Use Of Digital Image Correlation Research Articles

Development and evaluation of sub-element testing of SiC/SiC ceramic matrix composites at elevated temperatures

SiC/SiC ceramic matrix composites (CMCs) are being developed for use in aero-engines to replace nickel superalloy components. Sub-element testing acts as the key stepping stone in bridging understanding derived from basic coupon testing and more complex component testing. This study presents the development of high temperature C-shape sub-element testing with the use of digital image correlation to study damage progression. The specimen is designed with a bias towards a mixed mode-stress state more similar to what a CMC component may see in service. Both monotonic and fatigue tests were completed on C specimens and compared with predicted behaviour from modelling. Test data from both test types suggested that specimens were failing once they reached a critical radial stress level. However evidence from fractography of specimens showed that in both monotonic and fatigue tests radial cracks (driven by hoop stresses) are initiating prior to circumferential cracks.

Spinal Motion Segments — II: Tuning and Optimisation for Biofidelic Performance

Most commercially available spine analogues are not intended for biomechanical testing, and the few that are suitable for using in conjunction with implants and devices to allow a hands-on practice on operative procedures are very expensive and still none of these offers patient-specific analogues that can be accessed within reasonable time and price range. Man-made spine analogues would also avoid the ethical restrictions surrounding the use of biological specimens and complications arising from their inherent biological variability. Here we sought to improve the biofidelity and accuracy of a patient-specific motion segment analogue that we presented recently. These models were made by acrylonitrile butadiene styrene (ABS) in 3D printing of porcine spine segments (T12−L5) from microCT scan data, and were tested in axial loading at 0.6 mm·min−1 (strain rate range 6×10−4 s−1–10×10−4 s−1). In this paper we have sought to improve the biofidelity of these analogue models by concentrating in improving the two most critical aspects of the mechanical behaviour: the material used for the intervertebral disc and the influence of the facet joints. The deformations were followed by use of Digital Image Correlation (DIC) and consequently different scanning resolutions and data acquisition techniques were also explored and compared to determine their effect. We found that the selection of an appropriate intervertebral disc simulant (PT Flex 85) achieved a realistic force/displacement response and also that the facet joints play a key role in achieving a biofidelic behaviour for the entire motion segment. We have therefore overall confirmed the feasibility of producing, by rapid and inexpensive 3D-printing methods, high-quality patient-specific spine analogue models suitable for biomechanical testing and practice.

A novel methodology for estimating tensile properties in a small punch test employing in-situ DIC based deflection mapping

Determining tensile properties from small punch test is being pursued actively in the nuclear industry due to the limited volume of material such tests use compared with standard tests which can be critical when considering active or development samples. One of the crucial challenges in harnessing the full potential of this technique is formulating methodologies which correlate the small punch specimen’s deflection to equivalent uniaxial tensile properties. Existing approaches for correlation rely on deflection obtained from a single point on the small punch test specimen, used with empirical equations to make the correlation. However, the deflection and strain accumulation in a small punch specimen is highly heterogeneous and data from a single point does not represent the gross deformation evolving in the specimen. This data when used in conjunction with the empirical formulations for deriving equivalent uniaxial tensile properties, would not result in accurate identification of material properties. In this work we offer an alternative approach which uses the full field deflection of the specimen mapped through in-situ digital image correlation. The use of digital image correlation combined with inverse finite element analysis augments the existing method of material properties identification from single point deflection data thereby significantly improving the reliability of the measurements.

Understanding the Role of Friction and Adhesion in the Display of Tapestries on Slanted Supports

