Barely Visible Impact Damage Research Articles

In-situ detection of delamination reinitiation in carbon fiber reinforced polymers post barely visible impact damage

In this study, advancements are presented in the in-situ detection of delamination reinitiation from Barely Visible Impact Damage (BVID) in composite materials, utilizing enhancements in Digital Image Correlation (DIC) techniques during a Compression After Impact (CAI) test. The study measured strain fields in the longitudinal, transverse, and shear directions, focusing specifically on the point of highest out-of-plane displacement to identify the onset of delamination propagation from BVID sites generated at different impact energy levels. By correlating the measured strains with the peak out-of-plane displacement, a unique determination of onset damage reinitiation associated with BVID during CAI testing was achieved. This method introduces a refined in-situ assessment technique for structural integrity, allowing for the early detection of critical damage propagation in composite materials.

Characterisation and Application of Bio-Inspired Hybrid Composite Sensors for Detecting Barely Visible Damage under Out-of-Plane Loadings.

Traditional inspection methods often fall short in detecting defects or damage in fibre-reinforced polymer (FRP) composite structures, which can compromise their performance and safety over time. A prime example is barely visible impact damage (BVID) caused by out-of-plane loadings such as indentation and low-velocity impact that can considerably reduce the residual strength. Therefore, developing advanced visual inspection techniques is essential for early detection of defects, enabling proactive maintenance and extending the lifespan of composite structures. This study explores the viability of using novel bio-inspired hybrid composite sensors for detecting BVID in laminated FRP composite structures. Drawing inspiration from the colour-changing mechanisms found in nature, hybrid composite sensors composed of thin-ply glass and carbon layers are designed and attached to the surface of laminated FRP composites exposed to transverse loading. A comprehensive experimental characterisation, including quasi-static indentation and low-velocity impact tests alongside non-destructive evaluations such as ultrasonic C-scan and visual inspection, is conducted to assess the sensors' efficacy in detecting BVID. Moreover, a comparison between the two transverse loading types, static indentation and low-velocity impact, is presented. The results suggest that integrating sensors into composite structures has a minimal effect on mechanical properties such as structural stiffness and energy absorption, while substantially improving damage visibility. Additionally, the influence of fibre orientation of the sensing layer on sensor performance is evaluated, and correlations between internal and surface damage are demonstrated.

Low‐velocity impact characterization of polyurethane rubber/nano‐clay enriched sustainable sandwich composites: Synergy of experimentation and simulations

AbstractThis research proposes a novel sustainable composite using basalt (B), hemp (H) and polyurethane rubber (Pu) reinforced in nano clay functionalised epoxy matrix for sacrificial structural applications prone to low‐velocity impact (LVI). To this end, symmetric and asymmetric laminates such as HHHH, BBBB, BHHB, BHPuHB, BBPuHH and BPuBPuHPuH are fabricated using compression molding techniques and subjected to LVI at three different impact energies of 19.66, 39.39, and 59.05 J. The hemp fibers were treated with NaOH solution before fabrication. Material characterization such as X‐ray diffraction, Raman spectroscopy and morphological studies has been carried out. The impact and post‐impact properties of the proposed composites are experimentally evaluated and validated with the finite element (FE) results. The effect on the residual tensile strength degradation of laminates at different interlayers and energy levels is also investigated using the Caprino analytical model. The barely visible impact damages (BVID) are investigated through non‐destructive dye‐penetration tests, which facilitate easy identification of the prominent LVI damages like “Plateau” and “Cliff‐drop” impressions. Based on the impact energy absorption and residual tensile strength, proposed laminates followed BHPuHB > BBBB > BHHB > HHHH. The experimentation suggests that Polyurethane core laminates support maximum impact energy absorption by favoring a structural change in interlayers. Also, the residual tensile strength decreases as impact velocity increases.Highlights Low‐velocity impact behavior of sustainable composites is experimented. Basalt, hemp and polyurethane rubber are reinforced in nano clay epoxy matrix. A FE framework to validate the experimentation is proposed. Dye penetrant NDT is adopted to investigate the damages. Morphological studies are conducted to understand LVI responses.

