Damage identification in composite structures using high-speed 3D digital image correlation and wavelet analysis of mode shapes

In recent years, many efforts have been made to exploit full-field measurement optical techniques for modal identification. Three-dimensional digital image correlation using high-speed cameras has been extensively employed for this purpose. Modal identification algorithms are applied to process the frequency response functions (FRF), which relate the displacement response of the structure to the excitation force. However, one of the most common tests for modal analysis involves the base motion excitation of a structural element instead of force excitation. In this case, the relationship between response and excitation is typically based on displacements, which are known as transmissibility functions. In this study, a methodology for experimental modal analysis using high-speed 3D digital image correlation and base motion excitation tests is proposed. In particular, a cantilever beam was excited from its base with a random signal, using a clamped edge join. Full-field transmissibility functions were obtained through the beam and converted into FRF for proper identification, considering a single degree-of-freedom theoretical conversion. Subsequently, modal identification was performed using a circle-fit approach. The proposed methodology facilitates the management of the typically large amounts of data points involved in the DIC measurement during modal identification. Moreover, it was possible to determine the natural frequencies, damping ratios and full-field mode shapes without requiring any additional tests. Finally, the results were experimentally validated by comparing them with those obtained by employing traditional accelerometers, analytical models and finite element method analyses. The comparison was performed by using the quantitative indicator modal assurance criterion. The results showed a high level of correspondence, consolidating the proposed experimental methodology.

When combined with high-speed photography technology, the digital image correlation (DIC) method provides an excellent photographic image processing capability that can be used to convert the evolving full-field surface properties of structures to sets of two-dimensional (2D) or three-dimensional (3D) coordinate values. In this study, the applicability of the DIC method and high-speed cameras in underground mining was investigated by measuring the displacement and vibration of rock pillars caused by blasting. This technique is used extensively in engineering and is increasingly being applied to new fields. As a result of comparing the DIC results for blast vibration with the measured values of the contact sensor through field tests, the maximum displacement and vibration speed were found to be 86% and 93% accurate, respectively. In addition, the obtained values appeared similar to those predicted through numerical analysis. Field test results indicate that both methods yielded similar results. Therefore, it is concluded that it is feasible to use the DIC and high-speed camera to measure ground displacements and vibrations from blasting in underground mining. In addition, the system conditions required for blast vibration measurement were considered by comparing the accuracy with the existing measurement methods.

A bioinspired tethered MAV model is designed, constructed and studied for structural dynamic characteristics using novel materials and methods. The model consists of two flapping wings, a compliant flapping mechanism, and a supporting chassis. Large size insects such as dragonfly and cicada are considered as inspiration for the designing purposes. The wings and mechanism designed in this study are a simplified form of the natural counterparts. These idealized bioinspired structures are first studied for structural dynamic characteristics using finite element methods. Carbon nanotubes/polypropylene (CNTs/PP) nanocomposite stiffeners and a thin LDPE membrane are used to realize the designed wing model. The use of this novel material composition resulted in a lightweight, thin, and flexible wing capable of large amplitude passive bending-twisting motion. For constructing the flapping mechanism, a novel compliant mechanism is fabricated and attached with a piezoelectric actuator of cantilever form. The compliant mechanism is designed with CNTs/PP nanocomposite flexure joints supported using rigid carbon fiber/epoxy composite linkages. The fabrication of wing stiffeners and mechanism flexures from lab developed CNTs/PP nanocomposite materials is carried out using laser micromachining based manufacturing technique. The complete MAV model is constructed by assembling the wings and mechanism with a carbon fiber/epoxy composite chassis. The final assemblage is a two-winged MAV model of a total mass, body length, and wingspan of 0.6 1g, 60.46 mm, and 90.14 mm, respectively. An external customized power supply is developed and used with a microcontroller to actuate the flapping mechanism and wings during experiments. The computational structural analysis showed that no resonance occurs at the mechanism, all the first four fundamental modes are at the wings. This is advantageous for accurate and efficient motion transmission to the wings. The model generates large-amplitude wing deflections with a very small amplitude input excitation from the actuator - useful for higher aerodynamic performance. Experimentally, the structural dynamic analysis of the MAV model is carried out using an in-house high-speed 3D digital image correlation (DIC) technique, which reveals that the wings generate bending dominated deforming shape with marginal twisting and positive camber (during downstroke). Structural dynamic results from computations are in good agreement with the experiments that validates both of the approaches for further advancements in the developed MAV model. Overall outcome of the current work is a simplified biomimetic MAV design, a potential candidate for developing mechanically efficient MAV models.

