Digital Image Correlation Research Articles

A Method for Dynamic Kolsky Bar Compression at High Temperatures: Application to Ti-6Al-4V

Abstract An experimental apparatus for measuring the dynamic behavior of materials subjected to strain rates on the order of 10 $$^3$$ 3 s $$^{-1}$$ - 1 and temperatures up to 800°C with a unique triple actuation system is developed in this work. This system is based on the traditional Kolsky (or split-Hopkinson pressure) bar design, with the addition of an external furnace used to heat the specimen to the desired temperature. A synchronized triple pneumatic actuation system is used to control the motion and timing of the the sample, incident, and transmitted bars. The cold contact time (CCT), or the time during which the heated sample is in contact with the room temperature bars before compression, is measured experimentally and carefully controlled to minimize the development of a temperature gradient across the sample and avoid heating of the bars. Experiments are performed in conjunction with ultra high speed imaging and 2D digital image correlation (DIC), as well as high speed thermal imaging. To verify the viability of the proposed system, experiments were conducted on Ti-6Al-4V (wt.%) at temperatures from 25°C up to and 800°C at an average strain rate of approximately 1200 s $$^{-1}$$ - 1 .

A Novel Axial Load Inversion Method for Rock Bolts Based on the Surface Strain of a Bearing Plate

Anchor rock bolts are among the essential support components employed in coal mine support engineering. Measuring the axial load of the supporting anchor bolts constitutes an important foundation for evaluating the support effect and the mechanical state of the surrounding rock. The existing methods for measuring the axial load of rock bolts have difficulty meeting the actual demands in terms of accuracy and means. Therefore, we propose a novel inverse method for determining the axial load of rock bolts. On the basis of the dynamic relationship between the axial load of the anchor bolt and the strain of the plate, a calculation model for the inverse analysis of the axial load from the plate strain is presented, and it is verified and corrected through finite element analysis and indoor physical experiments. By combining the calculation model with the digital image correlation method, a low costinversion of the axial load of the anchor bolt in actual support engineering is achieved. The experimental results demonstrate that the average errors of the load inversion of anchor bolts in three different states via the theory and method proposed in this paper are less than 8.8% (4 kN), 3.6% (3.2 kN), and 14.7% (5.5 kN), respectively, and the average error of the axial load of the rock bolts in the proposed method is only 4.23 kN. It possesses relatively high accuracy and can be effectively applied in the actual production processes of mines.

Mechanical Behaviour of 7075-T6 Aluminium Alloy: Integrating Digital Image Correlation and Finite Element Analysis for Uniaxial and Biaxial Load Scenarios

The objective of this study is to investigate the mechanical properties of 7075-T6 Aluminium (Al) alloy under both uniaxial and biaxial loads. The study will be conducted using a combination of experimental and numerical methods. The experimental method used is the digital image correlation (DIC) technique, which was utilized to capture the deformation and strain fields of the material specimen under tensile test. A tensile machine was employed to apply uniaxial and biaxial loads on the sample. The strain and deformation distribution of the results was generated on the DIC and correlated software. The numerical method involved the use of a commercial finite element software to create a finite element model and simulate the mechanical behaviour of the material under the same loading conditions. The results obtained from the experimental and numerical methods were compared to validate the accuracy of the numerical model. The outcomes demonstrated that under uniaxial loading, localized necking and fracture were observed, while biaxial loading resulted in shear deformation and fracture. This research contributes to the development of more accurate models for predicting the mechanical behaviour of the model samples under different loading conditions.

