PBX材料宏细观断裂行为的数字散斑相关法实验研究

 采用半圆盘弯曲实验和数字散斑相关方法,对高聚物粘结炸药（PBX）的宏、细观断裂行为进行了实验研究。宏观上,带有预制裂纹的半圆盘试样发生拉伸破坏,利用数字散斑相关技术得到了试样的应变场和位移矢量场分布,定量分析了试样全场的变形特征,并测得了PBX材料的平面应变断裂韧性;细观上,用配有加载装置的扫描电子显微镜对含预制裂纹的半圆盘试样间接拉伸下的损伤演化和破坏过程进行了实时原位观察,借助于数字散斑相关方法,定量分析了试样损伤局部化特征。结果表明,将数字散斑相关方法用于研究PBX材料宏、细观尺度上的变形破坏问题是有效的。     

Strain mapping analysis of textile composites

The focus of the work is meso-scale analysis (scale level of the fabric unit cell) of textile composite deformation and failure. The surface strain measurement is used for: (1) experimental investigation, which includes study of strain distribution at various stages of deformation, plasticity detection, damage initiation; (2) numerical validation of the correspondent finite element (FE) models. Two examples are considered: carbon-epoxy triaxial-braided and glass polypropylene-woven composite. The surface strain measurement (by digital image correlation technique) accompanies the tensile tests, aiming at: (1) elastic anisotropic constants characterisation, (2) study of non-linear material behaviour (for the thermoplastic composite), (3) control of homogeneity of the macro-strain distribution, and (4) analysis of damage initiation in brittle composites. Validation of meso-FE models by strain measurements encounters difficulties arising from (1) resolution of the strain measurements, (2) irregularities of the initial structure such as random layer nesting, ply interaction, and deviation of yarns from their theoretical position, which affects the measured strain fields. The paper discusses these difficulties and demonstrates a qualitative agreement with the FE analysis of idealised composite configurations.     

Incremental calculation for large deformation measurement using reliability-guided digital image correlation

Conventional digital image correlation (DIC) technique using a fixed reference image provides high-accuracy measurements but normally fails when serious decorrelation effect occurs in the deformed images due to large deformation, serious illumination fluctuations or other reasons. In this paper, an incremental reliability-guided digital image correlation (RG-DIC) technique, by combining the recently developed RG-DIC technique and an automatic reference image updating scheme, is proposed for large deformation measurement. In the incremental RG-DIC technique, a seed point is defined in the original reference image and searched in the deformed images, if the estimated correlation coefficient is larger than a preset threshold, which means no serious decorrelation effect exists in the deformed image, the RG-DIC technique is used to continue correlation analysis to obtain full-field displacements. Otherwise, the image recorded just before the current deformed image is chosen as an updated reference image to proceed with correlation analysis. Afterwards, the incremental displacements extracted by comparing the current deformed image and the updated reference image can be cumulated to determine the overall deformation. The effectiveness of the proposed technique is demonstrated by retrieving the full-field deformation of a foam sample subjected to large compressive deformation.     

Optical measurement of the plastic zone size in a notched metal specimen subjected to low-cycle fatigue

This paper describes an optical method used for the determination of the plastic zone size generated on the surface of a notched metal specimen under conditions of low-cycle fatigue. The proposed optical method uses white-light scattering generated by plastically induced surface roughness on the specimen polished surface. By digital processing two images of the specimen surface, one recorded before and the other after the generation of the fatigue deformation, it is possible to evaluate the accumulated plastic damage. The plastic zone measured in the crack initiation stage is compared with the results obtained by a recrystallization technique.     

Digital speckle pattern interferometric (DSPI) and thermo-graphic investigations on the thermal responds in human teeth

Human dentine is a composite material made up of constituents of varying thermal properties, which when subjected to changes in temperature experiences complex thermal effects. In this study, the response of functionally adapted dentine to temperature changes in physiological range is analyzed using a digital speckle pattern interferometer (DSPI) and thermography in conjunction with an advanced digital fringe processing technique. The advanced digital fringe processing is conducted using an asymmetric and central peak Gaussian filter, in order to remove noise and, subsequently, enhance the quality of the images. This investigation demonstrates the responds of functionally adapted dentine to temperature changes in its own plane and out of plane. Distinct pattern of out-of-plane and in-plane displacement was observed on the dentine by the DSPI analysis. The thermo-graphic analysis was used to rationalize the above nature of deformation in the dentine. These analyses showed that the inner, cervical dentine exhibited conspicuous deformation at the maximum temperature rise and equilibrated last during temperature drop.     

