Digital Image Correlation Measurement System Research Articles

Mechanical properties and constitutive model of filling medium in hot casting socket at elevated temperatures

Hot casting sockets have been commonly applied in the prestressed structures to anchor steel cables. However, the study on the fire behaviour of hot casting sockets is limited, particularly in relation to the thermo-mechanical properties of the filling medium. This paper experimentally investigates the mechanical properties of a zinc copper alloy (employed as the filling medium) at varying target temperatures. A comprehensive range of stress–strain curves are obtained by using a digital image correlation measurement system. The elastic modulus, proportional limit, effective yield strength and ultimate strength are presented and compared to those of steel cables. It is found that the strain hardening effect diminishes as the temperature increases. Reduction factors of elastic modulus, effective yield strength and ultimate strength of the zinc copper alloy are smaller than those of steel cables at elevated temperatures, indicating the vulnerability of the filling medium in fire scenarios. Mathematical formulas of reduction factors of mechanical properties are proposed. Furthermore, a novel constitutive model is proposed by modifying the Johnson-Cook model. Parameters involved in the stress–strain relationship are determined by the smooth round bar test results, whereas those in the failure criterion are determined by the notched round bar test results. Finally, the modified constitutive model is applied in a finite element model, and the numerical results have good agreement with the test results.

A DIC method for thermal micro-deformation measurement of satellite high stability composite structure in simulated space environment

In order to ensure the stability and reliability of the entire satellite and onboard equipment indicators, the satellite highly stable structure can only produce small deformation or “near zero deformation” under the space on-orbit (thermal/vacuum) environment. The effective measurement and verification of its dimensional stability indicators directly determine the success or failure of the development of highly stable satellites. This work proposes a method for measuring the thermal deformation of highly stable satellite structures based on digital image correlation, (DIC), which designs a combined methodology of the digital image correlation measurement scheme, a measurement instrument protection method, a highly uniform temperature measurement and control system in the simulated space on-orbit thermal/vacuum environment. The methodology proposed in the work makes the digital image correlation measurement system suitable for space on-orbit measurement, and verifies that the measurement system can maintain high measurement accuracy. The research results indicate that this method can effectively solve the problems of measurement field interference, high uniform temperature control, speckle measurement camera, and measurement baseline protection. The thermal stability measurement experiment of a highly stable structure of a certain satellite has been successfully completed, and it has been verified that this method can meet the effectiveness and measurement accuracy of simulating thermal deformation measurement of structures in the simulated space on-orbit thermal environment.

Three‐dimensional deformation measurement of aero‐engine high‐speed rotating blade surface based on stroboscopic structure digital image correlation method

Under the working conditions of aero-engine, the three-dimensional composite deformation of the blade is caused by the centrifugal force generated by high-speed rotation and the load generated by gas scouring. Aiming at the problem of three-dimensional composite deformation on the surface of aero-engine blades under high-speed rotation, a three-dimensional deformation measurement method based on stroboscopic structure digital image correlation method is proposed, and a method for eliminating rigid body displacement induced by the trigger error of the stroboscope is proposed. First, the laser trigger-stroboscopic irradiation system is used to freeze the high-speed rotating blade under a specific phase. Then, the binocular vision system is used to collect the surface image of the rotating blade. Finally, the digital image correlation method is used to measure the surface deformation field of the high-speed rotating blade. In this paper, the experimental platform is built and the displacement field and main strain field of the blade surface under high-speed rotating state are measured. The experimental results show that the stroboscopic structure digital image correlation measurement system and method have realized displacement measurement accuracy of 2 μm and strain measurement accuracy of 100 μɛ and have realized the surface deformation measurement of the ducted turbine fan blade under the condition of 6000 rpm (CFM56-7 engine N1 speed). The results show that the method is effective and can provide a reliable measurement method for the deformation measurement of high-speed rotating blades in large engine manufacturing enterprises such as Aero Engine Corporation of China.