Measurement of dynamic deformation during shock loading using a fiber-optic loop-sensor

Abstract Characterizing materials under shock loading has been of interest in fields such as protective material development, biomechanics to study the injury mechanics and high-speed aerodynamic structures. However, shock loading of material is a very short duration phenomenon and it is extremely challenging to develop sensors for dynamic measurements under such loading conditions. Optical fiber sensors that present the possibilities to allow high resolution measurement of force or displacement in such high strain rate loading conditions. This work studies the possibility of developing a single mode optical fiber sensor based on the principle of power losses from the curved section for dynamic measurements under shock loading conditions. The strain measurement results obtained from the optical sensors are validated with the controlled digital image correlation measurements are found to be in close correlation. The sensor was able to provide time sensitivity of better than 1 ms. The results show that the fiber sensor has the characteristics of high sensitivity and high precision, which can be used to measure fine vibration in real-time.

Veneer-wrapped steel columns: Proof-of-concept experimental and numerical investigations

This paper presents and discusses the structural behaviour of a novel type of steel–timber composite column to be used in residential building applications. It consists of a circular hollow section wrapped by multiple beech veneer sheets and where the composite action is ensured by a structural bi-component glue applied over the contact surfaces. As part of a proof-of-concept study, five short and three long columns with lengths of 0.27 m and 2.00 m, respectively, were tested under vertical concentric load with fibres parallel to the load direction and inclined by 15°. The global response of the specimens in terms of load-displacement behaviour and failure modes was studied, along with localised strain measurements through strain gauges and a digital image correlation (DIC) system. Results showed a significant increase in buckling resistance due to the stiffness added by the veneer layers. Already with the still-moderate veneer layer thicknesses used in this study, an increase in strength of over 25 % could thus be achieved. This enhancement highlights the effectiveness of veneer wrapping in stabilizing the inner core and delaying the occurrence of flexural buckling failure. A 3D non-linear finite element model (FEM) validated these findings, showing a good correlation with experimental results in resistance and stiffness. A parametric study was conducted to explore different geometries and materials, thus creating an extended FEM-based database. A relatively good agreement with modest deviation between the FEM predictions and analytical formulations available in the European Standards was found, considering the appropriate introduction of the mechanical properties of timber.

CARACTERIZACIÓN DE LAS PROPIEDADES MECÁNICAS DEL ÁCIDO POLILÁCTICO (PLA) EN IMPRESIÓN 3D: PERSPECTIVAS A PARTIR DE TÉCNICAS AVANZADAS DE ANÁLISIS Y ORIENTACIONES DE IMPRESIÓN

This research examines the mechanical characteristics of polylactic acid (PLA) using a variety of experimental approaches, including analyses of printing orientations and the application of advanced methodologies such as digital image correlation (DIC) for accurate assessment of deformation, along with scanning electron microscopy (SEM) to evaluate fracture properties. Tensile strength, flexural, impact strength, compressive and shear strength were characterized in detail, highlighting the critical impact of layer deposition direction. These findings provide valuable information for optimizing orientation selection in specific additive manufacturing contexts, emphasizing the need to address anisotropy in 3D printed materials engineering. Keywords: Polymer; PLA; 3D-print; industrial process; Digital Image Correlation (DIC).

Fracturing Process Visualization of Cracked Chevron-Notched Brazilian Discal and Notch Semicircular Bend Rock Specimens and Comparative Analysis by Combined AE and DIC

Abstract A series of homogeneous granite specimens were prepared to investigate the Mode I fracture toughness (KIC) using the cracked chevron-notched Brazilian disk (CCNBD) method and semicircular three-point bending (NSCB) method recommended by the International Society for Rock Mechanics (ISRM). A combination of acoustic emission (AE) and digital image correlation (DIC) was utilized to monitor the crack propagation and analyze the fracture modes at various test stages. The results indicated that the average value of Mode I fracture toughness (KIC-) of the CCNBD specimen was approximately 10% higher than that of the NSCB specimen. The coefficient of variation in the NSCB test was small, which could provide more stable result of Mode I fracture roughness. Compared to the CCNBD test, the failure mode of NSCB specimens was mainly tensile damage with instantaneous damage during the experimental process. Furthermore, the failure process of the NSCB test was mainly controlled by tensile fractures, which exhibited instantaneous failure. The CCNBD test was first controlled by tensile fractures and then by shear fractures when the peak was reached, exhibiting progressive failure. The failure mode and the process may be the main factors controlling the difference between the results of the NSCB and CCNBD tests. This work helps select appropriate methods to measure the Mode I rock fracture toughness.

