DIC Technique Research Articles

Characterization and Uncertainty Analysis of the Interlaminar Inelastic Properties of Unidirectional Fibre-reinforced Composites

The compression failure mode has been achieved through changing the SBS (Short Beam Shear) experiment parameters, and the nonlinear constitutive relationship along thickness direction for the typical specimens loaded in 1-3 principal material plane has been computed iterative by FEMU with the reconstruction strain field and the numerical simulated stress data of FEM model. The uncertainly of the constitutive parameters is induced by the measurement system noise in the DIC technique and the approximation error in the displacements and strains smoothing algorithm. The covariance matrix of the extracted material constitutive parameters has been given explicitly. Three material constitutive parameters were identified from a customized short-shear experiment simultaneously using an estimated optimal reconstruction mesh size as an illustration. Sensitivity of measurement noise and reconstruction parameter on extracted material properties has been investigated. The effects of region of interest (ROI) and DIC image number on uncertainties of extracted material properties have been addressed. It is suggested that there exist an appropriate ROI and the number of images, from which reliable material parameters can be identified, but much more data used in identification process always lead to smaller standard deviation and COV. It is observed that the material constants used to characterize the interlaminar stress-strain behavior show strong robustness to the measurement noise. Another key finding is that the reconstruction parameter in the global finite-element based approximation approach is critical for reliable material properties identification. Its value has to stay close to optimum for guaranteeing reliable identification of material properties.

Flexible graphite as beam dumping material in the TDE blocks of the Large Hadron Collider

The Large Hadron Collider (LHC) is a high energy particle collider at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland. The LHC Beam Dumping System (LBDS) is composed by different equipment employed to extract and absorb the LHC circulating beam in case of need. At the end of the LBDS resides the LHC main dump (so called TDE, which stands for Target Dump External). The TDE block is constituted by several graphite blocks with different densities enclosed in a 318L stainless steel jacket. Among the different type of carbon-based materials, the flexible graphite is the one owning the lowest density (1-1.2 g/cm3). It differs from typical graphite forms such as polycrystalline and pyrolytic graphite in that no binder is added during the production process. The bonding frictional forces due to particle asperities give the typical flexibility to the material and contribute to the deformation mechanism. In order to predict the thermo-mechanical response to the proton beam-induced sudden energy deposition, the material behavior needs to be investigated in-depth in a wide range of temperature and strain-rates. In this preliminary work, the static properties of a commercial flexible graphite (Sigraflex® from SGL Carbon) have been observed at room temperature in the in-plane direction. Two sides DIC technique has been employed in order to get a reliable measurement of the strain on both front and edge specimen surfaces; the crosshead displacement-rate was varied between 0.01-10 mm/min. Finally, a discussion about the stress-strain behavior and the deformation mechanism has been given.

Effects of physical aging on thermomechanical behaviors of poly(ethylene terephthalate)-glycol (PETG)

ABSTRACTA series of experiments are performed to investigate the effects of physical aging on thermomechanical properties of poly(ethylene terephthalate)-glycol (PETG). The DMA, DSC and XRD tests are used to characterize the change of dynamic property, heat capacity and microstructure with annealing treatments. The uniaxial tension tests together with the DIC technique are employed to describe the failure behaviors of PETG. The results show that the annealed polymers exhibit a larger maximum local strain and fail at a smaller displacement. The type and size of the geometry defects can also affect the strain localization and final failure behaviors of PETG.

The effect of debinding and sintering parameters on the mechanical and microstructural properties of Fe-2Ni metal injection molded compacts

This paper investigates the effect of debinding and sintering parameters on the density, ultimate tensile strength (UTS), hardness and grain size of the Fe-2Ni MIM compact. For this purpose, a wax-based binder system containing PE, PP, PW and SA was adopted for developing the feedstock. After optimizing the feedstock powder loading based on the critical solids loading and studying the homogeneity of feedstock, green parts were injected. The samples were then immersed in the n-hexane solution for different times to investigate the effect of debinding time on the weight loss of binder system. After observing the samples shrinkage behavior, sintering temperatures were selected and the samples were sintered at temperatures of 1050, 1125, 1200, 1275 and 1350 °C. Finally, the microstructure, density and hardness of the sintered samples were evaluated and the UTS of the samples was measured using the DIC technique. The results showed that immersion of samples for 12 h in n-hexane with 50 °C temperature applied as optimum parameters for solvent debinding. In addition, appropriate thermal debinding cycle was designed through the TGA analysis to produce flawless samples. It was observed that the density increases with the sintering temperature increasing, until where in the temperature of 1350 °C the relative density was 97.2%. UTS and hardness values of the sintered samples increased to (∼548 MPa) and (∼144 HV) for the sintered specimen at 1275 °C and decreased at higher temperature. This phenomenon relates to the substantial grain growth in the sintered sample at 1350 °C, as proved by the results of EBSD analysis. Finally, proper sintering condition for Fe-2Ni samples derived through the density, UTS, hardness and microstructural evaluations, so that sintering of the samples at 1275 °C under hydrogen atmosphere showed the optimum condition due to an excellent balance between the mechanical properties and grain size in the samples.