Drop-weight impact and compressive properties of repeatable self-healing carbon fiber reinforced composites

Herein, repeated self-healing of impact damage in carbon fiber reinforced composites is successfully achieved by using hollow glass fibers containing self-healing resin and two types of microcapsules (HGFs-Ms). The HGFs-Ms and microcapsules containing hardener were incorporated to fabricate repeatable self-healing carbon fiber reinforced polymer composite laminates (RSH-CFRPs). Low-velocity drop-weight impact tests and compression tests were performed to investigate the impact resistance, energy absorption, residual compressive strength and repeated self-healing performances of the RSH-CFRPs. The results revealed that the energy absorption capacity of the RSH-CFRPs could be repeatedly recovered and the barely visible impact damage (BVID) zone was almost fully healed after every healing process. Moreover, the RSH-CFRPs successfully minimized the reduction in compressive strength after repeated drop-weight impact tests due to their efficient repeated self-healing ability in impact induced damage.

Advances in Embedded Sensor Technologies for Impact Monitoring in Composite Structures

Embedded sensor technologies have emerged as pivotal tools in redefining structural health monitoring (SHM) within composite materials, addressing a critical need in the composite structure industry. Composites, by their layered nature, are particularly vulnerable to internal delamination and micro-cracks from impacts, which can propagate and lead to catastrophic failures. Traditional inspection methods often fail to detect internal damage and these undetected damages can lead to reduced performance and potential system failures. Embedded sensors offer a solution capable of detecting a spectrum of damages, from barely visible impact damages (BVID) and subtle low-energy impacts to pronounced impact-related deformations, all in real-time. Key sensors, such as Piezoelectric transducers (PZTs), Fiber Bragg Gratings (FBGs), and other potential sensors, have been discussed as potential detection techniques in this review. This review discusses a comprehensive picture of the progress and current scenario of different embedded sensors for SHM of composite structures. The growth of embedded sensor technologies, current limitations, and future requirements focusing on sensor materials have been discussed in this review. Finally, challenges and opportunities for the development of a sustainable SHM system have been discussed in this paper.

Detecting of Barely Visible Impact Damage on Carbon Fiber Reinforced Polymer Using Diffusion Ultrasonic Improved by Time-Frequency Domain Disturbance Sensitive Zone.

Based on the decorrelation calculation of diffusion ultrasound in time-frequency domain, this paper discusses the repeatability and potential significance of Disturbance Sensitive Zone (DSZ) in time-frequency domain. The experimental study of Barely Visible Impact Damage (BVID) on Carbon Fiber Reinforced Polymer (CFRP) is carried out. The decorrelation coefficients of time, frequency, and time-frequency domains and DSZ are calculated and compared. It has been observed that the sensitivity of the scattered wave disturbance caused by impact damage is non-uniformly distributed in both the time and frequency domains. This is evident from the non-uniform distribution of the decorrelation coefficient in time-domain and frequency-domain decorrelation calculations. Further, the decorrelation calculation in the time-frequency domain can show the distribution of the sensitivity of the scattered wave disturbance in the time domain and frequency domain. The decorrelation coefficients in time, frequency, and time-frequency domains increase monotonically with the number of impacts. In addition, in the time-frequency domain decorrelation calculation results, stable and repetitive DSZ are observed, which means that the specific frequency component of the scattered wave is extremely sensitive to the damage evolution of the impact region at a specific time. Finally, the DSZ obtained from the first 15 impacts is used to improve the decorrelation calculation in the 16-th to 20-th impact. The results show that the increment rate of the improved decorrelation coefficient is 10.22%. This study reveals that the diffusion ultrasonic decorrelation calculation improved by DSZ makes it feasible to evaluate early-stage damage caused by BVID.