EXPERIMENTAL ANALYSIS OF POISSON RATIO ON METAMATERIALS MANUFACTURED FROM CRUMPLED ALUMINUM FOIL SUBJECTED TO UNIAXIAL COMPRESSION

In this work experimental results are presented concerning the auxetic behavior of a crumpled metamaterial manufactured with aluminum foil, so far such experimental evidence has not been reported by other authors. Thus, 2D digital image correlation (DIC) was used to make full field measurements of longitudinal and transverse strains on the surface of crumpled aluminum specimens under axial compression; it was no possible to use conventional methods on the measurements of strains, such as strain gages or extensometers, due to the irregular topography on the surface of the specimens. The obtained results exhibit the auxetic nature of the aluminum crumpled material, with Poisson´s ratio values that vary from ?=-0.17 to ?=-0.02 for relative densities in the range ?_r=0.07 to ?_r=0.21. Keywords: Auxetic, Digital Image Correlation, Crumpled material, Poisson, Computer vision system.

Investigation of scale effects of rock bridges based on Multi-Physical field monitoring

The presence of rock bridges is crucial for ensuring the stability of rock slopes. However, evaluating the mechanical properties of rock bridges solely based on the joint persistence coefficient (K) raises doubts due to potential variations in the scales of rock bridges within a rock mass with a constant K value. In this study, direct shear tests with different scales were carried out on sandstone specimens. Acoustic Emission (AE) and Digital Image Correlation (DIC) techniques were utilized to monitor the damage procession of the specimens. The evolution process of the specimen was divided into stages and three threshold points were determined based on AE parameters. As the scale decreased, the Joint Roughness Coefficient (JRC) increased, the tensile failure increased, and the shear failure decreased. As the normal stress increased, the JRC with the same scale decreased, the tensile failure decreased, and the shear failure increased. The change of fracture mechanism was the primary reason for the strength deterioration with decreasing scales. The decrease of the scale was not conducive to the failure prediction. By improving the rock bridge failure potential (RBP) index, the proposed RBPn index could appropriately characterize scale effects under different normal stresses.

Experimental study on the Mode l fracture toughness of frozen silty clay incorporating Digital image correlation

To investigate the effects of initial moisture content and temperature on the Mode I fracture toughness (KIc) of frozen silty clay, a series of three-point bending tests were conducted. Rectangular specimens with prefabricated cracks were tested, and digital image correlation (DIC) technology was employed to analyze the microscopic characteristics at the crack tip. The results indicate that the Mode I fracture toughness of frozen silty clay increases with decreasing temperature and increasing initial moisture content. Temperature governs the failure mode, with plastic failure predominantly occurring at high temperatures and brittle failure at low temperatures. Based on the load–displacement curves and DIC recordings, the macroscopic fracture process of the specimens can be categorized into three stages: elastic deformation, formation of the failure surface, and specimen failure. Additionally, the crack propagation process can be further divided into three stages: initiation and development of microcracks, transition from microcracks to macroscopic cracks, and rapid development of macroscopic cracks. These findings provide critical insights for slope stability research in cold regions and offer new perspectives for engineering design and construction in similar environments.

