Three-dimensional Digital Image Correlation Technique Research Articles

Dynamic shear behaviours of granite under coupled static and high-rate loadings

Understanding dynamic shear behaviours of rock under quasi-static confinement and dynamic loads is not only of great importance in underground engineering structures, but also beneficial to reveal earthquake mechanics regarding pre-stressed rocks fracturing from initial intact to rapid shear sliding. Here we propose a dynamic shear test setup and present full-field observations of stressed rock failure at high shear rate (up to γ˙ = 220 s−1) and sliding at high slip velocity over 10 m/s in the order of coseismic slip, with the integration of a novel triaxial Hopkinson bar (Tri-HB) and high-speed three-dimensional Digital Image Correlation (3D-DIC) techniques. Dynamic shear strength and friction coefficient are determined during the dynamic shear test under confinement, with different shear strain rates and pre-stresses. The dynamic shearing process can be divided into three stages, i.e. shear deformation, fracture degradation and dynamic frictional sliding. The high energy density of shear fracture band is observed, the rate and stress state dependences of rock shear strength and dynamic frictional coefficient are revealed. The dynamic shear strength of granite follows the Mohr-Coulomb criteria under varying pre-stresses and strain rates. The gouge materials with size of 1∼10 μm are generated within the high-rate shearing zone, and this appears to control the frictional resistance. The results could help determination of rock shear strength and post-shear failure, as well as explain the shear rupture and the resistance to slip mechanism of rock faults and strong near-surface earthquake events.

Design of Cold-Formed Z-Shaped purlin-to-rafter connections subject to pull-through failure

The current study is focused on understanding the pull-through behaviour of Z-Shaped Cold-Formed Steel (CFS) purlin-to-rafter connection. A total of forty tests were conducted using small-scale testing methods to investigate the influence of parameters such as thickness, depth, and flange width of the purlin section along with the size of the screw head diameter. The results indicated that the test specimens exhibited two modes of pull-through failure (i) rupture-type failure and (ii) bearing-type failure. A transition from rupture-type to bearing-type was observed with an increase in thickness from 1 mm to 3 mm. Both the failure modes were accompanied by tilting of the screw due to eccentricity in the applied load for Z-Shaped purlins. A full-field measurement approach called Three-Dimensional Digital Image Correlation (3D-DIC) technique was employed to better understand the failure mechanism. The DIC strain contours adequately captured the tilting of the screw that can be attributed to the increased strain and out-of-plane deformation at the vicinity of the screw head. A comparison of the test with existing design standards indicates that the design guidelines are unconservative for Z-shaped purlin sections. A modified design equation is proposed to determine the pull-through capacity of Z-shaped purlin-to-rafter connection. Further, reliability study was carried out to determine the resistance and safety factors for the preliminary design equations proposed.

Measurement and assembly methods for bolted flange joints using three-dimensional digital image correlation

Bolted flange joints are extensively used in pressure vessels and piping equipment. The assembly coaxiality accuracy and bolt pre-tightening force are particularly important in causing the risk of seal and leakage failure. This paper proposed a novel non-contact measurement and assembly adjustment method to reduce the coaxiality angle deviation efficiently. The flange joint is first bolted according to standard flange joint assembly guidelines and then adjusted using the proposed method. In the proposed method, the surface deformation and the coaxiality of the flange are evaluated using a 3D-DIC (three-dimensional digital image correlation) technique; RMS (root mean square) of the surface deformation is used to drive the correction of the bolts to further reduce the coaxiality angle deviation. Compared with existing bolting patterns, the established measurement and assembly adjustment method, which combines 3D-DIC measurement and adjustment based on RMS, can effectively reduce the coaxiality angle deviation. The experiments using a correction strategy demonstrated that the coaxiality angle deviation is reduced by 48.9 % and 60.1 % on average compared with the well-known star bolting pattern and modified star bolting pattern provided by ASME PCC-1, respectively, and even by 30.6 % and 51.9 % compared with their rotational pass patterns. Further experiments with an iterative strategy showed that the coaxiality angle deviation could be reduced by around 70 % and 75 %, tending to be stable around 0.01°–0.035°, after only 5 iterative adjustments.