Online measurement of the elastic recovery value of machined surface in milling titanium alloy

This paper aimed to obtain the elastic recovery value of the machined surface of the workpiece in the milling process of titanium alloy, because it can cause strong friction between the tool flank and the workpiece surface. A new online measurement system was designed to monitor the elastic recovery behavior of Ti6Al4V alloy in dry milling based on the digital image correlation (DIC). DIC measurement principle was intuitively analyzed by means of picture illustration. The orthogonal milling experiments were designed with different cutting parameters and the experiment processes were designed in detail, including the installation of DIC measurement system, the speckle disposal of the measured surface and the field calibration of test instrument. The displacement curve of the target region was analyzed according to the material deformation characteristics of metal materials in the cutting process and thus the calculation method of elastic recovery value of machined surface was obtained. Besides, the optimization method of cutting parameters with the minimum elastic recovery value in titanium alloy milling was also obtained, which can be expanded to the machining measurement of other difficult -to-machine materials.

Evolution of short-term creep strain field near fatigue crack in single crystal Ni-based superalloy measured by digital image correlation

This study measured the variation over time of the creep strain fields around a fatigue crack tip in a single crystal Ni-based superalloy in an open environment at temperatures up to 900 °C using digital image correlation (DIC). A high-temperature DIC measurement system was developed to overcome three difficulties: the degradation of random patterns, thermal radiation, and heat haze. Tension hold was applied to the specimens, and the time evolution of the creep strain fields around the crack tip was measured at different temperatures and under different loading conditions. The proposed DIC system measured the short-term creep strain fields with high reproducibility and reliability, and the strain and strain rate increased as the temperature and hold load increased. To confirm the utility of the measurement results, the stress distribution around the crack tip was calculated using the finite element method (FEM), and the calculation results were validated by comparing the calculated strain to the measurement results. The effect of creep deformation on the fatigue crack propagation (FCP) is discussed.

Influence of the stress-strain state on the jerky flow effects and kinetics of the strain bands nucleation in Al-Mg alloy (A 95456)

The aim of the work is to develop a technique on the experimental study of inelastic deformation of structural metal materials in various types of stress-strain state. A technique is proposed based on the use of original samples of specialized complicated geometry, in the working zone of which a controlled biaxial stress state is realized, and the use of a contactless 3D digital image correlation measurement system Vic-3D for recording displacement and deformation fields. The authors proposed the use of original samples of variable thickness in the form of plates located inside a rigid circular rim and a rim of reverse curvature. Under uniaxial tension of these samples in the central area of the plate (working part of the sample), the plane stress state is realized with principal stresses of different signs, depending on the shape and rigidity of the rim. In the working part of the plate with a ring-shaped rim, tensile forces in the longitudinal direction and compressive forces in the transverse direction are realized due to the curvature of the circular rim and its narrowing in the transverse direction. In a plate with a rigid rim of reverse curvature, biaxial tension is realized in the working section. High efficiency of the proposed experimental technique has been shown to solve problems in the mechanics of solids, in particular, to study the processes of macroscopic localization of plastic flow in Al-Mg alloys (AlMg6) under the conditions of the Portevin - Le Chatelier effect. Using a video system, experimental data of the realized ratio of the deformation components (longitudinal and transverse deformations) in the working zone of the samples were obtained. The analysis of the kinetics of band formation and discontinuous yield strain occurring under conditions of a plane stress state has been carried out. The use of the Vallen Amsy-6 system made it possible to obtain data on the occurrence of acoustic emission signals accompanying the observed processes of inelastic deformation of the material. The graphs of the dependence of the energy parameter and frequency characteristics of acoustic emission signals on time have been constructed and analyzed.

Mechanical Characterization of Porcine Skin Starting Material

Abstract Porcine skin has been used as a starting material in several released mesh medical devices. Although this controlled animal derived material is prevalent in tissue engineered medical devices, little is known about its mechanical properties. This study mechanically characterized porcine skin starting material (PSSM), provided by Midwest Research Swine. Uniaxial tensile tests were performed on samples cut from different regions (back and neck) and orientations (parallel and perpendicular to the spine) on the PSSM. The stress–stretch relationship was determined for each sample utilizing a load frame equipped with a Digital Image Correlation measurement system. The PSSM skin demonstrates the classic nonlinear and linear regions seen in other biologic tissues. A bilinear curve fit method was used to separate the nonlinear and linear regions of the tensile curve, and each region was analyzed with an Ogden and linear model, respectively. The results show that the tensile curve is better described with this method as opposed to analyzing the full curve with one model. A comparison was made between samples cut from the different regions and orientations. There were significant differences between the failure measures and mechanical indices from the two regions, and on average the back behaved anisotropically and the neck isotropically. The PSSM mechanical properties from this study could serve as a preliminary guide for those exploring devices or processes in the tissue engineering field. The methods demonstrated in this study could also help characterize other biologic materials, and be used toward the development of tissue specific industrial standards.

