Quasi-static tensile deformation and fracture behavior of a highly particle-filled composite using digital image correlation method

Polymer bonded explosives (PBXs) are highly particle-filled composite materials. This paper experimentally studies the tensile deformation and fracture behavior of a PBX simulation by using the semi-circular bending (SCB) test. The deformation and fracture process of a pre-notched SCB sample with a random speckle pattern is recorded by a charge coupled device camera. The displacement and strain fields on the observed surface during the loading process are obtained by using the digital image correlation method. The crack opening displacement is calculated from the displacement fields, the initiation and propagation of the crack are analyzed. In addition, the damage evolution and fracture mechanisms of the SCB sample are analyzed according to the strain fields and the correlation coefficient fields at different loading steps.     

Experimental analysis of crack tip fields in rubber materials under large deformation

A three-nested-deformation model is proposed to describe crack-tip fields in rubber-like materials with large deformation.The model is inspired by the distribution of the measured in-plane and out-of-plane deformation.The inplane displacement of crack-tip fields under both Mode I and mixed-mode(Mode I-II) fracture conditions is measured by using the digital Moire’ method.The deformation characteristics and experimental sector division mode are investigated by comparing the measured displacement fields under different fracture modes.The out-of-plane displacement field near the crack tip is measured using the three-dimensional digital speckle correlation method.     

基于数字图像相关性方法的脆性材料拉氏反分析实验技术

利用分离式Hopkinson压杆作为加载系统,借助超高速数字图像相关性分析方法,发展了长杆试件拉氏反分析实验技术,并用于研究脆性材料在小变形条件下的动态本构特性。通过超高速相机实时拍摄冲击加载下长杆试件变形的散斑图像,再对散斑图像进行数字图像相关性（digital image correlation,DIC）分析,获得长杆试件表面速度场和应变场。随后,以脆性材料PMMA（polymethyl methacrylate）为例,从DIC分析得到的速度场中提取出不同拉格朗日位置上质点速度时程曲线,构建路径线连接整个速度场,再结合零初始条件,数值求解得到了试件中的应力时程曲线,消去时间参数后,获得了脆性材料PMMA的动态应力-应变曲线,并与Hopkinson压杆实验和准静态压缩实验的结果进行了对比,揭示了PMMA材料在小应变条件下的黏弹性本构响应特征。     

基于显微CT图像的近场动力学建模与复合材料微结构破坏模拟

随着纤维增强复合材料应用领域的不断扩展且用量激增,亟需理清复合材料微观结构损伤对宏观力学性能影响的内在机制。因此,发展针对纤维增强复合材料微结构破坏过程的建模与高效模拟方法就显得十分重要。本文借助显微CT(Micro-computed Tomography)扫描技术,提出了一种基于显微CT图像中像素点离散的近场动力学建模与模拟方法。一方面,近场动力学作为一种由积分方程建模的非局部理论,便于采用基于空间点离散的数值计算方法,相比传统的连续介质力学能够更有效地模拟材料从连续变形到裂纹萌生与扩展(非连续变形)的全过程。另一方面,对显微CT图像使用像素点灰度阈值分割处理技术,能够快速建立含有复合材料原位微结构信息的空间点离散模型。该离散模型可以直接用于微结构破坏过程的近场动力学模拟,从而避免了传统的数值模拟技术需要依据像素点先建立光滑的几何模型、再划分成有限单元网格的复杂前处理过程,并且极大地保留了复合材料的原位组分分布信息。数值模拟结果表明,基于显微CT图像的近场动力学建模方法能够精确捕捉到复合材料微结构信息,并能准确模拟纤维增强复合材料的微结构破坏过程。     

Analysis of a multiaxial test on a C/C composite by using digital image correlation and a damage model

The “planar” digital image correlation technique needs a single CCD camera to acquire the surface patterns of a zone of a specimen in the underformed and deformed states. With these two images, one can determine in-plane displacement and strain fields. The digital image correlation technique used herein is based on Fast Fourier Transforms, which are very effective in reducing the computation cost. Its performance is assessed and discussed on artificial signals and in a real experimental situation. The technique is utilized to analyze experimental results of a plane shear experiment and validate a damage meso-model describing different degradations in a C/C composite material.     

<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.     

韧性金属裂纹尖端损伤区内应变场测量——数字散斑相关法应用

本文用数字散斑相关方法测量了五种不同幂硬化指数韧性金属材料(铝和铜),双边裂纹尖端细观区域内应变场.对所得结果用韧性损伤模型进行了分析.在此法中以金属自然表面结构为散斑场,不同加载状态的散斑场进行比较,得到相对变形与应变.图象之间相关性 C 是变形参数或是位移及其导数的泛函.使其相关性 C 取最大值的试凑变形即为其真实变形场.这一方法在细观测量中应用得到满意的结果.     

