately describe matrix cracking at both macro- and microscopic scales. However, it is necessary to enrich them to get a proper prediction of ultimate failure and fiber break density for Hi-Nicalon type S fiber-reinforced SiC/SiC minicomposites.     

A study on the damage evaluation of weld heat affected zone

From the micro- and macroscopic points of view, the damage evolution of weld-simulated heat affected zone (HAZ) is studied. In the framework of continuum damage mechanics (CDM), the ductile and low cycle fatigue (LCF) damage evolution laws of HAZ have been examined. Two alternative laws of damage are proposed in this paper, which may meet the need for describing damage evolution of ductile rupture and LCF fracture, respectively.     

<Emphasis Type="Italic">In-situ</Emphasis> AFM Experiments with Discontinuous DIC Applied to Damage Identification in Biomaterials

Natural materials (e.g. nacre, bone, and spider silk) exhibit unique and outstanding mechanical properties. This performance is due to highly evolved hierarchical designs. Building a comprehensive understanding of the multi-scale mechanisms that enable this performance represents a critical step toward realizing strong and tough bio-inspired materials. This paper details a multi-scale experimental investigation into the toughening mechanisms in natural nacre. By applying extended digital image correlation and other image processing techniques, quantitative information is extracted from otherwise prodominantly qualitative experiments. In situ three point bending fracture tests are performed to identify and quantify the toughening mechanisms involved during the fracture of natural nacre across multiple length scales. At the macro and micro scales, fracture tests performed in situ with a macro lens and optical microscope enable observation of spreading of damage outward from the crack tip. This spreading is quantified using an iso-contour technique to assess material toughness. At the nanoscale, fracture tests are performed in situ an atomic force microscope to link the larger-scale damage spreading to sliding within the tablet-based microstructure. To quantify the magnitude of sliding and its distribution, images from the in situ AFM fracture tests are analyzed using new algorithms based on digital image correlation techniques which allow for discontinuous displacement fields. Ultimately, this comprehensive methodology provides a framework for broad experimental investigations into the failure mechanisms of bio- and bio-inspired materials.     

活性粉末混凝土抗多次侵彻实验研究及数值预测

活性粉末混凝土（reactive powder concrete,RPC）具有超高的强度和优异的阻裂性能。为了研究RPC在多次冲击荷载下的损伤规律,采用25 mm口径滑膛炮对直径为600 mm、高600 mm的RPC圆柱形靶体进行了多次侵彻实验,得到了每次侵彻后靶体的破坏数据,并根据实验数据确定了Forrestal经验公式中的相关系数。基于K&C本构模型和现有RPC基本力学性能的实验数据,修正了K&C模型的强度面参数、损伤参数、状态方程参数、损伤演化模型以及应变率效应相关参数,系统地确定了RPC的K&C模型参数。采用LS-DYNA软件中的重启动功能模拟了弹体多次侵彻RPC靶体的破坏结果,模拟结果与实验结果基本一致,验证了模拟方法的有效性。对长2 200 mm、宽2 200 mm、高1 260 mm的RPC靶体抗侵彻实验进行了数值预测,得到了侵彻深度与弹速之间的关系、弹体贯穿靶体时的极限速度以及弹体侵彻过程中的峰值加速度。     

Results of studying creep and long-term strength of metals at the Institute of Mechanics at the Lomonosov Moscow State University (To Yu. N. Rabotnov’s Anniversary)

Basic results of experimental and theoretical research of creep processes and long-term strength of metals obtained by researchers of the Institute of Mechanics at the Lomonosov Moscow State University are presented. These results further develop and refine the kinetic theory of creep and long-duration strength proposed by Yu. N. Rabotnov. Some problems arising in formulating various types of kinetic equations and describing experimental data for materials that can be considered as statically homogeneous materials (in studying the process of deformation and rupture of such materials, there is no need to study the evolution of individual cracks) are considered. The main specific features of metal creep models at constant and variable stresses, in uniaxial and complex stress states, and with allowance for one or two damage parameters are described. Criterial and kinetic approaches used to determine long-term strength under conditions of a complex stress state are considered. Methods of modeling the metal behavior in an aggressive medium are described. A possibility of using these models for solving engineering problems is demonstrated.     