In situ observation of interactive failure modes in a single-edge notched symmetric cross-ply laminate using synchrotron X-ray tomography

Interactive failure mechanisms of a single-edge notched (SEN) symmetric cross-ply laminate subjected to tensile loading are studied in situ using synchrotron X-ray tomography and a DIC technique at the micro- and macro-scales. The SEN configuration guarantees the repeatability and consistency of the progressive and interactive failure behavior at the two different scales. The evolution of various fracture modes in different layers is observed through synchrotron radiation CT images. Due to the geometrically asymmetric configuration, all the initial cracks sequentially occurring near the notch tip can be captured in a narrow field of view. Stress relaxation due to the subsequent fracture initiations is indirectly captured by measuring the crack lengths and opening displacements from the 3D CT images at different loading steps. The stress relaxation is directly measured by DIC analysis using the macro-scale test data. DIC results show strains relaxed in the region between existing and emerging transverse cracks.

Complex deformation of naturally fractured rocks

Strain is a key parameter in the evaluation of rock mechanical properties and failure behavior. Strain is usually measured using strain gauges, however, this laboratory method has inherent uncertainty in the attachment of sensitive gauges to the rock surface and it is limited to one-point strain measurement. In complex geometries such as rock samples with natural fractures or lamination, that classical approach cannot provide the whole picture of strain development in such complex geometries. To overcome these challenges, the application of optical technique with image processing is proposed. The optical technique has the advantage of being a non-contact method and it provides quantitative visualization of full-field strain measurement.To examine quantitative visualization of strain development, DIC technique was first conducted on a homogeneous sandstone sample as a benchmark. The results verify that the maximum tensile strain is developed at the central vertical line of the sample as it was expected from theoretical solutions. This DIC measurement matches the post-failure fracture pattern of the specimen. Then the technique was applied on the Buda Limestone samples with horizontal, sub-horizontal or multiple complex natural fractures (sealed or partially open) and inclusions while the samples were under diametrical compression experiment. The results of this study provide further insight into the role of natural fractures on induced fracture initiation and propagation under this specific testing condition. Horizontal natural fractures are planes of maximum compressive strain which can delay the propagation of vertical tensile fractures. The maximum amount of tensile strain usually initiates at the boundary of these natural fractures and vertical tensile fractures usually deviate toward horizontal fracture for a short distance before turning back to propagate vertically following the maximum stress path. Full-field surface deformations are mapped optically in the crack–inclusion vicinity where DIC results showed that crack front is arrested by the inclusion for the complete fracture event. In contrast to the failure pattern in homogenous sandstone specimen, complex fracture pattern was observed on several specimens with multiple fracture network. Fracture slippages along the natural fracture, fracture deviation was observed in samples with steep inclined natural fractures.The results of this study can help us to better understand the complex failure mechanism in naturally fractured media and elaborate on various mechanisms of failure under diametrical compression tests. DIC is a powerful technique in providing the dynamic strain development over the surface of the sample in real-time, thus it can provide the fracture initiation and propagation in such heterogeneous rock media.

Frequency domain triangulation for full-field 3D operating-deflection-shape identification

Abstract The use of high-speed camera systems in vibration measurements is typically limited to identifying motion, transversal to the optical axis, due to an inherent limitation of 2D imaging systems. Depth information, lost in the imaging process, can be recovered by using the well-established 3D DIC technique, but is still limited to a single face of the object, observed by the stereo pair. In this research a full-field 3D operating-deflection-shape measurement technique, based on frequency-domain triangulation of image-data, is presented. A mathematical model of frequency-domain perspective transformation of small harmonic motion is introduced. This model is used to relate multiview image data to spatial amplitude spectra of the observed displacement. Using the developed method, spatial small harmonic motion of arbitrary-shaped specimen can be identified in the frequency domain using only a single, moving high-speed camera, extending the field-of-view of the established image-based vibration measurement methods.