Research on synchronous measurement technique of temperature and deformation fields using multispectral camera with bilateral telecentric lens

The hot-section parts easily occur the creep-fatigued interaction under the condition of mechanical-thermal coupled load during the period of service, which may lead to the damage of the parts, and therefore, the measurement and characterization of thermal-deformed fields of the parts are important to understand its damage process. Aiming at relevant demand, the bilateral telecentric-multispectral imaging system was established, the research of synchronous measurement technique of the temperature and deformation fields was developed. On the one hand, the measurement technology for surface temperature of the object was developed using the two-color images captured by the multispectral camera with bilateral telecentric lens and combined with colorimetric method. On the other hand, the 2D-DIC measurement technique of the multispectral camera was developed by conducting digital image correlation analysis using the blue light images before and after deformation, which can measure the high temperature deformation field of the object (the blue light images were filtered by multispectral camera). Results showed that the bilateral telecentric lens is used to replace the ordinary optical lens for imaging, which can effectively eliminate the distortion of the multispectral imaging system. Since the temperature measurement process of this measurement system is little affected by the emissivity of the object, therefore, it has excellent robustness. The thermal expansion coefficients of the nickel alloys are evaluated at the temperature ranges of 700–1000 ℃, indicating this system can achieve the synchronous and precise measurement of the temperature and deformation fields of the object.     

Modeling the effects of material non-linearity using moving window micromechanics

In the analysis of materials with random heterogeneous microstructure the assumption is often made that material behavior can be represented by homogenized or effective properties. While this assumption yields accurate results for the bulk behavior of composite materials, it ignores the effects of the random microstructure. The spatial variations in these microstructures can focus, initiate and propagate localized non-linear behavior, subsequent damage and failure. In previous work a computational method, moving window micromechanics (MW), was used to capture microstructural detail and characterize the variability of the local and global elastic response. Digital images of material microstructure described the microstructure and a local micromechanical analysis was used to generate spatially varying material property fields. The strengths of this approach are that the material property fields can be consistently developed from digital images of real microstructures, they are easy to import into finite element models (FE) using regular grids, and their statistical characterizations can provide the basis for simulations further characterizing stochastic response. In this work, the moving window micromechanics technique was used to generate material property fields characterizing the non-linear behavior of random materials under plastic yielding; specifically yield stress and hardening slope, post yield. The complete set of material property fields were input into FE models of uniaxial loading. Global stress strain curves from the FE–MW model were compared to a more traditional micromechanics model, the generalized method of cells. Local plastic strain and local stress fields were produced which correlate well to the microstructure. The FE–MW method qualitatively captures the inelastic behavior, based on a non-linear flow rule, of the sample continuous fiber composites in transverse uniaxial loading.     

Study of a Crack at a Fastener Hole by Digital Image Correlation

In this work the efficacy of using digital image correlation to determine stress intensity factors for a crack emanating from a fastener hole has been investigated. To this end a fatigue crack was grown in pure mode I from a 50 mm diameter hole in an Al 7010 alloy plate test-piece. This crack was then loaded elastically under several combinations of mixed mode (I + II) displacements. In each case, images of the sample surface before and after the deformation were recorded using a high resolution digital camera. The surface preparation consisted only of scratching the surface lightly with silicon carbide abrasive paper. The crack location and resulting displacements were then calculated using digital image correlation. The analytical displacement fields for a traction free crack under arbitrary loading conditions based on the Muskhelishvili’s complex function approach were fitted to the experimentally measured displacement fields and the mixed mode stress intensity factor was determined in each case. Good agreement with the nominal applied values was obtained. The uncertainty of the crack tip position has a major influence on the accuracy of the stress intensity factors and so the Sobel edge finding filter was successfully applied to experimental displacement fields to establish precisely the crack tip location. This paper was presented at the 2007 SEM Congress held in Springfield, Massachusetts, USA     

On strain and damage interactions during tearing: 3D in situ measurements and simulations for a ductile alloy (AA2139-T3)

Strain and damage interactions during tearing of a ductile Al-alloy with high work hardening are assessed in situ and in 3D combining two recently developed experimental techniques, namely, synchrotron laminography and digital volume correlation. Digital volume correlation consists of registering 3D laminography images. Via simultaneous assessments of 3D strain and damage at a distance of 1-mm ahead of a notch root of a thin Compact Tension-like specimen, it is found that parallel crossing slant strained bands are active from the beginning of loading in a region where the crack will be slanted. These bands have an intermittent activity but are stable in space. Even at late stages of deformation strained bands can stop their activity highlighting the importance of plasticity on the failure process rather than damage softening. One void is followed over the loading history and seen to grow and orient along the slant strained band at very late stages of deformation. Void growth and strain are quantified. Gurson–Tvergaard–Needleman-type simulations using damage nucleation for shear, which is based on the Lode parameter, are performed and capture slant fracture but not the initial strain fields and in particular the experimentally found slant bands. The band formation and strain distribution inside and outside the bands are discussed further using plane strain simulations accounting for plastic material heterogeneity in soft zones.     

