이미지 상관법을 이용한 3 점 굽힘 시험 계측 프로그램 개발 관한 연구

The results of experimental studies on the evaluation of the performance of the non-contact optical video system Vic-3D and the digital image correlation method during tensile tests of composites under normal and elevated temperatures are presented. The dependences of the recorded strain values are analyzed by two independent measurement methods: the digital image correlation method (DIC) and a mounted extensometer. The tests were carried out at the Center of Experimental Mechanics of the Perm National Research Polytechnic University using the Instron 5882 universal electromechanical testing system, which includes a thermal chamber with an operating temperature range of -100° to +350 °С. Samples of a composite material made of STEF fiberglass and carbon fiber were studied. Tensile tests were carried out at temperatures of 2°2, 100°, 120°, 200°, and 300°C. Recommendations for conducting temperature tests using the DIC method are proposed. Based on the test results, deformation diagrams were constructed. Fields and diagrams of longitudinal deformations are obtained at different temperature levels. A detailed evaluation of the finely dispersed coating required for the implementation of the DIC method in temperature tests has been carried out.

The spatial resolution and measurement accuracy of the digital image correlation (DIC) method are constrained by camera resolution. This limitation is primarily determined by hardware costs. However, in current stereo DIC measurements, only the gray level or its gradient from two images is used for integer-pixel matching and sub-pixel optimization. It implicitly treats the two images from different viewpoints as independent entities before correlating them. However, the inherent structural information has not been fully utilized. This previously overlooked structural information provides a novel approach to enhancing the accuracy of DIC by leveraging the inherent correlations between stereo image pairs. The realization of binocular super-resolution typically requires a relatively small parallax. Moreover, the DIC method can achieve image window pairing with small parallax through pre-matching. This implies that binocular super-resolution and Stereo-DIC can complement each other by sharing information. In this paper, the DIC method is employed for whole-pixel image matching, while the binocular super-resolution method, based on deep learning, is applied to process the matched image pairs. Building on previous experiments, extensive datasets containing diverse experimental scenes and various speckle patterns were compiled and utilized. Furthermore, the DIC method can establish training datasets with minimal parallax through integer-pixel matching, thereby achieving highly effective super-resolution results. Experimental results demonstrate that super-resolution images with a higher signal-to-noise ratio can be obtained. Additionally, it effectively provides more image details, which enhance the calculation accuracy and resolution of DIC.

Effects of Various Shape Functions and Subset Size in Local Deformation Measurements Using DIC

Application of the DIC (Digital Image Correlation) method for the analysis of crack initiation and propagation in RC beams reinforced by BFRP bars (Basalt Fibre Reinforced Polymer) is presented in the paper. Two dimensional, single camera DIC system was used to observe strain and crack distribution in such RC beams loaded using four point bending setup. BFRP bars have arose as an alternative to steel bars for RC (Reinforcement Concrete) members in aggressive environments. The low modulus of elasticity of BFRP bars lead on to large crack widths and, as a result, the design of BFRP Reinforced Concrete beams is often governed by SLS (Serviceability Limit State) criterion. Due to this it is necessary to explore specific crack propagation in concrete beams. Process of such beams degradation can be identified by comparison of strains and displacements obtained by DIC method, with the database of experiment results.

The purpose of this study was to investigate the applicability of the digital image correlation method, which uses image processing to obtain areal strain distributions, for the observation of rock fractures. In the results of the Brazilian tensile test, the strain increase process just prior to crack initiation was captured when the images were taken at 8,000 fps. The tensile testing of a rubber sheet coated with irregular spots confirmed that the accuracy of the analysis was sufficient, indicating that digital image correlation can be applied for the observation of crack propagation in rocks. By simulating the Brazilian tensile test with a manual mechanical jack, the experiment was conducted on gypsum specimens, and high-resolution images of the experiment were taken. The results of the analysis showed that the cracks started at the points of increased strain by comparing the normal strain distribution in the X direction and the shear strain distribution with the actual crack locations. INTRODUCTION It is important to evaluate the initiation and propagation of cracks in a rock mass during the cavity excavation. In the case of hard rock, the fracture initiation occurs in a short time, making it difficult to observe the process. However, recent advances in digital imaging devices have made it possible to capture high-speed, high-resolution images of rock fracture experiments. If the strain distribution in an area can be measured by image processing, it will be possible to track the initiation and progress of the fracture. Strain gauges have usually been used to measure the strain generated in structures. However, the strain value obtained from a strain gauge is the average strain at one point in one direction; and thus, many strain gauges need to be installed for multipoint measuring. This involves complicated wiring to the measurement device, which causes problems in terms of labor and cost. In addition, as cracks develop or crack widths increase, the strain gauges may break off, making measurement impossible. On the other hand, digital image correlation can obtain the strain distribution over a wider range than strain gauges. It is a simple method requiring only the image of the object to be captured. Digital image correlation (DIC) can measure complex cracks and large deformations. DIC uses a camera to capture the speckle pattern (irregular dots) on a sample surface and measures the displacement and strain distribution from the correlation of images before and after deformation.

Failure analysis and engineering mechanics modeling methods of a stitched ceramic sandwich structure

Nylon 66 is an engineering plastic with the advantageous characteristics of light weight, heat resistance, high strength, and high rigidity; it has been used in automobile parts as a substitute for aluminum alloys. There have also been many studies on the mechanical properties of nylon 66. However, the flow stress in nylon 66 after necking has not been thoroughly investigated yet due to the difficulty in identifying local stress. In this study, a flow stress optimization method based on the digital image correlation (DIC) method and response surface method, is proposed. The averaged true stress–true plastic strain curve is first obtained by a quasi-static tensile test via DIC. Subsequently, the flow stress is optimized via the response surface method to obtain the actual flow stress state after necking. The accurate flow stress in the specimen can be obtained based on the relationship between the global load force and distance between the two shoulders of the specimen grip parts that is unaffected by the cross-sectional area change during necking. The results indicate that the average true stress recorded from the experiment overestimates the flow stress after necking. The optimized flow stress exhibits the maximum decrease of 9%. The accuracy of the optimized flow stress is evaluated by comparison with the true strain distribution on the specimen that is accurately measured via DIC.