Attention mechanism enhanced spatiotemporal-based deep learning approach for classifying barely visible impact damages in CFRP materials

Most existing machine learning approaches for analysing thermograms mainly focus on either thermal images or pixel-wise temporal profiles of specimens. To fully leverage useful information in thermograms, this article presents a novel spatiotemporal-based deep learning model incorporating an attention mechanism. Using captured thermal image sequences, the model aims to better characterise barely visible impact damages (BVID) in composite materials caused by different impact energy levels. This model establishes the relationship between patterns of BVID in thermography and their corresponding impact energy levels by learning from spatial and temporal information simultaneously. Validation of the model using 100 composite specimens subjected to five different low-velocity impact forces demonstrates its superior performance with a classification accuracy of over 95%. The proposed approach can contribute to Structural Health Monitoring (SHM) community by enabling cause analysis of impact incidents based on predicting the potential impact energy levels. This enables more targeted predictive maintenance, which is especially significant in the aviation industry, where any impact incidents can have catastrophic consequences.

A simple image correlation technique for imaging subsurface damage from low-velocity impacts in composite structures

A robust computer vision system is proposed to visualize subsurface barely visible impact damage (BVID) in composite structures through a simple image correlation technique together with a damage imaging condition. This system uses a digital camera to record a video of the surface motion, capturing micron-scale dynamic movement from guided waves propagating on the surface of the structure generated via a sweeping frequency excitation (chirp) up to the ultrasonic frequency range. As the excitation frequency changes during the chirp, waves become trapped within this damaged region, forming standing waves to generate local resonance at specific frequencies. This localized resonance accumulates high wave energy in the region, generating a higher transverse displacement at the damage site compared to the remaining part of the structure. In this work, a simple image correlation technique is proposed to correlate each filtered video frame with the temporal mean of the filtered wavefield video to highlight standing waves at localized damage. The proposed image correlation technique is distinct from digital image correlation (DIC) since it does not use correlation for subset matching to track the movement of surface patterns between two images (video frames). Instead, it uses correlation to directly quantify similarities between corresponding image pixels (windows). Utilizing a single camera greatly simplifies system complexity and hence enhances the practicality and potential for real-time performance over a recently developed technique that utilized 3D DIC with a stereo camera for vision-based BVID detection. To realize the aim, work was conducted in two sequential steps: (1) off-axis 2D DIC was employed rather than 3D DIC to examine the potential of employing a single camera to capture images and extract not only in-plane displacement but also a fraction of transverse displacement in which local resonance is dominant and (2) image correlation was then employed, supplanting the off-axis 2D DIC image processing involved in the first step, to highlight the damage with significantly less processing complexity. This proposed technique using a zero-mean normalized cross-correlation imaging condition, rather than the total wave energy used for DIC-based approaches, is efficient and effective for identifying regions of minute surface movement from local resonance within the damage region without the use of the computationally intensive interpolation to get the sub-pixel gray level information followed by subset matching employed in DIC-based image processing. Two geometrically identical CFRP composite honeycomb panels that had been subjected to low-velocity impacts were used for verification and validation, and two excitation location configurations were tested for each panel. Damaged images produced with correlation for a 100 mm × 100 mm field of view using a single 3-s video, or a total of 19,200 video frames, show accurate damage imaging capabilities regardless of excitation location that exceed that of DIC techniques and is comparable to benchmark damage images obtained from laser Doppler vibrometry, ultrasonic C-scans, and X-ray CT scans. This success of correlation-based imaging of subsurface BVID demonstrates substantial improvements in efficiency and practicality and shows high potential for in situ and real-time computer vision-based nondestructive inspection or structural health monitoring of subsurface BVID in composite aircraft and other critical structures.