Multiscale evaluation of sand-geomaterial interface shear response in terms of material geometry effects

This study evaluates the effects of particle and surface geometric features on the interface shear behavior of sand-geomaterial. Particle shape and size were adjusted for the sand, and surface roughness form and height were set for the geomaterial part. 3D printing technology was used to produce geomaterials in the desired form. A custom-made transparent interface shear box was developed to perform digital image processing. For macroscale response, peak and residual strengths and dilation angle were measured. Sliding percentage and shear band thickness were determined from digital image correlation analyses for the mesoscale response. The experimental results showed that the macroscale response parameters increased as the overall regularity of the sand particles decreased. The increase in mean particle size and roughness height led to an increase in macroscale parameters. Digital image correlation analyses showed that the sliding percentage had an inverse correlation with the shear band thickness in terms of the particle shape and the roughness height effects. The roughness form controlled the trend of mean particle size effects on the sliding percentage. Overall, the mesoscale parameters revealed the traces of interlocking response, particle kinematics, and the failure mechanism in the interface region. Thus, a bridge was established between meso- and macro-scale responses.

A speckle projection-based 3D digital image correlation method for measuring dynamic liquid surfaces

A speckle projection-based 3D digital image correlation method for measuring dynamic liquid surfaces

Towards measuring absolute residual stress by HR-EBSD with simulated reference patterns

The High-Resolution Electron Backscatter Diffraction (HR-EBSD) technique has drawn attention in recent years, mainly thanks to its ability to assess the elastic strain/stress state with a sub-micron spatial resolution. Each diffraction pattern is correlated with a reference one, and the strain level relative to the reference is retrieved in the procedure. Several studies have attempted to use dynamically simulated diffraction patterns as references to access absolute strain/stress states to circumvent the difficulty of finding a strain-free reference pattern, which is often inaccessible. Numerous factors need to be considered and corrected to register better experimental and simulated diffraction patterns, such as the accurate positioning of the projection center, Kikuchi band (K-band) brightness asymmetry, K-band gray level reversal, optical distortion, or non-uniform electron energy. Here, an integrated digital image correlation method is proposed to register experimental patterns to a master pattern defined on a sphere, where the difficulties with experimental patterns are, for the most part, corrected efficiently. Due to the shallow inspection depth of EBSD, the free-surface property (vanishing stress vector at the observation surface) is also adopted to reduce the degrees of freedom and enhance precision. Through several tests on high-quality experimental patterns, the proposed method proves robust and precise. Both the systematic and random errors of the residual strain results are several 10−4. The method also allows the simultaneous determination of residual stress and the projection center coordinates.

A New Method for Compression Testing of Reinforced Polymers

Compressive testing of specimens taken from relatively thin composite plates is difficult, especially due to the occurrence of buckling. To prevent buckling, the central portion of the specimens used for the compression test has smaller dimensions, and the specimens can be guided along their entire length. For these reasons, optical methods, such as digital image correlation (DIC), cannot be used for the compression test and strain rosettes cannot be glued onto the samples to determine Poisson’s ratio. In this study, compression tests of a glass fiber-reinforced polymer (GFRP) were conducted using both the ASTM D695 (Boeing version) and a newly proposed method. The new method involves using special specimens that allow T-type rosettes to be bonded to determine Poisson’s ratio, whose value of 0.14 was thus determined. SEM images of the failure surfaces were presented and interpreted. A finite element analysis (FEA) of the specimens tested in compression is also presented. The first analyzed case considers the homogeneous and orthotropic composite, loaded with a uniformly distributed force. The normal stress in the central section of the specimen, determined with FEA, has an error of 6.52% compared to that determined experimentally. Additionally, the strain in the center of the strain gauge, determined with FEA, has an error of 4.76% compared to the measured one. In the second case studied with FEA, the sample is loaded with a quasi-concentrated force, which can move in the direction of the symmetry axes of the cross-section, to study the effect of the eccentricity of the compression force on the state of stress. It was shown that the eccentricity of the force has a great influence: the stress distribution in the section of the specimen becomes strongly non-uniform. For a force eccentricity of 0.4 mm in the direction of the OX axis, the minimum stress decreases by 53.7%, and the maximum stress increases by 55.4%. In order to analyze the influence of some manufacturing defects, two other cases were analyzed by FEA, in which it was assumed that the thicknesses of the outer resin layers were modified, making them asymmetrical. For this final FEA, the specimen was considered to be composed of laminates. These results demonstrate the special attention that must be paid to the centric application of force in compression testing.

