The present study is focused on the characterization of the fatigue damage features in carbon/epoxy laminates under mode-II loading conditions. To this aim, a sinusoidal cyclic load was applied to the End-Notched Flexural (ENF) specimens and the fatigue behavior of specimens was investigated. Scanning Electron Microscope (SEM) was used to identify the damage features on the fracture surface, i.e. fiber imprints, cusps, roller cusps, and striations. It was found that the fatigue damage features, such as cusps and striations, completely depended on the fatigue crack growth rate, da/dN. In addition, a linear relationship between the fatigue striation space and the strain energy release rate range (ΔGs) and the hysteresis loop area was established. The Acoustic Emission (AE) method was also employed to characterize the damage features. The obtained results showed that higher AE energy indicates larger and rougher cusps and striation features.@en

By analyzing the failure mechanisms, crashworthiness characteristics of FW composite tubes subjected to two modes of progressive damage and catastrophic failure are investigated using acoustic emission technique and numerical method. The AE signals of ±45° composite tubes were classified using hierarchical and wavelet transform methods, and based on the realistic and three-dimensional geometrical architecture of tubular structures, the microstructural finite element model was developed using Catia and ABAQUS software. Then deformation patterns and the impression of each mechanism on the crashworthiness characteristics were assessed. Results indicated that fiber breakage and fiber/matrix debonding could likely control the higher percentage of damage. By changing the type of modes from progressive damage to catastrophic failure, the percentage of matrix cracking increases, the fiber/matrix separation decreases, and the failure behavior become dominated by local buckling. Comparing the FE simulation with experimental results, we found the proposed 3D model can reasonably predict the pre-crushing, post-crushing, and material densification.@en

Composite structures in transportation industries have gained significant attention due to their unique characteristics, including high energy absorption. Non-destructive testing methods coupled with machine learning techniques offer valuable insights into failure mechanisms by analyzing basic parameters. In this study, damage monitoring technologies for composite tubes experiencing progressive damage were investigated. The challenges associated with quantitative failure monitoring were addressed, and the Genetic K-means algorithm, hierarchical clustering, and artificial neural network (ANN) methods were employed along with other three alternative methods. The impact characteristics and damage mechanisms of composite tubes under axial compressive load were assessed using Acoustic Emission (AE) monitoring and machine learning.Various failure modes such as matrix cracking, delamination, debonding, and fiber breakage were induced by layer bending. An increase in fibers/matrix separation and fiber breakage was observed with altered failure modes, while matrix cracking decreased Signal classification was achieved using hierarchical and K-means genetic clustering methods, providing insights into failure mode frequency ranges and corresponding amplitude ranges. The ANN model, trained with labeled data, demonstrated high accuracy in classifying data and identifying specific failure mechanisms. Comparative analysis revealed that the Random Forest model consistently outperformed the ANN and Support Vector Machine (SVM) models, exhibiting superior predictive accuracy and classification using ACC, MCC and F1-Score metrics. Moreover, our evaluation emphasized the Random Forest model's higher true positive rates and lower false positive rates. Overall, this study contributes to the understanding of model selection, performance assessment in machine learning, and failure detection in composite structures.@en

The focus of this study is to investigate the mechanical properties, of ±35° and ±55° filament wound (FW) composite tubes under axial compression loading using the acoustic emission technique. For this purpose, material failure, crashworthiness characteristics, and the effect of each mechanism on the energy absorption capacity were studied using numerical and experimental approaches. Also, to identify and estimate the contribution percentage of damage mechanisms as well as how the damage grows in the specimens, the analysis of acoustic emission signals recorded during loading was performed. Digital image correlation was additionally used to capture displacement/strain contour maps. Finally, to analyze the effect of the winding pattern in the experimental test, the tubes were simulated using finite element analysis (FEA). For modeling of damage mechanisms, a 3D continuum damage model was used. The results of signal processing showed that by increasing the weaving angle of fibers from ±35° to ±55°, the separation of fibers from the matrix decreases, and the percentage of matrix crushing and fiber failure increases. The assessment of damage percentages showed that the reason for the large drop in force at ±55° compared to ±35° is the increase in matrix crushing. Furthermore, the failure behavior of FW tubes appeared to be dominated by local buckling, and the FEA effectively predicted the linear behavior and maximum load value of the composite tubes.@en

Interleaving composite laminates with nanofibrous mat is one of the most reliable methods for increasing interlaminar fracture toughness. The present study seeks to find out how the damage mechanisms of carbon fiber reinforced polymers (CFRPs), subjected to the mode-I and mode-II fracture tests, are affected while those are modified by interleaved Polyamide 66 (PA66) electrospun layers. For this goal, acoustic emission (AE) and scanning electron microscope (SEM) techniques were used for assessing the damage mechanisms. The mode-I test results showed that adding nanofibers could decrease matrix cracking, fiber breakage, and fiber/matrix debonding by 92%, 27%, and 87%, respectively. The AE demonstrated that no fiber breakage occurred during mode-II loading in both non-modified and nanomodified specimens which was validated by SEM images. On the other hand, the two other damage modes, i.e. matrix cracking and fiber/matrix debonding, decreased about 97% in the nanomodified laminates.@en

