Digital Image Correlation Strain Research Articles

High-throughput measurement of coefficient of thermal expansion using a high-resolution digital single-lens reflex camera and digital image correlation.

High-throughput measurement of thermal deformation and determination of coefficient of thermal expansion (CTE) using a high-resolution digital single lens reflex (DSLR) camera and digital image correlation (DIC) is described. To mitigate the mosaic effect caused by the Bayer filter of DSLR cameras, a color image pre-processing method, which adjusts the brightness and equalizes the color channels of the raw image, is carried out. In addition, a Gaussian pre-filtering step is adopted for denoising the images captured with DSLR cameras to enhance the subpixel registration accuracy. Then, by processing the recorded images using the state-of-the-art DIC algorithm, full-field displacements and strains can be determined. Compared with conventional industrial cameras, a DSLR camera offers not only portability, compactness, and economy but also much higher resolution of recorded images, allowing CTE characterization with higher throughput. Real experiments, including a verification experiment of the color image pre-processing technique, a benchmark CTE determination of Al alloy, and a high-throughput CTE determination of 15 samples of three different metals, validated the feasibility and accuracy of the proposed technique. The proposed method is cost-effective and time-saving, showing great potential in the high-throughput CTE measurement and other high-throughput strain measurement scenarios.

In-situ fatigue life analysis by modal acoustic emission, direct current potential drop and digital image correlation for steel

Optimizing acoustic emission (AE) is useful to understand fatigue crack growth of metal structures. Experimental investigation of fatigue in steel with an edge notch was performed using AE to understand source and location of crack formation and growth. Non-Destructive Techniques like Direct Current Potential Drop (DCPD) and Digital Image Correlation (DIC) are useful in assisting to interpret the AE. The cumulation of AE correlated well with the initiation and propagation similar to potential drop values and DIC strain field. 2D AE source location technique was successfully implemented to monitor crack growth. Frequency content correlate with the mode of crack growth.

High resolution digital image correlation mapping of strain localization upon room and high temperature, high cycle fatigue of a TiAl intermetallic alloy

Microplasticity in a lamellar TiAl alloy upon high cycle tensile fatigue was measured by high resolution digital image correlation strain mapping at several stress and cycle increments, at 25 °C and 670 °C, for two lamellar thicknesses. Plastic deformation occurred primarily in soft-mode colonies at both temperatures, operating by slip parallel to the lamellae, and near lamellar interfaces. Plastic strains generally decreased to zero by the colony boundary; strain transfer across such boundaries was rare at both temperatures. At 25 °C, the maximum applied stress influenced the number of slip bands more so than did the number of loading cycles.

Coupled application of innovative electromagnetic sensors and digital image correlation technique to monitor corrosion process of reinforced bars in concrete

Corrosion monitoring of reinforcement is of great importance in evaluating the durability performance of concrete structures, especially under harsh environments. This paper presents a creative approach that combine using innovative electromagnetic monitoring apparatus (EMMA) and digital image correlation (DIC) technique in order to monitor the reinforcement corrosion inside concrete. The new-type electromagnetic sensor is capable of evaluating the corrosion degree of reinforced bars in concrete, meanwhile, DIC technique is always used to detect the tiny corrosion-triggered deformation on concrete surface. The result shows that joint use of these two techniques can monitor the entire process from corrosion initiation to further corrosion-induced strain variation on the surface of concrete. Besides, concrete beam with higher vertical load level leads to greater bending movement, which will accelerate the corrosion behavior and limit the horizontal strain. Moreover, the characteristics of DIC technique and strain gauges are compared respectively.

Effect of shear strain rate on interlaminar shear behavior of 2D-C/SiC composites: A damage transition from notch ends initiation to gauge section initiation

Two-dimensional carbon fiber reinforced silicon carbide composites (2D-C/SiCs) exhibit excellent mechanical properties at high temperature. However, the weak interfacial performance limits range of their applications. In the present work, interlaminar shear strength (ILSS) of 2D-C/SiC was investigated. By using an industrial camera and an acoustic emission (AE) detection system, quasi-static tests at the shear strain rates from 10−5/s to 10−3/s were conducted. Strain contours revealed the damage evolution process. Peak frequencies of AE signals were clustered into three groups, corresponding to matrix damage, interfacial debonding and fiber fracture. Dynamic tests at the shear strain rates of 200/s and 600/s were conducted using a modified split Hopkinson bar (SHPB). The dynamic deformation phenomenon was captured by a high-speed camera. The high-speed images and digital image correlation (DIC) strain contours revealed the damage initiation under dynamic loading. Damage morphologies were observed by a scanning electron microscope (SEM). The real-time images and damage morphologies explained the mechanisms of shear strain-rate effect on ILSS. The proposed experimental method elicited a fresh perspective on designing dynamic interlaminar shear experiments. Moreover, the interlaminar shear performance over a wide range of shear strain rates enhanced our understanding of the strain-rate sensitivity of compressive strength and tensile strength of 2D-C/SiCs.

