Three-dimensional Analysis of an In Situ Double-torsion Test by X-ray Computed Tomography and Digital Volume Correlation

The double-torsion (DT) test is commonly used to characterize the slow crack growth behavior of brittle materials. However, it relies on several mechanical hypotheses which still need to be deeply tackled. Measuring the full 3D displacement field experimentally through in situ experiments would improve the understanding of the mechanical behaviour. A dedicated experimental set-up was designed to perform in situ tests in a X-ray Computed Tomography (XCT) scanner on a porous brittle ceramic which is optically opaque. The analysis of the three-dimensional images with Digital Volume Correlation (DVC) allows a first study of the global kinematics and the stable crack growth propagation. This experimental and numerical methodology presents a strong potential for a future deep understanding of this complex mechanical test.     

A Parametric Study on the Influence of Internal Speckle Patterning for Digital Volume Correlation in X-Ray Tomography Applications

Experimental Techniques - Use of the Digital Volume Correlation (DVC) technique has grown steadily in the mechanics community as a way of quantifying internal mechanical response of complex...     

In-situ Analysis of Laminated Composite Materials by X-ray Micro-Computed Tomography and Digital Volume Correlation

The complex mechanical behaviour of composite materials, due to internal heterogeneity and multi-layered composition impose deeper studies. This paper presents an experimental investigation technique to perform volume kinematic measurements in composite materials. The association of X-ray micro-computed tomography acquisitions and Digital Volume Correlation (DVC) technique allows the measurement of displacements and deformations in the whole volume of composite specimen. To elaborate the latter, composite fibres and epoxy resin are associated with metallic particles to create contrast during X-ray acquisition. A specific in situ loading device is presented for three-point bending tests, which enables the visualization of transverse shear effects in composite structures.     

Digital Volume Correlation: Review of Progress and Challenges

3D imaging has become popular for analyzing material microstructures. When time lapse series of 3D pictures are acquired during a single experiment, it is possible to measure displacement fields via digital volume correlation (DVC), thereby leading to 4D results. Such 4D analyses have been performed for almost two decades. The present paper aims at reviewing the achievements of and challenges faced by such measurement technique. Ex-situ and in-situ experiments are discussed. A general and unified DVC framework is introduced. Various sources of measurement bias and uncertainties are analyzed. The current challenges are studied and some propositions are given to address them.     

New Development of Digital Volume Correlation for the Study of Fractured Materials

Experimental Mechanics - This study reports on Digital Volume Correlation and its limitation in the case of fracture mechanics. Due to its sensitivity, detecting the crack opening in sub pixel...     

An Error Criterion in Digital Image Correlation for Unknown Deformation Fields and Its Application of Parameters Selection

Digital image correlation (DIC) is a widely used optical metrology for surface deformation measurement. In DIC, the square root of the mean square error (RMS error) and standard deviation error (SD error) are used as quantitative criteria in order to evaluate the accuracy and robustness of a DIC method\algorithm. However, RMS and SD error criteria are computed from prescribed and measured displacements, which indicates that the prescribed displacement fields must be precisely generated. Therefore, it is difficult to quantitatively evaluate the accuracy and robustness of an algorithm\method in practical DIC measurements because imposed displacements are unknown (that’s why DIC measurements are needed). Moreover, the accuracy of DIC measurements highly relies on parameters selection, especially the selections of subset size and shape function. In practice, the subset size and shape function are usually selected according to experience because there are numerous factors (e.g. the quality of speckle image, local displacement field) and uncertainties (e.g. noise level, out-of-plane motion, illumination lighting fluctuation during image capturing) that affect the parameters selection, which makes it difficult to select optimal parameters based on previous works which mainly focused on theoretical deduction in ideal condition. In this paper, an error criterion for evaluating the accuracy of practical DIC measurements with unknown displacements is proposed. Numerical experiments are used to validate the effectiveness and feasibility of the proposed criterion for accuracy evaluation. It is concluded that the square root of the sum of squared forward and backward displacements difference (SFBD) error has a significant positive linear correlation with the widely used SD error in most practical DIC measurements where the mismatch between the frequently-used first- and second-order shape functions and the actual field is usually small. Also, an application of the proposed criterion is presented by real experiments for subset size and shape function selections, which verifies that the proposed error criterion can be effectively used for DIC parameters selection.     

