Digital Image Correlation Processing Research Articles

In-plane shear experimental method and mechanical behavior of ceramic matrix mini-composites

In-plane shear mechanical experiments of ceramic matrix mini-composites (CMMC) were realized in this paper by overcoming the difficulties of material preparation, specimen design, gripping and loading, deformation measurement, etc. The in-plane shear stress-strain responses of different matrix volume fractions were obtained based on the method. The stress-strain response of CMMC was strongly non-linear, and its elastic modulus and strength were positively correlated to the matrix volume fraction. The main factors affecting the digital image correlation (DIC) based micro-region shear deformation measurement are analyzed quantitatively, and the corresponding solutions are discussed. The DIC calculation time model is established, and the accurate estimation of DIC processing time is realized. The shear strain field evolution clearly captured the matrix crack initiation and propagation.

Application of optical measurement techniques during various stages of pregnancy

Optical measurement techniques have evolved since the mid-2000's due to the advancement in computer processing and digital camera sensors. These techniques allow for the digitization of full three-dimensional (3D) surfaces and provide time-domain surface information data with high details not previously possible with more traditional measurement techniques. A variety of industries are using these technologies with a focus on high accuracy industrial applications including aerospace and automotive. This paper introduces optical measurements techniques used to provide quantifiable data and visual information of a woman's abdomen during various stages of pregnancy. Two non-invasive techniques are used for this study including structured light (SL) scanning and digital image correlation (DIC). An overview of these techniques, the process for data collection, results of the data, and opportunities for future studies are presented in this paper. The structured light scanning was used to capture a 3D surface of the woman's abdomen at four to six week intervals starting at week 15 of maternity and ending six days prior to birth. The results quantify growth during each stage of pregnancy along with full surface data in addition to postnatal results. The DIC data was collected during week 30 when the baby was very active in the womb and external deformations on the woman's abdomen were observed visually. The results from these studies were not intended to be a formal medical study but to provide feasibility of the SL and DIC process during the stages of pregnancy and encourage unique uses of otherwise industrial measurements.

Motion-enhancement assisted digital image correlation of lithium-ion batteries during lithiation

For the first time, a motion enhancement and registration technique (MERt) – an advanced phase based motion enhancement method combined with an optical flow based digital image correlation process is used to investigate batteries. This, in combination with the non-invasive investigative nature of X-ray computed tomography, makes for a powerful tool to examine the mechanism behind (de)lithiation of Li-ion cells. Through MERt, which magnifies the displacement of electrodes and tracks any internal changes, it is generally observed that the electrode expands outwards, but once encountering the solid barrier that is the cell casing, the electrode has nowhere to expand but inwards, which is why the electrode deformations initiate at the cell core. The technique clearly affirms that the greatest movement of electrodes during lithiation occurs at pre-existing inflection points in the jelly roll, which are a result of the manufacturing process, and eventually leads to the large deformations observed in this and previous works. MERt also indicates a contortion of the cell casings during lithiation which may be caused by the uneven expansion of the electrodes, but requires further studies to confirm.

Study on the failure characteristics of fractured rock with different topologies under uniaxial compression

Rock masses are characterized by many discontinuities, the distribution and interactions of which are closely related to the mechanical properties and control the stability of the rock mass. In this work, uniaxial compression tests were conducted on granite with prefabricated cracks in the form of topological branches to study the related fracture network. A high-speed camera and an industrial camera were used to capture images of the crack initiation, propagation, coalescence, and complete failure process. Digital image correlation processing was then employed to obtain the global strain field evolution diagram of the specimens. The following results were obtained. (1) The initial strain localization zone of specimens with different forms of prefabricated fractures mainly shows two initiation modes, namely, T- and H-shaped fractures. (2) Macroscopic cracks propagate along the direction of the maximum principal stress.