Nano-Scale Residual Stress Profiling in Thin Multilayer Films with Non-Equibiaxial Stress State.

Silver-based low-emissivity (low-E) coatings are applied on architectural glazing to cost-effectively reduce heat losses, as they generally consist of dielectric/Ag/dielectric multilayer stacks, where the thin Ag layer reflects long- wavelength infrared (IR), while the dielectric layers both protect the Ag and act as an anti-reflective barrier. The architecture of the multilayer stack influences its mechanical properties and it is strongly dependent on the residual stress distribution in the stack. Residual stress evaluation by combining focused ion beam (FIB) milling and digital image correlation (DIC), using the micro-ring core configuration (FIB-DIC), offers micron-scale lateral resolution and provides information about the residual stress variation with depth, i.e., it allows depth profiling for both equibiaxial and non-equibiaxial stress distributions and hence can be effectively used to characterize low-E coatings. In this work, we propose an innovative approach to improve the depth resolution and surface sensitivity for residual stress depth profiling in the case of ultra-thin as-deposited and post-deposition annealed Si3N4/Ag/ZnO low-E coatings, by considering different fractions of area for DIC strain analysis and accordingly developing a unique influence function to maintain the sensitivity of the technique at is maximum during the calculation. Residual stress measurements performed using this novel FIB-DIC approach revealed that the individual Si3N4/ZnO layers in the multilayer stack are under different amounts of compressive stresses. The magnitude and orientation of these stresses changes significantly after heat treatment and provides a clear explanation for the observed differences in terms of scratch critical load. The results show that the proposed FIB-DIC combined-areas approach is a unique method for accurately probing non-equibiaxial residual stresses with nano-scale resolution in thin films, including multilayers.

Experimental stress state-dependent void nucleation behavior for advanced high strength steels

The influence of microstructure and stress state, as defined by the stress triaxiality and Lode parameter, on micro-void nucleation was evaluated experimentally for two 800 MPa Advanced High Strength Steels (AHSS), one a Complex-Phase CP800 alloy, with a ferritic-bainitic microstructure, and the other a Dual-Phase DP780 ferritic-martensitic steel. Four plane stress specimen geometries (simple shear, hole tension, v-bend and biaxial Nakazima) were adopted, providing stress triaxiality and Lode parameter values ranging from in-plane shear to biaxial tension under approximately constant stress states until failure. This approach facilitated determination of the relationship between void nucleation and macroscopic stress state. Damage histories were developed from interrupted samples using 3D micro-tomography and quantitative stereology measurement of void nucleation paired with in situ digital image correlation (DIC) strain measurements during the mechanical testing. The trends in damage evolution are strongly linked to the stress state, with very little void nucleation under shear deformation but extensive void damage under biaxial tension for both materials. A dependency of the nucleation rate on Lode parameter was also demonstrated. A higher rate of damage accumulation was observed for the DP780 steel compared to damage in the CP800 steel for all loading conditions highlighting the strong influence of initial microstructure. An analytical framework is proposed to obtain the local stress-state and equivalent plastic strain history from direction integration of the measured DIC strain histories, using a measured hardening law and assumed anisotropic yield function (Yld91) to develop the link between nucleation and the macroscopic stress state. A stress-state dependent nucleation model is proposed by introducing a nucleation strain surface as a function of stress-triaxiality and Lode parameter using a modified form of the strain-based Chu and Needleman nucleation criterion.

Mixed mode fracture characterization of brittle and semi-brittle adhesives using the SCB specimen

Fracture mechanics and damage mechanics are widely used to predict the mechanical behavior of adhesive joints. The critical energy release rate is one of the most important parameters in these models. In practice, most adhesive joints experience mixed mode loading conditions. Therefore, mixed mode fracture energy measurement plays an important role in adhesive joint failure analysis. A new test method for mixed mode fracture energy measurement of brittle and semi-brittle adhesive joints is presented in this paper using semi-circular bend specimens. A data reduction scheme based on a finite element analysis is suggested and finite element results are validated by digital image correlation strain measurement technique. The fracture envelops obtained from the proposed specimen are compared to double cantilever beam (DCB), single leg bending (SLB) and, end-notched flexure (ENF) test results. Good agreement between the new specimen and the classic energy measurement methods is observed.

