Image Correlation Measurements Research Articles

Identification of Stop Criteria for Large-Scale Laboratory Slab Tests Using Digital Image Correlation and Acoustic Emission

Advanced monitoring methods are required to identify stop criteria in proof-load tests. In this study, the combined methodology of two-dimensional digital image correlation and acoustic emission is investigated for its applicability for future implementation in field tests. The two monitoring systems are deemed to provide valuable insight with external measurements from digital image correlation and internal measurements from acoustic emission. Two overturned T-section reinforced concrete slabs (0.37 × 1.7 × 8.4 m) tested under laboratory conditions are used for the assessment. The first slab test served as a preliminary test to enable sensor placement and creation of a relevant loading protocol. The main scientific results lead to a proposal for a test procedure using the combined methodology based on results, observations, and experiences from an individual stop criteria assessment for the two methods. The results include full-field plots, an investigation of the time of crack detection and monitoring of crack widths with digital image correlation, and a qualitative assessment of activity vs. load followed by a quantitative evaluation of calm ratios using acoustic emission. The individual results show that both digital image correlation and acoustic emission can identify damage occurrence earlier than other secondary methods. At crack detection (415 kN), crack widths were measured at widths between 0.078 mm to 0.125 mm and can be monitored until reaching the stop criterion at 463 kN (Eurocode SLS threshold of wmax = 0.2 mm). The acoustic emission results were limited by the pre-defined loading protocol and thus, only indicated that damage occurred sometime between 300 kN and 500 kN (pre-defined load levels). Therefore, the proposal for test procedure involves a methodology, where the loading protocol may be updated during testing based on monitoring results and thus provide even more valuable data.

Super Resolution Digital Image Correlation (SR-DIC): an Alternative to Image Stitching at High Magnifications

BackgroundHigh-resolution Digital Image Correlation (DIC) measurements have previously been produced by stitching of neighboring images, which often requires short working distances. Separately, the image processing community has developed super resolution (SR) imaging techniques, which improve resolution by combining multiple overlapping images.ObjectiveThis work investigates the novel pairing of super resolution with digital image correlation, as an alternative method to produce high-resolution full-field strain measurements.MethodsFirst, an image reconstruction test is performed, comparing the ability of three previously published SR algorithms to replicate a high-resolution image. Second, an applied translation is compared against DIC measurement using both low- and super-resolution images. Third, a ring sample is mechanically deformed and DIC strain measurements from low- and super-resolution images are compared.ResultsSR measurements show improvements compared to low-resolution images, although they do not perfectly replicate the high-resolution image. SR-DIC demonstrates reduced error and improved confidence in measuring rigid body translation when compared to low resolution alternatives, and it also shows improvement in spatial resolution for strain measurements of ring deformation.ConclusionsSuper resolution imaging can be effectively paired with Digital Image Correlation, offering improved spatial resolution, reduced error, and increased measurement confidence.

Multi-instrumented dynamic loading experiments on laminated glass

Describing quantitatively the response of laminated glass to low-velocity (~5 m/s) impacts by rigid bodies is an important issue because of its significance in terms of structural degradation and integrity, key parameters for people safety and anti-intrusion performances. This study aims to address the formation of cracks during graveling and steel ball drop tests, so, two well-instrumented experimental set-ups are proposed to study cracking in reproducible conditions. The first device can be seen as a mini-Hopkinson bar system, which from two strain gauges, allows to estimate force and velocity at a sharp indent tip. The second device, reproducing a blunt impact, exploits stereo-Digital Image Correlation (D.I.C.) measurements of the laminated glass surface.

