Three-dimensional Digital Image Correlation Research Articles

Dynamic shear fracture behavior of rocks: insights from three-dimensional digital image correlation technique

The influence of the loading rate on the mode I fracture property of rocks has been extensively investigated through experiments. However, the rate-effect of the dynamic mode II fracture has rarely been reported. This study employed the modified split Hopkinson pressure bar (SHPB) system to conduct a series of dynamic mode II fracture tests under a wide range of loading rates using the modified short-core-in-compression (SCC) method. The complete dynamic fracture processes of rock were observed using the three-dimensional high-speed digital image correlation (3D HS DIC) technique. The results reveal that the dynamic mode II fracture toughness increases with the loading rate at a power relationship. By virtue of the evolution of the surface principal strain and displacement, the dynamic fracture and damage process and the deformation characteristic of the SCC specimen were analyzed in detail. The dynamic fracture processes are divided into the elastic stage, plastic deformation stage, unstable crack growth stage, and post fracture stage. The crack propagation path of the SCC specimen was obtained under impact loading. It was found that new cracks first initiate from two notch tips and then propagate toward the central area in the expected shear fracture band (ESFB); the path is nearly independent of the loading rate. Additionally, the macro–micro fracture surface morphology of the SCC specimen was measured using a 3D optical scanner as well as scanning electron microscope (SEM) respectively. It was found that the roughness and curvature in the fracture surface created by shearing generally decrease as the loading rate increases, in accordance with fractal theory. The SEM result indicates intergranular fracture occurs more frequently on the shear fracture surface under static loading, while transgranular fracture is more common on the shear fracture surface at high loading rates.

Three-Dimensional Digital Image Correlation Based on Speckle Pattern Projection for Non-Invasive Vibrational Analysis.

Non-contact vibration measurements are relevant for non-invasively characterizing the mechanical behavior of structures. This paper presents a novel methodology for full-field vibrational analysis at high frequencies using the three-dimensional digital image correlation technique combined with the projection of a speckle pattern. The method includes stereo calibration and image processing routines for accurate three-dimensional data acquisition. Quantitative analysis allows the extraction of several deformation parameters, such as the cross-correlation coefficients, shape and intensity, as well as the out-of-plane displacement fields and mode shapes. The potential of the methodology is demonstrated on an Unmanned Aerial Vehicle wing made of composite material, followed by experimental validation with reference accelerometers. The results obtained with the projected three-dimensional digital image correlation show a percentage of error below 5% compared with the measures of accelerometers, achieving, therefore, high sensitivity to detect the dynamic modes in structures made of composite material.

Shear-bond behaviour between concrete and hybrid fibre-reinforced cementitious composites for repairing: Experimental and modelling

Hybrid fibre-reinforced cementitious composites (HFRCC), with high toughness and ductility, have great potential for repairing damaged concrete in complex service environments. The performance of repairing largely depends on the bond between repairing materials and existing concrete. In this study, the direct shear-bond tests of cubic specimens were carried out to explore the shear-bond behaviour between existing concrete and HFRCC for repairing concrete structures. Then, the effects of polyvinyl alcohol (PVA)/steel fibre volume fraction and water/binder ratio of HFRCC on the bond behaviour were analyzed. The experimental results show that the failure mode of specimens changed from HFRCC fracture to interfacial fracture with the increase of PVA fibre content in HFRCCs. Additionally, the failure mode of specimens transformed from interfacial fracture to hybrid fracture with the increase of steel fibre content in HFRCCs. However, water/binder ratio of HFRCCs only marginally affected the failure mode. The shear specimens with a water/binder ratio of 0.35, PVA fibre volume fraction of 1.5% and steel fibre volume fraction of 0.75% (e.g., S0.75P1.5W0.35) exhibited the highest shear strength. With the increase in loading, the strain concentration on the interface obtained by three-dimensional digital image correlation (3D-DIC) technology moved from the loading end to the free end and the strain at the loading end increased gradually. Finally, the shear strength model and bilinear bond-slip model of HFRCC-concrete were proposed to predict shear strength and shear stress-slip curve, respectively.

Silent Speech Recognition with Strain Sensors and Deep Learning Analysis of Directional Facial Muscle Movement.