In situ fatigue monitoring investigation of additively manufactured maraging steel

This work describes an experimental validation set for assessing the real-time fatigue behavior of metallic additive manufacturing (AM) maraging steel structures. Maraging steel AM beams were fabricated with laser powder bed fusion (LPBF) and characterized with ex situ studies of porosity through X-ray computed tomography (CT), nano-indentation, and atomic force microscopy, as well as quasi-static testing to evaluate the as-printed state. Microscale evaluation showed void content of 0.34–0.36% with hardness and stiffness variation through the build direction on the order of 5.1–5.6 GPa and 139–154 GPa, respectively. The microscale inhomogeneities created an as-printed state where the compression and tension plasticity behavior at the macroscale was unequal in quasi-static loading, leading to greater yielding in tension. Specimens were subjected to cyclic loads, while the structural behavior was characterized through in situ magnetic permeability, digital image correlation (DIC) strain, and structural compliance measurements. In the range of 3 × 103 to 3 × 104 cycles to failure, magnetic permeability measurements were able to capture the mechanical state as early as 60% of life depending on failure location. Results are discussed with an emphasis on material-property-structure relationships in terms of the multi-scale material state and fatigue validation data for improving the durability of AM parts.

Method for conducting in situ high-temperature digital image correlation with simultaneous synchrotron measurements under thermomechanical conditions.

This work presents a novel method of obtaining in situ strain measurements at high temperature by simultaneous digital image correlation (DIC), which provides the total strain on the specimen surface, and synchrotron x-ray diffraction (XRD), which provides lattice strains of crystalline materials. DIC at high temperature requires specialized techniques to overcome the effects of increased blackbody radiation that would otherwise overexpose the images. The technique presented herein is unique in that it can be used with a sample enclosed in an infrared heater, remotely and simultaneously with synchrotron XRD measurements. The heater included a window for camera access, and the light of the heater lamps is used as illumination. High-temperature paint is used to apply a random speckle pattern to the sample to allow the tracking of displacements and the calculation of the DIC strains. An inexpensive blue theatrical gel filter is used to block interfering visible and infrared light at high temperatures. This technique successfully produces properly exposed images at 870 °C and is expected to perform similarly at higher temperatures. The average strains measured by DIC were validated by an analytical calculation of the theoretical strain. Simultaneous DIC and XRD strain measurements of Inconel 718 (IN718) tensile test specimens were performed under thermal and mechanical loads and evaluated. This approach uses the fact that with DIC, the total strain is measured, including plastic strain, while with XRD, only elastic strain is captured. The observed differences were discussed with respect to the effective deformation mechanisms.

AN ALTERNATIVE TO PERIODIC HOMOGENIZATION FOR DENTIN ELASTIC STIFFNESS

Dentin, the main tissue of the tooth, is made of tubules surrounded by peri-tubular dentin (PTD), embedded in a matrix of inter-tubular dentin (ITD). The PTD and the ITD have different relative fractions of collagen and hydroxyapatite crystals. The ITD is typically less rigid than the PTD, which can be seen as a set of parallel hollow cylindrical reinforcements in the ITD matrix. In this paper, we extend Hashin and Rozen’s homogenization scheme to a nonuniform distribution of hollow PTD cylinders, determined from image analysis. We relate the transverse isotropic elastic coefficients of a Representative Elementary Volume (REV) of dentin to the elastic and topological properties of PTD and ITD. The model is calibrated against experimental data. Each sample tested is consistently characterized by Environmental Scanning Electron Microscopy (ESEM), nanoindentation and Resonant Ultrasound Spectroscopy (RUS), which ensures that macroscopic mechanical properties measured are correlated with microstructure observations. Despite the high variability of microstructure descriptors and mechanical properties, statistical analyses show that Hashin’s bounds converge and that the proposed model can be used for back-calculating the microscopic Poisson’s ratios of dentin constituents. Three-point bending tests conducted in the laboratory were simulated with the Finite Element Method (FEM). Elements were assigned transverse isotropic elastic parameters calculated by homogenization. The tubule orientation and the pdf of the ratio inner/outer tubule radius were determined in several zones of the beams before testing. The remainder of the micro-mechanical parameters were taken equal to those calibrated by RUS. The horizontal strains found experimentally by Digital Image Correlation (DIC) were compared to those found by FEM. The DIC and FEM horizontal strain fields showed a very good agreement in trend and order of magnitude, which verifies the calibration of the homogenization model. By contrast with previous studies of dentin, we fully calibrated a closed form mechanical model against experimental data and we explained the testing procedures. In elastic conditions, the proposed homogenization scheme gives a better account of microstructure variability than micro–macro dentin models with periodic microstructure.

