Fracture Behaviour Investigation into a Polymer‐Bonded Explosive

Abstract: Polymer‐bonded explosive (PBX) is used widely in weapon systems. Failure of PBX caused by mechanical damage is one of the sources of accidental ignitions. A brittle crack of PBX produces local heating, creating a ‘hot spot’ finally. Investigation into the tensile fracture behaviour of PBX is one of the main works to determine the failure mechanism. Although many researchers have carried out the quasi‐static Brazilian test to understand the damage evolution of PBX, the fracture feature of PBX under dynamic impact is rarely reported. In this article, dynamic Brazilian tests were conducted. A single‐pulse loading apparatus was used to ensure that specimen was loaded only once during a dynamic Brazilian test. High‐speed camera, digital image correlation and micro‐observation techniques were adopted for strain measurement and microfracture observation. All the dynamic tensile crack exhibits transgranular fracture, which indicates more heat would released by the propagation of crack and more friction between fractured crystal surfaces. On the basis of the theories of interface debonding and transgranular fracture, larger crystals are more prone to crack, whereas smaller crystals simply debond with neighbouring binders. Discrete element method simulation results show that specimen with interface debonding microcracks was able to sustain additional load until transgranular fracture begins.     

Application of Stereo‐Digital Image Correlation to Full‐Field 3‐D Deformation Measurement of Intervertebral Disc

Abstract: This paper presents the application of a Stereo‐Digital Image Correlation (Stereo‐DIC) based approach for full‐field deformation measurements on a porcine intervertebral disc (IVD) under in‐vitro loading. Full‐field capabilities of Stereo‐DIC provide useful information on the IVD structure‐function relationship needed for designing novel disc replacement devices on the basis of biomimetic concepts. In this work, the use of a multi‐view Stereo‐DIC system allows full‐field measurement over more than 180° of the IVD surface. This is achieved by sequentially moving a single camera through seven fixed positions in order to cover the desired angle of vision. Ad hoc data processing and merging procedures are calibrated for a cylindrical sample. Strain maps are determined for a rubber cylinder subjected to rigid‐body motions and then to uniform compression. These preliminary procedures serve to evaluate strain‐mapping errors as well as to assess repeatability and accuracy of camera re‐positioning. Once the entire procedure is verified and calibrated, a fresh porcine functional spine unit is loaded under anterior, posterior and lateral compression. From displacement maps obtained with the multi‐view set‐up, it is possible to gather information on both IVD bulging and surface strains. Results are consistent with the IVD global behaviour under standard testing protocols reported in the recent literature. Furthermore, detailed information on local variations in structural response and stiffness properties occurring when load is applied in different regions of the IVD are obtained. The present approach allows to correlate structural response with the entire 3‐D deformation and strain field and not just with a single displacement/strain component and thus it could be effectively used for explaining in‐homogeneity observed in human discs degenerative patterns.     

Application of Digital Image Correlation Method to Study Dental Composite Shrinkage

Abstract:  Although resin-based composites are widely used in dental restoration, these materials shrink during polymerisation. Polymerisation shrinkage results in distortion of the restoration and bonded tooth and also generates internal stress at the resin–tooth interface. Digital image correlation (DIC) is used to determine the in-plane displacement field by matching different zones of two characterised pictures. The objective of this study was to examine the applicability of DIC in measuring the deformation of the composite restoration and the surrounding tooth. A preliminary experiment examined the shrinkage of composites in a simulated cavity using the DIC method. The measured shrinkage pattern was consistent with a corresponding finite element model. Subsequently the deformation of composite restorations on human molars was examined using this validated DIC method. The greatest deformation was found on the free occlusal surfaces, and the least on the gingival wall. The increased deformation on the post-cured images indicated that the shrinkage continued even after termination of light activation. DIC method facilitates a full-field measurement of shrinkage profile. These experimental results did not only demonstrate the spatial and temporal relationship of displacement in a dental restoration, but also provide validation of computational models to examine the polymerisation consequence.     