聊聊动态强度和损伤演化

材料强度在传统上常理解为材料在外载荷下抵抗流动/变形和破断的能力。由流变阶段到貌似突发的破断,其实源于一个隐含的应变率/时间相关的损伤演化过程。动态损伤演化研究的难点在于损伤与流变总是耦合在一起发展的。研究发现,热激活损伤演化模型可成功描述材料宏观损伤的动态演化。在此基础上,从实测的含损伤演化的表观应力应变曲线,可将两者解耦分开,并可确定各自相关的材料参数。这一思路可推广到中医脉诊的客观化研究,通过脉搏波信息定量反演脉搏波系统的正常及病态本构关系,可诊断生命体偏离正常状态的程度(病情),这可视为一种广义的损伤演化和强度问题。上述思路还可推广到地震预报研究,即“对地球把脉”。与加卸载响应比理论相结合,通过相邻3处的地震波信息来反演地球相关板块含损伤演化的非线性载荷-响应曲线,再区分出损伤演化程度,将有利于改进地震预报,这可视为另一种广义的损伤演化和强度问题。     

Modelling multi-scale deformation of amorphous glassy polymers with experimentally motivated evolution of the microstructure

Novel experimental data, obtained recently using advanced multi-scale experiments, have been used to develop a micro-mechanically motivated constitutive model for amorphous glassy polymers. Taking advantage of the experiments, the model makes use of a microstructural deformation gradient to incorporate the experimentally obtained deformation of the microstructure, as well as its evolving orientation. By comparing results from the model to experimental data, it is shown that the proposed approach is able to accurately predict glassy polymer deformation over a wide range of length-scales, from the macroscopic response (mm range) down to the deformation of the microstructure (nm range). The proposed model is evaluated by comparing the numerical response to experimental results on multiple scales from an inhomogeneous cold drawing experiment of glassy polycarbonate. Besides the macroscopic force–displacement response, a qualitative comparison of the deformation field at the surface of the specimen is performed. Furthermore, the predicted evolution of the fabric orientation is compared to experimental results obtained from X-ray scattering experiments. The model shows very good agreement with the experimental data over a wide range of length scales.     

QUANTIFICATION OF SHEAR DAMAGE EVOLUTION IN ALUMINIUM ALLOY 2024T3

Shear damage may occur in the process of metal machining such as blanking and cutting, where localized shear deformation is developed. Experimental findings indicate that microscopic shear damage evolution in aluminium alloy 2024T3 (A1 2024T3) is a multi-stage mechanism, including particle cracking, micro-shear banding, matrix microcracking and coalescence of microcracks. This study is an attempt to use a set of equations to describe the multi-stage shear damage evolution in Al 2024T3. The shear damage variables in terms of multi-couple parameters of a power-law hardening material have been defined. An evolution curve of shearing damage has been calculated from experimental data. The values of the shear damage variable at different stages of damage have also been calculated. By making use of the findings, the relation between the microscopic shear damage evolution and the macroscopic shear response of the material has been discussed.     

A local viewpoint for evaluating the influence of stress triaxiality and Lode angle on ductile failure and hardening

All damage and failure models, describing either the evolution of microvoids, the development of shear bands or local rupture, rely on the knowledge of the hardening function at large plastic strains which, then, becomes an essential prerequisite for any failure prediction.     

金属蠕变与蠕变破坏理论综述

本文对金属在均匀单向与复杂应力状态下的蠕变与蠕变破坏行为及结构物蠕变下持久性发展了系统的试验与理论研究。 在试验研究中本文考虑了各种金属在恒应力与变应力下的行为,应力集中,加载方式对蠕变破坏的影响;同时还提出了材料的结构损伤的新的量测方法。     

岩石变形演化诱致灾变破坏过程的同步实验观测

将试样变形场演化特征与试样的宏观载荷位移曲线的演化特征结合起来研究是揭示非均匀脆性介质变形演化诱致灾变破坏的一个重要途径.本文发展了一套实验系统,通过对试样表面变形场的演化、宏观载荷和位移信号的同步观测,对单轴加载下岩石试样变形演化和灾变破坏的过程进行了实验研究.揭示了试样变形场由加载初期的随机涨落到灾变破坏前出现明显的变形局部化的演化特征现象,试样最终在变形局部化区内形成宏观破裂面.     

