Development of the digital image correlation (DIC) method for mechanical testing at elevated temperatures

The test on the inhomogeneous strain field and creep rupture behavior of the weld joint at elevated temperature by digital image correlation method

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.

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.

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

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.

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

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

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

A digital image correlation (DIC) method was utilized to measure strain distributed within approximately 0.5 mm of the cutting edge during slow-speed orthogonal cutting of air-dried hinoki (Chamaecyparis obtusa), to clarify the relationships of the strain distribution and cutting conditions, including cutting angle ( $$\theta$$ ) and depth of cut ( $$d$$ ). The strain was measured in 0.04 mm steps, and the measurable minimum strain was approximately 0.08%. Tensile strain of 3% or larger normal to the cutting direction, $${\varepsilon _y}$$ , tended to extend 0.2 mm or further ahead of the tool when $$\theta \leq 60^\circ$$ and $$d \geq 0.1{\text{ mm}}$$ . This tensile $${\varepsilon _y}$$ corresponded to the occurrence of the fore-split in Chip Type I. The tensile $${\varepsilon _y}$$ detected along the path of the cutting edge decreased as $$\theta$$ and/or $$d$$ decreased. Positive shear strain, $${\gamma _{xy}}$$ , tended to be detected ahead of the tool in Type I. Negative $${\gamma _{xy}}$$ tended to be detected ahead of the tool in Type II and III $$\left( {\theta \geq 70^\circ ,\,\,d \geq 0.05{\text{ mm}}} \right)$$ . These $${\gamma _{xy}}$$ values were considered to be related to the elongation and shrinkage of the chip. The study confirmed the usability of the DIC method for the evaluation of cutting conditions and also to classify chip formation into chip types.

Tensile strength, tensile modulus, compressive modulus, and Poisson's ratio are important mechanical parameters for brittle explosives. Generally, these parameters are separately measured by several different tests in which the homogeneity of specimens cannot be guaranteed, thus requiring a simultaneous determination of these mechanical parameters by one test. In this paper, simultaneous determination of multiple mechanical parameters of a DNAN/HMX melt‐cast explosive using Brazilian disc test combined with digital image correlation (DIC) method is described. The method would allow tensile strength, tensile modulus, compressive modulus, and Poisson′s ratio to be obtained simultaneously by one test, when the influences of rigid body motion have been effectively removed. The method principle was introduced in detail in this paper. The effect of temperature on the mechanical properties and the difference between the tensile modulus and compressive modulus of the DNAN/HMX melt‐cast explosive were investigated. The elastic constants and tensile strength were quantitatively analyzed and are qualitatively correct, which demonstrates the effectiveness of the presented method.

The deep fissured rock mass is affected by coupled effects of initial ground stress and external dynamic disturbance. In order to study the effect of internal flaw on pre-stressed rock mechanical responses and failure behavior under impact loading, intact granite specimens and specimens with different flaw inclinations are tested by a modified split Hopkinson pressure bar (SHPB) and digital image correlation (DIC) method. The results show that peak strain and dynamic strength of intact specimens and specimens with different flaw angles (α) decrease with the increase of axial static pressure. The 90° flaw has weak reduction effect on peak strain, dynamic strength and combined strength, while 45° and 0° flaws have remarkable reduction effect. Specimens with 90° flaw are suffered combined shear and tensile failure under middle and low axial static pre-stresses, and suffered shear failure under high axial static pre-stresses. Specimens with 45° and 0° flaws are suffered oblique shear failure caused by pre-existing flaw under different axial static pre-stresses. Besides, based on digital image correlation method, it is found that micro-cracks before formation of macro fractures (include shear and tensile fractures) belong to tensile cracks. Tensile and shear strain localizations at pre-existing flaw tip for specimen with 45° and 0° flaws are produced much earlier than that at other positions.

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.

The present paper investigates crack growth in specially designed specimens of 6061 aluminum alloy sheets subjected to dynamic shear loads. The sheets were previously thoroughly tested using uni-axial static and dynamic tests, so the less-known shear fatigue tests are presented in this work. For this purpose, a special specimen and fixtures were designed, which are capable of fatigue shear testing at elevated temperatures. In addition, the shape of the specimen was designed for more predictable crack initiation and growth along the specimen surface. In this study, the specimens were tested with low frequency (0.2 Hz) alternating shear loading (amplitude 0.3 mm in the positive and negative directions) until complete fracture. Each time the peak amplitude was reached, a photograph of the shear area on the specimen surface was taken and used to investigate crack growth further. The main objective of this work was to observe the crack initialization and crack growth under alternating shear loading using Digital Image Correlation (DIC) method; this was achieved by analyzing the photographs taken with the DIC method by observing local in-plane fields of strains and displacements.

The study of glass multilayer columns made of ordinary glass using triplexing technology. The main disadvantage of using ordinary glass is the fragile nature of its destruction, it breaks instantly. To avoid this effect, triplexing technology is used: the glass is joined in several layers with an EVASAFE polymer film (Bridgestone, Japan), after which the columns were heated to a temperature of 130 °C and kept for 30 minutes. The film material is an elastomer (a polymer with highly elastic properties in a wide temperature range). This allowed restraining parts of fragments of glass structures, making them plastic inside the bonding plane, increased their reliability, prevented the instantaneous opening of cracks in the depth of the section. A method for studying multilayer glass columns for central-axial compression using the method of two-dimensional digital image correlation is developed. Two series of prototypes were manufactured and tested. The tests were performed on a hydraulic press. The model of the destruction of glass columns of different cross-section is investigated and described. The accuracy of the digital image correlation method using two-dimensional correlation for glass centrally compressed columns is estimated. The results of the digital image correlation method with the results of measurements of absolute deformations by mechanical devices are compared. Based on the analysis of the results, the dependences of relative deformations e on the applied load N were determined using the digital image correlation method. The dependences of relative deformations e on normal stresses σ=N/A and dependences of relative deformations e on the outer glass surfaces on the applied load N are determined

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.