Multiple‐view Shape and Deformation Measurement by Combining Fringe Projection and Digital Image Correlation

Abstract: We present a new method that combines the fringe projection and the digital image correlation (DIC) techniques on a single hardware platform to simultaneously measure both shape and deformation fields of three‐dimensional (3‐D) surfaces with complex geometries. The method in its basic form requires only a single camera and single projector, but this can be easily extended to a multi‐camera multi‐projector system to obtain complete 360° measurements. Multiple views of the surface profile and displacement field are automatically co‐registered in a unified global coordinate system, thereby avoiding the significant errors that can arise through the use of statistical point cloud stitching techniques. Experimental results from a two‐camera two‐projector sensor are presented and compared with results from both a standard stereo‐DIC approach and a finite element model.     

Volumetric Digital Image Correlation Applied to X-ray Microtomography Images from Triaxial Compression Tests on Argillaceous Rock

Abstract:  A set of triaxial compression tests on specimens of argillaceous rock were performed under tomographic monitoring at the European Synchrotron Radiation Facility in Grenoble, France, using an original experimental set-up developed at Laboratoire 3S , Grenoble. Complete 3D images of the specimens were recorded throughout each test using X-ray microtomography. Such images were subsequently analysed using a Volumetric Digital Image Correlation software developed at the Laboratoire de Mécanique des Solides in Palaiseau, France. Full-field incremental strain measurements were obtained, which allow to detect the onset of shear strain localisation and to characterise its development in a 3D complex pattern. Volumetric Digital Image Correlation revealed patterns which could not be directly observed from the original tomographic images, because the deformation process in the zones of localised deformation was essentially isochoric (i.e. without volumetric strain), hence not associated to density changes.     

Strain Localisation in iPP/MWCNT Nanocomposites Using Digital Image Correlation

Polypropylene/multiwall carbon nanotube nanocomposites with different rates of weight incorporation (0–1%) were prepared by melt compounding and cast extrusion. The effect of maleic anhydride (5 wt%) grafted on polypropylene is studied through mechanical tests at different scales and morphological observations. In particular, the micromechanism of deformation was investigated through instrumented tensile experiments (at a macro and micro scales) using of a non‐contact method known as digital image correlation. The objective of this paper is first to characterise global behaviour (Young modulus, tensile strength, and ultimate properties) and second to go further in local analysis. In particular, optical instrumentation enables estimation of strain profile distribution onto the sample in a constricted area. Statistical parameters extracted from these local profiles are promising tools to enhance mechanical properties in link with microstructural composition. Tensile tests confirm composite reinforcement at a low level of nanocomposite incorporation, and local analysis enables quantitative measurements of adding maleic anhydride in formulations. The results reveal that addition of maleic anhydride delays strain localisation in the necked area.     

Formulation of Lucas–Kanade Digital Image Correlation Algorithms for Non‐contact Deformation Measurements: A Review

Digital image correlation (DIC) metrology has been increasingly used in a wide range of experimental mechanics research and applications. The DIC algorithm used so far is however limited mostly to the classic forward additive Lucas–Kanade type. In this paper, a survey is given about the formulation of other types of Lucas–Kanade DIC algorithms that have been appeared in computer vision, robotics, medical image analysis literature and so on. Concise notations consistent with the finite deformation kinematics analysis in continuum mechanics are used to describe all Lucas–Kanade DIC algorithms. An intermediate image is introduced as a frame of reference to clarify the so‐called compositional algorithms in a two‐frame DIC analysis. Explicit examples about the additive and compositional updating of deformation parameters are given for affine deformation mapping. Extensions of these algorithms to the so‐called consistent or symmetric types are also presented. The equivalency of final numerical solutions using additive, compositional and inverse compositional algorithms is shown analytically for the case of affine deformation mapping. In particular, the inverse compositional algorithm for affine image subset deformation is highlighted for its superior computational efficiency. While computationally less efficient, consistent and symmetric algorithms may be more robust and less biased and their potentials in experimental mechanics applications remain to be explored. The unified formulation of these Lucas–Kanade DIC algorithms collected all together in this paper can serve as a useful guide for researchers in experimental mechanics to further evaluate the merits as well as limitations of these non‐classic algorithms for image‐based precision displacement measurement applications.     