ABSTRACT This paper reports on an investigation into slanted support, an increasingly common method of displaying tapestries in mainland Europe. Two factors contribute to the efficacy of slanted supports for tapestry: the angle of display and the degree of friction between the tapestry and the support. Initial analysis demonstrated that a small degree of slope, of 5°, has only marginal impact on reducing the load experienced by the tapestry. Experiments were carried out to compare the friction between small tapestry fragments and a range of fabrics commonly used to cover slanted supports. Lined and unlined tapestry fragments were placed on a fabric-covered board and tilted from horizontal to vertical. In most cases it was found that the board could be tilted beyond vertical before the tapestry began to slip; this meant it was not possible to calculate a coefficient of friction. It was clear that the simple Coulomb model of friction does not apply in this situation and that adhesive forces are even more significant than friction. Loading the tapestry samples demonstrated that the coefficient is dependent on load, confirming this observation. The paper also reports on the use of digital image correlation (DIC) to calculate strain data. A tapestry fragment was suspended on a vertical board half covered in cotton molton fabric. The strain values were very low on the half of the tapestry against the molton; they were higher on the half against the uncovered board. It was also apparent that the tapestry with board-covering fabric was less subject to expansion and contraction caused by fluctuating relative humidity. The research concluded that the friction/adhesion forces are so effective that a tapestry is well supported on a vertical fabric-covered board; there is no additional benefit in providing a small degree of slant.

Inverse structural damage identification problem in CFRP laminated plates using SFO algorithm based on strain fields

Damage detection methods are an important field of engineering and crucial in terms of structural safety. However, in many practical cases, the process of monitoring and identifying damage is extremely difficult or even impractical due to the conditions of access and operation of a given component/structure. In this study, an inverse algorithm based on strain fields for damage identification in composite plate structures is presented. The inverse analyses combine experimental tests and digital image correlation (DIC) with numerical models based on finite element update method with great advantage of being a non-contact method. The proposed technique identifies the location and dimension of damages in a CFRP plate using static strains formulated as an objective function to be minimized. By model updating, the discrepancies between the experimental and the numerical results are minimized. For the success of the model updating, the efficiency of the optimization algorithm is essential. A powerful new metaheuristic sunflower optimization (SFO) is employed to update the unknown model parameters. Experimental results showed the excellent efficiency in the combined use of DIC, numerical modeling and SFO optimization to accurately identify the location of damage in numerical and experimental tests. The obtained results indicate that the proposed method can be used to determine efficiently the location and dimension of structural damages in mechanical structures.

On the combined use of Digital Image Correlation and Micro Computed Tomography to measure fibre orientation in short fibre reinforced polymers

Measurement of fibre orientation in short fibre reinforced polymer is crucial for the description of their microstructure and subsequent multiscale modelling. Micro Computed Tomography is becoming the standard method for the reconstruction of the inner microstructure of short fibre reinforced polymers and different methods exist for the analysis of orientations. Applications of Digital Image Correlation to X-rays Micro Computed Tomography data sets to measure fibre orientation in continuous fibre reinforced polymers exist. In this work, the possibility of extending the application of the Digital Image Correlation technique to measure fibre orientation in short fibre reinforced polymers is investigated and a new procedure is proposed. The method is applied to two different data sets obtained by scanning the same sample using both a synchrotron micro CT equipment and an industrial scanner. Results are compared with other existing methods and discussed to identify possible future improvements.

Initial research of changes in mechanical properties of polymer-graphene composite with use of Digital Image Correlation

The problem of the possibility of using digital image correlation to assess changes in the mechanical properties of a polymer composite with the addition of graphene (1% by weight) in the form of flakes was taken up in the paper. Samples for uniaxial tensile testing were produced by selective laser sintering from polyamide powder particles and multi-flake graphene. In the tests carried out using the three-camera system, a variable strain distribution was recorded in the heterogenic PA-G composite material. Maps developed in the evaluation process – in particular, tangential deformations – allowed to conclude the impact of axial load on PA-G behaviour in time in the area of the polymer matrix, graphene agglomerates, and their interfaces. The research showed that the digital image correlation technique is a perfect complement to traditional mechanical tests when determining the properties of new heterogeneous composites, in particular, polyamide-graphene composite. Real-time observation of changes in the material map allows for a detailed analyzes of complex micromechanical aspects.

A bio-inspired functional film embedded with fluorescent elastic microspheres for pressure sensing

Developments in global pressure and shear sensors are invaluable for flow visualization and force measurement and are indispensable tools for research in fluid mechanics. Here, we introduce a bio-inspired pressure-sensitive film that has high spatial resolution and a wide measurement range. The film is embedded with fluorescent elastic microspheres that emit light under appropriate excitation. These microspheres can be compressed under external pressure, and they fully recover as pressure is released. This pressure-induced deformation causes variations in microsphere emission intensity, establishing a quantitative relationship between pressure and emission intensity. Furthermore, this film has the potential for shear-force sensing via the use of digital image correlation to measure the displacements of microspheres under shear force. This functional film has an impressively broad scope of sensing applications, such as in air-flows, underwater, and in tactile-based settings.

