DIGITAL IMAGE CORRELATION: PERFORMANCE AND POTENTIAL APPLICATION IN PHOTOGRAMMETRY

Within a research project on the deformation capacity of in-plane loaded unreinforced masonry structures, 10 tests were performed on full-scale large walls subjected to in-plane static-cyclic loading. During testing, the measurements included all applied forces together with an overall and a local picture of the deformation state of the specimens. To achieve this, in addition to the traditional hard-wired instruments, i.e., LVDTs and potentiometers, an in-house developed 2D digital image correlation (DIC) measurement system was applied. DIC is a cutting-edge, non-contact, optical measurement technique that provides the complete deformation field by comparing the digital images of the test object's surface before and after the deformation. This paper reports on the applied measurement procedure, discusses important DIC issues and the applicability of DIC for deformation field measurements when testing large masonry walls at full scale. The obtained results proved that in-house developed DIC system based on conventional DSLR cameras can be considered as an effective system to measure the complete deformation field with high level of accuracy and spatial resolution even in the case of large specimens and complicated deformation fields.

Ply level strain measurement in composites using digital image correlation

Characterizing the mechanical behavior of rocks plays a crucial role to optimize the fracturing process in unconventional reservoirs. However, due to the intrinsic anisotropy and heterogeneity in unconventional resources, fracture process prediction remains the most significant challenge for sustainable and economic hydrocarbon production. During the deformation tracking under compression, deploying conventional methods (strain gauge, extensometer, etc.) is insufficient to measure the deformation since the physical attachment of the device is restricted to the size of the sample, monitoring limited point-wise deformation, producing difficulties in data retrieval, and a tendency to lose track in failure points, etc. Where conventional methods are limited, the application of digital image correlation (DIC) provides detailed and additional information of strain evolution and fracture patterns under loading. DIC is an image-based optical method that records an object with a camera and monitors the random contrast speckle pattern painted on the facing surface of the specimen. To overcome the existing limitations, this paper presents numerical modeling of Brazilian disc tests under quasi-static conditions to understand the full-field deformation behaviors and finally, it is validated by DIC. As the direct tensile test has limitations in sample preparation and test execution, the Brazilian testing principle is commonly used to evaluate indirectly the tensile strength of rocks. The two-dimensional numerical model was built to predict the stress distribution and full-field deformation on Brazilian disc under compression based on the assumptions of a homogenous, isotropic and linear elastic material. The uniaxial compression test was conducted using the DIC technique to determine the elastic properties of Spider Berea sandstone, which were used as inputs for the simulation model. The model was verified by the analytical solution and compared with the digital image correlation. The numerical simulation results showed that the solutions matched reasonably with the analytical solutions where the maximum deviation of stress distribution was obtained as 14.59%. The strain evolution (normal and shear strains) and displacements along the central horizontal and vertical planes were investigated in three distinguishable percentages of peak loads (20%, 40%, and 90%) to understand the deformation behaviors in rock. The simulation results demonstrated that the strain evolution contours consistently matched with DIC generated contours with a reasonable agreement. The changes in displacement along the central horizontal and vertical planes showed that numerical simulation and DIC generated experimental results were repeatable and matched closely. In terms of validation, Brazilian testing to measure the indirect tensile strength of rocks is still an issue of debate. The numerical model of fracture propagation supported by digital image correlation from this study can be used to explain the fracturing process in the homogeneous material and can be extended to non-homogeneous cases by incorporating heterogeneity, which is essential for rock mechanics field applications.

Framework to optimise two-dimensional DIC measurements at different orders of accuracy for concrete structures

Digital Image Correlation (DIC) is a vital optical measurement technique that finds diverse applications in the domain of mechanics of materials. In aerospace applications, DIC has excellent scope in structural health monitoring of aircraft components. Aircraft wings, one of the critical components are subjected to different loads during flight. Ground testing and In-flight testing of wings can benefit substantially by DIC monitoring. DIC can be utilized to analyze the time-based variation in the speckle pattern or grid, applied to the wing’s surface. High-resolution images processed through a suitable correlation software helps decipher the data into stress and strain contours. Thus, any potential material failure or component defects can be identified. DIC also finds a role in flutter analysis, enabling the scrutiny of wing vibrations and deformations. In this review, the applications of DIC in analysis of aircraft components has been taken up, as in-flight structural health monitoring is a critical activity for a safe flight.

Assessment of digital image correlation for tensile characterization of fique woven textiles used as matrix-reinforcement

The paper presents application on digital image correlation (DIC) and microindentation for investigation of plastic flow under Brinell ball indenter applied on steel specimen made of two parts screwed together. Specimens in two different material states (a) as delivered (b) annealed were investigated. This approach enables internal surface to act as the external one and to be examined by aforementioned methods. Results obtained by application of DIC on scanning electron microscope (SEM) images are compared to microhardness maps and agreement is demonstrated.

The prospective aspects of the application of digital image processing in the field of monitoring the coking process inside the boilers in thermal power plants are discussed. Timely monitoring the dynamics of the coking is an important issue to the safety of thermal power plants. Digital image processing is an effective technique to fulfill this monitoring. Edge detection and wavelet transform serve as accurate tools to investigate the ash images captured by the high-speed camera to inspect the coking. Edge detection finally presents the boundary contour of the coking zone. The advantage of wavelet transform over Fourier transform is that the former obtains not only the information on the frequency but also the information on time.