Composite structures are generally more susceptible to impact damage than non-composite structures, and early identification of damage is the primary goal of structural health monitoring (SHM). If such damage remains undetected or reaches a critical size, it can lead to sudden collapse and catastrophic failure. Modern SHM methods aim to preserve the integrity of composite structures through continuous inspection, monitoring, and damage assessment, including detection, localization, quantification, classification, and prognosis. These methods use sensor-based technologies to assess vibration, extension, and acoustic and thermal emission. This paper provides a review of various computational methods including physics-based methods (signal processing techniques, modal analysis, and finite element model updating) and optimization methods (inverse problems, particle swarm optimization, topology optimization, genetic algorithms, time series analysis, and hybrid techniques), alongside machine learning methodologies employing neural networks as well as deep learning for damage identification in composite structures. These computational and learning-based techniques are widely applied in the development of algorithms, optimization strategies, and hybrid frameworks for SHM. The review further summarizes the applications, advantages, and limitations of each method according to structure type and damage characteristics. The key emphasis of this review is on integrating computational approaches, as well as machine learning, to enhance the efficiency of damage identification. The conclusion is drawn based on an overview of the literature, focusing on the contributions of different computational methods and machine learning for damage identification in composites.

The damage identification problem is one of crucial problems during operation of machines' elements made of polymeric composites. Therefore the appropriate non-destructive techniques should be developed in order to detect and identify the damages with the best possible accuracy. Moreover, such methods should be applicable in various testing conditions. One of the intensively developed directions in non-destructive damage assessment is a class of methods based on wavelet analysis of modal shapes of vibration applied for a tested structure. The effectiveness of an algorithm is strongly dependent on the type of applied wavelet and its parameters. The proposed approach uses a combination of the wavelet-based damage identification algorithm with multi-objective meta-optimization in order to select optimal parameters of applied wavelets and determine a front of optimal non-dominated solutions. Based on these solutions the operator can choose the desired accuracy of damage identification with respect to the suitable computation time.

As humans continue to develop the undersea engineering ecosystem of systems, the consequences of catastrophic events must continue to be investigated and understood. Almost every undersea pressure vessel, from pipelines to sensors to unmanned vehicles, has the potential to experience a catastrophic collapse, known as an implosion. This collapse can be caused by hydrostatic pressure or any combination of external loadings from natural disasters to pressure waves imparted by other implosion or explosion events. During an implosion, high-magnitude pressure waves can be emitted, which can cause adverse effects on surrounding structures, marine life, or even people. The imploding structure, known as an implodable volume, can be in a free-field or confined environment. Confined implosion is characterized by a surrounding structure that significantly affects the flow of fluid around the implodable volume. Often, the confining structure is cylindrical, with one closed end and one open end. This work seeks to understand the effect of fluid flow restriction on the physics of implosion inside a confining tube. To do so, a comprehensive experimental study is conducted using a unique experimental facility. Thin-walled aluminum cylinders are collapsed inside a confining tube within a large pressure vessel. High-speed photography and 3D Digital Image Correlation are used to gather structural displacement and velocities during the event while an array of dynamic pressure sensors capture the pressure data inside the confining tube. The results of this work show that by changing the size of the open end, referred to as the flow area ratio, there can be a significant effect on the structural deformations and implosion severity. It also reveals that only certain configurations of holes at the open end of the tube play a role in the dynamic pressure pulse measured at the closed end of the tube. By understanding the consequences of an implosion, designers can make decisions about where these pressure vessels should be in relation to other pressure vessels, critical infrastructure, marine life, or people. In the same way that engineers design for earthquakes and analyze the impact their structures have on the environment around them, contributors to the undersea engineering ecosystem should design with implosion in mind.