激光冲击喷丸与激光辐照LY12铝合金材料拉伸力学行为的实验研究

为了研究激光冲击喷丸与激光辐照处理后LY12铝合金材料的微尺度变形特点和失效机理,对经过不同激光功率密度处理后的LY12铝合金材料,在扫描电镜下进行了原位拉伸力学行为和失效机理研究。在扫描电子显微镜（SEM）下,得到各组试件的拉伸载荷曲线和不同载荷下的微观区域图像,并利用数字图像相关技术进行不同载荷下的微观区域全场变形分析,并对拉伸断口形貌也进行了分析。研究表明：激光处理的功率大小对LY12铝合金拉伸最大载荷有明显的影响,激光处理的交界处有较强的应变集中。     

Mechanically Regularized FE DIC for Heterogeneous Materials

In situ tensile tests in a scanning electron microscope (SEM) have been conducted on a 8-layer 5-harness satin carbon fibre and epoxy matrix composite to observe the first stages of damage at the scale of fibres and matrix. A speckle pattern based on a suspension of alumina particles was applied onto the surface of the specimen to facilitate the use of digital image correlation (DIC). Local and finite element (FE) DIC are compared on pictures acquired during the tensile tests, with and without a speckle pattern. FE DIC with mechanical regularization was found to be the only approach able to measure displacement fields at a fine enough resolution in both cases. This method, initially created for homogeneous materials, was then adapted to heterogeneous materials. First, a microstructure consistent mesh was created and used for correlation purposes. Second, the difference between the mechanical properties of the constituents is taken into account in the mechanical regularization. Last, the accuracy of the method is analysed. The adaptation presented herein was proved to be able to measure displacement fields in the matrix between fibres with an error of 10 nm (a fifth of a pixel) and to detect the initiation of the first damage mechanisms by means of the mechanical residuals.

     

竹材层合板弯曲破坏机理的实验研究

竹材层合板是为了满足一定的使用要求，由多层单向板按同一方向整齐排列、胶合、加热固化处理而成的复合结构体。由于竹材层合板的各向异性和非均匀性，它的力学行为十分复杂，特别是弯曲时不仅各个单层产生损伤破坏，而且伴随着层间胶接层的开裂。本文对竹材层合板的弯曲破坏特征进行实验研究；一系列包含变形信息的数字散斑图像被记录；采用数字散斑相关技术(DSCM)提取了三点弯曲变形过程中的面内位移场信息，分析了不同变形阶段的位移场；给出了不同弯曲变形阶段试件的应变场分布和应变集中现象；分析了中性轴在胶层开裂、层内纤维撕裂等复杂应力情况下的演化过程及规律。最终一些宏微观破坏机制被分析，例如基体开裂，界面损伤与撕裂，界面与裂纹作用等。这些研究结果将为竹材层合板的破坏机理等力学行为的研究和工程应用开发提供实验依据。     

含孔复合材料层合板静拉伸损伤演化的实验和表征方法

为了深入探究复合材料层合板结构的损伤机理和损伤演化,应用声发射技术和图像相关技术同步实时监测含孔碳纤维复合材料层合板试样在静拉伸过程中的损伤演化。实验结果表明,试样表面应变场呈现局部化特征。对应变集中带在加载方向的应变值进行了统计分析,获得了应变场的特征统计量(标准差)随加载的演化模型。层合板损伤时产生声发射信号的峰值频率大小能够有效区分复合材料的损伤模式,由此,建立了基于损伤模式累积声发射数的损伤演化模型。通过对应变场演化模型和声发射损伤演化模型的分析,可以将复合材料的损伤演化分为损伤初始阶段、损伤平稳扩展期、损伤严重阶段三个部分。统计分析结果表明:在损伤严重阶段,基于声发射事件数的各种损伤的损伤变量和局部应变场标准差快速增长,因此局部应变场统计标准差可以作为后期局部损伤严重程度的识别指标。     

应用微相关技术研究裂端弹塑性变形

测量裂端附近区域的弹塑性变形，对于研究材料的损伤或微裂纹的产生，获得材料早期的损伤信息，为预防宏观裂纹的产生及扩展具有十分重要的意义。本文用A3钢材料制作边缘带裂纹的显微拉伸试件，试件表面进行了金相处理。试件是用由计算机控制的显微拉伸台进行加载试验。在加载过程中，利用环境扫描电镜对裂端附近的显微结构进行原位观察以及扫描记录。应用数字散斑相关技术对扫描电子显微镜记录的显微结构图进行分析处理，求出裂端附近的弹塑性变形。借助扫描电子显微镜，不仅可以定性观察应力集中区微裂纹的萌生及扩展过程，还可以结合数字散斑相关技术，对微区内的弹塑性变形进行测试，是研究微区域内材料变形的一个有效手段。     