桥梁结构劣化与损伤过程的多尺度分析方法及其应用

研究桥梁结构多尺度模拟和分析方法对于发展重大桥梁结构损伤检测与状态评估方法以确保其安全运营是至关重要的.本文综述和总结了作者及其课题组多年来在桥梁结构劣化与损伤过程分析领域的研究工作.包括:系统地提出了针对大型桥梁结构损伤劣化过程分析需求的结构多尺度问题,研究证实了结构损伤与劣化过程分析可以通过结构一致多尺度模拟和时空多尺度分析来实现;提出了针对大型桥梁结构多尺度模型的多因素、多目标同步模型修正技术,指出多尺度模型修正必须对各分析目标敏感的多种因素同步优化,同时交叉验证修正后模型对该级修正目标和对其它模拟目标的复现与预测能力.提出了基于结构一致多尺度模拟和结构健康监测信息进行结构劣化分析的方法并应用于两个重大桥梁工程结构的疲劳损伤分析与评估.最后,指出在此领域中还要很多问题亟待解决,尤其是如何模拟损伤从材料与结构中的细观层次的缺陷演化发展到结构局部乃至整体失效的过程,建立结构损伤演化与失效过程的跨尺度分析理论与方法.     

Relationship between strain localization and catastrophic rupture

In order to explore a prior warning to catastrophic rupture of heterogeneous media, like rocks, the present study investigates the relationship between surface strain localization and catastrophic rupture. Instrumented observations on the evolution of surface strain field and the catastrophic rupture of a rock under uniaxial compression were carried out. It is found that the evolution of surface strain field displays two phases: at the early stage, the strain field keeps nearly uniform with weak fluctuations increasing slowly; but at the stage prior to catastrophic rupture, a certain accelerating localization develops and a localized zone emerges. Based on the measurements, an analysis was performed with local mean-field approximation. More importantly, it is found that the scale of localized zone is closely related to the catastrophic rupture strain and the rupture strain can be calculated in accord with the local-mean-field model satisfactorily. This provides a possible clue to the forecast of catastrophic rupture.     

CHARACTERISTIC DIMENSIONLESS NUMBERS IN MULTl-SCALE AND RATE-DEPENDENT PROCESSES

Multi-scale modeling of materials properties and chemical processes has drawn great attention from science and engineering. For these multi-scale and rate-dependent processes, how to characterize their trans-scale for-mulation is a key point. Three questions should be addressed:How do multi-sizes affect the problems?How are length scales coupled with time scales?How to identify emergence of new structure in process and its effect?For this sake, the macroscopic equations of mechanics and the kinetic equations of the microstructural transforma-tions should form a unified set that be solved simultaneously.As a case study of coupling length and time scales, the trans-scale formulation of wave-induced damage evolution due to mesoscopic nucleation and growth is discussed. In this problem, the trans-scaling could be reduced to two inde-pendent dimensionless numbers: the imposed Deborah number De=(ac)/(LV) and the intrinsic Deborah num-ber D = (nNc5)/V* ,where a. L, c, V and nN are wave speed, sample size, micr     

复合材料层合板低速冲击损伤分析的连续介质损伤力学模型

针对复合材料层合板低速冲击损伤问题,提出了一种各向异性材料连续介质损伤力学模型,模型涵盖损伤表征、损伤起始判定和损伤演化法则3 个方面. 通过材料断裂面坐标下的损伤状态变量矩阵完成损伤表征,并考虑断裂面角度的影响,建立了主轴坐标系下的材料损伤本构关系. 损伤起始由卜克(Puck) 失效准则预测,损伤演化由断裂面上的等效应变控制,服从基于材料应变能释放的线性软化行为. 模型区分了纤维损伤和基体损伤,并根据冲击载荷下层内产生多条基体裂纹继而扩展至界面形成层间裂纹(分层) 的试验观察,引入基体裂纹饱和密度参数表征层间分层. 以[0₃/45/-45]_S 和[45/0/-45/90]_4S 两种铺层的复合材料层合板为例,预测了不同冲击能量下复合材料层合板的低速冲击损伤响应参数,试验结果证明了连续介质损伤力学模型的有效性.模型在不同网格密度下的计算结果表明单元特征长度的引入可以在一定程度上降低损伤演化阶段对网格密度的依赖性.