Experimental Study on the Floor Heave and Failure Process of Rock Samples under Biaxial Step Loading

Floor heave is a typical tunnel issue in tunnelling engineering. To gain deep insights into the deformation mechanism and failure processes of floor heave at the bottom of a tunnel in layered rock, biaxial step-loading tests were conducted on rock samples (including schist and sandstone) with and without prefabricated invert arches. The failure processes of the samples were observed by the three-dimensional digital image correlation technique (3D-DIC) during the test. The test results showed that the deformation evolution processes of the floor heave of the sample included the following steps: (1) crack initiation at the interlayer weak planes; (2) separation of the rock matrix into platy structures along the bedding planes and flexures; and (3) fracture and uplift of the platy structures in the middle part. As the stress redistributes on the bottom plate of the sample, and stress concentration zones shift toward locations far away from the arching surface, the deformation evolution shows a similar variation trend with the stress. Continuous buckling fracturing takes place progressively from the vicinity of the arch surface to certain distant regions. Based on the test results, the key location of internal surrounding rock deformation was determined, and the mechanism of floor heave was clarified. The schist sample SC-BI-10 began to experience floor heave at 1064.4 s, and the deformation curve (the relationship between Y and U) showed a convex shape in the range of 0–20 mm in the Y-coordinate. The displacement reached its maximum value at y = 11.7 mm, corresponding to the position where the rock slab was broken. In addition, the influence of the interlayer properties and cover depth of rocks on bottom uplift was also studied. The design of tunnel supports and the monitoring and prevention of floor heave can benefit from this study.

Dynamic responses of kerosene-filled riveted tanks subjected to hydrodynamic ram caused by ballistic impacts

The hydrodynamic ram (HRAM) is a hydrodynamic pressure phenomenon that occurs when high-speed projectiles impact a liquid-filled tank. During combat missions, aircraft are prone to HRAMs caused by projectile impacts, which can damage the structural integrity of riveted tanks, posing a threat to flight safety. In this study, kerosene-filled riveted tanks were chosen as the research object. The dynamic response histories of the rear panel were observed using the three-dimensional digital image correlation (3D-DIC) technique. Additionally, numerical simulations of the projectile's impact on the tank were performed using the fluid–structure interaction finite element method. The simulation results had a good fit with the experimental results. The spatial and temporal distribution of the HRAM and the effect of the projectile impact velocity on the deformation of the tank structure were further analyzed. The impulse exerted by the drag wave on the rear panel was greater than that of the pioneer wave. However, the drag wave's region was limited to a radius of 6 cm around the exit point with the impact velocity range of the projectile of 955 m·s−1–1607 m·s−1 in both the experiments and simulations. When the impact velocity of the projectile reached 955 m·s−1, radial plastic hinge lines appeared on the rear panel along the diagonal direction. When the impact velocity increased to 1607 m·s−1, radial plastic hinge lines along the diagonal direction, and traveling plastic hinges at the root of the central cross-shaped cracks appeared on the rear panel, accompanied by a petal-shaped hole. Based on the spatial distribution of the impulse on the rear panel, a dimensional relationship was established between the hole size and the impulse on the rear panel. The petal model for describing petal-shaped cracks was proposed based on the law of conservation of energy, which could predict the hole size in the case of projectile impact at high velocity. The research results are of great significance for the study of the damage of riveted tanks with HRAMs and provide support for the vulnerability analysis of aircraft tanks.