Enhancing indentation and impact resistance in auxetic composite materials

Auxetic materials exhibiting a negative Poisson's ratio are of great research interest due to their unusual mechanical responses and a wide range of potential deployment. However, due to the cellular structure and the bending or rotation deformation nature of the elements in auxetic materials, they usually have relative low stiffness, limiting their applications where high stiffness, strength, hardness, and energy absorption are simultaneously desired. To overcome this limitation, we apply the auxetic lattice structures as the reinforcements combining with the nearly incompressible soft materials as the matrix to create a class of high-performance composites. This coupled geometry and material design concept is enabled by the state-of-the-art additive manufacturing technique. Guided by static and dynamic experimental testing, we systematically study the indentation behavior of the 3D printed auxetics reinforced composites and achieve a significant enhancement of their indentation stiffness and impact resistance compared with the non-auxetic reinforced composites. By digital image correlation processing of experimental tests and numerical simulation, this improved mechanical performance is found due to two deformation mechanisms. The first mechanism is the negative Poisson's ratio effect of the auxetic reinforcements, which makes the matrix in a state of biaxial compression during indentation and impact and hence provides additional support. The second mechanism is the negative Poisson's ratio effect of the overall composites, which makes the auxetic reinforced composites denser at the site of the indentation and therefore are more resistant to indentation. The results show that auxetic structures can lead to design stiffer, harder and tougher composite materials. The material design strategy provides insights into the development of classes of novel auxetic composites with a wide range of mechanical and structural applications.

Closed-Loop Optimization of DIC Speckle Patterns Based on Simulated Experiments

Stereo digital image correlation (DIC) spreads widely in the last years as one of the most flexible and accurate full-field displacement-strain measurement techniques. The development of novel applications based on DIC makes the design of 3-D setups a challenging task, given the complex and nonlinear nature of stereophotogrammetric methods. The present literature allows the design and optimization of DIC experimental setup only with approximated uncertainty models or upon image quality metrics that are linked loosely with the actual value of uncertainty. In this paper, a closed-loop optimization method based on 3-D experiment simulation is presented. The approach may be used to optimize several parameters (from the camera setup to the DIC processing parameters). This paper features a case study on the problem of optimizing a regular speckle pattern for different measurement tasks. The whole approach is validated experimentally in Section V .

Investigation of microstructure-property relantionships of magnesia-hercynite refractory composites by a refined digital image correlation technique

Industrial magnesia-spinel bricks destined for thermal shock applications often show more flexibility and improved crack growth resistance. Components from the spinel structure group are usually added to promote microcracking coming from thermal expansion mismatch. This leads to the development of toughening mechanisms that are very effective in improving the crack propagation resistance.Magnesia-hercynite composites were investigated in order to highlight their fracture process, with regard to their microstructure, by using Digital Image Correlation (DIC). The direct measurement of displacement fields between digital images of the reference state and the deformed one has provided valuable information on material deformation during loading. The aim of this work was to investigate the fracture behaviour of refractories through the coupling of the Wedge Splitting Test (WST) and DIC. By using a refined DIC process transformation taking into account a discontinuity of displacement, called 2P-DIC, a more effective characterisation of the fracture behaviour was achieved.

Etching Method to create a Random Speckle Pattern on Semi-Solid Cast A356 Aluminium for DIC in-situ strain measurements

Aluminum semi-solid casting is constantly evolving, as it offers a combination of reduced shrinkage porosity and gas entrapment defects together with high productivity and an extended die-life. The relationship between the microstructure and stress-strain behavior is not well-understood due to its non-conventional microstructure. In-situ tensile testing, combined with optical microscope and Digital Image Correlation (DIC), has been used for local strain distribution measuements in cast irons. The critical capability was an etching technique to generate a micro-scale random speckle pattern with a sufficiently high speckle density to enable the sufficient spatial resolution of displacement and strain. The current paper focuses on the development of a pit etching procedure for the semi-solid cast A356 aluminum alloy to study local strain accommodation on the microstructure during tensile loading. The critical challenge of this procedure was the generation of homogeneously distributed pits on both the primary aluminum and eutectic regions. Therefore, a heated solution used for wet-etch aluminum in microfabrication was modified as well as a process adapted to generate pits with suitable characteristics. In-situ tensile tests were performed attached to an optical microscope to record the microstructure and displacements during loading. DIC software was used for analysis. The procedure was validated through a comparison between the resulting Young´s moduls using standard tensile testing and the DIC process on the speckle pattern generated. A good fit between the two methods for Young´s modulus was found. The spatial resolution obtained was, however, not sufficient to fully resolve the strain gradients in the microstructure, but it did reveal large strain variations in the microstructure.