Silent communication based on biosignals from facial muscle requires accurate detection of its directional movement and thus optimally positioning minimum numbers of sensors for higher accuracy of speech recognition with a minimal person-to-person variation. So far, previous approaches based on electromyogram or pressure sensors are ineffective in detecting the directional movement of facial muscles. Therefore, in this study, high-performance strain sensors are used for separately detecting x- and y-axis strain. Directional strain distribution data of facial muscle is obtained by applying three-dimensional digital image correlation. Deep learning analysis is utilized for identifying optimal positions of directional strain sensors. The recognition system with four directional strain sensors conformably attached to the face shows silent vowel recognition with 85.24% accuracy and even 76.95% for completely nonobserved subjects. These results show that detection of the directional strain distribution at the optimal facial points will be the key enabling technology for highly accurate silent speech recognition.

Space–time evolution law of progressive failure area and mechanical behaviour of rock under different seepage conditions

In engineering geological disasters, the presence of water can accelerate rock mass fracture, and the state of the seepage field has a varying influence on the elastic behaviour of rock mass. A three-dimensional digital image correlation (3D-DIC) system is used to conduct triaxial compression tests on sandstone under different seepage conditions. The variation law of rock mechanical behaviour and the space–time evolution law of the surface radial strain field in the progressive failure process are analysed. The results show that, as the seepage pressure increases, the rock peak stress decreases and the Young's modulus increases, and the mechanical properties of the rock are easily affected by the varying seepage conditions. Meanwhile, a constitutive equation based on the Gumbel distribution, which can accurately reflect the axial stress–strain relationship in the rock failure process is established. By analysing the spatial distribution of virtual data points with large radial strain values at different times on the sample surface, a patent strain localisation phenomenon is identified in the progressive failure process of the rock. The localisation degree and extent of the failure area can be quantified using the average nearest-neighbour analysis (ANNA) and user-defined parameter S, which is used to normalise the average distance from the virtual data points to the fitted fracture surface. The results show that, with the continuous compression of samples under different seepage conditions, ANN decreases as S increases, and the large strain regional distribution is increasingly concentrated. The research results can be applied to predict surrounding rock failure areas in underground spaces and provide monitoring references with respect to engineering geological disasters under different seepage conditions.

Statistical Characterization of Boundary Kinematics Observed on a Series of Triaxial Sand Specimens

This paper follows up on a reference paper that inspired MDPI’s topic “Stochastic Geomechanics: From Experimentation to Forward Modeling”, in which the authors populated a spatio–temporal database of boundary displacement fields from a series of triaxial sand specimens using three-dimensional (3D) digital image correlation analysis. The database was curated and is currently available to the scientific community for further study. This paper uses a subset of this database, in which the experimental conditions were similar, to statistically investigate the dominant kinematic phenomena observed on the boundary of triaxial sand specimens under compression. The first-order 3D kinematic operators under the cylindrical coordinates, comprised of the divergence, curl, and gradient, were applied to the boundary displacement fields to illustrate the localization deformation patterns including the translational, rotational, shearing, and volumetric behaviors throughout the triaxial compression processes. Subsequently, the first-order statistics of the kinematic results are estimated, with the aim of revealing the evolution of associated localization effects as well as their corresponding uncertainties in space and time. The results of this research provide an innovative statistical interpretation of the localization effects on soil specimens under three-dimensional stress conditions. The proposed approach advances the interpretation of granular material’s responses under triaxial compression experimental conditions, while opening an opportunity to reproduce the material’s kinematic responses under the triaxial experimental conditions through constitutive modeling or machine learning techniques.

Subcritical crack growth and fracture behavior of rocks and long-term strength estimation