Blast Absorption Capacity of Brittle Mineral Foam: an Experimental Evaluation

Cellular materials, such as aluminum foam, have proven to be effective energy absorbents. They can be used as the crushable core for blast mitigation sacrificial cladding. In this paper insights into the blast response of a brittle mineral foam-based sacrificial cladding are presented. The experimental set-up used consists of a rigid steel frame (1000 mm × 1000 mm × 15 mm) with a square cavity of 300 mm x 300 mm in the center. An aluminum plate, representing the structure, with a total surface of 400 mm × 400 mm and a thickness of 2 mm is clamped into the steel frame. Two synchronized high-speed cameras in a stereoscopic configuration are used to capture the dynamic response of the aluminum plate at a frame rate of 20000 frames per second. The transient deformation fields are computed using a three-dimensional digital image correlation technique. The blast load is obtained by detonating 20 g of C4 placed at a distance of 250 mm from the center of the tested aluminum plate. The absorption capacity of the brittle mineral foam is assessed by comparing the out-of-plane displacement, the velocity, the acceleration and the major principal strain of the thin aluminum plate with and without the protective mineral foam. Two foam configurations with different thicknesses are considered: 60 mm and 120 mm. It is shown that adding the brittle mineral foam reduces the out-of-plane displacement together with the displacement velocity and acceleration of the aluminum plate at least by a factor of two. Post-mortem analysis of the foams shows that mitigation of the blast load in the present set-up and under the considered loading is partially obtained by crushing of the cell walls but mostly though the growth of cracks in the specimen.

Experimental investigation on the axial compression behaviour of perforated glass fibre reinforced plastic medium-length tubes

This paper aims to investigate the axial compression behaviour of perforated glass fibre reinforced plastic (GFRP) circular tubes. A total of 12 GFRP medium-length tubes with and without circular holes were tested under axial compression. The influence of hole size and hole quantity on the failure mode and mechanical behaviour of perforated GFRP tubes were carefully evaluated. The overall buckling failure and material strength failure, mainly determined by the hole size, were observed in the axial compression tests. The critical load and axial stiffness of the perforated tubes decreased further with the increase of hole sizes and quantities, compared to those of the intact GFRP tubes. To reveal the damage mechanism of perforated GFRP tubes, three-dimensional digital image correlation (3D-DIC) technique was used to analyse the evolution of full-field strain around the hole. It was demonstrated that the damage mechanisms of the overall buckling failure and material strength failure were respectively compression-shear destruction and tension-shear destruction in the high strain concentration regions around the hole. Finally, design-oriented equations were proposed to predict the critical load and axial stiffness of perforated GFRP tubes under axial compression, and 95% confidence interval was used for interval estimation. The proposed equations were in good agreement with the test results. In addition, the prediction results of axial stiffness are generally more accurate than that of critical load.

Free-edge effect on the tensile properties of 3D woven composites

Free-edge effect is one of the factors affecting the mechanical properties of three-dimensional woven composites under tensile load. However, current research is relatively poorly understood regarding the effect of free-edge on the stiffness and strength of the material. This paper aims at examining the influence of free-edge effect on the mechanical properties of 3D woven composites under tension through experimental and simulation methods. The three-dimensional digital image correlation (DIC) technique is used to collect the full-field strains on the specimen surface during the test, and the stress-strain differences in different regions in the width direction are analyzed, and the overlap of the curves in each region is found to be high, indicating that the boundary effect has a small influence on the tensile properties of 3D woven composites. Experimental studies are conducted on specimens of different widths (within the range of 15–20 mm), and the results indicate that the differences in mechanical properties of 3D woven composites under tension loading in this width range are not significant. A progressive damage finite element model is developed for calculation and compared with experimental results. It is found that the tensile properties of the material decreased when the width of the specimen is less than twice the size of the single cell. This study can provide certain data support for the study of the mechanical properties of 3D woven composites and enable the subsequent more in-depth study to provide a certain foundation.

Experimental Investigation of the Effect of Delay Time on Rock Fragmentation in Multi-Hole Bench Blasting