Effect of graphene nano-platelets on the low-velocity-impact and compression-after-impact strength of the glass fiber epoxy composite laminates

Detecting the low-velocity impact damage on the FRP composite structures can be challenging because the visible signs on the surface may be subtle. Mitigating the effects of BVID (Barely Visible Impact Damage) is crucial in ensuring the safety and reliability of composite components, especially in industries where these materials are commonly used, such as aerospace and automotive. This article explores the experimental investigations of the impact and post-impact damage propagation under compression in the Graphene Nano-Platelets (GNPs) infused glass fiber epoxy composite laminates. A comparative study of varying the percentage of the nano-platelets (0, 0.25, 0.5, and 0.75%) in the matrix material of the composite laminate is presented. The impact tests at low energies (15 J, 20 J, and 25 J), followed by the quasi-static compression tests, are carried out as per the procedures mentioned in the ASTM D7136/D7137. The digital image correlation technique (DIC) is used to measure strains and plot the strain field on the impacted face of the specimen under compression. Damage envelopes of the impact and compression tests are studied using X-ray CT scans. Micro-level failures are observed through Scanning Electron Microscopy (SEM) to get better insights into the failure mechanism of the composite materials under impact and compression loadings. Upon completing the study, it is found that the addition of the GNP into the matrix materials not only increased the impact performance but also increased the load-carrying capacity of the laminate under compressive loading. A decrement of 2–21% in the energy absorbed in the damage formation is seen with the incorporation of GNPs into composite materials. A maximum increment of 17% in the compressive strength is observed for the composite laminates with the 0.75% GNPs. However, for a particular percentage of the GNP, the compressive strength of the laminate is reduced with an increase in impact energy compared with its virgin counterpart. From X-ray CT scans, it is seen that the laminate under the compression tests fails due to excessive ply-splitting and the delamination at the damage site created during the impact. Fiber breakage, fiber brooming, matrix cracking, fiber-matrix debonding, etc., are some of the other failure modes observed in SEM studies.

Assessment of low-velocity impact response of a hybrid carbon-aramid braided composite by algorithmic quantification of volumetric structures

This work investigates the impact response of a hybrid two-dimensional (2D) tubular braided composite through an ex-situ examination of its internal structure. An inter-ply hybrid laminate of biaxial braided carbon and aramid layers was manufactured. The sample underwent split Hopkinson pressure bar (SHPB) impact testing, where the applied energy induced barely visible impact damage (BVID). Volumetric representations of the sample were created before and after impact testing using micro-computed tomography (µCT). Novel algorithms were developed to assess properties of the braided composite sample. First, voids in the sample before and after impact were identified through a series of image processing techniques, after which various void properties were extracted. Each void was then matched between datasets to track property change caused by impact. Next, damage in the sample after impact was identified through another series of image processing techniques, then quantified and characterized in the context of the full sample. Statistical analyses in the form of paired-sample t-tests were performed for void properties and overall surface topologies. Additionally, measurement of 3D strain within selected regions of the sample was performed using digital volume correlation (DVC). Results showed that the impact damage did not cause significant void enlargement, but instead caused surface topology shifts, particularly at the inner surface, owing to plastic deformations caused by impact damage, which were primarily caused by delamination and developed from large voids at the site of impact. The hybrid material configuration exhibited a phenomenon where the inner and outer layers minimized cracking in the middle layer, causing plastic deformation to be the primary impact response of the middle layer. Through the analysis processes developed in this work, quantitative assessment of tubular braided composites was achieved by measuring changes in voids caused by impact at the individual level, and by examining the damage profile while accounting for the voids. The methodology can be applied to various material configurations to provide meaningful insight into the effects of hybridization in the impact response of braided composites.