Thermal damage of CFRP laminate in fiber laser cutting process and its impact on the mechanical behavior and strain distribution

Fiber laser cutting is a promising alternative to the conventional methods in machining CFRP with high efficiency, while the influence of machined quality on strain distribution and mechanical behavior of CFRP laminates is yet not fully understand. The aim of this paper is to study the effect of laser cutting parameters on thermal defects and related mechanical performance. DIC technique was successfully employed to assess full-field strain distribution under tensile loading. The relationship between strain distribution and failure evolution/mode was also investigated. Results showed that various thermal defects including matrix recession, resin decomposition, fiber burrs and delamination were observed on entry and exit surfaces, while microcracks, cavities and striations were prevalent on machined surface. Tensile strength of CFRP laminate with open hole was highly related to applied laser processing parameters. DIC technique indicated that the crack propagation and failure mode were in good agreement with the high level of strains developed around the hole. Matrix-fiber interface cracking, fiber breakage and splitting were the main failure modes at test cessation.

Temperature and strain rate dependent large tensile deformation and tensile failure behavior of transparent polyurethane at intermediate strain rates

Transparent polyurethane has been widely applied in laminated windshield glasses as the interlayer material to enhance the reliability due to its outstanding impact resistance. Under impact loading such as bird strike, the interlayer undergoes large tensile deformation mainly at intermediate strain rates (at the order of magnitudes from 100 to 103, excluding 1000/s). In addition, the interlayer is on service over a wide range of temperatures for a plane traveling around the world. The mechanical behavior of transparent polyurethane under these conditions is not fully understood. In this study, systematical experiments were performed on transparent polyurethane. The viscoelasticity of the material was firstly verified by several quasi-static cyclic tests. Then a series of large tensile deformation and tensile failure experiments were conducted under intermediate strain rates and at temperatures of −40°C to 40°C using a servo-hydraulic high-speed tensile machine. All strain data were acquired by the DIC technique. The experimental results show that tensile stress–strain curves and failure behaviors are significantly temperature and strain rate dependent. The rate-temperature equivalence was also observed. Finally, a phenomenological analysis of mechanical quantities of the material was carried out.

Numerical and experimental study on deformation of 3D-printed polymeric functionally graded plates: 3D-Digital Image Correlation approach

This paper presents a novel experimental and simulation investigation of quasi-static transverse deformation of 3D printed polymeric Functionally Graded (FG) plates, which obey the distribution of material properties through the length. Graded solid elements with continuous property distribution at different Gauss points were implemented by a user material subroutine (UMAT) in ABAQUS Finite Element (FE) software. In order to validate the proposed graded FE solutions, the original experimental deflection measurements using 3D-Digital Image Correlation (DIC) technique performed to capture transverse deformation of designed and manufactured 3D printed polymeric FG plates. DIC technique involved tracking the motion of geometric features on a specimen surface over the course of an experiment to generate the displacement field experimentally. It was found that the deviation between FE and experimental out of plane deflection for nonlinear FG plates is higher than that of linear FG plates, which is potentially due to the high gradient distribution of the material inclusions. It can be concluded that the presented 3D-digtal image correlation technique provides for transverse deformation of polymeric FG plates. The deflection contours for the FG plate are not symmetric, differences being more prominent for the linear high stiffness ratio combination. The outcomes of this study can be applied to determine the optimum material distribution to produce a controlled-stiffness polymeric FG plate corresponding to prescribed structural characteristics.

Material flow analysis in indentation process by 3D Digital Image Correlation

Focusing on the application of the 3D Digital Image Correlation technique, this work proposes a material flow analysis of an indentation process. The study establishes the methodology for the calibration and implementation of the 3D image sensing technology for deformation measurements. The purpose is to continue with the validation of DIC application on the indentation processes, where a deep penetration is achieved and extensive material flow is produced. With the 3D DIC technique is possible to perform accurate deformation measurements in non planar specimens and study the material emerging towards the exterior of the tested specimen, which is not possible with the 2D DIC technique. Although previous 2D studies were efficient detecting the flow field and von Mises strains on the specimens tested, the bulge emerging under the punch on the front surface (dead zone) could not be studied due to its predominantly 3D character. Therefore, present work implements a 3D methodology that carries out a complete deformation study, including the material flow that occurs on the Z axis, towards the exterior of the tested specimen, optimizing previous analyses.