The aim of this study is to investigate the failure of cracked aluminum plates repaired by one-side composite patches under fatigue loading using Acoustic Emission (AE) and fractography images. Rectangular specimens made of 6061 aluminum alloy with central through thickness pre-cracks were repaired using glass/epoxy laminated patches. The specimens were subjected to the fatigue loading and AE technique was employed to monitor the effect of the repair patch on the damage progression. First, different stages of damage evolution were studied based on the mechanical data and fractography images. Then, the AE energy utilized to characterize failure process of the specimens. To this aim, AE signals of the aluminum cracking and adhesive layer failure were discriminated according to their energy content. The effect of patch thickness and layup on the failure behavior of the specimens were also studied. Finally, it is concluded that AE is a powerful technique to characterize the failure process of a repaired cracked aeronautic structure by composite patches.@en

This study introduces a comprehensive set of designed and tested glass/epoxy composites, AE monitoring and signal processing techniques; (i) to investigate the effect of multiple delaminations on buckling and post-buckling behaviors of laminated composites and (ii) to evaluate Acoustic Emission (AE) technique ability to monitor the buckling delamination growth and to classify the occurred damage mechanisms. The pre-delaminations were made by inserting a Teflon film at the plies interfaces during fabrication. Three different types of specimens were fabricated and subjected to compression loading to study the effects of the location, the number of delaminations, and the thickness of the Teflon film on buckling and post-buckling behaviors of the specimens. The mechanical results showed that the number of delaminations has a major effect on the critical and maximum loads and the location of delamination and the thickness of the Teflon film have minor effects on the critical and maximum loads. The AE signals of the specimens were then classified using Gaussian Mixture Model (GMM) and the evolution of different damage mechanisms was investigated. The AE results showed that AE is a robust technique to classify damage mechanisms in buckling of laminated composites and could identify delamination propagation earlier and with a lower standard deviation, compared with the conventional methods.@en

The aim of this study was to investigate the applicability of acoustic emission (AE) technique to evaluate delamination crack in glass/epoxy composite laminates under quasi-static and fatigue loading. To this aim, double cantilever beam specimens were subjected to mode I quasi-static and fatigue loading conditions and the generated AE signals were recorded during the tests. By analyzing the mechanical and AE results, an analytical correlation between the AE energy with the released strain energy and the crack growth was established. It was found that there is a 3rd degree polynomial correlation between the crack growth and the cumulative AE energy. Using this correlation the delamination crack growth was predicted under both the static and fatigue loading conditions. The predicted crack growth values was were in a good agreement with the visually recorded data during the tests. The results indicated that the proposed AE-based method has good applicability to evaluate the delamination crack growth under quasi-static and fatigue loading conditions, especially when the crack is embedded within the structure and could not be seen visually.@en

The aim of this research is to investigate the failure mechanisms of the filament-wound composite tubes under axial compressional loading by using an acoustic emission approach. First, the mechanical properties of ±45°C composite tubes were obtained experimentally. Then, failure due to the buckling phenomenon and crashworthiness characteristics were studied utilizing numerical simulation and experimental methods. Tubes were next simulated in ABAQUS software, and a continuum damage mechanics model was implemented in a progressive framework to assess the failure modes. From the macroscale view, results showed that the damage behavior of composite tubes turned out to be dominated by local buckling followed by a post-buckling field, which is generated by longitudinal cracks along the winding direction. On the micro-scale, the acoustic emission-based procedure based on the wavelet packet transform method was adopted. The hierarchical modeled assessment resulted in the identity of four clusters of AE signals. In GFRP tubes, the fiber breakage and fiber/matrix separation could mostly control the higher percentage of damage and cause to increase the energy absorption. Finally, by comparing the results obtained from micro and macro scales, the local buckling failure mode was attributed to the low content of fiber/matrix debonding in the structure.@en

This study investigates the mechanical behavior and damage mechanisms of thin-walled glass/epoxy filament wound tubes under quasi-static lateral loads. The novelty is that the tubes are reinforced in critical areas using strip composite patches to provide a topology-optimized tube, and their damage mechanisms and mechanical performance are compared to that of un-reinforced (reference) tubes. To detect the types of damage mechanisms and their progression, the Acoustic Emission (AE) method is employed, accompanied by data clustering analysis. The loading conditions are simulated using the finite element method, and the results are validated through experimental testing. The findings confirm that the inclusion of reinforcing patches improves the stress distribution, leading to enhanced load carrying capacity, stiffness, and energy absorption. Compared to the reference tubes, the reinforced tubes exhibit a remarkable increase of 23.25% in the load carrying capacity, 33.46% in the tube’s stiffness, and 23.67% in energy absorption. The analysis of the AE results reveals that both the reference and reinforced tubes experience damage mechanisms such as matrix cracking, fiber-matrix debonding, delamination, and fiber fracture. However, after matrix cracking, delamination becomes dominant in the reinforced tubes, while fiber failure prevails in the reference tubes. Moreover, by combining the AE energy and mechanical energy using the Sentry function, it is observed that the reinforced tubes exhibit a lower rate of damage propagation, indicating superior resistance to damage propagation compared to the reference tubes.@en