数字散斑相关方法与应用研究进展

数字散斑相关方法(digital speckle correlation method, DSCM)作为固体实验力学领域材料表面变形场测量的一种非接触式方法与其它技术相比具有一些独到的优点,其有效性已经在近20年中被众多的应用实例所证明.近年来该方法在理论上逐步完善,一些现代的数学理论和数学方法(例如:小波变换、遗传算法、神经网络等)逐渐被引入到该方法中;在实验中的各个技术环节上正在逐步的改进,其结果的精度逐步的提高,结果收敛的速度也逐渐的加快;其应用的领域正逐渐从常规材料的测试向一些新型材料测试、从宏观场逐渐向细微观尺度、从常规环境向比较恶劣的环境、从实验室测试逐步向工程现场应用、从静态准静态向动态准动态等方面发展.本文简要介绍了DSCM的基本原理和数学模型,在相关搜索的改进、相关系数的选择、灰度级的重建和图像识别方面的进展以及在材料力学性能测试,微尺度变形场测量等方面的应用.最后就这一实验测量技术的几个可能的发展趋势进行了预测.      

基于红外热成像的编织复合材料低速冲击和冲击后压缩试验研究

针对二维三轴编织复合材料（two-dimensional triaxially braided composite, 2DTBC）在低速冲击和冲击后压缩（compression after impact, CAI）载荷下的损伤失效机理,开展了2DTBC试样的不同能量低速冲击试验以及相应的CAI试验,并采用红外热像仪监测在低速冲击和CAI试验过程中的温升现象。通过C扫描表征了不同能量低速冲击后试样的分层损伤情况,讨论了试样背面温度场分布特性及其随冲击能量的演化规律;对比分析了2DTBC冲击后剩余压缩强度与冲击能量的对应关系,基于数字图像相关（digital image correlation, DIC）技术监测了CAI试验中的全局应变场,结合热成像、变形场和光学图像数据,阐明了不同能量冲击后2DTBC的压缩失效特性,讨论了基于红外热成像技术表征编织复合材料损伤失效行为的有效性。试验结果显示:编织复合材料低速冲击和CAI试验中的温度场分布图与编织几何构型有明显关联度;低速冲击试验的温升幅值随冲击能量的增加而快速上升,CAI试验的温升现象随着冲击能量的增加而减弱;分层面积随冲击能量的增大而增大,冲击后剩余压缩强度随冲击能量的增大而降低。研究结果表明:红外热成像技术能够很好地捕捉试样破坏瞬间释放断裂能所产生的温升现象,温度场图像相较于全局应变场能更好地捕捉破坏的起始位置和失效特征。     

非连续数字图像相关方法在裂纹重构中的应用

数字图像相关方法作为一种新的非接触式位移测量方法,在力学工程中有广泛的应用前景,然而受限于标准方法对图像变形的连续性要求,这种高效的测量方法在断裂力学领域的推广受到了限制. 为解决这一问题,提出采用引入子区分离数学模型,代替标准方法的连续模型,来对非连续区域进行精确识别和匹配的非连续数字图像相关方法. 研究子区被裂纹等非连续分割后原始像素点的位移情况,并引入裂纹张开向量用以表征被分割子区的主区和副区的位移关系;从而建立子区分离模型的数学表达式,并且为所提出的模型设计相应的图像相关算法;然后将所提出的非连续数字图像相关方法应用于重构平板拉伸试验开裂过程中图像的位移. 研究结果表明,相比于标准的数字图像相关方法,所提出的非连续数字图像相关方法解决了图像相关法在非连续区域失效的问题,提高了数字图像相关方法对位移测量的正确率,特别是能够准确重构裂纹面及附近的位移场,其测量精度能够达到亚像素级别.      

压电复合材料粘接界面断裂有限元模拟

根据数字化FRMM(Fix-Ratio Mix-Mode)断裂试验,得到了压电复合材料试件的断裂韧性和位移及应变场。本文在试验的基础上,通过非线性有限元软件ABAQUS及用户子程序UMAT进行了模拟分析,采用基于损伤力学的粘聚区模型(CZM)对压电复合材料界面的起裂和脱胶扩展进行了分析,并与VCCT方法进行了比较。计算得到的荷载位移曲线更接近于试验结果,但在裂纹扩展路径上的吻合需要对粘聚区法则进一步修正。通过进一步对CZM参数进行分析,表明界面粘结强度和界面刚度对计算结果的影响很大。研究结果表明,粘聚区模型可以很好地表征压电复合材料弱粘接界面脱胶断裂问题。