Static Residual Tensile Strength Response of GFRP Composite Laminates Subjected to Low-Velocity Impact

The dependency of the static residual tensile strength for the Glass Fiber-Reinforced Plastic (GFRP) laminates after impact on the impact energy level and indent shape is investigated. In this study, two different laminates, unidirectional, [0°2]s) and TRI (tri-axial, (±45°/0°)2]s), were prepared using the vacuum infusion method, and an impact indent on the respective laminates was created at different energy levels with pyramidal and hemispherical impactors. Impact damage patterns, such as matrix cracking, delamination, debonding and fiber breakage, could be observed on the GFRP laminates by a scanning electron microscope (SEM), and it is found that those were dependent on the impactor head shape and laminate structure. Residual in-plane tensile strength of the impacted laminates was measured and the reduction of the strength is found to be dependent upon the impact damage patterns. Furthermore, in this study, stress concentrations in the vicinity of the indents were determined from full-field stress distribution obtained by three-dimensional Digital Image Correlation (3D DIC) measurement. It was found that the stress concentration was associated with the reduction of the residual strength for the GFRP laminates.

Effect of Laser Parameters on Microstructure and Fracture Properties of Repaired Cracks with Micro/Nano Material Addition

Abstract Compact tension specimens with prefabricated cracks are repaired by laser with micro/nano materials added at the crack tip. Different combinations of laser parameters, i.e., laser power, laser spot diameter and heating time, were applied to investigate their effects on the microstructures and fracture properties of repaired specimens. J-integrals were calculated according to the digital image correlation (DIC) measurements, and microstructures were examined using scanning electron microscopy (SEM). When suitable laser parameters were used, homogeneous and compact equiaxed grains with no cracks, pores, and agglomeration are observed in the repaired layer, and the J-integral of repaired specimens is approximately 20% less than that of unrepaired specimen. The paper reveals relations among laser parameters, J-integrals, and microstructures, and provides a guideline for the selection of laser repair parameters.

Learning Localized Spatial Material Properties of Substrates in Ultra-Thin Packages Using Markov Chain Monte Carlo and Finite Element Analysis

Thinned silicon dies and thin substrates using thin core and coreless structures have enabled thin packages. For robust manufacturing and reliability of these parts, solving the warpage problem is key. While current finite element methodologies can provide some insights at the design stage, these simulations are only as accurate as the inputs such as the material properties and the stress-free temperatures. Electronic substrates are especially challenging to characterize and model as they are laminates consisting of a core with layers of resin and metal lines on either side. In this work, a hybrid approach using Markov Chain Monte Carlo (MCMC) and Finite Element Analysis (FEA) is used to learn the spatially varying properties of the substrate from Digital Image Correlation (DIC) measurements of the warpage. The analysis is carried out at room temperature and at an elevated temperature point. Image analysis on electrical artwork is also carried out to correlate the material properties to the substrate metal density. These results will be useful to package and substrate designers to understand how material properties vary over the substrate and how temperature and metal density affect material properties so that robust design for future packages to minimize warpage can be initiated by careful routing of metal lines depending on the locally desired properties of the stack.

Monitoring of the nano-structure response of natural clay under mechanical perturbation using small angle X-ray scattering and digital image correlation

This paper introduces a new experimental method to monitor the evolving intra-particle, nanometre-scale response during hydro-mechanical tests on undisturbed wet clay samples using small angle X-ray scattering (SAXS). The method uses a newly developed miniature plane-strain one-dimensional compression cell that facilitates simultaneous full-field surface displacement measurements using digital image correlation and SAXS measurements. The 60–120 s acquisition times offered by SAXS at synchrotron facilities are beneficial to study time-dependent mechanisms in clays. The experimental results presented indicate that, for the natural sensitive clay tested in this work, no significant structural changes occur at the intra-particle scale during loading, even when large strains are measured at the macro-scale. In addition, changes that are observed at this scale occur after the end of perturbation, i.e. the creation of new intra-particle structures. Further information is obtained on the orientation evolution of the fabric by the comparative analysis of the individual mineral component response.