Progressive failure behaviors and crack evolution of rocks under triaxial compression by 3D digital image correlation

In the study, a series of monotonic triaxial compression tests were carried out on Emochi andesite to evaluate the effects of confining pressure on progressive failure behaviors. The three-dimensional digital image correlation (3D-DIC) system consisting of six cameras was used to capture the images based on the transparent pressure cell. The effects of confining pressures on stress thresholds, energy evolution, field strain patterns and crack evolution were investigated qualitatively or quantitatively. The crack initiation (σci), crack damage (σcd) and peaks stress (σc) increased with confining pressure and the ratios σci/σc and σcd/σc respectively varied from 57.74% and 75.11% to 64.02% and 85.33%. Total energy U, elastic energy Ue, and dissipation energy Ud at different characteristic points increased with confining pressures. Field strain patterns indicated that localization developed rapidly in the post-peak region due to the acceleration of crack propagation and damage evolution. The failure mechanisms of Emochi andesite under the uniaxial and triaxial condition are respectively tension-shear destruction and shear destruction. Furthermore, the stress-strain response inside and outside the localization zone are characterized by strain accumulation or inelastic unloading. Finally, a method of confirming the stress level of localization initiation is proposed based on the DIC technique. The stress level decreases with confining pressure. The study provides a useful reference for understanding the failure process of rocks and improving the stability and safety of underground structures.

Underwater mechanical behavior of closed cell PVC foams under hydrostatic loading through 3D DIC technique

The underwater constitutive behavior of poly vinyl chloride foams with varying densities was investigated in this study. The experiments were conducted in an optically clear acrylic tube, which allowed for visualization of the specimen and the application of 3D Digital Image Correlation. A series of calibration experiments was conducted to investigate the applicability of the Digital Image Correlation technique for measuring the deformation of objects underwater inside of a curved acrylic tube of considerable thickness. The results of the calibration experiments demonstrated that a submerged object located in the middle of the acrylic tube appears magnified in the radial direction. This apparent magnification was taken into account during the analysis of the deformation for all underwater experiments. The hydrostatic loading was achieved by fitting the acrylic tube with a nylon piston, and compressing the piston with an Intron testing machine. Hydrostatic load of up to 5 MPa was achieved during quasi-static compression of the piston. The load applied by the Instron machine was coupled with the Digital Image Correlation data to analyze the constitutive behavior of the PVC foams. The hydraulic crush pressure, bulk modulus, and energy stored up to densification strain were determined for each foam density.

Investigations of material behaviour under monotonic tension using a Digital Image Correlation system

The paper reports behaviour of engineering materials for different kinds of notches, i.e. having U, V shapes. Their variants with respect to dimensions are illustrated in static and fatigue tests. The influence of these types of geometrical imperfections on material fatigue is presented using variations of the W¨ohler curve, number of cycles to failure and the fatigue notch factor. Results of experiments conducted by the use of the Digital Image Correlation system called 4M Aramis are illustrated. Courses of tensile characteristics of the 41Cr4 steel, obtained by means of both techniques: extensometer and DIC are compared. They indicated that the DIC technique can be a good tool for determination of mechanical properties. The equivalent strain full-field distributions on specimens tensioned with and without imperfections up to material fracture are presented. The influence of U and V notches on variations of tensile curves is shown.

Experimental results on the elevated temperature tensile response of SiC/SiC ceramic matrix notched composites

Abstract Results from a series of experimental studies on the elevated temperature response of notched SiC/SiC ceramic matrix composites are presented. SiC/SiC ceramic matrix composite notched specimens were subjected to quasi-static monotonic tensile loading at room and, 1093 ∘ C , 1204 ∘ C , and 1316 ∘ C temperature. Full field digital image correlation was employed to investigate failure of the specimens, most likely emanating from the notch. The effectiveness of a novel DIC technique at temperatures of up to 1316 ∘ C is also demonstrated. Strength measurements and differences in crack initiation and propagation are discussed.