The performance of energetic materials subjected to dynamic loading significantly depends on their micro- and meso-scale structural morphology. The geometric versatility offered by additive manufacturing opens new pathways to tailor the performance of these materials. Additively manufactured energetic materials (AMEMs) have a wide range of structural characteristics with a hierarchy of length scales and process-inherent heterogeneities, which are hitherto difficult to precisely control. It is important to understand how these features affect AMEMs’ response under dynamic/shock loading. Therefore, temporally and spatially resolved measurements of both macroscopic behavior and micro- and meso-level processes influencing macroscopic behavior are required. In this paper, we analyze the shock compression response of an AMEM simulant loaded under several impact conditions and orientations. X-ray phase contrast imaging (PCI) is used to track features across the observed shock front and determine the linear shock velocity vs particle velocity equation of state, as well as to quantify the interior deformation fields via digital image correlation (DIC) analyses. Photon Doppler velocimetry is simultaneously used to measure the particle velocities of the specimens, which are consistent with those obtained from x-ray PCI. The DIC analyses provide an assessment of the average strain fields inside the material, showing that the average axial strain depends on the loading intensity and reaches as high as 0.23 for impact velocities up to 1.5 km/s. The overall results demonstrate the utility of x-ray PCI for probing “in-material” equation of state and interior strains associated with dynamic shock compression behavior of the AMEM simulant.

In this work, we present a two-step procedure for damage identification in beam structures exploiting modal curvature changes. The reconstruction of modal curvatures requires the knowledge of several mode shape components along the analyzed beam. This requirement is practically unachievable when mode shapes are identified via vibration-based monitoring using a limited number of accelerometers. To overcome this limitation, in the first step of the proposed procedure, we perform a mode shape expansion employing a reduced subset of measured modal components. The remaining measured components are used as control parameters to formulate a first hypothesis on damage location and extent. For this purpose, the expansion procedure is performed considering a number of possible damage scenarios, consisting of a location and a severity (loss of stiffness) of the damage. Using the Total modal assurance criterion (TMAC), we select the expanded modes with the highest degree of correspondence with the measured control components. These expanded modes are thus associated with a first guess of the damage location and severity. In the next step, this initial damage identification is verified through the computation of a modal curvature-based damage index. If the curvature-based damage identification confirms the previous identification, the damage location and extent are determined. The procedure can be easily extended to identify multiple simultaneously damaged elements. The approach is numerically validated using a benchmark beam modeled via finite elements, investigating the influence of different parameters such as noise, position of the control components and beam discretization on the identification success rate. Finally, the procedure is tested on two experimental specimens: a steel beam, with three different damage configurations and a concrete beam progressively damaged with multiple damage locations.

Electronics may be subjected to shock, vibration, and drop-impact during shipping, handling and during normal usage. Measurement of transient dynamic deformation of the electronics assemblies during the shock and vibration can yield significant insights in understanding the occurrence of failure modes and the development of failure envelopes. Failure-modes include solder-joint failures, pad cratering, chip-cracking, copper trace fracture, and underfill fillet failures. Previous researchers have measured the transient-dynamics of board assemblies with high-speed imaging in conjunction with high-speed image analysis for measurement of relative displacement, angle, velocity, and acceleration [Lall 2006, Che 2006]. In addition, high-speed data-acquisition systems with discrete strain gages have been used for measurements of transient strain [Lall 2004, 2005, Liang 2005] and with accelerometers for measurement of transient acceleration [Dunford 2004, Goyal 2000, Seah 2005]. Development of accurate models requires better understanding of full-field strain deformation in board assemblies. In this paper, the use of digital image correlation (DIC) with ultra high-speed imaging has been used for full-field measurement of transient strain in various board assemblies subjected to shock in various orientations. Measurements have been taken on both the package and the board side of the assemblies. Accuracy of high-speed optical measurement has been compared with that from discrete strain gages. Package architectures examined include flex ball-grid arrays, tape-array ball-grid arrays, and metal lead-frame packages. Explicit finite-element models have been developed and correlated with experimental data. Models developed include, smeared property models, Timoshenko-beam models, and explicit sub-models. The solder strains have been computed from the explicit finite element models for life prediction in shock.