The use of digital image correlation for identifying failure characteristics of cross-laminated timber under transverse loading

The localised crack induced by external load on an individual cross laminated timber (CLT) structural element will generate stress concentrations, which leads to the failure of a structural member. A better understanding of the structural performance of CLT in terms of its strength and failure modes under transverse loading needs to be developed to ensure structural stability, safety and resilience of mass timber buildings. The monitoring and prediction of crack initiation and the failure modes of structural elements before and after localised deformation could enable the mitigation of severe damage in timber buildings. In this study, four-point bending tests were conducted and the digital image correlation (DIC) technique was used as a cost-effective solution to monitor the crack initiation and growth behaviour and predict the failure types of Australian radiata pine CLT panels under out-of-plane loading. Both a speckled surface pattern and the natural grain of the timber were used in the DIC analysis. This will aid with the development of efficient performance-based design guidelines for timber buildings. Based on the crack growth rates and strain fields, the DIC technique can predict the failure locations and modes, namely rolling shear or delamination. This study also highlighted that DIC analysis with a natural grain of timber can be used to measure the crack growth rates in CLT.

Development of a mini-tensile approach for sheet metal testing using Digital Image Correlation

The miniature uniaxial tensile testing is a simple yet powerful method to characterise the behaviour of a material. Based on this technique, an experimental system was developed aiming the determination of sheet metal mechanical properties, using the Digital Image Correlation (DIC) technique. A tensile-compression loading device was designed with a maximum load capacity of 2.5 kN. The device includes custom-made clamps that guarantee the alignment of the sample with respect to the loading axis. To evaluate the aptitude of miniature samples and the use of Digital Image Correlation (DIC) in this type of scale, some tensile tests were performed using different types of miniaturised geometries and the results were compared with a macro specimen reference. Additionaly, some cyclic tests were also performed, in order to evaluate the possibilities of developed prototype on the tensioncompression loading paths.

Analysis of Printed Circuit Boards strains using finite element analysis and digital image correlation

This paper investigates the use of digital image correlation (DIC) and finite element analysis for strain measurement on Printed Board Circuits (PCBs). Circuit boards (PCBs) are designed to mechanically support and electrically connect an electronic component assembly. Due to screw assemblies, the surface level differences on which the PCB is placed, the process of assembling the electronic components induces a certain state of stress and deformation in the PCB. The main components affected are microprocessors due to the way they are glued to PCBs with BGA - Ball grid arrays (BGA). Digital Image Correlation (DIC) is a full-field contactless optical method for measuring displacements and strain in experimental testing, based on the correlation of images taken during test. The experimental setup is realized with Dantec Q-400 system used for image capture and Istra 4D software for image correlations and data analyses. The maximum level of the obtained strain is compared with the allowable limit. Finite element analysis (FEA) is a numerical method of analysis for stresses and strain in structures of any given geometry.

Development and Validation of a Miniature Tensile Specimen for Determination of Mechanical Properties

Abstract In this work, a miniature tensile specimen with nominal dimensions of 3.0-mm gage length (GL), 1.5-mm gage width, and 0.5-mm thickness that was carved out of a 10.0-mm-diameter disk has been developed and standardized using analytical and experimental methods. The geometry of the miniature specimen, called ultra sub-size (USS), was optimized using finite element analysis to determine the fillet radius and appropriate geometrical tolerances for specimen dimensions like gage width and thickness. The methods of specimen preparation, gripping, and tensile testing with the use of digital image correlation for strain measurement were standardized. The 0.2 % offset yield strength, ultimate tensile strength, and uniform strain of different materials obtained using this specimen geometry were analyzed and compared with the results of ASTM sub-size (GL: 25 mm) and further sub-size geometry (GL: 12.5 mm). The results of this study show that USS tensile specimen geometry developed in this work can be reliably employed for mechanical property evaluation in situations where tensile testing using standard-size specimens is practically not possible.