Understanding the potential subcritical fracture behavior is very essential to assess the perennial stability of rock engineering. The purpose of this study is to reveal the fracture characteristics and subcritical crack propagation of rocks with different lithologies, emphasizing that the influence of time-dependent fractures on the long-term strength of rocks cannot be ignored. The fracture toughness, KI-V curves and subcritical growth index of five types of rocks were obtained by the double-torsion (DT) technique. Subcritical fracture propagation exists in the rock as the stress intensity level is between K0 and KIC, and the ratio of the two values is between 0.73 and 0.83. Using a three-dimensional digital image correlation (3D-DIC) technique and a plane mirror, the evolution of tensile strain and crack propagation process on the bottom surface of specimens were observed successfully. The granite and basalt showed a faster growth rate of subcritical crack propagation, with fracture rate ranging from 10-2.5 to 10-6.9 m/s. The long-term strength of rocks was estimated based on the tensile strength, fracture toughness, SCI and A values. In a short term, basalts with the highest tensile strength have the highest long-term strength, while granites show the highest long-term strength when the time-to-failure exceeds 408.2 years. Therefore, the time-dependent cracking behavior of rock engineering must be considered when assessing the long-term stability.

Identification of crack initiation and damage thresholds in sandstone using 3D digital image correlation

In this study, a non-contact image correlation measurement technique, Three-Dimensional Digital Image Correlation (3D-DIC), is employed to identify and quantify the evolution of rock crack initiation and propagation. With the aim of carefully investigating the cracking progress, a uniaxial compression test was conducted on sandstone specimens with a single pre-existing flaw. The surface and internal micro-fracture characteristics of the sandstone specimen were detected by combining 3D-DIC and acoustic emission techniques from both optical and acoustic perspectives. On the basis of the full-field deformation data derived by 3D-DIC, a method for determining the principal strain of cracks was defined, which aids in identifying the spatial propagation characteristics of rock cracks. Additionally, the crack principal strain method (CPSM) was proposed for determining the characteristic stress threshold of rocks based on the crack principal strain. Then, the results obtained from CPSM are compared to the conventional acoustic emission method (AEM) and axial stiffness method (ASM) tests. The research results suggest that AEM is suitable for identifying crack closure stress and crack initiation stress, whereas ASM and CPSM are superior for identifying damage stress. This work provides a novel method and concept for the measurement of non-contact characteristic stress thresholds, as well as has an insight into the detection and evaluation of the evolutionary process of rock damage and fracture.

Damage evolution and failure mechanism of asymmetric composite laminates under low-velocity impact and compression after impact

The damage mechanisms of the balanced symmetrical laminates under low-velocity impact (LVI) and compression after impact (CAI) loading conditions are not fully applicable to composite stiffened panels. However, studies on LVI and CAI damage evolution and failure mechanism for asymmetric composite laminates are very rare, which poses challenges to the theoretical study, strength prediction, and application of composite structures, and increases the risk of sudden failure under load. In this study, the damage evolution and failure mechanisms of asymmetric composite laminates under LVI and CAI loading conditions were studied experimentally and numerically. Ultrasonic phased array C-scan technique was used to analyze damage state, damage size, and delamination damage induced by LVI under different impact energies in impact tests. Moreover, a three-dimensional digital image correlation (3D-DIC) technique was employed to monitor the evolution of full-field displacement and strain in the asymmetric composite laminates during the CAI tests. A three-dimensional damage model considering the interaction among the interlaminar delamination damage, intralaminar matrix damage, and intralaminar fiber damage was proposed, and an interface-based cohesive behavior embedded framework in ABAQUS/Explicit was used to define and capture the interlaminar damage. The damage initiation, evolution, and propagation behaviors of different damage modes were simulated to study the complex damage and failure mechanisms of the asymmetric laminates in the LVI and CAI processes. The relationships among the different impact energies and the impact damage modes, delamination morphologies, and compression damage propagation and failure modes were discussed.

Experimental investigation blast resistance of CFRP/polyurea composite plates under blast loading

With the development of composite structures, polyurea elastomer can be utilized to enhance the protective performance of composite laminated structures under blast or impact loading. Three types of polyurea coated CFRP (CFRP/polyurea) composite plates were experimentally evaluated. In order to study the blast resistance of CFRP/polyurea composite plates subjected to free air blast loading, a three-dimensional digital image correlation (3D-DIC) full-field measurement method was used to obtain the dynamic response of composite structures. The effect of polyurea coating position and scaled distance were analyzed and discussed. In addition, the failure modes and failure mechanisms of CFRP/polyurea composite plates were analyzed. The results show that 3D-DIC technique is a good tool for the assessment of the dynamic response of CFRP/polyurea composite plates under blast loading. From the deformation and damage level of structures, the polyurea coating on rear surface of CFRP exhibited superior blast resistance. With the decrease of scaled distance, the deformation level of CFRP/polyurea composite plates increases. The failure modes of CFRP/polyurea composite plates varied significantly with the change of structural types. Brittle fracture, structural warpage and delamination are the main failure modes of CFRP materials. Moreover, cracking, melting and the deformation of the micropore are the main failure mechanism of polyurea coating. Overall, the combination of CFRP plate and polyurea has an obvious role in the aspect of blast resistance.