基于单幅数字散斑投影及图像相关的离面振动测量

采用单幅数字散斑图投影及高速数字图像采集技术,研究了动态离面位移的测量。采用商用液晶投影仪将计算机产生的模拟散斑图投影到待测动态变形物体表面,由高速数字图像采集设备摄取并保存变形散斑图,采用时间序列数字图像相关软件计算出物体表面各点随时间变化的离面位移。这种方法用于振动分析时不仅可以获得振型分布,而且还可以获得各点的振幅值。与现有的激光频闪照相测振及激光多普勒测振等方法相比,具有光学系统简单,可全场定量测量。悬臂梁振动实验结果证实了该方法的有效性。     

基于散斑干涉的光滑表面变形快速检测

针对目前光滑表面力学性能测试困难的情况,建立了一种改进的数字激光散斑干涉测量系统。首先通过新的散斑干涉光路设计实现散斑照射,同时采用空间载波傅里叶变换法,对光滑零件加载变形的动态散斑干涉图像进行处理,最后得到光滑表面的变形场分布情况。该方法不对被测表面进行任何处理,可实现光滑表面的高精度全场变形测量。实验结果表明:最大变形处为镜面板的中央,测得最大变形量分别为1.936、1.861和1.797μm,与中心变形预设值接近。该方法光路简单、测量方案切实可行,能够实现光滑表面变形的快速动态测量。     

数字散斑技术在混凝土单轴压缩试验中的应用

数字散斑技术作为一种新兴的测量方法,它具有非接触、全场量测、精度高、操作简单等特点,随着计算机技术的迅猛发展,数字散斑技术得到了广泛的应用与研究。针对混凝土在外荷载作用下具有明显的非均匀变化的特征,借助传统的测量方法很难得到混凝土表面的全场变形结果的问题,结合数字散斑相关方法测量技术,开展了混凝土单轴压缩试验,拍摄了混凝土表面在单轴压缩过程中的位移与应变矢量场,为分析混凝土试件的损伤区域奠定了基础;与电测法测量结果进行了对比,其测量结果表明,这两种测量方法误差相对较小,且数字散斑技术能更早的发现混凝土表面的破坏,验证了数字散斑相关方法的准确性和可靠性。借助数字散斑技术可以更加方便准确的得到材料表面的变形场,这也将在一定程度上促进土木工程领域的发展,具有广阔的应用前景。     

Dynamic deformation image de-blurring and image processing for digital imaging correlation measurement

This paper proposes a method for de-blurring of images captured in the dynamic deformation of materials. De-blurring is achieved based on the dynamic-based approach, which is used to estimate the Point Spread Function (PSF) during the camera exposure window. The deconvolution process involving iterative matrix calculations of pixels, is then performed on the GPU to decrease the time cost. Compared to the Gauss method and the Lucy–Richardson method, it has the best result of the image restoration. The proposed method has been evaluated by using the Hopkinson bar loading system. In comparison to the blurry image, the proposed method has successfully restored the image. It is also demonstrated from image processing applications that the de-blurring method can improve the accuracy and the stability of the digital imaging correlation measurement.     

Experimental study on the micromechanical behavior of a PBX simulant using SEM and digital image correlation method

The micro-scale mechanical behavior of a polymer-bonded explosive (PBX) simulant was experimentally studied using a scanning electron microscope (SEM) imaging system and digital image correlation (DIC) method. The semi-circular bend (SCB) test was chosen for the study. During the testing, a series of SEM images of the specimen was acquired in situ. The natural micro-structural features of the specimen were used as random speckle pattern for DIC analysis. The displacement and strain fields at the area of interest were obtained by DIC. The deformation and damage of PBX were analyzed. Heterogeneous strain fields demonstrated the damage evolution underneath the specimen surface and predicted possible micro-crack growth. Based on the contour plots of the correlation coefficient, the formation and extension of microscopic cracks were quantitatively analyzed.     