Damage Quantification via Digital Volume Correlation with Heterogeneous Mechanical Regularization: Application to an In situ Meso-Flexural Test on Mortar

Experimental Mechanics - The mechanical response and damage mechanisms of heterogeneous cementitious materials result from the mechanical properties of their constituents at the mesoscale. In this...     

3D Digital Volume Correlation of Synchrotron Radiation Laminography Images of Ductile Crack Initiation: An Initial Feasibility Study

A feasibility study of measuring 3D displacement fields in the bulk during ductile crack initiation via combined Synchrotron Radiation Computed Laminography (SRCL) and Digital Volume Correlation (DVC) is performed. In contrast to tomography, SRCL is a technique that is particularly adapted to obtain three-dimensional (3D) reconstructed volumes of objects that are laterally extended (i.e., in 2 directions) and thin in the third direction, i.e. sheet-like objects. In-situ laminography data of an initiating crack ahead of a machined notch are used with a voxel size of 0.7 μm. The natural contrast of the observed 2XXX Al-alloy caused by intermetallic particles and initial porosity is used to measure displacement fields via a global DVC technique assuming a continuous displacement field. An initial performance study is carried out on data of the same undeformed material but after a substantial shift of the laminography rotation axis with respect to the imaged specimen. Volume correlations between different loading steps provide displacement fields that are qualitatively consistent with the remote loading conditions. Computed strain fields display a strain concentration close to the notch tip.     

数字图像相关方法中匹配及过匹配形函数的误差分析

基于图像子区的数字图像相关方法需采用合适的形函数来近似目标图像子区的真实变形。由于实际测量时目标子区的局部变形往往是未知的,实际采用的不同阶次(零阶、一阶和二阶)的形函数不可避免地产生误匹配(欠匹配和过匹配)问题,从而引入位移测量的系统或随机误差。尽管由欠匹配形函数引起的系统误差已被充分认识,由过匹配形函数引起的位移误差仍缺少理论解释。本文首先推导出采用一阶和二阶形函数的数字图像相关方法的随机误差理论公式,随后采用一系列数值实验验证了理论公式的准确性。结果显示:过匹配形函数不会引入额外的系统误差,但会增加随机误差,且二阶形函数的随机误差是一阶形函数的二倍。考虑到由欠匹配一阶形函数引入的系统误差往往远大于过匹配二阶形函数的随机误差,因此在未能确知变形的情况下,推荐使用二阶形函数。     

Assessment of Digital Image Correlation Measurement Accuracy in the Ultimate Error Regime: Improved Models of Systematic and Random Errors

The literature contains many studies on assessment of DIC uncertainties, particularly in the ultimate error regime, when the shape function used to describe the material transformation perfectly matches the actual transformation. For pure sub-pixel translations, bias and random errors obtained for experimental or synthetic images show more complex evolution versus the fractional part of displacement than those predicted by the existing theoretical models. Indeed, small deviations arise, mainly around integer values of imposed displacements for noisy images, and they are interpreted as the unrepresentativeness of the underlying hypotheses of the theoretical models. In a first step, differences between imposed and measured displacements are analysed: random error is independent of fractional displacement, and systematic error does not decrease for values close to integer displacements whatever the noise level. Therefore, new prediction models are proposed based on the analysis of identified phenomena from synthetic speckle-pattern 8-bit images. The statistical approach used in this paper generalizes the methods proposed in the literature and mimics the experimental methodology usually used for displacement measurements performed in different subsets in the same image. Two closed-form expressions for the systematic and random errors and a linear interpolation scheme are developed. These models, depending only on image properties and the imposed displacement, are built with a very limited number of parameters. It is then possible to predict the evolution of bias and random errors from one to four images.     