Investigation on out-of-plane displacement measurements of thin films via a mechanical constraint-based 3D-DIC technique

Thin film is functioning as a key material in surface engineering, and accurate measurement of its deformation are particularly important for the reliability and stability assessment of coating-substrate structures. However, the displacement field measured by the conventional three-dimensional subset-based digital image correlation (3D-DIC) method has no spatial continuity; thus, it cannot precisely reflect the true deformation of thin films. To improve the measurement accuracy, a 3D-DIC method considering mechanical constraints (3DMC-DIC) was proposed in this paper. In this method, a mesh with 8-noded quadrilateral shell elements satisfying spatial continuity was employed to determine the out-of-plane displacements of all nodes based on trusted points, which are the speckle points with higher measurement accuracy obtained by the conventional subset-DIC method. The whole out-of-plane displacement field was then inversely obtained based on the optimized displacements of these nodes. A blister test was performed, and the results show that the out-of-plane displacement field of thin film with spatial continuity and high accuracy on specimen edge can be obtained by the 3DMC-DIC method.

Experimental Evaluation of Hoop Stress–Strain State of 3D-Printed Pipe Ring Tensile Specimens

Data on the strain and stress status of the pipe in the circumferential direction are required for various pipe manufacturing procedures (e.g., in the oil business, the process of manufacturing seamless pipes with a conical shaft). The aim of this study is to develop a procedure to determine the strain and stress behavior of Pipe Ring Tensile Specimens (PRTSs) in the hoop direction, as there are a lack of official standardized methods for testing PRTS. This paper discusses the application of the Digital Image Correlation method for testing plastic PRTSs. PRTSs are tested using a specially designed steel tool with two D blocks. A 3D-printed PRTS is placed over two D-shaped mandrels, which are fixed on a tensile tool and tensile testing machine. The strain evolution in the gage length of the specimens is captured using the three-dimensional Digital Image Correlation (3D DIC) method. To check the geometry of the cross-section of the PRTS after fracture, all the specimens are 3D scanned. For the study, six groups of PRTS are analyzed, consisting of three filling percentages (60, 90, and 100%) and two geometry types (Single and Double PRTS). The results show that the type and percentage of filling, as well as the method of printing, affect the material behavior. However, the approach with the DIC system, 3D printer, and scanner shows that they are effective instruments for mapping complete strain fields in PRTS, and thus are effective in characterizing the mechanical properties of pipes.

Height deviation detection of rail bearing platform on high-speed railway track slab based on digital image correlation

The security and convenience of high-speed railways depend on the high reliability and high stability of ballastless tracks. To ensure track regularity, it is necessary to measure the height deviation of each rail bearing platform before laying the steel rail. By adjusting the fastening system, the rail could reach the design height and be at the same level. Conventionally, the height deviation of the bearing platform is measured through the contact walking measurement system. However, there are shortcomings such as few measurement points, long preparation time and low efficiency. In this paper, a fusion measurement method based on digital image correlation method and total station is proposed to solve these shortcomings. A measuring equipment consisting of dual cameras and reflecting prisms is assembled to measure the height deviation of bearing platforms (675 mm × 275 mm × 28 mm) on the high-speed railway track slab (5600 mm × 2500 mm). The three-dimensional contour of each bearing platform is measured using three-dimensional digital image correlation method and unified by locating the position of the reflecting prisms using the total station. In real applications, the height deviation of each bearing platform is measured with high accuracy, the contour measurement accuracy of a single bearing platform is better than 0.1 mm. More importantly, the range of ballastless tracks with bearing platforms to be detected in a single day is increased from the original 500 m to 5000 m without increasing the measurement error. The efficiency is increased by 10 times. The paper achieved the height deviation detection of rail bearing platform with high precision and high efficiency, contributing to improving the construction efficiency.