Evaluating J‐integral from Displacement Fields Measured by Digital Image Correlation

Path integral, domain integral and least squares methods for evaluating J‐integral from measured displacement fields for a power‐law hardening material are described in this paper. The values of the J‐integral are evaluated by applying the path and domain integral methods to the displacement fields obtained by elastoplastic finite element analysis and the displacement fields obtained through the measurement using digital image correlation. Results show that the values obtained by the domain integral method are slightly better than those by the path integral method, because the domain integral method efficiently uses the full‐field measurement data. The values of the J‐integral are also evaluated by the least squares method with the Hutchinson, Rice, and, Rosengren displacement fields. Results show that the J‐integral can be obtained by the least squares method simply and easily without any calculation of the integration. The J‐integral values obtained by the least squares method agree well with the values obtained using other methods. Because J‐integral can be evaluated easily by any method described in this paper, it is expected that these methods are applicable to various fracture problems during experimental evaluation of structural components.     

The Enhanced Digital Image Correlation Technique for Feature Tracking During Drying of Wood

Abstract: The enhanced digital image correlation (EDIC) technique is proposed as an improvement of the digital image correlation (DIC) technique in that it utilises monogenic filtering as a prefilter for extracting intrinsic local phase information from the low‐contrast images and normalized cross‐correlation (NCC) as a feature‐tracking algorithm. The monogenic filtering separates local structural information that is the local phase of an image, which is invariant with respect to the local energy of the image. Therefore, it improves the image, permitting the DIC technique to produce stable and accurate measurements of deformation for a heterogeneous and hygroscopic material like wood during drying.     

应用数字图像相关方法测试航空复合材料的弹性常数EI北大核心CSCD

通过数字图像相关方法对三种航空复合材料的弹性常数进行了测试。通过对DICM理论分析,依据ASTM标准提出合理的测试方案使得DICM的应变测试精度达到25με,满足航空复合材料测试要求。同时应用DICM和电测法对比测试了三种不同航空复合材料的弹性常数,实验结果表明这两种方法得到的实验结果相对误差较小,应力-应变曲线基本重合,DICM能够很好地反映航空复合材料的应变响应和精确测试其弹性常数。     

非接触数字图像相关应变测量的仿真与实验研究

数字图像相关法测量全场位移和应变是一种新的实验力学方法.该测量算法中相关函数和子区大小的选择是影响最终应变测量精度的重要因素.本文采用已知应变变形的仿真散斑图,研究了测量算法中主要相关函数和子区大小在正常光照与高斯不均匀光照条件下,对应变测量精度的影响.得出了不同测量要求下,可选择的相关函数和最佳计算窗口.通过实验验证与分析了此算法的测量精度.研究结果表明,数字图像处理技术测量应变的测试精度满足基本要求.     

A Point‐wise Approach to the Analysis of Complex Composite Structures Using Digital Image Correlation and Thermoelastic Stress Analysis

Thermoelastic stress analysis (TSA) and digital image correlation (DIC) are used to examine the stress and strain distributions around the geometric discontinuity in a composite double butt strap joint. A well‐known major limitation in conducting analysis using TSA is that it provides a metric that is only related to the sum of the principal stresses and cannot provide the component stresses/strains. The stress metric is related to the thermoelastic response by a combination of material properties known as the thermoelastic constant (coefficient of thermal expansion divided by density and specific heat). The thermoelastic constant is usually obtained by a calibration process. For calibration purposes when using orthotropic materials, it is necessary to obtain the thermoelastic constant in the principal material directions, as the principal stress directions for a general structure are unknown. Often, it is assumed that the principal stress directions are coincident with the principal material directions. Clearly, this assumption is not valid in complex stress systems, and therefore, a means of obtaining the thermoelastic constants in the principal stress directions is required. Such a region is that in the neighbourhood of the discontinuities in a bonded lap joint. A methodology is presented that employs a point‐wise manipulation of the thermoelastic constants from the material directions to the principal stress directions using full‐field DIC strain data obtained from the neighbourhood of the discontinuity. A comparison of stress metrics generated from the TSA and DIC data is conducted to provide an independent experimental validation of the two‐dimensional DIC analysis. The accuracy of a two‐dimensional plane strain finite element model representing the joint is assessed against the two experimental data sets. Excellent agreement is found between the experimental and numerical results in the adhesive layer; the adhesive is the only component of the joint where the material properties were not obtained experimentally. The reason for the discrepancy is discussed in the paper.     

