Digital Image Correlation Methodology Research Articles

A Digital Image Correlation Technique for Laboratory Structural Tests and Applications: A Systematic Literature Review

Digital image correlation (DIC) is an optical technique used to measure surface displacements and strains in materials and structures. This technique has demonstrated significant utility in structural examination and monitoring. This manuscript offers a comprehensive review of the contemporary research and applications that have leveraged the DIC technique in laboratory-based structural tests. The reviewed works encompass a broad spectrum of structural components, such as concrete beams, columns, pillars, masonry walls, infills, composite materials, structural joints, steel beams, slabs, and other structural elements. These investigations have underscored the efficacy of DIC as a metrological instrument for the precise quantification of surface deformation and strain in these structural components. Moreover, the constraints of the DIC technique have been highlighted, especially in scenarios involving extensive or complex test configurations. Notwithstanding these constraints, the effectiveness of the DIC methodology has been validated as a strain measurement instrument, offering numerous benefits such as non-invasive operation, full-field measurement capability, high precision, real-time surveillance, and compatibility with integration into other measurement instruments and methodologies.

Development and characterisation of novel three-dimensional axisymmetric chiral auxetic structures

Novel three-dimensional (3D) axisymmetric chiral structures with negative and zero Poisson's ratios are presented based on the existing 3D conventional chiral unit cell. The conventional tetra-chiral unit cell is mapped to the axisymmetric space to form the new 3D axisymmetric chiral structure. Two different structure designs are characterised depending on the period delay of the sine curve representing the horizontal struts of the structure. The structures are fabricated using additive manufacturing technology and experimentally tested under compression loading conditions. The digital image correlation methodology is used to determine the Poisson's ratio dependence on the axial strain. The computational model of axisymmetric chiral structures is developed and validated using the experimental data. The computational model is then used to evaluate the new virtual axisymmetric chiral structures with graded cell structures. The newly developed axisymmetric structures show enhanced mechanical properties when compared to the existing 3D chiral structures.

Multiscalar DIC analyses of granular string under stretch reveal non-standard deformation mechanisms

Mechanical tests designed to measure classical behavior do not provide information underlying the macroscale response. Multiscalar digital image correlation (DIC) can be employed in these experiments to expose the micromechanisms that lead to the emergent macroscale behavior. A multiscalar DIC methodology was applied to study the extensional behavior of a particular granular string with chiral properties. DIC was performed at the macroscale in which the underlying microstructure is overlooked and the construct is treated as a continuum bar. Further, the underlying microstructure was exploited in a microscale DIC analysis to expose the rich deformation regimes. In addition, a mesoscale analysis was carried out to extract the rigid body motions that characterize a granular material system. These analyses were performed for two types of surface patterns and measurement uncertainties were quantified. The microscale results show that the grain string can be treated as a granular material composed of a set of nearly rigid grains that store elastic energy through intergranular mechanisms. Further, the results reveal non-standard deformation mechanisms that underlie the chiral granular string subjected to extension. These mechanisms include grain rotations that are coherent as opposed to gear-like grain rotation. More notably, grain transverse displacements are observed that are controlled by the behavior of the grain interconnections.

Dynamic structural testing of a composite structure by 3D projected digital image correlation

Complex composite structures have unexpected mechanical behaviour, specially under high dynamic loads or in bucking conditions. The complexity and the number of sensors of these types of structural testing require non-standard instrumentation in order to detect local phenomena before critical failure appears. The main problem introduced by accelerometers or strain sensors is the cabling, due to the weight and complexity introduced. Digital Image Correlation (DIC) is a strain and deformation measurement technique well know and extendedly applied in composite structural testing. In this paper it is proposed a new DIC methodology based on project the aleatory patter over the structure instead of paint it. Furthermore, an UAV wing full made of composite will be tested at high frequency and vibrations modes will be observed with a stroboscope and, simultaneously, the aleatory patter will be projected in order to measure the shape of the vibration mode applying the DIC technique. Additionally, strain and shape sensing at high frequencies will be validated with Distributed Fiber Optic at high frequency rates. In this paper, the different technologies will be tested and compared in order to validate their capabilities and limitations.

Formation of a complete stress-strain curve of concrete using digital image corellation

This paper reports the development and verification of a new procedure for formation of a complete stress-strain curve of concrete with a downward region of strain by using a digital image correlation method. A new technique to build spectle patterns on the surface of concrete is described. That makes it possible to accurately enough reproduce the spectle patterns on the surface of concrete and perform a high-quality analysis of strains involving digital image correlation. The advantages of this research technique have been established when predicting the formation of internal cracks in concrete followed by their propagation. In addition, using the digital image correlation methodology makes it possible to obtain strains of the entire studied plane of the sample at each stage of loading. This procedure provides an opportunity to investigate a change in strains and the movement of individual points or areas when studying concrete surfaces. That is a relevant issue as it enables more detailed diagnostics of existing reinforced concrete structures. To check the accuracy of this procedure application, a mechanical gauge with an accuracy of 0.001 mm was additionally installed. 2 high-speed monochrome CCD cameras with different lenses were used in determining concrete strains involving the digital image correlation technique. The deformations were controlled with a period of time every 250 ms. The load was controlled by an additional third camera with a speed of 50 frames/second. The result of the experimental study is the formed full concrete destruction diagram with a downward region of strain. The deviation of the results of strains based on the mechanical gauge with an accuracy of 0.001 mm with a base of 200 mm from those acquired by the digital image correlation procedure was mainly up to 10 %, which confirms the reliability of the results. The results of this work allow a more accurate calculation of reinforced concrete structures in the practice of design, inspection, or reinforcement of existing structures

Investigation by Digital Image Correlation of Mixed-Mode I and II Fracture Behavior of Polymeric IASCB Specimens with Additive Manufactured Crack-Like Notch.