Experimental Study on Mechanical Properties and Failure Laws of Granite with Artificial Flaws under Coupled Static and Dynamic Loads.

Rock is the main construction material of rock engineering, such as the engineering of mines and tunnels; in addition, its mechanical properties and failure laws are of great significance to the stability evaluation of rock engineering, especially under the conditions of coupled static–static stresses. In this study, granite specimens were manufactured with artificial flaws. Coupled static and dynamic loads tests were carried out with a modified split Hopkinson pressure bar (SHPB) apparatus; and six typical levels of axial pre-stresses and three crack inclination angles were designed. Three-dimensional digital image correlation (3D-DIC) was also applied to record and analyze the fracturing process and damage evolution of the specimens. The test results show that there was no compaction stage in the stress–strain curve under combined dynamic and static loading. The dynamic strength of the specimens increased first and then decreased with the increase in the static pressure; moreover, the specimens reached the maximum dynamic strength when the static pressure was 10% UCS. The dynamic strength decreased first and then increased with the increase in the crack inclination angle; and the lowest strength appeared when the inclination angle was 45°. The change in axial compression had a significant influence on the failure mode, and the failure mode gradually transformed from shear–tensile failure to shear failure with the increase in the pre-stress. The tensile strain was usually generated at the end of the fractures or near the rock bridge. When the axial pressure was small, the tensile strain zone parallel to the loading direction was easily generated; and when the axial pressure was large, a shear strain zone developed, extending along the diagonal direction. The research results can provide a theoretical reference for the correct understanding of the failure mechanisms of granite and its engineering stability under actual conditions.

An Experimental Investigation of the Influence of the State of Stress on the Ductile Fracture of 2024-T351 Aluminum

Abstract This paper investigates the influence of stress state on the equivalent plastic fracture strain in 2024-T351 aluminum alloy. Eighteen unique stress states at failure—with triaxialities ranging from 0.388 (compressive) to −0.891 (tensile) and Lode parameters ranging from −0.978 to 1.000—are explored through mechanical experiments on 2024-T351 aluminum specimens with various geometries under multiple loading conditions. These include tension tests of plane stress (thin), plane strain (thick), and axisymmetric specimens, as well as pure shear and combined axial–torsional loading on thin-walled tubes. Using a hybrid numerical–experimental approach, the dependence of fracture strain on stress triaxiality and Lode parameter is quantified for each experiment. Fracture strains are measured using three-dimensional digital image correlation. Equivalent plastic fracture strain for 2024-T351 generally increases with stress triaxiality (moving toward a more compressive state). Fracture strain decreases modestly as the Lode parameter decreases from 1 to 0, although there is a significant increase in ductility as the Lode parameter decreases below −0.8. Compression–torsion tests extend the data’s stress-space coverage well into the compressive triaxiality region (up to 0.388) and to Lode parameters approaching −1. This experimental program provides the most extensive set of ductile fracture data from a single 12.7-mm-thick 2024-T351 aluminum plate in the literature. These data can be used to calibrate ductile fracture models used in finite element simulations of dynamic events such as bird strikes, automotive collisions, and debris containment.

Deformation Evolution Law of Surfacing Welding on Thin Bending Plates Based on the Three-Dimensional Thermal Digital Image Correlation Method

Surfacing on the surface of thin metal bending plates will cause significant deformation, and current numerical simulation and experimental methods cannot fully and truly reflect the deformation state of the bending plate. In this paper, a non-contact detection method based on the three-dimensional (3D) thermal digital image correlation (DIC) method is proposed. The proposed method can be used for the 3D full-field dynamic measurement of metal thin bending plate surfaces. In addition, the evolution law of in-plane and out-of-plane deformation of thin bending plates during surfacing welding and cooling was studied. Moreover, the influence of curvature on the shrinkage deformation of thin bending plate weld was explored, and the correlation between the curvature of thin bending plates and the weld shrinkage was established. Results show that the proposed detection method based on the 3D thermal DIC method can rapidly and accurately detect bending deformation online. The out-of-plane deformation of the surfacing welding of the thin bending plate transits from the disk to the saddle. Furthermore, the curvature of the thin bending plate is inversely proportional to the transverse shrinkage of the weld bead. After the curvature reaches a certain value, it has little effect on the longitudinal shrinkage of the weld bead. This detection method solves the problem of welding deformation simulation verification, truly clarifies the law of welding dynamic deformation, and provides a theoretical basis for welding lightweight manufacturing.