This study focuses on the clustering of the indentation-induced interlaminar and intralaminar damages in carbon/epoxy laminated composites using Acoustic Emission (AE) technique. Two quasi-isotropic specimens with layups of [60/0/-60]4S (is named dispersed specimen) and [604/04/-604]S (is named blocked specimen) were fabricated and subjected to a quasi-static indentation loading. The mechanical data, digital camera and ultrasonic C-scan images of the damaged specimens showed different damage evolution behaviors for the blocked and dispersed specimens. Then, the AE signals of the specimens were clustered for tracking the evolution behavior of different damage mechanisms. In order to select a reliable clustering method, the performance of six different clustering methods consisting of k-Means, Genetic k-Means, Fuzzy C-Means, Self-Organizing Map (SOM), Gaussian Mixture Model (GMM), and hierarchical model were compared. The results illustrated that hierarchical model has the best performance in clustering of AE signals. Finally, the evolution behavior of each damage mechanism was investigated by the clustered AE signals with hierarchical model. The results of this study show that using AE technique with an appropriate clustering method such as hierarchical model could be an applicable tool for structural health monitoring of composite structures.@en

Despite the key advantages of Fiber Reinforced Polymer (FRP) composites, they are susceptible to Barely Visible Impact Damage (BVID) under transverse loadings. This study investigates BVID in two quasi-isotropic carbon/epoxy laminates under quasi-static indentation and Low-Velocity Impact (LVI) loadings using Acoustic Emission (AE). First, the evolution of interlaminar and intralaminar damages is studied by analyzing the AE signals of the indentation test using b-value and sentry function methods. Then, the specimens are subjected to the LVI loading and the induced damages are compared with the indentation test and the percentage of each damage mechanism is calculated using Wavelet Packet Transform (WPT). In consistent with the mechanical data, ultrasonic C-scan and digital camera images of the specimens, the AE results show a considerable similarity between the induced BVID under quasi-static indentation and LVI tests. Finally, the obtained results show that AE is a powerful tool to study BVID in laminated composites under quasi-static and dynamic transverse loadings.@en

This research focused on fracture and fatigue response of carbon/epoxy laminates modified with electrospun nylon 66 nanofibers. For this purpose, AS4/8552 composite laminates interleaved by nanofibrous mat were considered under mode-I quasi-static and fatigue loading conditions. According to test outcomes, the fracture toughness increased about 133% under quasi-static tests. On the other hand, the plotted fatigue curves showed that the crack growth rate significantly decreased in modified samples and the threshold energy release rate enhanced about 128%. Finally, scanning electron microscopy (SEM) was also conducted to investigate the damage mechanisms.@en

The advantages of applying electrospun nylon 6,6 as a toughening agent in epoxy-based composite laminates have been considered already under quasi-static mode-I and mode-II fracture loads. In the present study, the fatigue behavior of unidirectional carbon/epoxy laminates interleaved by nylon 6,6 is investigated under mode-II loading. To this aim, a 50-μm nylon 6,6 nanofibrous mat was put between mid-layers of a carbon/epoxy laminate, and then quasi-static and cyclic loadings were applied on End Notched Flexure (ENF) samples to investigate the onset of delamination growth and crack propagation rate (da/dN) under different ratios of GIIin/GIIC. The results showed that the mode-II fracture toughness increased about 161% under quasi-static loading tests. Additionally, the fatigue test results proved that the crack propagation was 14–27 times slower in the modified sample (with the same GIImax) because of the bridging between composite layers caused by nanofibers. Different fractography techniques including optical microscopy, microscopic 3D surface scanning device, and scanning electron microscopy (SEM) were also used to explore further the toughening mechanism.@en

The advantages of applying electrospun nylon 6,6 as a toughening agent in epoxy-based composite laminates have been considered already under quasi-static mode-I and mode-II fracture loads. In the present study, the fatigue behavior of unidirectional carbon/epoxy laminates interleaved by nylon 6,6 is investigated under mode-II loading. To this aim, a 50-μm nylon 6,6 nanofibrous mat was put between mid-layers of a carbon/epoxy laminate, and then quasi-static and cyclic loadings were applied on End Notched Flexure (ENF) samples to investigate the onset of delamination growth and crack propagation rate (da/dN) under different ratios of GIIin/GIIC. The results showed that the mode-II fracture toughness increased about 161% under quasi-static loading tests. Additionally, the fatigue test results proved that the crack propagation was 14–27 times slower in the modified sample (with the same GIImax) because of the bridging between composite layers caused by nanofibers. Different fractography techniques including optical microscopy, microscopic 3D surface scanning device, and scanning electron microscopy (SEM) were also used to explore further the toughening mechanism.@en