The paper aims to characterise the strain concentration of welded junctions subjected to elastic loads using digital image correlation (DIC). When measuring such small strains and heterogeneous strains fields with DIC, a compromise between noise and accuracy arises where a suitable choice of the image processing settings becomes crucial to obtain accurate results. An extensive study of the influence of the different processing parameters was carried out to ensure the best trade-off between accuracy and noise. A practical application was later performed with the found optimal settings; welded specimens were subjected to uniaxial and biaxial loads to assess possible influence of the strain state on the strain concentration. Results show a great variability of the strain concentration along the weld bead base, evidencing a significant influence of local geometrical characteristics. On the other hand, the strain state influence was found to be moderate, or almost inexistent for lower strain concentrations.

Optical methods are becoming commonplace in investigations of the physical and mechanical behavior of biological tissues. Digital image correlation (DIC) is a versatile optical method that shows tremendous promise for applications involving biological tissues and biomaterials. We present the fundamentals of DIC with an emphasis on the application to biological materials. An approach for surface preparation is described that facilitates its application to hydrated substrates. Three examples are presented that highlight the use of DIC for biomedical research. The first example describes the use of DIC to study the mechanical behavior of arterial tissues up to 40% elongation. The second example describes an evaluation of the mechanical properties of bovine hoof horn in the dehydrated and fully hydrated states. Uniaxial tension experiments are performed to determine the elastic modulus (E) and Poisson's ratio (nu) of both the arterial and dermal tissues. Spatial variations in the mechanical properties are evident from the full-field characterization of both tissues. Finally, an application of DIC to study the evolution of loosening in cemented total hip replacements is described. The noncontact analysis enables measurement of the relative displacement between the bone/bone cement and bone cement/prosthesis interfaces. Based on the elementary optical arrangement, the simple surface preparation, and the ability to acquire displacement/strain measurements over a large range of deformation, DIC should serve as a valuable tool for biomedical research. Further developments will enable the use of DIC for in vivo applications.

Application of digital image correlation (DIC) to sloshing liquids

Digital image correlation (DIC) technique with the aid of scanning probe microscopes has become a very promising tool for deformation analysis of micro- and nanoscale components. The scanner drift of the atomic force microscope (AFM) is a great disadvantage to the application of digital image correlation to micro/nanoscale deformation measurements. This chapter has addressed the image distortion induced by the scanner drifts and developed a method to reconstruct AFM images for the successful use of AFM image correlation. The proposed AFM/DIC method is to generate a corrected image from two correlated AFM images scanned at the angle of 0° and 90°, respectively. The method has been validated by two simple AFM/DIC experiments. The application of this AFM/DIC technique was demonstrated by the deformation measurement on the micro-interconnection in a micro thermoelectric cooler. AFM images of the scan region of interest were obtained separately when the microelectronic device was before and after operating at both its cooling and heating stages. The AFM images were then used to obtain the in-plane deformation fields in the observed region of the micro-assembly. AFM image correlation is performed for nanoscale deformation analysis using the authors’ AFM–DIC program. The results show that the observed region was subjected to cyclic strains when the device worked between its cooling and heating stages, and cyclic strain in the vertical direction was found to be significant deformation mode. The thermally induced deformation behavior of the micro-assembly device was modeled by finite element analysis (FEA). Both thermal-electric analysis and thermal stress analysis were conducted on a 3D finite element model of the device. It is shown that the experimental results were able to validate the finite element analysis results.

Review of digital image correlation application to large-scale composite structure testing

The development of measurement methods, and in particular digital image correlation (DIC) systems, which are designed to measure of entire displacements and deformations fields, opens up new areas of research. In general, the materials constitutive relations are formulated in such a way that material parameters could be determined with relatively simple experimental tests carried out on samples with uniform (approximately) stress and strain fields. Then it is possible to apply them to complex boundary value problems formulated e.g. in the small or large deformation theories. The application of DIC allows to verify the accuracy of their predictions by comparing the results of the experiment with solutions to boundary value problems obtained using the finite element method (FEM).

The paper presents the assessment of the possibility and reliability of the digital image correlation (DIC) system for engineering and scientific purposes. The studies were performed with the use of samples made of the three different materials—mild S235JR + N steel, microalloyed fine-grain S355MC steel, and high strength 41Cr4 steel subjected to different heat-treatment. The DIC studies were focused on determinations of dangerous zones with large stress concentrations, plastic deformation growth, and prediction of the failure zone. Experimental tests were carried out for samples with different notches (circular, square, and triangular openings). With the use of the DIC system and microstructure analyses, the influence of different factors (laser cutting, heat treatment, material type, notch shape, and manufacturing quality) on the material behavior were studied. For all studied cases, the stress concentration factors (SCF) were estimated with the use of the analytical formulation and the finite element analysis. It was observed that the theoretical models for calculations of the influence of the typical notches may result in not proper values of SCFs. Finally, the selected results of the total strain distributions were compared with FEM results, and good agreement was observed. All these allow the authors to conclude that the application of DIC with a common digital camera can be effectively applied for the analysis of the evolution of plastic zones and the damage detection for mild high-strength steels, as well as those normalized and quenched and tempered at higher temperatures.