The use of a consumer grade photo camera in optical-digital correlator for pattern recognition and input scene restoration

In this work an optical-digital correlator for pattern recognition and input scene restoration is described. Main features of the described correlator are portability and ability of multi-element input scenes processing. The correlator consists of a consumer grade digital photo camera with a diffractive optical element (DOE) inserted as a correlation filter. Correlation of an input scene with a reference image recorded on the DOE are provided optically and registered by the digital photo camera for further processing. Using obtained correlation signals and DOE’s point spread function (PSF), one can restore the image of the input scene from the image of correlation signals by digital deconvolution algorithms.The construction of the correlator based on the consumer grade digital photo camera is presented. The software procedure that is necessary for images linearization of correlation signals is described. Experimental results on optical correlation are compared with numerical simulation. The results of images restoration from conventionally and specially processed correlation signals are reported. Quantitative estimations of accuracy of correlation signals as well as restored images of the input scene are presented.     

应用光测技术对热载荷下金属表面裂纹扩展的研究

材料在加工中由于夹渣、孔洞、缝隙等原因会造成材料内部的非均匀性,在机械或热载荷下内部的缺陷将通过变形表现在外表面,即使这种变形量十分微小,数量级小于0．1μm,一般测量仪器无法测出,但对于精密微电子器件（MEMS）、精密机械等,这种缺陷是致命的。针对上述问题设计了一套专门检测材料在热载荷、机械载荷或两者耦合情况下变形的系统,并应用该系统对金属材料（LY12）表面裂纹在热载荷场下的变形进行了测量,得到了裂纹扩展的变形矢量图。该系统为一套集显微观测、数字图像处理、加载及照明等关键技术为一体的非接触、全场测量,结果可数字化输出的系统。     

Application of aluminum foil for “strain sensing” at fatigue damage evaluation of carbon fiber composite

Surface layer of a loaded solid is an individual structural level of deformation that was shown numerously within concept of physical mesomechanics.This gives rise to advance in its deformation development under loading as well as allows using this phenomenon to sense the strain induced structure changes.It is of specific importance for composite materials since they are highly heterogeneous while estimating their mechanical state is a topical applied problem.Fatigue tests of carbon fiber composite specimens were carried out for cyclic deformation estimation with the use of strain sensors made of thin(80μm)aluminum foil glued to the specimen’s surface.The surface images were captured by DSLR camera mounted onto an optical microscope.Strain relief to form during cyclic loading was numerically estimated using different parameters:dispersion,mean square error,universal image quality index,fractal dimension and energy of Fourier spectrum.The results are discussed in view of deformation mismatch in thin foil and bulk specimen and are offered to be applied for the development of Structural Health Monitoring(SHM)approach.     

Experimental and theoretical study of multiscale damage-failure transition in very high cycle fatigue

Multiscale mechanisms of failure of metals (Armco iron, titanium, aluminum) are studied for high cycle and very high cycle fatigue. By correlating with the results of structural studies, a theoretical approach is developed to describe fatigue crack kinetics in damaged material under high cycle and very high cycle fatigue loading conditions. Stages of crack nucleation and propagation are analyzed using the profilometry data from the fracture surface. The scale invariance of fracture surface roughness is established, which allows an explanation of the self-similar nature of fatigue crack kinetics under high cycle and very high cycle fatigue. Variation of elastic-plastic properties of Armco iron under very high cycle fatigue is studied using an acoustic resonance method. It is found that the material density decreases during fatigue damage accumulation, with the minimum of the material density in the bulk of the specimen.     