On the Use of Digital Image Correlation (DIC) for Evaluating the Tensile Behaviour of BFRCM Strips

Fibre Reinforced Cementitious Matrix (FRCM) composites are becoming largely adopted for retrofitting masonry structures. These materials offer several advantages in comparison to Fibre Reinforced Polymer (FRP) composites, such as good resistance to fire and high temperatures, vapour permeability, possibility to be applied on wet surfaces, higher compatibility with the masonry substrate. However, the tensile behavior of FRCM materials is more complex compared to FRP composites, due to the limited tensile strength of the cement-based matrix. For this reason, FRCM materials require appropriate tensile characterization and, in this context, the use of non-conventional measurement systems, such as the Digital Image Correlation (DIC), can offer numerous advantages. This work presents an experimental study on the application of the DIC technique for the tensile characterization of Basalt Fibre Reinforced Cementitious Matrix (BFRCM) strips. Tensile tests were carried out on three series of specimens reinforced with one, two or three layers of basalt grid in order to investigate the effect of the reinforcement ratio on the tensile response of the composite strips. The test setup and the calibration of the DIC analyses are discussed. It is shown as the DIC allows obtaining detailed information on the tensile response, including the evaluation of the full strain field on the surface of the BFRCM strips and the location of cracks. Results are discussed also in terms of stress-strain curves and failure modes.

The effects of stress triaxiality and strain rate on the fracture strain of Ti6Al4V

The fracture strain is known to be affected by the stress state and strain rate. To measure the fracture strain over a wide range of stress triaxiality and strain rate, a rectangular hat-shaped specimen is designed. This specimen can be tested at low rates with a conventional load frame and at high rates with a Split Hopkinson Pressure Bar (SHPB). The specimen’s flat face facilitates the use of Digital Image Correlation (DIC) technique. The fracture strain of Ti6Al4V was determined for a stress triaxiality varying from −0.31 to 0.50 at strain rates ranging from quasi-static to 103/s. The results indicate that the fracture strain decreased with increasing either the stress triaxiality or the strain rate. A fracture strain model incorporating the effects of stress triaxiality and strain rate was established.

The Effect of Localized Vibration during Welding on the Microstructure and Mechanical Behavior of Steel Welds.

The use of vibratory welding is treated with some caution in the industry due to inconsistent beneficiary results. Here, a partial explanation is suggested by the differentiation between global vibrational effects (GVEs) and local vibrational effects (LVEs), and the latter is investigated experimentally. Two structural plates of steel are welded at three frequency/amplitude combinations using manual gas metal arc welding in an experimental setup that ensures only LVEs. After welding, tensile tests, microhardness tests, and metallurgical characterization are performed locally in the different welding zones and the results are compared to the non-vibrated welds. Novel use of digital image correlation (DIC) is implemented in tensile testing of welded samples, thus enabling the separate determination of local mechanical properties of the base metal, heat-affected zone and fusion zone of the same weld. LVE is found not to promote any distinct difference in weld properties, at least within the vibrational regimes studied. Nevertheless, depending on geometry and structural response, it is explained how vibratory welding may promote residual stress relief due to GVEs of the welded structure.

Measurement of Hysteresis Energy Using Digital Image Correlation With Application to Energy-Based Fatigue Life Prediction

A modified experimental method using digital image correlation (DIC), a noncontact optical method for measuring full-field displacements and strains, is used to interrogate accumulated fatigue damage for low and high cycle fatigue at continuum scales. Previous energy-based fatigue life prediction methods have shown that cyclic strain energy dissipated during fatigue acts as a key damage parameter for accurate determination of total and remaining fatigue life. DIC enables the collection of accurate strain energy measurements or damaging energy of complex geometries that would otherwise be exceedingly difficult and time consuming using traditional strain measurement techniques. Thus, the use of DIC to obtain strain energy measurements of gas turbine engine (GTE) components is highly advantageous for energy-based fatigue life prediction methods. Presented in this study is the experimental characterization of the cyclic strain energy dissipation as a means of predicting fatigue performance and assessment of damage progression of Aluminum 6061 subjected to fully reversed axial fatigue loading utilizing DIC. Validation of total and cyclic strain energy dissipation DIC measurements is accomplished with the simultaneous use of axial extensometery for direct comparison and implementation to strain energy-based life prediction methods.