Failure mechanisms of asymmetric sandwich panels subjected to low-velocity impact: An explicit wave dominating damage model

Asymmetric sandwich panels (ASPs) with tapered transitions are effective configurations for connecting them with other structural elements. However, fiber reinforced composite panels are susceptible to impact events and thus it is imperative to gain a comprehensive understanding of the impact failure mechanisms of ASPs. This article investigates the failure mechanisms of ASPs subjected to low-velocity impacts experimentally and numerically. Low-velocity impact tests are conducted at two energy levels to induce barely visible impact damage (BVID) and visible impact damage (VID). A non-linear finite element (FE) model is developed to elucidate the damage behavior and failure mechanisms of ASPs. The numerical results are compared with experimental data to validate the FE model. Additionally, a parametric study is undertaken to explore the failure mechanisms of ASPs subjected to penetration. This study also investigates the influence of impactor diameter, impactor angle, foam core thickness, face sheet stacking sequence, and face sheet thickness on impact response through numerical simulations. The reported results hold potential significance for safety design ASPs.

Impact damage detection in woven CFRP laminates based on a local defect resonance technique with laser ultrasonics

This paper presents a local defect resonance (LDR)-based approach to detect barely visible impact damages (BVIDs) in woven CFRP laminates using laser-ultrasonic guided waves. In the experiments, broadband guided waves were excited by an Nd:YAG pulsed laser to induce resonant vibrations of BVIDs. The laser was scanned on the designed area of the woven CFRP laminates by using a galvano mirror system. A fixed broadband PZT sensor received the waves propagated from all the laser irradiation points and the datasets of the recorded waveforms were further processed to visualize the wavefield in the scanned area. A robust post-processing method was proposed to extract automatically the LDR frequencies and to obtain the image of the BVID. The feasibility of the proposed method was well validated on a woven CFRP laminate with a single BVID. In order to evaluate the detection performance, we compared our proposed method with other traditional methods, i.e., root mean square (RMS) and weighted RMS. As a result, it was found that our proposed post-processing procedure could visualize the BVID with higher contrast. Furthermore, the detection of multiple BVIDs using the developed method was demonstrated successfully.

Experimental assessment of barely visible impact damage carbon fiber reinforced epoxy composite using ultrasound method

Background Carbon fiber reinforced epoxy (CFRP) is susceptible to impact damage which could resulted in reduction of the mechanical properties. This paper studies the architecture of barely visible impact damage (BVID) to comprehend the extent of damage on quasi-isotropic CFRP laminates of varying thickness (i.e. 16, 24 and 32-ply laminates of 3, 4 and 5 mm respectively). Methods Quasi-static indentation is chosen to produce BVID on CFRP laminates, followed by using non-destruction evaluation method, namely conventional contact-type ultrasonic testing (UT) and C-mode scanning acoustic microscopy (C-SAM) method. Results The findings revealed (1) the size and shapes of the BVID on CFRP laminates, (2) no damage was found at the center of the impact point, and (3) the bridging between the point of impact to the outer damaged diameter due to the consequence of diverse orientation of carbon fiber strips which exhibit excellent mechanical properties before structural failure. Conclusions The results concluded that the UT and C-SAM method can identify both the pristine region and the internal damaged structures in CFRP laminates.

The effects of impactor shape on the compression after impact strength of carbon/epoxy face sheet foam core sandwich structure

This study presents experimental results of compression after impact (CAI) strength testing of foam core sandwich structure with carbon/epoxy face sheets impacted at three different energies with either a blunt or sharp tip impactor. The impact energies used were chosen to span the barely visible impact damage (BVID) thresholds for the sharp (4.1 J) and blunt (12.9 J) impactors with an impact energy approximately halfway between these (8.1 J) also used. While most impact testing on composites utilize a hemispherical (blunt) impactor, actual damage to a part may be due to an object impacting the part that is not blunt, but sharp and the impact response and resulting CAI strength values may be different for a given impact energy level. In this study, with regards to the impact response, the sandwich specimens showed larger transverse displacements (by about a factor of two) during the impact event when impacted by a sharp impactor versus a blunt impactor. The maximum load of impact was larger for the blunt impactor by about a factor of three. The barely visible impact damage (BVID) threshold energy was lower by about a factor of three for the sharp impactor. The absorbed energy of impact was higher for the sharp impactor. The CAI strength results showed that the sharp impactor gave lower average CAI strength values at the lowest impact energy level used, slightly lower average CAI strength values for the medium impact energy used and about the same average CAI strength values for the highest impact energy used.