New insights on the proximal femur biomechanics using Digital Image Correlation

Finite element analyses (FEAs) of human femurs are mostly validated by ex-vivo experimental observations. Such validations were largely performed by comparing local strains at a small subset of points to the gold standard strain gauge (SG) measurements. A comprehensive full field validation of femoral FEAs including both strains and displacements using digital image correlation (DIC) full field measurements, especially at medial and lateral surfaces of the neck that experience the highest strains, provide new insights on femurs’ mechanical behavior.Five cadaver femurs were loaded in stance position and monitored at the shaft and neck using two DIC systems simultaneously. DIC strains were compared to SG measurements at a limited number of locations so to corroborate DIC measurements by the gold standard technique. These were used to quantitatively assess the validity of FEA strains prediction especially at the neck where fracture usually occurs.Strains measured by DIC correspond well to the SG observations. An excellent agreement was observed between DIC and FEA predicted strains excluding the superior neck surface: FE=1.02×DIC-17,r2=0.977. At the superior neck however, strains were not well predicted by FEA models: Although the FEA predicted high strains at the ’saddle region’, these were not observed experimentally. On the other hand, strain concentrations were measured by DIC at numerous vessel holes which were not represented by FE models. Since fractures usually initiate at the subcapital region in stance position ex-vivo experiments, where numerus vessel holes exist, these vessel holes may be required to be accounted for in future FE models so to allow a better estimation of the fracture load. Full field measurements are mandatory to allow a better validation of fracture load and location predictions which are of high clinical importance.

Investigation of various necking criteria for sheet metal formability analysis using digital image strain data

The precise determination of the forming limit strains of a sheet metal is necessary in order to accurately predict the onset of failure in sheet metal forming processes. The method used to detect the onset of necking (i.e. the necking criterion) is a most important factor in formability analysis as it significantly affects the forming limit strains. Many different necking criteria have been developed, which take advantage of the flexibility and high resolution of digital image correlation (DIC) strain measurements. Several time-dependent and time/geometry-dependent necking criteria were carefully investigated in order to evaluate their ability to reliably and consistently detect the onset of necking of TRIP780 sheet specimens that were stretch-formed over a hemispherical punch. It was found that the geometry-dependent, surface slope criterion was the most robust and consistent criterion of those evaluated.

Warm Forming Characteristics of AA7050-T6

The formability of AA7050 aluminum alloy sheet is characterized under isothermal forming conditions at room temperature and 150 °C. The material was tested in the T6 condition. Nakazima limiting dome height (LDH) tests in conjunction with stereoscopic digital image correlation (DIC) strain measurements are used for the formability assessment. Four-point forming limit curves (FLCs) were developed at the two test temperatures. AA7050-T6 is shown to respond positively to elevated temperature forming with a 0.19 major true limit strain for a 76.2 mm Nakazima specimen, for example, compared to 0.13 at room temperature. The current results for AA7050-T6 are compared to the formability of AA7075-T6 sheet tested under similar conditions; a slightly higher formability was observed for the AA7075-T6 alloy at the elevated temperature.

Determination of In-Plane Shear Properties of Laminate with V-Notch Rail Shear Test and Digital Image Correlation

Abstract This article presents the results of the application of Digital Image Correlation (DIC) to measurements of in-plane shear modulus and strength of three different carbon fiber reinforced laminates. Three different approaches to shear strain calculations via DIC are evaluated and compared with standard strain gage measurements. Calculation of shear strain based on averaging DIC strain values of strain gages area in most cases yielded results closest to strain gages, while measurements based on single point strain measuring differed the most from strain gages. These results are attributed to shear strain distribution in the center area of the specimen. Thermoplastic matrix fabric reinforced composite had the lowest shear strength at 5% of shear strain, but the highest ultimate shear strength and strain at failure. Of thermosetting materials, laminate reinforced with unidirectional carbon fiber had shear modulus about 10% lower, than fabric reinforced laminate, but higher ultimate strength and strain at failure. This behavior is attributed to the presence of weaves in fabric reinforcing the laminate, causing shear stiffening of the material, but lowering its ability to deform under shear loading.