Evaluation and optimization of digital image correlation processing variables using genetic algorithm

In the surface strain measurement, different settings of the variables applied in digital image correlation procedure give rise to different measurement error levels. This article discusses how the variable settings affect the error, and meanwhile put forward a method to optimize the variables. The deformation of the synthetic reference image for digital image correlation procedure is determined by a finite element simulation of a tensile test under certain loading condition. The digital image correlation variables concerned in this article contain the subset size, step size, shape function and strain window size. Moreover, a genetic algorithm is developed to optimize the setting of these variables, and their sensitivities are further analysed to clarify the different impacts of each parameter on the total measurement error. As demonstrated by the results, the measurement can be improved to a more satisfactory level after the optimization to the digital image correlation processing variables.

Towards the design of a new standard for composite stiffness identification

This paper presents a step towards the design of a novel test for simultaneous identification of all the stiffness components of orthotropic composite materials. A simulator was adopted to numerically simulate the whole identification process. Synthetic images were generated and then processed by Digital Image Correlation (DIC) to calculate the strain fields. The Virtual Fields Method (VFM) was used to identify the material stiffness parameters and error functions were finally defined to evaluate the identification error. Two steps of optimization were applied to obtain the best design variables of different specimens and the optimal DIC processing parameters. Four types of test configuration were simulated including short off-axis tensile test, short off-axis open-hole tensile test, off-axis Brazilian disc and off-axis unnotched Iosipescu test and the most promising configuration was identified.

Optimised Experimental Characterisation of Polymeric Foam Material Using DIC and the Virtual Fields Method

AbstractThis article presents a methodology to optimise the design of a realistic mechanical test to characterise the material elastic stiffness parameters of an orthotropic PVC foam material in one single test. Two main experimental techniques were used in this study: Digital Image Correlation (DIC) and the Virtual Fields Method (VFM). The actual image recording process was mimicked by numerically generating a series of deformed synthetic images. Subsequent to this, the entire measurement and data processing procedure was simulated by processing the synthetic images using DIC and VFM algorithms. This procedure was used to estimate the uncertainty of the measurements (systematic and random errors) by including the most significant parameters of actual experiments, e.g. the geometric test configuration, the parameters of the DIC process and the noise. By using these parameters as design variables and by defining different error functions as object functions, an optimisation study was performed to minimise the uncertainty of the material parameter identification and to select the optimal test parameters. The confidence intervals of the identified parameters were predicted based on systematic and random errors obtained from the simulations. The simulated experimental results have shown that averaging multiple images can lead to a significant reduction of the random error. An experimental determination of the elastic coefficient of a PVC foam material was conducted using the optimised test parameters obtained from the numerical study. The identified stiffness values matched well with data from previous tests, but even more interesting was the fact that the experimental uncertainty intervals matched reasonably well with the predictions of the simulations, which is a highly original result and probably the main outcome of the present paper.

Displacement Discontinuity or Complex Shape of Sample: Assessment of Accuracy and Adaptation of Local DIC Approach

AbstractIn this paper, we study the impact of a complex shape of sample and the appearance of a displacement discontinuity on performances of measurement by local Digital Image Correlation (DIC). Both cases are modelled from synthetic images into two parts: one with a speckle field and another part without speckle or two parts with similar speckle fields but with different kinematics. The accuracy of displacement measurements assessed for DIC subsets crossing the boundary between the two parts is lower than the one obtained without discontinuity and justifies adapting DIC method. A new local DIC process is presented in detail. It is an alternative way able to take into account any shape of sample and several curved lines of discontinuity in the same subset. Two experimental examples are presented: displacement and strain fields obtained by DIC and adapted DIC are compared. We can conclude that it is possible to lead a mechanical analysis until the limits of the sample and even during the appearance and the propagation of a crack.

Deformation-induced speckle-pattern evolution and feasibility of correlational speckle tracking in optical coherence elastography.