Low-velocity impact damage detection in cfrp laminates based on ultrasonic phased-array ndt technique

To investigate the delamination characteristics of CFRP laminates in Barely Visible Impact Damage (BVID) state, Low-Velocity Impact Tests (LVIT) and Non-Destructive Testing (NDT) were employed. The energy thresholds of six diverse ply-stacking sequences of laminates in the BVID state were established utilizing visual inspection and contact measurement methods. Ultrasonic phased-array NDT techniques were employed to evaluate quantitatively the magnitude of delamination damage from different viewpoints, including delamination area, shape, and trend variation. Furthermore, the impact resistance of laminates was qualitatively evaluated by analyzing the distinctive delamination damage forms of various specimens to identify the effects of the ply-stacking sequence. The findings of this study demonstrate that reducing the angle disparity between adjacent plies and minimizing the repetition of identical plies can mitigate delamination damage. Additionally, enhancing the ratio of ±45 and 0° plies can improve the flexural stiffness of the structure. Specifically, the ultrasonic phased-array NDT technique was shown to effectively detect delamination damage within the laminate in the BVID state.

Determination of optimal combination of pumping and probing inputs in vibro-acoustic modulation for sensitive BVID detection of CFRP

In this work, the potential of using defect resonance-based vibro-acoustic modulation (VAM) spectroscopy to determine pumping and probing frequencies to maximize the nonlinear VAM response of carbon fiber reinforced polymer (CFRP) plates is investigated. An integrated 3D finite element (FE) model which implements the barely visible impact damage (BVID) due to impact force and the loading of ultrasonic signals is proposed. Based on this model, the scaling subtraction method (SSM) is used to determine the low-frequency global plate resonance that leading strongest defect excitation for pumping input (GPRpump) and high-frequency local defect resonance for probing input (LDRprobe). A scanning laser Doppler vibrometer (SLDV) is adopted to experimentally validated the proposed numerical methodology. Then, the maximum response amplitude (MRA) of nonlinear VAM response is proposed to evaluate the influence of defect resonance on the nonlinear VAM response. The numerical and experimental results are in good agreement and show that MRA of the VAM sideband can be improved when the GPRpump frequency is taken as the pumping input and the LDRprobe frequency is taken as the probing input, respectively. The best BVID detection effect is obtained when the pumping input of the VAM test is selected as the GPRpump frequency and the probing input is selected as sum or difference of the GPRpump frequency and LDRprobe frequency.

Delamination link-ups in composite laminates due to multiple hail impacts

Damages to aircraft fuselages by ice impactors are categorized as barely visible impact damage (BVID). As the application of composite laminates in manufacturing aircraft fuselage is increasing rapidly, studying hail impact on composite laminates seems crucial. Investigation of variation of delamination area and link-up area for different impact scenarios between ice and carbon fibre prepreg composite plates is the main goal of this study. Link-up refers to joining of delaminated areas created by separate impacts at relatively close impact locations. To this aim, multiple impacts at 1 and up to 6 impact locations and with different impactor energies were simulated using SPH formulation, and delamination areas were quantified. The results showed that as the spacing L between two impact locations increases from 0 to 4R (where R denotes the impactor radius), the total delamination area first shows an increase up to a peak point (usually at R<L<2R) after which it shows a large drop, and after L≈2R, the total delamination area level remains almost constant. The large drop is related to disappearance of the link-up phenomenon beyond a certain spacing. Moreover, it was observed that increasing the number of impact locations increases the delamination area almost linearly. As for the impactor energy influence, the results showed that as the impact energy increases, the threshold spacing for link-up increases. Moreover, for the case of impact at two locations and constant total energy level, two impactors with identical energy level lead to higher delamination area as compared to impactors with non-identical energy levels.