GMDIC: a digital image correlation measurement method based on global matching for large deformation displacement fields

The digital image correlation method is a non-contact optical measurement method, which has the advantages of full-field measurement, simple operation, and high measurement accuracy. The traditional DIC method can accurately measure displacement and strain fields, but there are still many limitations. (i) In the measurement of large displacement deformations, the calculation accuracy of the displacement field and strain field needs to be improved due to the unreasonable setting of parameters such as subset size and step size. (ii) It is difficult to avoid under-matching or over-matching when reconstructing smooth displacement or strain fields. (iii) When processing large-scale image data, the computational complexity will be very high, resulting in slow processing speeds. In recent years, deep-learning-based DIC has shown promising capabilities in addressing the aforementioned issues. We propose a new, to the best of our knowledge, DIC method based on deep learning, which is designed for measuring displacement fields of speckle images in complex large deformations. The network combines the multi-head attention Swin-Transformer and the high-efficient channel attention module ECA and adds positional information to the features to enhance feature representation capabilities. To train the model, we constructed a displacement field dataset that conformed to the real situation and contained various types of speckle images and complex deformations. The measurement results indicate that our model achieves consistent displacement prediction accuracy with traditional DIC methods in practical experiments. Moreover, our model outperforms traditional DIC methods in cases of large displacement scenarios.

Method for luminescent digital image correlation deformation measurement in non-illuminated environments

This study presents a luminescent digital image correlation (DIC) method that utilizes long afterglow materials to prepare speckle patterns, overcoming the limitations of classical DIC in achieving high-precision deformation measurements, such as the issues of specular reflections from specimens and insufficient contrast of speckle patterns. While fluorescent DIC has some advantages in overcoming these limitations, it relies on active ultraviolet light sources, making it challenging for luminescent measurements. Long afterglow materials, capable of maintaining brightness for extended periods, serve as a viable alternative. Through sphere reconstruction experiments, the accuracy of this method was validated, demonstrating a relative error of 0.04% under well-illuminated conditions and 0.025% under non-illuminated conditions. Finite element simulations and a comparison with DIC experimental results showcased excellent consistency, suggesting the potential for this method to further replace fluorescent DIC measurements. Furthermore, the study revealed that speckle patterns prepared using this approach ensure measurement validity in both well-illuminated and non-illuminated scenarios. This luminescent DIC method holds promising potential for broader applications in non-illuminated measurement environments.

Analytic modeling and validation of strain in textile-based OLEDs for advanced textile display technologies

In the IoT era, the demand for wearable displays is rapidly growing, catalyzing the advancement of research into textile-based organic light-emitting diodes (OLEDs). This growing interest stems particularly from the inherent flexibility of textile-based OLEDs1,2, allowing for seamless integration into the dynamic and interactive functionalities of cutting-edge wearable technology, alongside their superior electrical performance. The durability and mechanical robustness of these displays, especially under physical stress and deformation, are critical to their practical application and longevity. Thus, understanding and enhancing the mechanical properties of textile-based OLEDs is paramount for their successful integration into wearable technologies. However, many studies assessing the mechanical properties of OLEDs have predominantly relied on simplistic bending test outcomes determined by the radius, often neglecting or insufficiently analyzing the strain exerted on the OLEDs atop textile substrates in relation to curvature of these devices. Existing analyses typically presume pure bending, though such an assumption leads to considerable errors in strain estimations, making such approaches problematic if the goal is practical application in actual wearable display products. To address these limitations, an analytic model that includes a comprehensive energy equation is introduced, considering the stretching energy, bending energy, and shear energy of each layer composing the textile substrate. This holistic approach provides a novel formula specifically designed to calculate the top surface strain of textile substrates. Robust validation of this formula is conducted by comparing its results with strain measurements obtained from digital image correlation (DIC) and finite element analysis (FEA) outcomes from ANSYS across various bending radii (or equivalently, curvatures). The close alignment of the calculated strain values with those derived from DIC and FEA not only underscores the precision of this formula but also highlights its significant potential for enhancing the designs and functionalities of future wearable display technologies under real-world conditions.