Feasibility of speckle tracking in optical coherence tomography (OCT) based on digital image correlation (DIC) is discussed in the context of elastography problems. Specifics of applying DIC methods to OCT, compared to processing of photographic images in mechanical engineering applications, are emphasized and main complications are pointed out. Analytical arguments are augmented by accurate numerical simulations of OCT speckle patterns. In contrast to DIC processing for displacement and strain estimation in photographic images, the accuracy of correlational speckle tracking in deformed OCT images is strongly affected by the coherent nature of speckles, for which strain-induced complications of speckle “blinking” and “boiling” are typical. The tracking accuracy is further compromised by the usually more pronounced pixelated structure of OCT scans compared with digital photographic images in classical DIC applications. Processing of complex-valued OCT data (comprising both amplitude and phase) compared to intensity-only scans mitigates these deleterious effects to some degree. Criteria of the attainable speckle tracking accuracy and its dependence on the key OCT system parameters are established.

Quantitative analysis of deformation in hollow cylinder tests on anisotropic clay formations

A series of triaxial laboratory experiments are performed on thick-walled hollow cylindrical samples of boom clay. The aim of this testing program is to better understand the anisotropic deformation during the excavation. The testing conditions are similar to those to be experienced by host rocks around disposal galleries for radioactive waste. X-ray computed tomography is performed at different steps for each test with the samples remaining inside the loading cell. Initial analysis of the tomography images allows of the observation of the deformation of the central hole. In addition, particles manual tracking and 3D volumetric digital image correlation processing methods are considered being used to analyze the particles displacements and the boundary deformation of the sample quantitatively. An unsymmetrical damaged zone is induced around the hole, with a reverse deformation trend being found at the boundary after unloading, which indicates that the significant anisotropic deformation of boom clay can be induced by mechanical unloading.

STUDIES ON THE DYNAMIC FRACTURE PROPERTIES AND FAILURE MODES OF A PBX

Polymer-bonded explosives (PBXs) are particulate composite materials composed of crystalline explosive grains bound in a relatively soft polymeric binder. It is important to optimize the fracture properties, while still maintaining the low sensitiveness and high explosiveness of PBX. This paper describes a study on the fracture properties and failure modes of a PBX by adopting a newly proposed dynamic fracture experimentation method — notched semi-circular bend (NSCB) specimen loaded with split Hopkinson pressure bar (SHPB) which was used in this study. This method offers the advantage of simultaneously determining the fracture initiation toughness, fracture energy, fracture propagation toughness and fracture velocity. The crack propagation is monitored by using a synchronous high-speed camera, which allows the observation of strain field history via digital image correlation process. The experimental results indicate that both the initiation toughness and the propagation toughness linearly increase with loading rate. The propagation fracture toughness is found to increase with fracture velocity, and a limiting fracture velocity is obtained. The failure modes are interpreted by using various theoretical models. Results suggest that the debonding strength of the binder is much smaller than the crystal fracture strength. The tensile strength is similar to the debonding strength, while the compression strength is somewhere intermediate between them.

Design of an Orthodontic Torque Simulator for Measurement of Bracket Deformation

The design and testing of an orthodontic torque simulator that reproduces the effect of archwire rotation on orthodontic brackets is described. This unique device is capable of simultaneously measuring the deformation and loads applied to an orthodontic bracket due to archwire rotation. Archwire rotation is used by orthodontists to correct the inclination of teeth within the mouth. This orthodontic torque simulator will provide knowledge of the deformation and loads applied to orthodontic bracket that will aide clinicians by describing the effect of archwire rotation on brackets. This will also impact that design on new archwire\bracket systems by providing an assessment of performance. Deformation of the orthodontic bracket tie wings is measured using a digital image correlation process to measure elastic and plastic deformation. The magnitude of force and moments applied to the bracket though the archwire is also measured using a six-axis load cell. Initial tests have been performed on two orthodontic brackets of varying geometry to demonstrate the measurement capability of the orthodontic torque simulator. The demonstration experiment shows that a Damon Q bracket had a final plastic deformation after a single loading of 0.022 mm while the Speed bracket deformed 0.071 mm. This indicates that the Speed bracket plastically deforms 3.2 times more than the Damon Q bracket for similar magnitude of applied moment. The demonstration experiment demonstrates that bracket geometry affect the deformation of orthodontic brackets and this difference can be detected using the orthodontic torque simulator.