Combination of Cross-Correlation-Based Analysis and Ultrasonic GW Tomography for Barely Visible Impact Damage Detection Preliminary Assessment

Statistical based reconstruction methods and signal processing tooling techniques are implemented and used to detect delaminations or debondings within composite complex items with very high precision. From the literature, it appears that although a single procedure for the estimation of structural health is a fast solution, multiple analyses based on different reconstruction methods or different damage parameters are the way to achieve maturation assessments of the methodology. This highlights the fact that the hardware and software parts of an SHM system need two different assessment and maturation ways. This work focuses on the software part by proposing a way to start assessing the outcomes. In this paper, the damage detection and localization strategy in CFRP plate-like structures with elastic guided waves excited and acquired with a circular array PWAS network is considered. Previous outcomes are compared by new analyses using a new post-processing approach based on a cross-correlation-based technique in terms of the BVID (Barely Visible Impact Damage) surface position and its center of mass. The advantage of this specific study is hopefully to enable confidence in the transition from R&amp;D to field implementation. In addition, this work tries to evidence an improvement in terms of cost efficiency and reduced complexity while maintaining the same accuracy.

Detection of barely visible impact damage in composite plates using non-linear pump-probe technique

In the last decades, composite materials have been increasingly used in aircraft structures as a mean to reduce the weight. For safety reasons, periodic checks of impact-induced damage need to be performed. In that framework, ultrasound non destructive testing has proven its ability to detect macro-defects such as delamination. However, at early stages, Barely Visible Impact Damage (BVID) may not be detected via linear ultrasound techniques. In recent years, non-linear ultrasound has gained traction for the inspection of such defects. However, detection of the weak non-linear defect signature buried in ambient noise remains challenging, and depends on an empirical choice of excitation frequencies. This study focuses on the evaluation of a non-linear pump probe ultrasound inspection technique applied to BVID in multilayer carbon fibre reinforced composites used in aeronautics. In particular, the influence of various experimental parameters on the measured non-linear response is studied. A low frequency pump wave (via a shaker) and a high frequency wave (via a piezoelectric transducer) are transmitted to the medium while a second transducer records the ultrasonic response. Both pump and probe waves are sent in the form of a few secondslong frequency sweep, in the audible and ultrasonic range respectively. Appropriate post processing is then applied to the recorded signals to retrieve the non-linear response of the defect. The procedure is repeated on several composite plates with and without impact damages. The nonlinear response appears in the form of modulation sidebands in the frequency spectrum of the post-processed signals. The relative amplitudes of the side bands obtained for the various samples with various defect sizes and excitation amplitude or frequency content are studied. Optimal experimental parameters were obtained and led to a good detection of defects. The limits of the method are also discussed.

Barely visible impact damage detection in composite structures using deep learning networks with varying complexities

Visual inspection is one of the most common non-destructive testing (NDT) methods that offers a fast evaluation of surface damage in aerospace composite structures. However, it is highly dependent on human-related factors and may not detect barely visible impact damage (BVID). In this research, low velocity impact tests with different energy levels are conducted on two groups of composite panels, namely ‘reference’ and ‘sensor-integrated’ samples. Then, the results of impact tests, together with C-scan and visual inspection images, are analysed to define the BVID range and create an original image dataset. Next, four different deep learning models are trained, validated and tested to capture the BVID only from the images of the impacted and non-impacted surfaces. The results show that all four networks can learn and detect BVID quite well, and the sensor-integrated samples reduce the training time and improve the accuracy of deep learning models. ResNet outperforms other networks with the highest accuracy of 96.2% and 98.36% on the back-face of reference and sensor-integrated samples, respectively. The proposed damage recognition method can act as a fast, inexpensive and accurate structural health monitoring tool for composite structures in real-life applications.