Single-step scanner-based digital image correlation (SB-DIC) method for large deformation mapping in rubber

The incremental digital image correlation (DIC) method has been applied in the past to determine strain in large deformation materials like rubber. This method is, however, prone to cumulative errors since the total displacement is determined by combining the displacements in numerous stages of the deformation. In this work, a method of mapping large strains in rubber using DIC in a single-step without the need for a series of deformation images is proposed. The reference subsets were deformed using deformation factors obtained from the fitted mean stress-axial stretch ratio curve obtained experimentally and the theoretical Poisson function. The deformed reference subsets were then correlated with the deformed image after loading. The recently developed scanner-based digital image correlation (SB-DIC) method was applied on dumbbell rubber specimens to obtain the in-plane displacement fields up to 350% axial strain. Comparison of the mean axial strains determined from the single-step SB-DIC method with those from the incremental SB-DIC method showed an average difference of 4.7%. Two rectangular rubber specimens containing circular and square holes were deformed and analysed using the proposed method. The resultant strain maps from the single-step SB-DIC method were compared with the results of finite element modeling (FEM). The comparison shows that the proposed single-step SB-DIC method can be used to map the strain distribution accurately in large deformation materials like rubber at much shorter time compared to the incremental DIC method.     

Dental fiber-reinforced composite analysis using optical coherence tomography

An optical coherence tomography (OCT) system with 6 μm spatial resolution was employed to image the sites of fracture initiation and slow crack propagation in a fiber reinforced composite. Bar specimens (2 mm × 3 mm × 25 mm) of fiber reinforced composite were mechanically and thermally cycled to emulate oral conditions. The interior of the samples were analyzed prior and after the emulations. We analyzed the specimens that were intact after the loading cycling. The results demonstrated the capacity of the OCT technique to generate images of the sites fracture initiation, crack propagation, and regions surrounding the fracture, and it can be used in the future for quantitative analysis thus complementing other existing methods, with the main advantage of being non-destructive and non-invasive.     

数字图像相关中基于位移场局部最小二乘拟合的全场应变测量

为了从含噪声的位移场中计算得到可靠的应变场,提出一种基于位移场局部最小二乘拟合的全场应变求解方法。介绍了数字图像相关方法的原理,阐述了基于位移场局部最小二乘拟合的全场应变求解方法,并讨论了计算区域边界、孔洞及裂纹附近区域等情况下的应变计算。对均匀变形和中心带圆孔的薄铝板拉伸实验的计算结果表明,该方法能有效地从原始位移场数据中提取全场应变信息。在均匀变形情况下应选择大的应变计算窗口,计算结果更逼近真值;在非均匀变形情况下,如果位移场中包含较强的噪声,则应选择较大的应变计算窗口,而位移场精度很高时可选择更小的应变计算窗口。     

Large deformation measurement scheme for 3D digital image correlation method

Difficulties often arise for digital image correlation (DIC) technique when serious de-correlation occurs between the reference image and the deformed image due to large deformation. An updating reference image scheme could be employed to deal with large deformation situation, however that will introduce accumulated errors. A large deformation measurement scheme, combining improved coarse search method and updating reference image scheme, is proposed in this paper. For a series of deformation images, the correlation calculation begins with a seed point and spreads out. An improved coarse search method is developed to calculate the initial correlation parameters for the seed point, which guarantees that the correlation calculation can be carried out successfully even in large deformation situation. Only for extremely large deformation, the reference image is updated. Using this method, not only extremely large deformation can be measured successfully but also the accumulated error could be controlled. A polymer material tensile test and a foam compression test are used to verify the proposed scheme. Experimental results show that up to 450% tensile deformation and 83% compression deformation can be measured successfully.     

Initial damage induced by thermal residual stress and microscopic failure analysis of carbon-fiber reinforced composite under shear loading

In order to study the mechanical properties and the progressive failure process of composite under shear loading, a representative volume element (RVE) of fiber random distribution was established, with two dominant damage mechanisms – matrix plastic deformation and interfacial debonding – included in the simulation by the extended Drucker–Prager model and cohesive zone model, respectively. Also, a temperature-dependent RVE has been set up to analyze the influence of thermal residual stress. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the in-plane shear fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation. The progressive damage process and final failure mode of in-plane shear model which are based on constitute are very consistent with the observed result under scanning electron microscopy of V-notched rail shear test. Also, a transverse shear model was established as contrast in order to comprehensively understand the mechanical properties of composite materials under shear loading, and the progressive damage process and final failure mode of composite under transverse shear loading were researched. Thermal residual stress changes the damage initiation locations and damage evolution path and causes significant decreases in the strength and fracture strain.