Thermal radiation elimination method for high-temperature digital image correlation using polarization camera

Digital image correlation (DIC) is a material displacement and strain measurement technology based on visible light illumination. At high temperatures, the problem of thermal radiation seriously affects the quality of acquired images and restricts the development of high-temperature DIC technology which is increasingly applied in the field of high-temperature measurement due to stringent measurement temperature requirements. A thermal radiation elimination method based on the use of a polarization camera for high-temperature DIC measurements is proposed in this study. This method uses a polarization camera combined with a filter set to achieve clear image acquisition at 1200 °C and effectively eliminates the effects of thermal radiation on image acquisition. The gray average method and an image inverse filtering algorithm are adopted in this study to eliminate high-temperature thermal disturbances. Finally, a high-temperature DIC measurement system is independently designed, and a rigid-body displacement experiment is carried out on an FV566 steel specimen to obtain time–displacement curves. A set of uniaxial tensile tests is also performed on FV566 steel material to explore its strain field at 1200 °C.

Experimental investigation of OSB sheathed timber frame shear walls with strong anchorage subjected to cyclic lateral loading

Sheathed timber frame shear walls (STFSW) with strong anchorage have recently been studied to be used as the main lateral force resisting system in timber buildings. The first part of this research project highlighted the enhancement of the shear stiffness and strength of the STFSWs with strong anchorage in comparison with the established configuration of light-frame timber shear walls under monotonic horizontal loading. In this paper, the in-plane seismic behavior of STFSWs with strong anchorage was further investigated by means of quasi-static monotonic and cyclic loading tests on large-scale timber shear walls. The boundary conditions of the specimens were considered more realistically by applying a combination of vertical force and bending moment on top of the walls. In addition to the ISO 21581, the walls were tested under a new horizontal cyclic loading protocol especially developed for the low-to-medium seismicity regions. The STFSWs with strong anchorage showed a marginally higher shear resistance under cyclic loading. Nevertheless, in terms of ultimate displacement, they did not evidently perform as well as under monotonic loading when subjected to cyclic loading. Withdrawing of staples was the main failure of the stapled connections resulting in the out-of-plane displacement of the sheathing panels, which was clearly observed by the 3D digital image correlation measurement system. The low-to- moderate seismicity loading protocol was not highly influential on their shear resistance and displacement capacity.

Experimental Investigation on Buckling and Post-buckling Behavior of Superelastic Shape Memory Alloy Bars

This paper examines the buckling and post-buckling behavior of superelastic shape memory alloy (SMA) bars. A NiTi SMA bar with a diameter of 12 mm was used in all the experimental tests. First, the tensile and compression responses of NiTi bar were characterized under monotonic loading up to failure. A total of 15 specimens with slenderness ratios that range from 25 to 115 were tested to study the critical buckling load and post-buckling behavior of SMA bars. Digital image correlation (DIC) system was implemented to monitor full-field surface displacements. The interaction between material nonlinearity due to phase transformation and geometric nonlinearity was explored. Data obtained from the DIC measurement system were further analyzed to identify the onset of buckling and to extract experimental critical buckling loads. The effect of loading rate on buckling response of SMAs was investigated by conducting additional testing at higher loading rates on the specimens with three selected slenderness ratios. The temperature field on the surface of the specimens was recorded by an infrared camera. The analytical critical buckling loads were computed and compared with experimental results. All specimens exhibited a unique buckling behavior characterized with almost a complete shape recovery upon unloading.

Experimental and Numerical Investigations of Thin-Walled Stringer-Stiffened Panels Welded with RFSSW Technology under Uniaxial Compression.

