Digital Image Correlation Principle Research Articles

Design and application testing of imaging probe for strain measurement

To address the challenge of obtaining key device strain parameters within narrow spaces, this paper presents the design of a small-sized optical imaging probe with an independent optical system based on the basic principle of digital image correlation (DIC). To validate the strain measurement accuracy of the probe, a high-temperature heating system was built under in a laboratory setting, and experimental tests involving room temperature strain measurement and high temperature thermal expansion coefficient measurement were conducted. The findings demonstrate the potential of the developed probe for strain measurement in critical components situated in restricted spaces.

Model Test Study on Deformation Characteristics of a Fissured Expansive Soil Slope Subjected to Loading and Irrigation

Expansive soils are characterized by repeated swelling and shrinkage. They cause great damage to engineering projects because of their expansiveness, over-consolidation and propensity to crack. However, the impact of cracks on the stability of an expansive soil slope during loading and irrigation is not yet fully understood. This study aimed to investigate the relationship between slope state and crack development in fissured expansive soils. A series of physical model tests with different types of cracks were conducted, in which the fissured expansive soil slopes were subjected to different loadings (1.6, 3.2, 4.8, 6.4, 16 MPa), and irrigated at a flow rate of 25 mL/min. The VIC-2d software, which utilizes the digital image correlation principle, was used to quantitatively obtain the horizontal and vertical strain data of the slope model. The closure and opening of cracks, and the slope state after loading and irrigation were monitored by strain data analysis using VIC-2d software. The results indicate that the excessive overlying stress revives the existing cracks and produces sliding along the crack interface. The sliding surface of the fissured expansive soil slope became shallower due to the water infiltration. It was demonstrated that the middle and foot of the fissured expansive soil slope were the key positions for reinforcement from the perspective of the mutual transformation of tensile strain and compressive strain on the surface of the slope. It is of great importance to study the relationship between the crack strain state and deformation trend of a slope subjected to loading and infiltration to understand the progressive surface- or shallow-layer sliding mechanisms, and reinforce key areas of the slopes in areas containing moderately or strongly expansive soils with abundant cracks.

Accurate and dynamic 3D shape measurement with digital image correlation-assisted phase shifting

Phase-shifting profilometry (PSP) has been widely used in structured-light (SL) systems for three-dimensional (3D) shape measurements, but the speed of the PSP technique is limited by increased phase-shifting patterns. This paper proposes an accurate and dynamic 3D shape measurement method by projecting only four patterns, including three-step phase-shifting patterns and one speckle pattern. Three-step phase-shifting images are used to obtain the initial unwrapped phase map with phase ambiguity. Based on the principle of digital image correlation and multi-view geometry, the absolute phase can be recovered reliably without requiring any embedded features or pre-defined information of the object. To improve the measurement accuracy, the projector coordinate is used as the measuring coordinate to establish a novel stereo SL system model. By solving a least square solution using the triple-view information, accurate 3D surface data can be reconstructed. The experimental results indicate that the proposed method can perform high-speed and accurate 3D shape measurements with an accuracy of 10.64 μm, which is superior to conventional methods and has certain instructive significance for 3D profilometry and measurement engineering.

In Situ Monitoring of Additive Manufacturing Using Digital Image Correlation: A Review.

This paper is a critical review of in situ full-field measurements provided by digital image correlation (DIC) for inspecting and enhancing additive manufacturing (AM) processes. The principle of DIC is firstly recalled and its applicability during different AM processes systematically addressed. Relevant customisations of DIC in AM processes are highlighted regarding optical system, lighting and speckled pattern procedures. A perspective is given in view of the impact of in situ monitoring regarding AM processes based on target subjects concerning defect characterisation, evaluation of residual stresses, geometric distortions, strain measurements, numerical modelling validation and material characterisation. Finally, a case study on in situ measurements with DIC for wire and arc additive manufacturing (WAAM) is presented emphasizing opportunities, challenges and solutions.