Digital image correlation strain measurement of ultra‐high‐performance concrete‐prisms under static and cyclic bending‐tensile stress

AbstractThis paper provides experimental results on investigations for the validation of photogrammetric strain measurements of ultra‐high‐performance concrete (UHPC)‐prisms subjected to static and cyclic bending‐tensile stress. For this purpose, 4 static and 5 cyclic test series were performed. Damage progresses during loading are monitored by means of a digital image correlation (DIC) system and a clip gauge. The control of the DIC by trigger lists and the measurement noise as a function of the measurement rate are examined. All static tests were performed force controlled with the same testing speed and the same measuring rate of DIC and clip gauge. All cyclic tests were performed with the same upper and lower stress levels but with different loading rates.During the static tests, the DIC can be used to make accurate strain measurements before UHPC failure. In the cyclic tests, the measurement noise of the DIC decreases with an increasing measuring rate. The tests performed confirm the control of the DIC by trigger lists for cyclic tests on UHPC‐prisms and show that the measurement noise is negligible in static and cyclic tests.

Direct Visualization of Cross-Sectional Strain Distribution in Flexible Devices.

For flexible devices that inevitably undergo repetitive deformations, it is important to evaluate and control the mechanical strain imposed on the flexible systems for enhancing the reliability. In this paper, a novel experimental method to directly visualize cross-sectional strain distribution in the thin flexible devices is proposed. Digital image correlation (DIC) is effectively adapted by using microscopic images of the cross section for accurate analysis of the microscale deformations. To conduct the DIC strain analysis, speckle patterning is accomplished by using microparticles from diamond-abrasive suspensions with optimized fabrication conditions. First, the cross-sectional micro-DIC analysis is performed successfully for 100 μm-thick substrates. Full-field strain quantification and easy inspection of a neutral plane are demonstrated and compared with results of finite element analysis simulation. Using the presented method, generation of multiple neutral planes is clearly visualized for a trilayer structure with a very soft adhesive midlayer, where strain decoupling occurs by severe shear deformation of the soft adhesive layer. Furthermore, bending strain distribution in a flexible fabric-reinforced polymer (FRP) substrate is also investigated to analyze and predict fatigue fracture in thecomplex inner structure under repetitive bending loading.

Slip bands in lamellar TiAl during high cycle fatigue microcompression by correlative total strain mapping, diffraction orientation mapping and transmission electron imaging

Microcompression testpieces of a lamellar γ-TiAl alloy were high cycle fatigued up to 105 cycles at stress ratios of 0.5–0.7, to maximum stresses above and below the yield stress. The localised deformation was studied by nano-scale digital image correlation strain mapping of a Pt patterned side-face of the pillars. With crystal orientation mapping, the fatigue deformation lines were found to locate at the γ/γ and γ/α2 lamellar interfaces. The deformation lines were closely spaced slip lines in transmission electron imaging. Gradients of shear strain developed along the deformation lines, increasing in magnitude as cycling progressed. Extensive transverse twinning also occurred.

Digital image correlation strain measurement of thick adherend shear test specimen joined with an epoxy film adhesive

Structural bonding and bonded repairs of composite materials become more and more important. Understanding the strain within the bondline leads to suitable bonding design. For new design approaches the strain distribution within the bondline has to be analyzed. Thus, often finite element analysis (FE) are used. However, a huge challenge is the availability of reliable material properties for the adhesives and their validation. Previous work has shown that it is possible to measure the small displacements resulting within thin epoxy film adhesives using high resolution digital image correlation (DIC). In this work a 2D DIC setup with a high resolution consumer camera is used to visualize the strain distribution within the bondline over the length of the joint as well as over the adhesive thickness. Therefore, single lap joints with thick aluminum adherends according to ASTM D 5656 are manufactured and tested. Local 2D DIC strain measurements are performed and analyzed. Two different camera setups are used and compared. The evaluation provides reliable material data and enables a look insight the bondline. The results of the full field strain data measured with DIC are compared with numerical simulations. Thus, material models as well as chosen parameters for the adhesive are validated. Compared to extensometers, giving only point-wise information for fixed measuring points, the DIC allows a virtual point-wise inspection along the complete bondline. Furthermore, it allows measuring close to the bondline to reduce the influence of adherend deformation.

Fatigue behaviour of carbon/epoxy Non-Crimp Fabric composites for automotive applications

Facing the stringent requirements for fuel economy in automotive industry, composite materials are seen to be utilized for structural lightweighting. This paper focuses on the fatigue test development and fatigue behavior of a biaxial [0/90] carbon/epoxy non-crimp fabric composites for automotive applications. A new fatigue test specimen was successfully developed for fatigue testing under fully reversed loading, i.e. stress ratio R=-1, at room temperature and 130 ℃. The developed test specimen prevents buckling during testing without use of an anti-buckling fixture. 3D Digital image correlation strain mapping also revealed a uniform strain distribution on the surface of this specimen geometry. Fatigue test results indicate that the fatigue behavior at both room temperature and 130 ℃ is matrix dominated. The fatigue data is a good fit to the Basquin equation σa=σf(2N)b.