Many aircraft structures are thin walled and stringer-stiffened, and therefore, prone to a loss of stability. This paper deals with accurate and validated stability analysis of the model of aircraft skin under compressive loading. Both experimental and numerical analyzes are conducted. Two different methods of joining panel elements are considered. In the first case, the panel is fabricated using rivets. In the second variant, the refill friction stir spot welding technique is used. Both types of panels are loaded in axial compression in a uniaxial tensile testing machine. The geometrically and physically nonlinear finite element analyzes of the panels were carried out in ABAQUS/Standard. The Digital Image Correlation measurement system ARAMIS has been utilized to monitor the buckling behavior and failure mode in the skin-stringer interface of the stiffened panels. The results of experiments and the digital image correlation system are presented and compared to the numerical simulations.

Laboratory Observations of Repeated Interactions between Ruptures and the Fault Bend Prior to the Overall Stick-Slip Instability Based on a Digital Image Correlation Method

Fault geometry plays important roles in the evolution of earthquake ruptures. Experimental studies on the spatiotemporal evolution of the ruptures of a fault with geometric bands are important for understanding the effects of the fault bend on the seismogenic process. However, the spatial sampling of the traditional point contact type sensors is quite low, which is unable to observe the detailed spatiotemporal evolution of ruptures. In this study, we use a high-speed camera combined with a digital image correlation (DIC) method to observe ruptures during stick-slip motions of a simulated bent fault. Meanwhile, strain gages were also used to test the results of the DIC method. Multiple cycles of the alternative propagation of ruptures between the two fault segments on the both sides of the fault bend were observed prior to the overall failure of the fault. Moreover, the slip velocity and rupture speed were observed getting higher during this process. These results indicate the repeated interactions between the ruptures and the fault bend prior to the overall instability of the fault, which distinguishes the effect of the fault bend from the effect of asperities in straight faults on the evolution of ruptures. In addition, improvement in the temporal sampling rate of the DIC measurement system may further help to unveil the rupture evolution during the overall instability in future.

Experimental study on alarming of concrete micro-crack initiation based on wavelet packet analysis

Wavelet packet analysis and digital image correlation (DIC) have been applied extensively owing to their respective advantages. In this work, a novel approach is presented to realize micro-damage alarming and identification of concrete by the coupling of wavelet packet transform and DIC technology. DIC measurement system is utilized to measure the damage strain field of static three-point bending of concrete specimen. According to excellent characteristic of time-frequency localization of wavelet packet analysis, the strain nephograms are decomposed by wavelet packet transform. Through wavelet packet analysis, the eigenvectors of the component energy are constructed, based on which the alarming index of concrete micro-crack initiation is obtained. The experimental results show that our novel approach can effectively predict the micro-crack initiation and can automatically achieve alarming. Furthermore, the initiation position of micro-cracks and micro-damage degree can be accurately determined by damage strain field. It is beneficial to damage prediction and safety control of concrete structures.

Acoustic emission analysis of cyclically loaded superelastic shape memory alloy fiber reinforced mortar beams

Superelastic shape memory alloys (SMAs) are metallic alloys that can recover their nonlinear deformations upon unloading. The use of SMAs in cementitious composites as fibers can enable crack recovery and re-centering capabilities. In this study, the crack recovery characteristics of SMA fiber reinforced mortar specimens subjected to flexural cyclic loading are studied through acoustic emission analysis. SMA fiber reinforced mortar specimens with fiber volume ratios of 0.3%, 0.5% and 1.0% are tested under cyclic flexural loads. Acoustic emissions (AE) signals are captured and analyzed to characterize the crack formation and crack width development at each cycle of loading. In addition, the crack width propagation of the specimens is monitored by Digital Image Correlation measurement system and is used as a basis in AE analyses. Results show that the SMA fiber reinforced mortars exhibit considerable re-centering and crack recovery characteristics at large deformation amplitudes.

Interrelation between local strain jumps and temperature bursts due to the jerky flow in metals under the complicate loading conditions: experimental study by using the DIC-technique and the IR-analysis

The present work focused on the complex study of the influence of the stress concentrators, the loading system stiffness, and the additional cyclic impact on the behavior patterns of the Al-Mg alloy and the carbon steel during uniaxial tension tests with the use of the original samples with complicated geometry. The aim of the investigation was the complex study of the temporal instabilities and spatial localization due to the Lüder’s behavior, the Portevin–Le Chatelier effect and the shoulder or necking effect with the use of the non-contact 3-D Digital Image Correlation measurement system Vic-3D (Correlated Solutions) and the infrared system FLIR SC7700. This study contains a brief description of the test procedure and the experimental results of providing mechanical tests on the flat dog-bone specimens and the specimens with stress concentrators, with the additional deformable parts and the complicated samples with two L-shaped holes.