Impulse saturation in metal plates under confined blasts

The phenomenon of impulse saturation is revisited for metal plates subjected to confined blasts through a combination of experiments, numerical simulations, and analytical modeling. Internal blast experiments were performed on the fully-clamped aluminium square plate where its transient inelastic (mode I) response was recorded using three-dimensional digital image correlation (DIC). A methodology was proposed to idealise the confined blast induced pulse-pressure through a validated numerical model. An analytical model was developed to predict the transient central deflection of the aluminium plate under the idealised pulse pressure, where the predictions were shown to be in reasonable agreement with the DIC measurements. Parametric studies were performed to elucidate the effect of standoff distance and confinement geometry on the saturation duration, saturated impulse and deflection. It was found that both the pulse-shape and saturated impulse strongly influence the saturated deflection during internal blast loadings.

Mechanical behaviors of inner and outer sidewalls of honeycomb cores subjected to out-of-plane compression

The mechanical behaviors of honeycomb cores subjected to out-of-plane compressive loading were studied based on the deformation of cell walls. Four types of honeycomb structures, with and without panels, were experimentally, theoretically, and numerically investigated, respectively. In addition to the deformation of outer sidewalls, the stereovision three-dimensional digital image correlation (3D-DIC) method was first applied to determine the deformation process of the inner sidewalls. A coaxial 3D-DIC was developed to obtain the full-field displacements and strain distributions of the inner sidewalls. The compression strength of the structures was predicted successfully by the theoretical elastic buckling strength. Moreover, the onset of the damage process was simulated with finite element calculations, which agreed well with experimental data. Therefore, this study provides a better understanding on the behavior of honeycomb structures from the deformation of outer and inner surfaces.

Monitoring the slip of helical wires in a flexible riser under combined tension and bending

When a flexible pipe is subjected to combined tension and bending, the helical wires in the external tension armour layer are most failure prone. Understanding the true relative slippage of helical wires is very important to accurately predict the mechanical behaviour of flexible risers under complex loads. This study proposes a monitoring method for the relative slippage of helical wires in flexible risers under combined tension and bending, using three-dimensional digital image correlation technology. For the flexible riser test specimen with an internal diameter of 8 inches, a four-point quasi-static test was carried out, and full-field displacement vectors of helical wires marked by a range of scattered spots were accurately monitored. Thereafter, the relative slip character of the helical wires is obtained by processing the measurement data. The results indicate that the measured relative slip vectors are essentially tangential to the surface of the internal cylinder, while the measured total slip distances and angles are compared with the results from the existing theoretical model and the good agreement is achieved, thus verifying the correctness of the measurement method. Additionally, the laws of slip responses, such as the total slip distance, slip direction, and slip components along the axial and lateral wire directions, of helical wires in flexible risers are investigated, and the slip responses of the helical wire under different axial tensions are discussed.

Experimental investigation into dynamic mechanical properties and explosion responses of polyurea elastomer

To reduce the threat of explosions to personnel safety, it can be considered to retrofit the existing buildings in high-risk areas of process industry in an economical and effective way. The mechanical behaviors of two types of polyureas were studied through a series of laboratory tests and field gas explosion tests, and the full-field explosion responses of polyurea were measured by a high-speed, three-dimensional digital image correlation (DIC) technique. It was found that polyurea exhibited a significant strain-rate-hardening effect, and the fracture strain of polyurea decreased significantly with the increase of strain rate. The failure criterion of the polyurea-coated fiber-reinforced cement board (FRCB) under gas explosion load conformed to the overpressure criterion. Polyurea could significantly improve the explosion resistance of the FRCB, and the reinforcement ability could be further improved by increasing the polyurea thickness of the rear face and strengthening on both sides. Dynamic thermodynamic and microscopic observations revealed that under gas explosions, the polyurea was in the rubber–glass transition zone, which was beneficial for improving the ability of the material to attenuate shock waves.