An optical method for rapid examination of check development in decorative plywood panels

Common methods for assessment of surface checking in decorative plywood panels rely on manual handling and visual inspection of specimens, a laborious procedure practically limiting the number of materials and variables that may be considered within one project. In this study, a new automated optical method for detection and measurement of checks has been developed. This method was based on the digital image correlation principle, which allowed identification of checks as small as 0.2 mm wide and 1 mm long. Continuous measurement allowed reliable check counts, and measurement of check dimensions as they develop during exposure to drying conditions. A check severity index has been proposed. The method has been validated in exposure tests conducted in harsh but realistic conditions, to increase the likelihood of checking and reduce the test duration to 4 h. In addition, an innovative test setup allowed near simultaneous monitoring of check development in up to 48 panel specimens sized 30 × 30 cm. The efficiency of the method allows studies to examine an unprecedented number of treatments and replicates.

Micromechanics of the internal bond in wood plastic composites: integrating measurement and modeling

In this study, an integrated approach combining experimental measurements and numerical modeling was used for characterization of load transfer in the wood/matrix interface in wood plastic composites (WPCs). The experimental methodology was based on optical measurement of surface displacements and strains in model WPC specimens subjected to tensile loads. The model specimens consisted of thin HDPE films with single embedded wood particles. The optical measurement of surface strains was based on the digital image correlation principle. The material point method was used for morphology-based numerical modeling of the loaded specimens. The exact location and morphology of the embedded particle determined by X-ray computed tomography were used as input for the numerical model. Imperfect interface characteristics, reflecting the efficiency of the load transfer through the interface in the numerical model of the composite, were determined using inverse problem methods. Good agreement was obtained between the simulated and measured strain maps determined on a number of specimens including particles with various orientations to the loading direction using the same values of interface parameters.

A Mesoscopic Mechanics Research on Deformation of 7B04 High Strength Aluminum Alloy

In this paper, a new method mesoscopic been studied for metal material deformation to fracture process. In the polycrystalline material plastic flow process, mesoscopic structural features was obtained for translation - vortex by digital image correlation principle. A preliminary analytical method of material deformation is established. The behavior of plastic deformation of 7B04 high strength aluminum alloy was analyzed. Grain groups deformation, amount of rotation and the energy dissipation distribution were analyzed. An important deformation features is that micro cracks were bred in severe plastic strain region on mesoscopic scale. Based on the stress concentration, the formation of rotational deformation can be regarded as the beginning of an irreversible damage criterion. The Rotation of grain groups is the reason of the formation of microcracks.

Exploring a Multi‐modal Experimental Approach to Investigation of Local Embedment Behaviour of Wood under Steel Dowels

AbstractA multi‐modal experimental approach for analysing the embedment behaviour of timber connections with steel dowels is proposed in this study. In this approach, a standard mechanical embedment test on single‐dowel connections is combined with an optical measurement of surface deformations of the connection based on digital image correlation principle and an X‐ray micro‐computed tomography examination of the deformations in the dowel‐wood interface. The latter is conducted on cylindrical cores including the dowel hole, physically extracted from the loaded specimen at three characteristic points of the load‐deformation curves. The major challenge of this procedure is disrupted load transfer between the cylindrical core specimens and the external material they were plugged in for further analysis. Despite its challenges and limitations, the method revealed a potential for an unprecedented insight into the micromechanics of dowel connections and for effective correlation of the micro‐level observations with the external macroscopic load‐deformation characteristics.

Optical Investigation of Component Made by FDM Technology

3D printed plastic components are nowadays frequently used parts in all areas of industrial sphere. These components are often made by FDM technology. The main advantage of this technology is quick manufacturing process, price and also possibility of producing complex parts. This paper is aimed to the component made by FDM technology from the view of their strength. Specially designed chair was loaded by different force and the maximum load before destruction was measured. Chair was under inspection of two different optical strain measurement systems working on different principles. System PONTOS working on the principle of digital photogrammetry and system ARAMIS working on the principle of digital image correlation were used. These systems were used in order to investigate and identify weak places of this chair.