In this work, the fracture mechanics properties of polyamide (PA) specimens manufactured by the selective laser sintering (SLS) technology are investigated, in which an embedded crack-like notch was inserted in the design and produced during the additive manufacturing (AM) phase. To cover a wide variety of mode I/II mixity levels, the inclined asymmetrical semicircular specimen subjected to three points loading (IASCB) was employed. The investigation was carried out by analyzing the full displacement field in the proximity of the crack tip by means of the digital image correlation (DIC) technique. To characterize the material, which exhibits a marked elastic-plastic behavior, the quantity J-integral was evaluated by two different methods: the first one exploits the full fields measured by the DIC, whereas the second one exploits the experimental load–displacement curves along with FEM analysis. The DIC methodology was experimentally validated and proposed as an alternative method to evaluate the J-integral. It is especially suited for conditions in which it is not possible to use the conventional LDC method due to complex and possibly unknown loading conditions. Furthermore, results showed that the AM technique could be used effectively to induce cracks in this type of material. These two aspects together can lead to both a simplification of the fracture characterization process and to the possibility of dealing with a wider number of practical, real-world scenarios. Indeed, because of the nature of the additive manufacturing process, AM crack-like notches can be sintered even having complex geometry, being three-dimensional and/or inside the tested structure.

Effect of Local Post Weld Heat Treatment on Tensile Properties in Friction Stir Welded 2219-O Al Alloy

To improve the formability of the aluminum alloy welds and overcome the size limitation of the bulk post weld heat treatment (BPWHT) on large size friction stir welded joints, a local post weld heat treatment method (LPWHT) was proposed. In this method, the resistance heating as the moving heat source is adopted to only heat the weld seam. The temperature field of LPWHT and its influence on the mechanical properties and formability of FSW 2219-O Al alloy joints was investigated. The evaluation of the tensile properties of FSW samples was also examined by mapping the global and local strain distribution using the digital image correlation methodology. The results indicated that the formability was improved greatly after LPWHT, while the hardness distribution of the FSW joint was homogenized. The maximum elongation can reach 1.4 times that of as-welded joints with increase the strength and the strain of the nugget zone increased from 3 to 8% when annealing at 300 °C. The heterogeneity on the tensile deformation of the as-welded joints was improved by the nugget zone showing large local strain value and the reason was given according to the dimple fracture characteristics at different annealing temperatures. The tensile strength and elongation of LPWHT can reach 93.3 and 96.1% of the BPWHT, respectively. Thus, the LPWHT can be advantageous compared to the BPWHT for large size welds.

A Dedicated DIC Methodology for Characterizing Plastic Deformation in Single Crystals

This paper focuses on the development of an appropriate Digital Image Correlation (DIC) methodology based on Image Registration and dedicated for characterizing the plastic deformation in single crystals. A pure nickel single crystal specimen is plastically deformed in tension and investigated by DIC technique. Based on the measured kinematic fields, the proposed method enables to identify the slip activity on the crystal surface and to locate precisely the slip band interfaces at microscale which behave as kinematic discontinuities. The computed displacement data are projected on a well-defined physical basis containing slip details, then the strain fields can be derived directly from a set of analytical functions. The possible errors in displacement induced by this projection approach are evaluated. Finally, some results of the evaluated strain fields are presented. It demonstrates that the developed DIC methodology allows quantitative characterization of a heterogeneous deformation process and promotes further relationships to be established between slip activity and strain field evolution in single crystals.

Microscopic Strain Mapping Using Scanning Electron Microscopy Topography Image Correlation at Large Strain

Measuring the distribution of local strain at the microscopic level is a challenging problem, especially for materials subjected to large overall strain. In the present study, a novel microscopic strain mapping technique has been developed based on the analysis of surface topography using digital image correlation (DIC) software. The input is a series of scanning electron microscopy (SEM) images. The method uses topographic features (such as surface slip traces) found in these images as the input. A commercially available optical strain measurement system (ARAMIS®, which is a trade name of the equipment from GOM mbH, Braunschweig, Germany) that utilizes the DIC methodology is used for this purpose. It was found that the best results were obtained using an incremental approach in which DIC is used to map the local strain increments following a modest amount of macroscopic deformation. This is essential when using topographic features such as slip traces that are not static. The accuracy and scale of the measurements are affected by image and facet size. The method has been validated, based on in situ deformation of an aluminium alloy within an SEM, using strains measured independently by means of surface indents. The results clearly reveal the details of the local shear on a sub-grain-size scale and the evolution of shear bands within the necking area, leading to local strains that exceed the average strain by a factor of 2.3.

Accuracy of Digital Image Correlation for Measuring Deformations in Transparent Media

Experimental models to measure spatial deformation patterns within a soil mass are typically limited by the fact that soil sensors do not provide a continuous image of the measured continuum. Additionally, soil sensors exhibit static and dynamic characteristics that are different from those of the surrounding soils and therefore can change the response of the measured continuum. The fundamental premise of this research is that transparent synthetic soil surrogates can be used to overcome these difficulties using digital image correlation (DIC). A system consisting of a laser source and a line-generating lens was used to optically slice the transparent synthetic soil models. A digital camera was used to capture images of the slices before and after deformation. This paper presents a new technique for quantifying spatial deformation throughout transparent synthetic soil models using DIC. The accuracy of the DIC technique was evaluated based on a scheme of predefined digital movement of synthetic soil images. Finally, a model consisting of a strip footing on a synthetic transparent soil is presented. The spatial deformations in the model are evaluated using the proposed DIC methodology and compared with the result of finite-element analysis.