Identification of Material Parameters of PVC Foams using Digital Image Correlation and the Virtual Fields Method

This paper presents an effective methodology to characterize all the constitutive (elastic) parameters of an orthotropic polymeric foam material (Divinycell H100) in one single test using Digital Image Correlation (DIC) in combination with the Virtual Fields Method (VFM). A modified Arcan fixture is used to induce various loading conditions ranging from pure shear or axial loading in tension or compression to bidirectional loading. A numerical optimization study was performed with different loading angles of the Arcan test fixture and off-axis angles of the principal material axes. The objective is to identify the configuration that gives the minimum sensitivity to noise and missing data on the specimen edges, which are the two major issues when identifying the stiffness components from actual DIC measurements. Two optimized Arcan test configurations were chosen. The experimental results obtained for these two optimized test configurations show a significant improvement of the measurement accuracy compared with a pure shear load configuration. The larger sensitivity of the pure shear test to missing data as opposed to the tensile test is also evident from the experimental data and confirms the analysis from the optimization study. The recovery of missing data along the specimen edges is a promising way to further improve the identification results.     

数字散斑相关法在聚偏氟乙烯压电薄膜力学性能测试中的应用

利用数字散斑相关方法对PVDF透明压电薄膜进行了单轴拉伸力学性能研究。首先比较了喷漆与不喷漆两种条件下不同厚度薄膜的单轴拉伸曲线,发现当薄膜厚度超过一定值时,样品表面喷漆与否对测试结果无显著影响。然后,利用数字散斑相关方法对PVDF挤出薄膜进行了单轴拉伸下的应力-应变关系研究,得到了PVDF薄膜的应力-应变曲线和反映PVDF薄膜全场应变信息的三维信息图,以及线性阶段的弹性模量和泊松比。实验结果表明:数字散斑相关方法用于PVDF压电薄膜的力学性能研究是一种行之有效的方法。     

Correction to: A Novel Approach for the Determination of Surface Tilt Angles in Two-Dimensional Digital Image Correlation Experiments

Experimental Mechanics - The original article has been corrected on the line “These two parameters are the average…” and on Eqs. 19, 20, 21 and 23.     

Micrographic Digital Image Correlation Coupled with Microlithography: Case Study of Strain Localization and Subsequent Cracking at an FIB Notch Tip in a Laminated Ti-6Al-4V Alloy

This study presents a microlithography-based approach to increase the spatial resolution of strain mapping by micrographic digital image correlation. A micro-mesh with a lattice size of 500 nm was added on the surface of a Ti-6Al-4V alloy specimen with a coarse lath size of 1.1 μm. Although the micro-mesh pattern was not random, a combination of the laminated microstructure and the micro-mesh enabled sub-micrometer strain mapping through digital image correlation even for coarse lath larger than 1 μm. Specifically, the strain mapping technique used in this study was applied to characterize the strain component and distribution near an artificial sharp micro-stress concentration site introduced by a focused ion beam. The strain characterization under tensile deformation clarified that cracking occurred via shear strain localization at the micro-stress concentration site, indicating that accumulation of damage (such as vacancy or dislocation) plays an important role in the cracking mechanism of the Ti-6Al-4V alloy.     

On the Cover of this Issue: A Robust Finite Element-based Filter for Digital Image and Volume Correlation Displacement Data by T.H. Becker and T.J. Marrow

Experimental Mechanics -     

Quantitative Strain Analysis of the Large Deformation at the Scale of Microstructure: Comparison between Digital Image Correlation and Microgrid Techniques

A comparative study has been carried out to assess the accuracy of the Digital Image Correlation (DIC) technique for the quantification of large strains in the microstructure of an Interstitial Free (IF) steel used in automotive applications. A microgrid technique has been used in this study in order to validate independently the strain measurements obtained with DIC. Microgrids with a pitch of 5 microns were printed on the etched microstructure of the IF steel to measure the local in-plane strain distribution during a tensile test carried out in a Scanning Electron Microscope (SEM). The progressive deformation of the microstructure with microgrids has been recorded throughout the test as a sequence of micrographs and subsequently processed using DIC to quantify the distribution of local strain values. Strain maps obtained with the two techniques have been compared in order to assess the accuracy of the DIC measurements obtained using the natural patterns of the revealed microstructure in the SEM micrographs. The results obtained with the two techniques are qualitatively similar and thus, demonstrate the reliability of DIC applied to microstructures, even after large deformations in excess of 0.7. However, an average error of about 16?% was found in the strain values calculated using DIC.