Role of molecular damage in crack initiation mechanisms of tough elastomers

Tough soft materials such as multiple network elastomers (MNE) or filled elastomers are typically stretchable and include significant energy dissipation mechanisms that prevent or delay crack growth. Yet most studies and fracture models focus on steady-state propagation and damage is assumed to be decoupled from the local stress and strain fields near the crack tip. We report an in situ spatial-temporally resolved 3D measurement of molecular damage in mechanophore-labeled MNE just before a crack propagates. This technique, complemented by digital image correlation, allows us to compare the spatial distribution of both damage and deformation in single network (SN) elastomers and in MNE. Compared to SN, MNE have a wide-spread damage in front of the crack and, surprisingly, delocalize strain concentration. A continuum model, where damage distribution is fully coupled to the crack tip fields, is proposed to explain these results. Additional measurements of time-dependent molecular damage during fixed grips relaxation in the presence of a crack reveal that the less localized damage distribution delays fracture initiation. The observations and exploratory modeling reveal the dynamic fracture mechanism of MNE, providing guidance for rational design of high-performance tough elastomers.

Interpretation of mechanical properties gradient in laser-welded joints: Experiments and grain morphology-dependent crystal plasticity modeling

The grain size and morphology of laser welds show a gradient distribution, leading to variations in mechanical properties. This study proposes a microscopic crystal plasticity constitutive (CPC) model to examine the effect of ER4047 welding wire on the mechanical properties of laser-welded aluminum alloy joints. A finite element model updating strategy was used to fully invert the morphological mechanical parameters, marking a significant advancement. This study combines mesoscale electron backscatter diffraction (EBSD) observations with macroscopic uniaxial tensile tests to obtain crystal plasticity finite element analysis and strain field data from digital image correlation. The mechanical property gradient of the welded joints was systematically analyzed from a crystal perspective. The CPC parameters were inverted for different grain sizes and morphologies. The study found that using ER4047 welding wire significantly enhanced the mechanical properties of the welded joints, especially in the welding zone. As grain size increased, the yield strength of the material decreased, while the hardening modulus increased. Additionally, the cast characteristics of columnar crystal morphology reduced both yield strength and hardening modulus.

Temperature and rate dependent shear characterization and modeling of spread-tow woven Flax/PP composite laminates

This paper investigates the in-plane shear behavior of spread-tow woven structured flax fiber-reinforced polypropylene composites, focusing on temperature and displacement rate effects. For this aim, the study includes bias-extension experiments to characterize the shear behavior of the material. Several experiments were conducted at different displacement rates and temperature levels. The temperature levels were chosen to encompass the material's behavior during solid and molten-state thermoforming, given the melting and decomposition temperatures acquired from differential scanning calorimetry and thermogravimetric analysis tests, respectively. A new fixture was designed and manufactured to allow pre-heating of the oven and rapid placement of the samples, which in turn, prevents their degradation for the time duration required to reach a uniform temperature distribution in the oven. During the experiments, the 3D digital image correlation technique accurately measured local deformations and strains on the specimen's surface under varying conditions. Further, a finite element model is developed, incorporating a fiber reorientation algorithm with a hypo-elastic shear model to simulate the material's temperature and rate-dependent behavior. The finite element shear angle distributions, as well as the shear angle-force and shear angle-displacement curves, are compared with the Digital Image Correlation (DIC) data and load measurements for different test conditions, showing good agreement between the numerical and experimental approaches.