Flattened Brazilian Disc Method for Determining the Dynamic Tensile Stress-Strain Curve of Low Strength Brittle Solids

An indirect tensile testing method is proposed to measure the full dynamic tensile stress-strain curve of low strength brittle solids. In this method, the flattened-Brazilian disc (FBD) sample is loaded by modified split Hopkinson pressure bars (SHPB) system. Low amplitude dynamic forces were measured with a pair of piezoelectric force transducers embedded in the incident bar and the transmitted bar. The evolution of tensile stress at the center of the disc sample was determined through finite element analyses using the measured stress in SHPB as inputs. In a traditional Brazilian test, a strain gauge is mounted at the center of the specimen to measure the tensile strain, which is difficult to apply for low strength brittle materials. Thus, two types of non-contact methods, the Digital Image Correlation (DIC) technique and the Laser Gap Gauge (LGG), were used to measure the strain. The DIC method was used to monitor the displacement and the strain map of the specimen during the test, from which the strain at the center of the specimen can be obtained. The accuracy of the DIC results was assessed, and the displacement and strain uncertainties of our system were 0.003 mm and 0.003, respectively. LGG was used to monitor the expansion of the disc perpendicular to the loading axis, from which the average tensile strain is deduced. The numerical simulation revealed that the tensile strain at the center of the specimen is proportional to the average tensile strain and that the ratio is not sensitive to the material elastic parameters. The strain measured through LGG was compared with that measured by the DIC method using photos captured with a synchronized high-speed camera. The result of the LGG method was 20 % smaller than that of the DIC process. However, the latter was limited by the number of frames of the high-speed camera. The feasibility of this methodology was demonstrated using a polymer-bonded explosive (PBX).

Digital Image Correlation for Specimens with Multiple Growing Cracks

Measuring the surface displacements of specimens having multiple, growing cracks is difficult with most implementations of the digital image correlation (DIC) method. This difficulty arises from the need to exclude the cracked area from the analysis, a process that oftentimes requires significant and time-consuming user input to achieve successful results. This work presents a set of modifications to the Newton–Raphson based DIC process that allows the method to automatically analyze specimens with multiple growing cracks. The modifications combine a relatively simple crack identification process that takes advantage of the consistency of quasi-regular speckle patterns with a method to reestablish the analysis in areas segregated by the crack growth. The use of a regular dot pattern does, however, introduce a greater chance for registration error in the correlation process. A method to minimize possible registration problems is also presented. Finally, the effectiveness of the method is demonstrated using images of concrete specimens with a complex and growing crack pattern.

Damage and fracture prediction of plastic-bonded explosive by digital image correlation processing

By digital correlation processing of Scanning electronic microscopy (SEM) images, the paper presents the deformation and damage analysis of an energetic material—the plastic-bonded explosive (PBX) on mesoscopic scale. The analysis is made by observing the deformation field resulted from the digital image correlation (DIC) processing of the images corresponding to the loading steps and comparing with the surface profiles of the composite material so as to visualize the matter damage near a preset crack. The results show that the local deformation disturbance can reveal the material damage even happened underneath the specimen surface. The strain distribution in the front of the preset crack, can be used to predict the propagating route of the microcrack initiated from the tip of the pre-crack, which is related to the splitting fracture of the granular-based composite under compressive loading.

Displacement and Strain Fields at a Stone/Mortar Interface by Digital Image Processing

Abstract This work is part of a study of historical masonry behavior. It deals with the homogenization of periodic and heterogeneous structures like masonry, which is composed of stones linked by joints of mortar. Combined compressive-shearing tests were performed in order to study the mortar interface behavior. Deformation properties were measured, locally, by an optical extensometer. Like the speckle method, this principle uses digital image correlation processing. Compared to the volume of the joint, the size of aggregates induces heterogeneities. An experimental protocol is proposed to determine homogenous deformation according to joint width and localized deformation according to joint height. This approach accurately characterizes the interface at a microscale level and enhances high compaction strain at the interface.