Strain Measurement of Nickel Thin Film by a Digital Image Correlation Method

Abstract:  Digital image correlation is a whole-field and non-contact strain-measuring method. It provides deformation information of a specimen by processing two digital images captured before and after the deformation. To search the deformed images, a hybrid genetic algorithm, in which a simulated annealing mutation process and adaptive mechanisms are combined with a real-parameter genetic algorithm, is adopted. This method is used to measure the strain during the microtensile testing of nickel thin film. In addition to the conventional single region, a double region in which the strain is inferred from the distance change of two regions is proposed to calculate the strain by digital image correlation. The results indicate that while the strain values obtained by single-region method are reasonable, those obtained by the double region method are more accurate. Moreover, the mechanical properties of nickel thin film could be obtained.     

Strain Localisation and Damage Measurement by Full 3D Digital Image Correlation: Application to 15‐5PH Stainless Steel

Abstract: The objective of this paper is to propose a method for measuring damage in ductile materials, from its inception to rupture. In the first stage of damage, which occurs before localisation, the usual method for determining damage through the measurement of stiffness variation is used. A damageable elastic–plastic model of the modified Lemaitre/Chaboche type is identified from these tests. An original method is proposed for measuring damage following the initiation of strain localisation. This method is based on a full 3D image correlation analysis using four cameras. The principle of the method consists in identifying the damage through tensile experiments on thin, flat‐notched specimens subjected to tensile loading. Speckles are applied on both faces of each specimen in order to follow the strain fields on the two faces at the same time. These two strain fields are digitised simultaneously by two synchronised sets of two digital cameras. This paper shows how this method enables one to identify strain localisation and deduce the evolution of damage directly. Here, the method is developed for 15‐5 PH stainless steel.     

Digital Image Correlation and Noise‐filtering Approach for the Cracking Assessment of Massive Reinforced Concrete Structures

This paper describes the use of Digital Image Correlation (DIC) techniques for the cracking assessment of reinforced concrete (RC) massive beams and walls. DIC is known to provide accurate and detailed information on displacement and strain fields. Non‐contact measurements can be used to evaluate concrete cracking of destructive tests carried out on a wide range of specimen scales. When applied to large RC structures tested outdoors or in difficultly controllable conditions, DIC‐based methods may lead to erroneous results. In this study a post‐processing procedure is presented to cope with noisy full‐field measurements. The proposed cracking assessment approach is validated on a large experimental campaign. Four points bending tests are carried out on RC beams: firstly on full‐scale rectangular beams and then on mock‐ups scaled down by 1/3. In addition, fours RC walls are tested under in‐plane cyclic shear up to failure. Digital images taken throughout the tests are processed by DIC techniques to provide in‐plane displacement and strain fields. Full‐field measurements are post‐processed by the noise‐filtering technique and the cracks patterns are identified. Crack widths are measured and compared with measurements obtained from conventional point‐based sensors (linear variable differential transformer LVDT and fibre‐optic FO transducers). The proposed DIC‐based post‐processing provides accurate estimation of cracks width for most of the tests. The analyses carried out on the two groups of RC beams show a scale‐effect on the cracks width.     