Accurate measurement of elastic modulus of specimen with initial bending using two-dimensional DIC and dual-reflector imaging technique

Common optical methods are not suitable for accurate measurement of elastic modulus of specimens with initial bending because it is rather difficult to separate the axial strain caused by the axial load from the surface strain which consists of the axial strain and the bending strain caused by additional bending moment. Inspired by the strain-gauge technique, averaging the two strains on the opposite surfaces can eliminate the effect of bending strain of the specimen with initial bending, which is easily implemented by dual-reflector imaging and two-dimensional digital image correlation (2D-DIC). With dual-reflector imaging, the front and rear surfaces of the specimen are recorded by using a single digital camera. Consequently, the strains of two optical extensometers constructed on these two surfaces are obtained with common 2D-DIC, and averaging these strains can eliminate the effect of bending moment and out-of-plane motion of the specimen. Therefore, the elastic modulus can be determined with high measurement accuracy. The effectiveness of the proposed method was first verified using self-manufactured tensile equipment which leads to large out-of-plane motion during the test. Furthermore, uniaxial tensile tests of stainless-steel specimens, including static and continuous tensile tests, were conducted to evaluate the accuracy of the proposed method. The experimental results show good agreement between elastic modulus obtained using the proposed method and a strain gauge and the relative error between them is less than 0.5%, which shows excellent performance of the proposed method on accurate measurement of elastic modulus of specimens with initial bending.

A study of the back stress and the friction stress behaviors of Ti-6Al-4V alloy during low cycle fatigue at room temperature

Low cycle fatigue (LCF) tests were performed on Ti-6Al-4V alloy at room temperature. The fatigue hardening/softening of the Ti-alloy were determined. Both the back stress (X) and the friction stress (σF) were evaluated from analysis of fatigue hysteresis loops and their cycle-dependent behaviors during fatigue tests were obtained experimentally. It is shown that the cyclic stress response or the fatigue hardening/softening of the Ti-alloy actually results from the competition effect of the back and the friction stress behaviors during fatigue cycling. A two-slope behavior in both the εpa-2Nf and the X-2Nf plots was also obtained. The micro-mechanisms associated with the back and friction stress behaviors as well as the two-slope behavior in the εpa-2Nf and X-2Nf plots were further discussed with the help of OM and TEM observations of fatigue failure specimens at representative strain amplitudes. In this study, the DIC technique was also introduced to characterize quantitatively the cyclic deformation heterogeneity of the Ti-alloy at the grain scale.

Concrete cover effect on the bond of GFRP bar and concrete under static loading

Abstract Paper presents assessment of bond behaviour between GFRP bars and concrete, investigated through set of centric and eccentric pull-out specimens. Main parameters under investigation are 1) bar external surface, 2) concrete mechanical properties and 3) concrete cover. Corresponding tests with steel reinforcement are performed for comparison in some cases. DIC technique was used for recording and evaluating of strain field on frontal side of eccentric specimens. Consequently, cracking patterns and local bond behaviour are described in details. Increasing of concrete mechanical properties always enhanced bond strength and delayed cracking of concrete cover. Ribbed GFRP bars showed excellent bonding performance when combined with low concrete cover. Their low splitting tendency and specific rib geometry developed better bond behaviour in case of eccentric tests, which showed the possibility of a proper prediction of the bonding behaviour of structural components.

Viscoelastic mechanical measurement of the healthy and atherosclerotic human coronary arteries using DIC technique

PurposeAtherosclerotic is a specific form of vascular disease showed to be in charge of the 30% of mortalities in the United States alone. Many studies so far have been reported on the linear and nonlinear mechanical properties of the human and animal coronary arteries. However, the Quasilinear Viscoelastic (QLV) mechanical behavior of the healthy and atherosclerotic human coronary arteries have not been well quantified in spite of the time-dependent mechanical behavior of the arterial walls. This study was aimed to set up a new relaxation viscoelastic tests to characterize the QLV parameters of the healthy and atherosclerotic human coronary arteries. MethodsTen healthy and atherosclerotic human coronary arteries were subjected to relaxation test and the QLV parameters were calculated by comparing the QLV model to that of stress-relaxation data. ResultsThe findings showed the highest stress in the atherosclerotic coronary samples (292.02 ± 18.14 kPa) (Mean ± SD) which is found to be higher than that of the healthy ones (18.12 ± 2.88 kPa) (p < 0.05). In addition, the stress-relaxation diagrams showed that the healthy coronary arteries can reach to a balance in slightly a lower time (1400 ± 24.15 sec) compared to the atherosclerotic ones (1800 ± 38.12 sec) (p < 0.05). ConclusionsThese data might provide a deep understanding not only for the viscoelastic time dependent mechanical behavior of the healthy and atherosclerotic human coronary arteries but also for the biomechanical experts in different fields of research including, tissue engineering, intervention and bypass surgery and stenting.

DIC Technique to Investigate Crack Propagation in Grid-Reinforced Asphalt Specimens

AbstractCrack growth data are among the key parameters needed to characterize the materials used in asphalt concrete fatigue and fracture tests. The purpose of this study is to investigate the use ...