Rock fragmentation by blasting influences ore recovery and the cost of downstream operations. The development of electronic detonators makes it possible to improve fragmentation by controlling the initiation timing in blasting projects, and the effect of the mechanism of delay timing on rock fragmentation should be studied. Fragmentation of granite bench specimens with different initiation timing was investigated in blast experiments. Conclusions are obtained by studying the surface strain field and post-blast specimens. A total of six blasting tests were carried out on granite bench specimens with four boreholes each having a diameter of 10 mm and a length of 450 mm. Each borehole used pentaerythritol tetranitrate (PETN) as the explosive charge, which was approximately 4.84 g with a charge diameter of 5.5 mm. Delay times between adjacent boreholes in the same row were set as 0, 50, 100, 150, 200, and 250 µs. The surface strain field of the bench specimen under blast loading was analyzed using three-dimensional digital image correlation (3D-DIC) techniques based on two cameras that captured high-speed images. Additionally, the post-blast specimen was also observed and recorded. Fragments of each bench specimen were carefully collected, weighed, and sieved with a set of sieves, including very fine particles. According to the 3D-DIC analysis for bench specimens, the propagation pattern of the main strain concentration zone transformed from horizontal to vertical with the increase in inter-hole delay. The maximum blast excavation weight was obtained by the bench specimen with an inter-hole delay of 100 µs, while the bench specimen with the longest inter-hole delay (250 µs) obtained the minimum blast excavation weight. By combining the results for blast excavation weight with the results from fragment size distribution analysis of all specimens, the optimal inter-hole delay was 200 µs. Compared to simultaneous detonation, the median size was decreased by about 14.5% for the inter-hole delay of 200 µs. The results of experiments show that delay time significantly influences rock fragmentation, but the stress wave superposition in short delays cannot improve rock fragmentation. For long delays, the blast-induced crack propagation time should be regarded as an influential factor when choosing the proper delay time. The experimental findings of this study could provide a better understanding of the effect of the mechanism of delay time on rock fragmentation.

Single-Camera Three-Dimensional Digital Image Correlation with Enhanced Accuracy Based on Four-View Imaging

Owing to the advantages of cost-effectiveness, compactness, and the avoidance of complicated camera synchronization, single-camera three-dimensional (3D) digital image correlation (DIC) techniques have gained increasing attention for deformation measurement of materials and structures. In the traditional single-camera 3D-DIC system, the left and right view images can be recorded by a single camera using diffraction grating, a bi-prism, or a set of planar mirrors. To further improve the measurement accuracy of single-camera 3D-DIC, this paper introduces a single-camera four-view imaging technique by installing a pyramidal prism in front of the camera. The 3D reconstruction of the measured points before and after deformation is realized with eight governing equations induced by four views, and the strong geometric constraints of four views can help to improve the measurement accuracy. A static experiment, a rigid body translation experiment, and a four-point bending experiment show that the proposed single-camera 3D-DIC method can achieve higher measurement accuracy than the dual-view single-camera 3D-DIC techniques and that the single-camera 3D-DIC method has advantages in reducing both random error and systematic error.

Evaluation of impact properties and residual strength of quasi-isotropic carbon-fiber reinforced laminates using digital image correlation

The paper demonstrates the impact and compression after impact behavior of carbon-fiber reinforced composites with epoxy matrix. Three specimens have been struck with the energies of 8.55 J, 17.1 J and 25.65 J which resulted in the formation of barely visible impact damages. The damaged area has been evaluated using digital shearography. Corresponding residual strength after impact has been measured. Deformation behaviour and buckling processes have been investigated via three-dimensional digital image correlation technique. Discussion has been further supported by theoretical estimations.

Mechanical Performance Study of Pipe-Liner Composite Structure Based on the Digital Image Correlation Method

Repair of reinforced concrete drainage pipes is often performed using polyethylene (PE) lining. To investigate the bearing capacity characteristics of a repaired composite structure, this study conducted experiments on three groups, namely, PE pipes, reinforced concrete pipes, and composite-reinforced concrete PE pipes (count = 9). The cracking and ultimate loads of the three groups under plate loads were evaluated and analyzed. A displacement monitoring method based on two-dimensional digital image correlation (2-D-DIC) and three-dimensional digital image correlation (3-D-DIC) techniques is proposed, and the quantitative laws of strain distribution at the end face of the specimen are obtained. Furthermore, the damage mechanism of the reinforced concrete pipe and the interface spalling damage mechanism of the composite-reinforced concrete PE under load were investigated to reveal the effect of PE lining improvement on the bearing capacity of the reinforced concrete pipe. The maximum deviation of the specimen vertical displacement results obtained using the new noncontact 3-D-DIC method is 2.6% compared with that of the traditional physical sensor method, and this method can successfully capture the deformation and strain data of the target specimen. Therefore, the noncontact DIC method has a broad application prospect in municipal pipeline early warning monitoring systems.