An Evaluation of Convergence Criteria for Digital Image Correlation Using Inverse Compositional Gauss–Newton Algorithm

A fast, robust and accurate digital image correlation (DIC) method, which uses a robust zero‐mean normalized sum of squared difference correlation criterion, a sophisticated reliability‐guided displacement tracking strategy and an efficient inverse compositional Gauss–Newton (IC‐GN) algorithm, was recently proposed for full‐field deformation measurement. As an iterative local optimization algorithm, IC‐GN algorithm iteratively solves for the incremental warp assumed on the reference subset until the preset convergence criteria are satisfied. In the literature, different convergence criteria have been set for iterative optimization algorithms. However, on the one hand, stringent convergence criteria lead to increased number of iterations and lessen the computational efficiency. On the other hand, too loose convergence conditions enhance the computational efficiency but may decrease the registration accuracy. Understanding the impact of prescribed convergence criteria on DIC measurement and how to choose proper convergence criteria are therefore fundamental problems in realizing high‐efficiency yet high‐accuracy DIC analysis. In this paper, the convergence characteristics of IC‐GN algorithm are investigated in terms of convergence speed and radius of convergence using real experimental images. The effect of various convergence criteria on the efficiency and accuracy of IC‐GN algorithm are carefully examined. Recommendations are given to select proper convergence criteria for more efficient implement of IC‐GN algorithm.     

Assessment of Analytical Models of Pure Bending of Sheet Materials Using the Digital Image Correlation Method

Bending of sheet materials is a common forming mode for shaping sheet components. Although many numerical models of bending, both analytical and numerical simulations based, are available in the literature, extensive experimental validations have been rather limited. A new bend test method and complementary three‐dimensional finite element (FE) simulation of the experiments are employed to assess the predictions from an advanced analytical and FE model of pure bending of aluminium sheet materials. The experimental set‐up developed and utilised is an open concept design that allows access to the tensile surface and through‐thickness region in the vicinity of the specimen bend line to continuously record images of the deforming specimen with two cameras. The specimen images are analysed for strains using an online strain mapping system based on digital image correction method. Tangential strain distribution results from the models in terms of material thinning in the bend region are compared with those from the experiments on AA2024 aluminium sheet material by considering the responses from the specimen edges and mid‐width regions at the bend line. Furthermore, the tangential and radial stress distributions on the through‐thickness section of the specimen from the analytical model are compared with those from the FE model. The results from experiments, FE model and analytical model are compared and discussed in the light of the experimental data and the assumptions involved in the development of the models.     

Application of High Magnification Digital Image Correlation Technique to Micromechanical Strain Analysis

Abstract: This paper describes an experimental apparatus and its application for the full‐field measurement of heterogeneous strains at high magnifications. The apparatus consists of an image acquisition and analysis system, an optical microscope and a stable tensile stage. Magnified images of the specimen surface are acquired and analysed using the digital image correlation (DIC) method. The response of the heterogeneous microstructure of a nodular cast iron is investigated during a tensile test. Strains obtained by using the DIC method and averaged over the observation window correlate with strain measurements simultaneously obtained by using an extensometer. The strain maps of DIC reveal the heterogeneous development of plasticity in the nodular cast iron microstructure. The apparatus has the potential to investigate material behaviour at the microscopic scale.     

Modelling the Effects of Geometric Imperfections on the Buckling and Initial Post‐buckling Behaviour of Flat Plates Under Compression Using Measured Data

Abstract: Recent developments in optical techniques have allowed accurate representations of the geometry of test specimens to be obtained. These enable the nature of geometric imperfections resulting from manufacture or set‐up to be captured in a form which allows them to be incorporated directly into models generated to predict the behaviour of the structure. This study examines the effect of such imperfections on the behaviour of a series of panels, simply supported along all four edges, and subject to uniaxial compressive in‐plane loading. In each case, digital image correlation is used to determine the initial profile and set‐up of the panel and to monitor its behaviour during test. The data are used to automatically generate a series of meshes representative of each of the specimens tested, suitable for finite element analysis. Comparison of the results obtained from these analyses with those found during the experiments modelled shows an improved correlation when compared with standard techniques for assessing imperfection sensitivity. Set‐up is straightforward, and models can be obtained quickly based on the data collected.