Damage quantification in composite laminates based on sparsity-difference estimation of the normal time–frequency representation

Deep learning approach for delamination identification using animation of Lamb waves

Early damage identification in composite structures has attracted a lot of attention in post-failure examinations in recent years. In the research, torsional fatigue test of composite drive shafts has been done and numerical methodologies have been employed for the validation for premature failure using modal flexibility and modal curvature methods. Initially, the eigenvalue and eigenvectors of healthy and fractured drive shafts are evaluated using a numerical model and verified by fatigue experimental approach. Flexibility variance and the absolute curvature difference between the drive shafts that are fractured and those that are healthy are evaluated after mass normalization. Variations in absolute modal curvatures and variations in local flexibilities that are utilized to identify the existence, location, and severity of a drive shaft fracture. The position and severity of the fracture are predicted more precisely by the modal curvature approach than by the modal flexibility approach.

This chapter reviews blast experiments that have been carried out on composite sandwich panels with glass-fiber reinforced polymer (GFRP) face-sheets, carbon-fiber reinforced polymer (CFRP) face-sheets and face-sheets containing a mixture of fiber fabrics. The panels were subjected to explosive charges ranging from 1.28 to 100 kg TNT equivalent during the air and underwater blast tests. The difference in panel response was recorded using high-speed photography and digital image correlation (DIC) during the air blast tests. More conventional instrumentation using strain gauges was required for the underwater blast tests. Following each experiment, the panel damage was analyzed and compared either visually or using X-ray computed tomography (CT) scanning. The addition of polypropylene (PP) Innegra interlayers into a GFRP front face-sheet has been shown to be advantageous during air blast loading. Due to the increased thickness of the front face-sheet, the panel deflects less and experiences less front face-sheet damage. The replacement of GFRP plies with aramid plies during underwater blast loading was shown to be detrimental to the panel performance. The panel suffered from more severe damage compared to a fully GFRP panel. The comparison of CFRP against GFRP panels during underwater blast revealed that the CFRP has a greater stiffness but this result in greater face-sheet debonding due to its lower strain to failure. These experiments have shown that an optimal blast resilience response could be achieved through the combination of different fiber fabrics.

Due to ecological requirements, the bypass-ratio of future civil turbofan engines will be increased. This leads to ultra-high bypass ratio (UHBR) engines, which bring in new challenges concerning the blade material, structural integration, and aeroelastic behaviour of the fan. The ambition of the project Composite fan Aerodynamic, Aeroelastic, and Aeroacoustic Validation Rig (CA3ViAR) is to design and test an open-test-case fan that experiences instability mechanisms, which are representative for UHBR fans of civil aircrafts. The optical measurement technique Digital Image Correlation (DIC) allows for the spatial measurement of deformations. The aim is to apply this technique to measure rotor blade deformation under loading and its vibration due to aeroelastic phenomena to get a better understanding of the structural and aeroelastic behaviour of the composite fan. However, the high rotational speeds, blade vibration frequencies, and expected amplitudes pose challenges for the measurement setup. In this work, a high-speed DIC system is prepared and tested to measure deformation and vibration of a fast rotating blade. Additionally, the requirements for the DIC setup are defined and presented. The expected blade tip speeds in the CA3ViAR project are up to 295 m/s and the blade frequencies of interest up to 650 Hz. Therefore, particular emphasis must be given to the setup in order to eliminate motion blur due to the rotation and to achieve the required frequency. This leads to a setup with synchronised high-speed cameras and a laser to measure frequency vibrations up to 1 kHz with an exposure time below 210 ns. Test measurements are conducted on a stationary beam and an axial blower with a max. blade tip speed of 50 m/s. A data analysis method is developed and described to eliminate the rigid body rotation, analyse the deformation of each blade compared to a reference condition, and analyse the spatial vibration in the frequency domain for a high number of data points per time step. The results of the structural beam are in agreement with the reference measurements and numerical simulations. By analysing the spatial vibration modes of the axial blower, the 1F-flap mode is identified at 38 Hz. In conclusion, this DIC setup shows promising results for future deformation and vibration measurements on a scaled UHBR fan.

Laboratory visualization of supercritical CO2 fracturing in tight sandstone using digital image correlation method