Mechanically Regularized FE DIC for Heterogeneous Materials

In situ tensile tests in a scanning electron microscope (SEM) have been conducted on a 8-layer 5-harness satin carbon fibre and epoxy matrix composite to observe the first stages of damage at the scale of fibres and matrix. A speckle pattern based on a suspension of alumina particles was applied onto the surface of the specimen to facilitate the use of digital image correlation (DIC). Local and finite element (FE) DIC are compared on pictures acquired during the tensile tests, with and without a speckle pattern. FE DIC with mechanical regularization was found to be the only approach able to measure displacement fields at a fine enough resolution in both cases. This method, initially created for homogeneous materials, was then adapted to heterogeneous materials. First, a microstructure consistent mesh was created and used for correlation purposes. Second, the difference between the mechanical properties of the constituents is taken into account in the mechanical regularization. Last, the accuracy of the method is analysed. The adaptation presented herein was proved to be able to measure displacement fields in the matrix between fibres with an error of 10 nm (a fifth of a pixel) and to detect the initiation of the first damage mechanisms by means of the mechanical residuals.

Effect of mix component on the mechanical properties of coral concrete under axial compression

Typical influences of mix component on the properties of coral concrete (CC) were analyzed in this paper. Different types of fine aggregates (sea sand and river sand), mixing water (seawater and fresh water) and cementitious materials (cement and fly ash) were considered. An experimental system was developed involving uniaxial compression tests and the use of digital image correlation (DIC) method to investigate the deformation distribution and crack propagation of CC. The variations in failure pattern, strength and deformation of CC were studied. Test results indicated that the failure of CC adopting sea sand and seawater was brittle compared to CC using fresh water and river sand. It was found that axial stress-strain curve (SSC) of ordinary concrete (OC) was different from that of CC. The curvature and the descending slope of SSC obviously changed with the variation of mix components. The seawater improved the strength and elastic modulus of CC compared to fresh water, however, the effect of sea sand was negative. It was verified that prismatic compressive strength of CC with fly ash (FA) developed slowly in early stage, while it increased quickly in late stage. After using different types of aggregates, mixing water and cementitious materials, obvious changes were found in the crack propagation and the axial deformation distribution. However, typical influence of mix component on horizontal displacement distribution was negligible. The cracking of CC was seldom observed before the peak stress when compared to OC, and the seawater and sea sand accelerated the CC crack growth after the peak point. Finally, a numerical stress-strain expression of CC considering the influences of mix component was suggested, which could be used in the theoretical analysis and practical design.

Calibrating thermoelastic stress analysis with integrated digital image correlation: Application to fatigue cracks

This article discusses the dual use of digital image correlation and thermoelastic stress analyses for the study of propagating cracks. It is shown that a few critical parameters such as emissivity and the thermoelastic constant required for the latter can be calibrated with the former. A unified framework is introduced, which treats both experimental techniques equally to locate crack tips, and then evaluate stress intensity factors. This framework allows for a detailed and quantitative comparison between both methods. It is found that, for the case at hand, the thermoelastic stress analyses outputs were less noise sensitive while the digital image correlation method was less dependent on calibration. The proposed procedure was very robust for finding the crack tip location for both experimental methods.

Experimental characterization of fiber-reinforced cementitious mortar under tension

This work focuses on the mechanical characterization of the inorganic matrix used for Fiber-Reinforced Cementitious Matrix (FRCM) composites, nowadays widely used to retrofit existing reinforced concrete and masonry structures. While several works in technical literature investigate the experimental behavior of the whole FRCM composite, few information are available on the mechanical characterization of the mortar, which contains polymers and synthetic fibers in its admixture. However, the knowledge of its behavior in tension, especially after crack formation, is an important feature for the calibration of constitutive models to be adopted in the study of structural elements strengthened with FRCM. To this aim, an experimental program was performed on mortar specimens characterized by different shapes and dimensions, tested under direct tension or three-point-bending. From the performed tests, it was possible to characterize inorganic matrix behavior both in the uncracked stage, through the determination of the elastic parameters (elastic modulus and Poisson’s coefficient), and in the cracked stage. The use of digital image correlation (DIC) technique also allowed the study of the evolution of crack propagation in the specimens. Lastly, a correlation factor between axial and flexural tensile strength is proposed, for both design and numerical modelling purposes.