PS0025-360 DIC計測を用いた均質化法のマルチスケールV&V

In this study, tensile tests of plate-fin structures with a digital image correlation (DIC) measurement system are conducted to investigate validity of the homogenization analysis method developed by our research group. For this, tensile tests using plate-fin test specimens made of an epoxy are performed, and their microscopic deformations are measured by a DIC measurement system. Then, corresponding elastic-viscoplastic analysis of plate- fin structures is carried out using the homogenization theory for nonlinear time-dependent materials. The analysis results obtained are compared with the experimental results to validate the present method.

291 プレートフィン構造体を用いた均質化法のマルチスケール的妥当性評価

In this study, tensile tests of plate-fin structures with a digital image correlation (DIC) measurement system are conducted to investigate validity of the homogenization analysis method developed by our research group. For this, tensile tests using plate-fin test specimens made of an epoxy are performed, and their microscopic deformations are measured by a DIC measurement system. Then, corresponding elastic-viscoplastic analysis of plate- fin structures is carried out using the homogenization theory for nonlinear time-dependent materials. The analysis results obtained are compared with the experimental results to validate the present method.

Study on the Improvement of the Image Analysis Speed in the Digital Image Correlation Measurement System for the 3-Point Bend Test

Machine material and structural strain are critical factors for appraising mechanical properties and safety. Particularly in three and four-point bending tests, which appraise the deflection and flexural strain of an object due to external force, measurements are made by the crosshead movement or deflection meter of a universal testing machine. The Digital Image Correlation (DIC) method is one of the non-contact measurement methods. It uses the image analyzing method that compares the reference image with the deformed image for measuring the displacement and strain of the objects caused by external force. Accordingly, the advantage of this method is that the object's surface roughness, shape, and temperature have little influence. However, its disadvantage is that it requires extensive time to compare the reference image with the deformed image for measuring the displacement and strain. In this study, an algorithm is developed for DIC that can improve the speed of image analysis for measuring the deflection and strain of an object caused by a three-point bending load. To implement this algorithm for improving the speed of image analysis, LabVIEW 2010 was used. Furthermore, to evaluate the accuracy of the developed fast correlation algorithm, the deflection of an aluminum specimen under a three-point bending load was measured by using the universal test machine and DIC measurement system.

Detection and Quantification of Slip Along Non-Uniform Frictional Discontinuities Using Digital Image Correlation

A deformation measurement system based on the principles of digital image correlation (DIC) has been developed to evaluate the process of slip along frictional discontinuities. A biaxial compression apparatus is used to impose shear failure on perfectly mated gypsum specimens with nonhomogeneous contact surfaces. The contact surfaces are made by casting gypsum against flat surfaces with different frictional characteristics and consisted of a smooth surface with low frictional strength on the upper half and a rough surface with high frictional strength on the lower half. Design, implementation, and verification of the DIC measurement system are presented in this paper. DIC successfully identified slip as a jump in the displacement field across the discontinuity. Slip is observed to initiate from the smooth surface with minimum frictional resistance and as the shear load is increased, propagates to the rough surface that has higher frictional resistance. DIC clearly exhibits a reduction in fracture’s shear stiffness based on an increase in the rate of relative vertical displacement across the discontinuity, which initiates from the smooth surface and propagates to the rough surface.

Singularities in using Vic-3D™ Digital Image Correlation software and its application to the study of inelastic strain fields

The present paper deals with the numerical processing of experimental data recorded by the Vic-3D digital image correlation measurement system, which provides estimation of the strain fields in the tension tests of flat Al-Mg alloy specimens. The influence of the subset and the step size selection on the inelastic strain field configuration has been studied by the algorithms for motion estimation in successive images. It is shown that these parameters have different effects on the accuracy of the digital image correlation analysis, the size of the marginal zone and the refinement of the strain fields. The paper contains the results of estimation of inhomogeneous strain fields due to the Portevin-Le Chatelier effect in the tension tests of the Al-Mg alloy flat specimens obtained using the Vic-3D digital image correlation measurement system.