Basic principle of digital image correlation for in-plane displacement and strain measurement

The basic principle and the algorithm of a digital image correlation method, and the procedure for obtaining displacements and strains are described. In order to describe the basic principle precisely, only in-plane displacement and strain measurement of a planar object are explained. Gray levels between integer pixels of a digital image after deformation are interpolated to obtain displacements with subpixel resolution. Displacements are then determined by solving nonlinear simultaneous equations taking the deformation of a subset into consideration. Strains are obtainable by differentiating the measured displacements. In addition to the basic principle of digital image correlation, the example of the measurement and its results are shown.

Analysis of Reliability of Modal Parameters Estimation Using High-speed Digital Image Correlation Method

This paper deals with description of modification and using high-speed digital image correlation method in experimental modal analysis. In its theoretical part the basic principle of digital image correlation and description of modification of correlation system Q-450 Dantec Dynamics for the purposes of SIMO (Single Input / Multiple Output) assumption of modal parameters is introduced. The practical part – analysis of reliability of program Modan3D, developed for modal parameters estimation – was realized on a simple steel specimen in laboratory conditions. The obtained outputs are present in a form of graphs and tables.

Extension of reflection-mode digital gradient sensing method for visualizing and quantifying transient deformations and damage in solids

The reflection-mode Digital Gradient Sensing (r-DGS) method is extended to visualize and quantify dynamic deformations in solids due to stress wave propagation and for damage/disbond detection in layered materials due to impact loading. The r-DGS technique employs digital image correlation principles to quantify two orthogonal surface slopes simultaneously in specularly reflective solids. Here, for the first time, r-DGS has been implemented in conjunction with ultrahigh-speed photography to quantitatively map surface slopes during stress wave propagation in a thin plate due to an impact event. The measured surface slopes have also been used to successfully evaluate instantaneous topographic information. The r-DGS method has been subsequently demonstrated to be an effective tool for detecting disbonds in layered structures subjected to impact loading. Baseline experiments in each category have been carried out to demonstrate the methodology.

Mapping Static and Dynamic Crack-Tip Deformations Using Reflection-Mode Digital Gradient Sensing: Applications to Mode-I and Mixed-Mode Fracture

The reflection-mode digital gradient sensing (r-DGS) method is extended for visualizing and quantifying crack-tip deformations in solids under quasi-static and dynamic loading conditions. The r-DGS technique employs digital image correlation principles to quantify two orthogonal surface slopes simultaneously in specularly reflective solids by measuring small deflections of light rays. Here, for the first time, r-DGS is implemented to study both mode-I and mixed-mode (I/II) problems and evaluate fracture parameters. Under dynamic loading conditions, r-DGS is employed in conjunction with high-speed digital photography to map surface slopes in edge cracked plates subjected to one-point impact. The measured surface slopes are used to successfully evaluate instantaneous stress intensity factor histories by combining measurements with the corresponding asymptotic crack-tip fields and performing over-deterministic least-squares analyses. Finite element computations are also used to complement experimental measurements as needed.

Stress Analysis Performed in the Near Surrounding of Small Hole by a Digital Image Correlation Method

The paper deals with the description of the whole process of stress fields identification performed in the near surrounding of a small hole using digital image correlation. At first the authors simply describe the principles and possibilities of modern optical devices working on digital image correlation principle. After then they focus on the analysis of strain fields as well as investigation of the influence of some correlation parameters such as facet size, virtual grid, smoothing and size of the calibration target on the accuracy and quality of the obtained results. As the correlation systems usually serve only for strain analysis, the stress analysis with properly set correlation parameters was done in Q-STRESS v.1.0 - a program developed at the authors department. The obtained results were processed in a form of graphs and tables and finally compared with the results achieved numerically using FEM.