In vitro biomechanical comparison of headless compression versus cortex screws for fixation of simulated midbody proximal sesamoid bone fractures in horses

A midbody fracture of a proximal sesamoid bone (PSB) is a severe injury, especially in racehorses. Screw fixation may be the only option for horses to return to athletic activity. In a cadaveric biomechanical study, fixation of simulated transverse midbody PSB fractures with a 4.5 mm headless compression screw (HCS) was compared to the fixation using a 4.5 mm cortex screw (CS) inserted in lag fashion. The front limbs of 8 horses were prepared with a standardized midbody transverse osteotomy in each medial PSB. The left or the right limb of each pair was randomly assigned to the CS or HCS group. Fracture reduction and fixation was controlled by radiography. Markers were fixed proximal and distal to the osteotomy to document gap opening by an image correlation measurement. Cyclic compressive loading was applied with a hydraulic cylinder in an axial manner leading to tensile cyclic loading in the PSBs. After mechanical testing all limbs were radiographed to document the mode of failure. The mean cycles to total failure were 27,803 for the HCS group and 36,624 for the CS group, respectively. A mean of 14,444 cycles (HCS group) versus 27,464 cycles (CS group) was recorded at a fracture gap opening of 10 %. The mean value of initial stiffness was 300 N/mm (HCS group) versus 590 N/mm (CS group). On post-testing radiographs, failure by screw breakage was detected in 3/8 limbs of the HCS group and in 2/8 limbs of the CS group. Screw pullout and bone failure was noted in 5 limbs in each group. In one limb of the CS group, the medial suspensory branch was disrupted and the fracture fixation was intact. None of the differences between HCS and CS constructs reached statistical significance. However, the lower absolute numbers of cycles sustained before failure, at 10 % gap opening and lower initial stiffness of HCS constructs as well as limitations of the study warrant further investigation before clinical use of the HCS for this indication can be recommended.

Displacement field denoising for high-temperature digital image correlation using principal component analysis

ABSTRACTPrincipal component analysis (PCA) was extended to minimize the noise effect in digital image correlation (DIC) measurement under a high-temperature atmosphere environment. First, the principle of PCA was introduced, and the singular vectors and singular values for each component of the displacement fields from DIC were obtained. Then, the simulated high-temperature speckle images were developed to investigate the influences of noise on the DIC method under a high-temperature environment. Finally, the displacement fields of several special conditions were extracted from the simulated speckle images and experimental images; the effects of noise on the PCA were also analyzed.

Combining Digital Image Correlation and Thermal Measurements

In this paper we present a calibration method, which enables a combining of 3D Digital Image Correlation and termographic systems for simultaneous measurements of displacements, strains and temperature. It concerns a model of a calibration target, that works for visible and infrared light spectrum and calibration method. The working system was applied for three interesting cases: monitoring of a tank filled with liquid, a heated plexi board and a human arm during muscle contraction.

Measurement of Strain Gradients Using Digital Image Correlation by Applying Printed-Speckle Patterns

Digital image correlation (DIC) measurements were obtained using speckle patterns produced using two different methods. In the first case, speckles were produced on the surface of a test specimen using white and black spray paint, which is probably the most common approach. In the second case, a computer code was written that creates a random-speckle pattern digitally. The digital-speckle pattern was then printed on white paper using a standard laser printer. The speckled paper was subsequently bonded to the surface of a test specimen using a commercial strain gage adhesive. Strains were measured using both types of speckle patterns for a thin plate with hole and for a U-shaped beam loaded in tension. Strain fields measured using the painted and digital-speckle patterns are essentially identical and are well predicted by the corresponding elasticity solutions.

Fatigue Crack Growth Rates and Tensile Strength of Titanium Produced by Means of Selective Laser Melting

Mini-samples technique was utilized to determine mechanical properties of technically pure titanium produced by means of selective laser melting (SLM). Full-field digital image correlation (DIC) measurements and inverse method were applied for crack tip position and stress intensity factors calculations in the case of fatigue crack growth rate tests. DIC was also used for strain measurement during tensile tests on sub sized samples. There was studied the influence of samples orientation on the mechanical properties of mini-samples. Samples were cut out from rectangular cubes and were oriented with 0°, 45° or 90° angle to the direction of laser beam travel. There were also tested samples directly produced via SLM. Additionally microstructure observations were performed to verify the quality of SLM processed materials and explain mechanical properties variations.