In this study, a 2-dimensional digital image correlation (DIC) method is used with smaller speckles to produce displacement and strain field near weld polymer ends of a Thermoset Composite Welding (TCW) joint. The shear and peel strain distribution across the overlap area obtained from the DIC method is compared to the numerical results from finite element modelling. Compared with the traditional single lap joint test, the DIC method provides extra insight into the strain distribution immediately prior to crack propagation, as well as its crack propagation path.

Designing highly stressed offshore renewable energy composite structures (e.g. wind and tidal turbine blades) necessitates characterisation of woven fabric composite under off axial loading. In this work a combined method of finite element analysis, digital image correlation and microscopy is used to assess the effect of ply orientation on the tensile/shear properties and failure modes of woven glass fibre reinforced polymer composites. Full field strain maps obtained by the digital image correlation method were used to evaluate the damage development and the inhomogeneity of strain localisation. The development of finite element models of mechanical test specimens is based on the analysis of micro-mechanical models of representative volume elements using a homogenisation technique in order to calculate the effective orthotropic properties. The agreement between numerically and experimentally calculated strains obtained in the elastic regimes indicates that stress analysis conducted by numerical methods is useful when characterising the effect of ply orientation on mechanical behaviour. Strain measurement conducted by the digital image correlation method indicated that there is a strong relationship between the strain distribution and the microstructure/ply orientation. In addition, it was found that the levels of localised tensile strain are higher than the global strain indicating the structural heterogeneity of the composite material. Finally, microstructural analysis of tension and shear test specimens showed that the main failure modes are de-bonded fibres, fibre pull out, in-plane/inter-laminar shear cracks and delamination.

Applying new composite material for strengthening and repairing existing structures is an important research topic. Carbon Fiber Reinforced Polymer/Plastics (CFRP) and Carbon Textile Reinforced Mortar (CTRM) are two common structural external reinforcement materials. 18 concrete specimens strengthened with CFRP and CTRM are prepared in this study. The quasi-static single-sided shear tests combined with the Digital Image Correlation (DIC) method is applied. The results show that the interface bonding strength of CFRP strengthening (0.76-0.96 MPa) is 65.0% to 74.8% higher than the CTRM-concrete interface (0.43-0.63 MPa). The ductility and energy dissipation capacity of CTRM strengthening is better than that of CFRP strengthening, and the effective bonding length is 125 to 300 mm. In practical work, CFRP is preferred for improving the strength of concrete components, while CTRM is preferred for improving ductility and seismic resistance. Keywords: Carbon fibre-reinforced composites (CFRP); Carbon textile-reinforced mortar (CTRM); Digital image correlation method (DIC); Single-sided shear test

Experimental determination of the notch stress intensity factor for sharp V-notched specimens by using the digital image correlation method

Attempts on developing the method for qualifying digital image correlation (DIC) method results, especial the displacement field, is presented in this paper. By taking a series images from center and margin of imaging field, DIC method is implemented to calculate displacement fields with respect to center of imaging field as origin of movement. Concurrently, residual displacements are obtained by subtracting mean value from the obtained displacement fields. The residual displacements are considered to be errors and mainly introduced by the imaging system of DIC. In this paper, modulation transfer function (MTF) is adopted to relate the residual displacement, DIC quality, with imaging system performance. By investing the MTFs cross-correlation values of all positions with respect to the reference image, a potential qualitative method for inspecting the DIC results based on sub-image MTF cross-correlation is proposed in this paper.

This work describes a method in which the digital image correlation (DIC) method and finite element analysis (FEA) were used to create a quasi-static mixed-mode fracture envelope for bonded joints consisting of 2024-T3 Al adherends and a tough structural epoxy adhesive. Symmetric and asymmetric versions of double cantilever beam, single-leg bend, and end-notched flexure tests are used to populate the mixed-mode fracture envelope with results at several mode mixities. Experiments are conducted in a universal testing machine while recording images for subsequent DIC analysis. Finite element analysis is used to implement cohesive zone models (CZMs) of the adhesive fracture and to account for plastic deformation of adherends. Mode I and mode II traction separation laws (TSLs) are determined from a property identification method with a Benzeggagh–Kenane mixed-mode coupling law used to model mixed-mode behavior. FEA results are shown to provide a good agreement to both the crosshead displacement and DIC data. The methods in this paper serve as a potential framework for future calibration of mixed-mode fracture envelopes for joints bonded with very tough adhesives that complicate assessment with traditional data analysis methods.

The objective of the article is the discussion the different behaviors of several concrete types based on the DIC method. The three various concrete mixes were tested. They include two ordinary Portland cemented concrete mixtures having difference concrete grades and another concrete in which the steel slag replaced for coarse aggregate. During the beam-bending test, strain contour plots on the surface of reinforcement concrete beams and the crack development were captured of the digital image correlation (DIC) technique. The DIC method was verified by comparing the actual strain measured from the strain gauge and that determined from DIC. The results show the feasibility of using this method for present the strain contour plots of the reinforcement concrete beams. The crack development in various mixtures of concrete was compared and discussed within the article.

Experimental determination of deformation homogeneity and shear states using the digital image correlation method