Stereo digital image correlation using binocular super-resolution

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.

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.

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.

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.

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

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

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.

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

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.

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

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.

This paper reviews recent demands and developments of microscopic strain mapping towards understanding plasticity-related fracture mechanisms in metals. After development of the most solid strain mapping method, i.e., micro-grid method, various strain mapping methods with different demands have been proposed. For instance, to satisfy the requirements of strain accuracy, spatial resolution, temporal resolution, view field range, strain limit, etc., digital image correlation (DIC) method, sampling moiré method, and photonic opal crystal film have been developed and improved. Furthermore, to understand true origins of fractures, regressive strain mapping, three-dimensional strain mapping, and post-mortem strain mapping have been increasingly required for the situations of “nobody knows the crack nucleation site before cracking”, “stress-driven cracking occurs on a plane strain condition (specimen interior)”, and “in some instances, only a fractured sample is available”. For these demands, DIC method on replicas, computed-tomography-based strain mapping technique, and orientation-based strain analysis have been developed. In this paper, based on our recent works, the state-of-the-art techniques of strain mapping in metals are briefly introduced.

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.