Interfibre Failure Characterisation of Unidirectional and Triax Glass Fibre Non-Crimp Fabric Reinforced Epoxy Laminates

The in-plane failure envelopes of unidirectional (UD) laminae in a UD and a Triax (0°, ±45°) laminate configuration have been investigated. The two laminate configurations have been characterised by testing off-axis specimens in uniaxial tension and compression at different angles relative to the fibre direction and further by Iosipescu shear tests. Strain gauge and Digital Image Correlation (DIC) measurements were used to measure the deformation states during loading, and to record the stress-strain responses to identify the initiation of failure and investigate the heterogeneity of the material and possible parasitic effects. A novel analysis methodology to determine the so-called ‘failure initiation strength’ based on the second derivative of the stresses with respect to the strains has been adopted. The experimentally determined ‘failure initiation stresses’ were compared with predictions from the commonly applied Maximum Stress, Tsai-Wu, and Puck failure criteria. From this work, a thorough comparison of the UD and Triax failure envelopes has been facilitated. It is shown that failure prediction for the Triax laminate based on the failure envelope derived from UD lamina tests may be too conservative in comparison with fitting a failure criterion directly to the Triax laminate test data. The latter approach implies that the Triax laminate is considered as a single lamina with homogenised properties, which in principle violates the theoretical background of the considered failure criteria, since these are established to predict failure for a UD lamina. However, the simple homogenisation is shown to be a useful design oriented approach for providing a simple estimation of the onset of failure in laminate configurations composed of e.g., multiple layers of Triax. Thus, a reliable and efficient approach is offered for the structural integrity assessment, which takes the non-crimp fabric configurations directly or ‘as delivered’ into account.

Temperature and Displacement Fields Measurements to Assist the Numerical Simulation of the Welding-Brazing Process of Thin Automotive Parts

Car manufacturers use many different mechanical or thermo-mechanical processes to assemble thin metal sheets. Each of these processes may induce –more or less localizedundesirable deformations of the assembled sheets. The numerical simulation of these deformations via the use of predictive thermo-mechanical finite element analyses may help and reduce development time and cost. To validate such complex non-linear numerical simulations by comparison of the numerical results with experimental evidence, it is necessary to develop well-controlled and highly instrumented tests. Welding-brazing is one the various assembly processes involving a heat source used by car manufacturers to assemble thin –typically 0.7 to 1mm thick- metal sheets. Predictive heat transfer simulations of this process require accurate modelling of both the heat source and the boundary conditions imposed onto the assembled sheets –including the contact between the sheets. Using ABAQUS general-purpose finite element code for this study, the welding-brazing heat source is modelled as the combination of a 3D volumetric source corresponding to the welding-brazing joint, and a 2D surface source corresponding to the heat flux emitted by the laser beam onto the sheets in the vicinity of the joint. Besides, the contact between the sheets is modelled by a thermal resistance depending on the pressure between the sheets when they are in contact, or the distance between them when they are apart. Identification of heat source and contact models is derived from the results of weldingbrazing tests instrumented with thermocouples and infrared cameras. Micrographic observations in the vicinity of the welding-brazing joint are used to (partially) validate the numerical predictions of the Heat Affected Zone (HAZ). After validation of this heat transfer numerical analysis, the thermo-mechanical analysis of the process will be validated by comparison of the predicted displacement fields with the results of 3D Digital Image Correlation (stereo-correlation) measurements made during the welding-brazing tests.

Non-Woven Flax Fibre Reinforced Polypropylene: Static and Low Velocity Impact Behaviour

Non-woven flax fibre reinforced new and recycled polypropylene were manufacturing by an industrial process of carding, batt, needle-punching and thermo-pressing. Polypropylene fibres were mixed with flax fibres in the carding process forming a thick multi-layered mat which was used for thermo-compression process. Effects of multi-layered non-woven manufacturing on mechanical properties were analyzed in different composites with three fibre areal weights. The results highlight the anisotropic behaviour of the obtained composites and a constant ratio of absorption energy to impact energy. In addition, cyclic tests and acoustic emission method coupled with image correlation measurement were specially employed in order to detect the damage development process in the composites and to qualify the deformation process in tensile test.