Experimental Study on the Influence of Delay Time on Rock Fragmentation in Bench Blasting

Delay time is an important factor in the quality of bench blasting. The development and application of electronic detonators make it possible to control the timing of detonation by a highly precise delay time. It is an easily achievable way to seek a better blast fragmentation by controlling the delay time. In order to investigate the influence of delay time on rock fragmentation, eight experiments on bench blasting models with double holes were carried out. The average weight of 4.59 g of pentaerythritol tetranitrate (PETN) was charged into each blast hole with a diameter of 10 mm. Delay times between the double holes were set as: 13.69, 27.36, 54.72, 60, 90, 120, 180, and 339.6 µs. During blast loading, the breakage processes of specimens were captured in detail by high-speed cameras. Full-field strains on the bench slope were analyzed by the three-dimensional digital image correlation (3D-DIC) technique. After each experiment, the coarse fragments were backfilled into the post-blast specimen to observe and record the bench slope’s final breakage pattern. The experimental results showed that the fragmentation of the bench slope transformed from horizontal crack dominance to vertical crack dominance as the delay time increased. In addition, post-blast fragments were collected and sieved, including fine materials. In the premise of approximate excavation, the optimum delay time was 180 µs. Compared to short delay times such as 27.36 µs, x50 was improved by approximately 25% at the delay time of 180 µs. The results showed a significant difference and great improvement in fragmentation when the delay times were in the time range of no-shock-wave interaction compared to interactions. When determining the optimum delay time in multi-hole blasting, except for the stress wave interaction, factors such as crack propagation should also be considered. The results of experiments could contribute to references for relevant research.

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.

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.

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 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.

Experimental and numerical studies of the impact breakage of granite with high ejection velocities.

The impact-induced fragmentation of rock is widely and frequently encountered when natural hazards occur in mountainous areas. This type of fragmentation is an important and complex natural process that should be described. In this study, laboratory impact tests under different impact velocities were first conducted using a novel gas-driven rock impact apparatus. The three-dimensional digital image correlation (3D DIC) technique was used to monitor the dynamic fragmentation process upon impact. Then, coupled 3D finite-discrete element method (FDEM) numerical simulations were performed to numerically investigate the energy and damage evolutions and fragmentation characteristics of the sample under different impact velocities. The laboratory test results show that as the impact velocity increases, the failure pattern of the rock sample gradually changes from shear failure to splitting failure, and the fragmentation intensity increases obviously. The strain localization area gradually increases as the impact velocity increases and as the location gradually deviates away from the impacting face. In the numerical simulation, the proposed model is validated by quasi-static uniaxial compression tests and impact tests. The numerical simulations clearly show the progressive fracture process of the samples, which agrees well with the experimental observations. The evolutions of energy and damage variables were also derived based on the simulation results, which are markedly affected by the impact velocity. The fragment size distributions based on mass and number can be well fitted using a generalized extreme value law. Finally, the distribution of the fragment flying velocity and angle are analyzed.

Vibration-based damage identification in composite plates using 3D-DIC and wavelet analysis

High-speed three-dimensional digital image correlation (3D-DIC) techniques can acquire full-field vibration responses under a single excitation, which is not restricted by the number of degrees of freedom (DOFs) in measurement compared with accelerometers and laser Doppler vibrometers (LDVs). Therefore, 3D-DIC exhibits compelling capacity in dynamic testing, especially for nondestructive damage detection. Based on experimental modal analysis (EMA), this study presents a damage detection method without using a baseline model, which combines the advantages of 3D-DIC with two-dimensional continuous wavelet transform (2D-CWT) for damage detection and recognition. The carbon fiber reinforced plastic (CFRP) composite plates with prefabricated artificial damages, including spatial notches and local crack, were investigated. The mode shapes of damaged CFRP composite plates were obtained by singular value decomposition (SVD) from the matrix of frequency response functions (FRFs). Then the mode shapes were analyzed by using wavelet transform, and the damage index was derived from the wavelet coefficients. For comparison, two other damage indices, namely mode shape curvature and polynomial fitting difference, were also derived. It is found that 3D-DIC could provide different measurement DOFs in line with the damage form for post-processing; and the Mexican-hat wavelet analysis can accurately detect the location and size of damage by suppressing the effects of measurement noise.