Application of High Speed Image Correlation for Measurement of Mode Shapes of a Car Bonnet

For the analysis of vibrations and mode shape extraction in particular the use of optical full-field measurement techniques has grown during the last years. Beside techniques like Digital Speckle Pattern Interferometry, Moiré, Thermography or Photoelasticity the Digital Image Correlation techniques have already been successfully proven to be an accurate displacement analysis tool for a wide range of applications.

Scanning Laser Image Correlation (SLIC) Measurements in Zebra Fish Larvae

3003‐Pos Scanning Laser Image Correlation (SLIC) Measurements in Zebra Fish Larvae Molly J. Rossow, William W. Mantulin, Enrico Gratton. University of California Irvine, Irvine, CA, USA. Scanning Laser Image Correlation (SLIC) is a technique to measure the flow of small particles and to observe local flow patterns over an area. SLIC can be applied to situations including blood cells flowing through blood vessels or tracer particles flowing through microfluidic channels. The main advantage of SLIC over other flow measurement techniques is that SLIC can be scaled to measure flow in areas ranging from microns to centimeters wide. To accomplish this, an image is first acquired through laser scanning and analyzed with number and brightness analysis (N&B) to identify regions of flow. This is used as a guide to manually (or potentially automatically) select a pattern within the image, such as a line along the center of a channel, that is then scanned repeatedly with the laser beam. Since the entire image is not scanned in each measurement SLIC measurements can be obtained quickly and efficiently. The results of these scanned patterns are analyzed with the recently developed pair correlation technique to extract the rate of flow and to identify characteristic flow patterns such as turbulence, particles that adhere to the channel walls, and variable velocity along the length of the channel. In the work presented here we demonstrate the effectiveness of SLIC by measuring blood flow in a zebra fish model. With SLIC, we are able to obtain blood flow measurements equivalent to those obtained with other techniques. We are also able to map the rate of flow and to observe variability in flow rate over time. This indicates that SLIC has potential to measure blood flow in other animals as well and may hold potential as the basis of a medical device. This research was supported by the National Institutes of Health (PHS 5 P41‐RR003155.)

Free edge effects study in laminated composites using Digital Image Correlation: effect of material and geometrical singularities

Composite materials are today used for various ind ustrial applications. However, delamination on free edges, where stress gradients are strong, s till remain a problem. In the aim of a better under standing of such phenomenons, Digital Image Correlation (DIC) measurements have been carried out on ((15 n/- 15n)2)s laminates under uniaxial tensile strain. Three dif ferent composites with different mechanical properties and microstructure have been tested as w ell as two samples geometries: flat and with ply dr op. Experimental results show high shear strain concent rations near 15°/-15° interlaminar interfaces on fr ee edges which depend on material mechanical properties and microstructure and increase in the vicinity o f a geometrical singularity.

Assessment of High Speed Imaging Systems for 2D and 3D Deformation Measurements: Methodology Development and Validation

Ultra high-speed and moderate speed image acquisition platforms have been characterized, with special emphasis on the variability and accuracy of the measurements obtained when employed in either 2D or 3D computer vision systems for deformation and shape measurements. Specifically, the type of image distortions present in both single channel cameras (HS-CMOS) and multi-channel image intensified cameras (UHS-ICCD) are quantified as part of the overall study, and their effect on the accuracy of experimental measurements obtained using digital image correlation have been determined. Results indicate that established methods for noise suppression and recently developed models for distortion correction can be used effectively in situations where the primary intensity noise components are characterized by minimal cross-talk and stationary spatial distortions. Baseline uniaxial tension experiments demonstrate that image correlation measurements using high speed imaging systems are unbiased and consistent with independent deformation measurements over the same length scale, with point-to-point strain variations that are similar to results obtained from translation experiments. In this study, the point-to-point variability in strain using the image intensified system is on the order of 0.001, whereas the non-intensified system had variability of 0.0001. Results confirm that high speed imaging systems can be utilized for full field two and three-dimensional measurements using digital image correlation methods.