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The growing demand of laminated composite materials made of natural fibers and resins with a lower environmental impact in different applications requires a more detailed understanding about the performance during its service considering the delamination in curved parts are one of the most critical failure mechanisms. The present study focuses on an experimental investigation and calculation of the interlaminar tensile strength of unidirectional flax/epoxy laminated curved beams featuring varying thicknesses. The assessment is conducted through a four-point-bending test, in accordance with the ASTM D6415 standard, and the outcomes are subsequently correlated with the employed formulae. The study also involves an examination of the formulation and methodology applicability for the ILTS calculation of unidirectional flax/epoxy laminated curved beams. The analysis includes the impact on maximum interlaminar tensile stress and the evolution of delamination in the flax/epoxy laminated curved beams post-failure behavior. These findings have significant implications for accurately predicting the interlaminar tensile strength and post-failure behavior of such laminated curved beams. [ABSTRACT FROM AUTHOR]

Real service requirements of the assembly performance and joining properties of design components are critical for composite usage in the aerospace industry. This experimental study offers a novel and comprehensive analysis of dry drilling optimization for glass-reinforced, high-performance epoxy matrix composites used in aerospace structures, focusing on thrust force and delamination. The study presents a first-time investigation into the combined effects of spindle speed (1000, 2250, 4000 and 5750 rpm), feed rate (0.2, 0.4, 0.6 and 0.8 mm/rev) and tool diameter (3 and 5 mm) using a custom-designed drill tool specifically developed for this application, filling a gap in the current literature. By employing the Taguchi design of experiments, the study identified that medium spindle speeds (2250–4000 rpm), lower feed rates (0.2 mm/rev) and smaller tool diameters (3 mm) provided optimal conditions for minimizing thrust force and delamination. These results present actionable insights into improving the structural integrity and performance of drilled aerospace-grade composite components, offering innovative advancements in both the aerospace and defense industries. [ABSTRACT FROM AUTHOR]

Despite significant progress, a comprehensive analysis of Machine learning algorithms for detecting delamination and crack damage in composite laminate plates is noticeably absent in the literature. This study aims to bridge this gap by meticulously help spectrum of six regression algorithms. By utilizing simulated displacement-time responses from a 16-layer laminated composite plate structure subjected to a random force, a robust dataset is constructed to evaluate the efficacy of these machine learning paradigms. Performance assessments rely on critical metrics: coefficient of determination (R2), mean absolute error (MAE), root mean squared error (RMSE), and mean absolute percentage error (MAPE). [ABSTRACT FROM AUTHOR]

Delamination is a prevalent type of damage in composite laminate structures. Its accumulation degrades structural performance and threatens the safety and integrity of aircraft. This study presents a method for the quantitative identification of delamination identification in composite materials, leveraging distributed optical fiber sensors and a model updating approach. Initially, a numerical analysis is performed to establish a parameterized finite element model of the composite plate. Then, this model subsequently generates a database of strain responses corresponding to damage of varying sizes and locations. The radial basis function neural network surrogate model is then constructed based on the numerical simulation results and strain responses captured from the distributed fiber optic sensors. Finally, a multi-island genetic algorithm is employed for global optimization to identify the size and location of the damage. The efficacy of the proposed method is validated through numerical examples and experiment studies, examining the correlations between damage location, damage size, and strain responses. The findings confirm that the model updating technique, in conjunction with distributed fiber optic sensors, can precisely identify delamination in composite structures. [ABSTRACT FROM AUTHOR]

Delamination is a common type of damage in composite laminates that can significantly affect the integrity and stability of structural components. This study investigates the post-buckling behavior of carbon fiber-reinforced epoxy composite laminates with embedded delamination under quasi-static compression. Experimental tests were conducted using an electronic universal material testing machine to measure deformation and load-bearing capacity in the post-buckling stage. The specimens, prepared from T300 carbon fiber and TDE-85 epoxy resin prepreg, were subjected to axial compressive loads with delamination simulated by embedding Teflon films. Finite element analysis (FEA) was performed using ABAQUS software, incorporating a four-part model to simulate delaminated structures, with results validated against experimental data through comprehensive convergence analysis. The findings reveal that increasing delamination depth and length decrease overall stiffness, leading to an earlier onset of buckling. Structural instability was observed to vary with the size of delamination, while the post-buckling deformation mode consistently exhibited a half-wave pattern. This research underscores the critical impact of delamination on the structural integrity and load-bearing performance of composite laminates, providing essential insights for developing more effective design strategies and reliability assessments in engineering applications. [ABSTRACT FROM AUTHOR]

Highlights The extensive use of multidirectional composite laminates requires to understand the delamination behavior at interfaces between plies with different fiber orientations. In this study, double cantilever beam testing was performed to investigate the mode I interlaminar fracture of carbon fiber reinforced polymer laminates with different central interfaces (0//0, 0//+45 and +45//−45). X‐ray microtomography revealed the curved crack front shape that was incorporated to calculate fracture toughness. The fracture toughness varies with the crack length in a typical delamination resistance curve, which can be quantified by an empirical equation. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate, compared to constant toughness. It was found that the delamination mechanism is independent of central interfaces. However, compared to unidirectional laminates, multidirectional laminates are less resistant to crack initiation, but more resistant to propagation with higher toughness and shorter fiber bridging zone. Mode I interlaminar fracture of CFRP laminates with different central interfaces was investigated experimentally and numerically. Curved crack front shape was incorporated to calculate fracture toughness. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate. Multidirectional laminates are less resistant to crack initiation but more resistant to propagation with higher toughness and shorter fiber bridging zone. [ABSTRACT FROM AUTHOR]

A ply-level based modeling strategy for predicting the delamination behavior of laminated composites under pure and mixed mode loading conditions is implemented within the framework of the Finite Element Method. Each ply and each interface of the laminate is explicitly modeled, with the plies represented by various element types such as conventional shell, continuum shell, and continuum elements, and the interfaces are discretized using cohesive zone elements. The comparison between all models is examined in terms of delamination onset and growth including load-displacement curves, delamination area, computation time, and mode-mixity. The results show that all ply-level based modeling strategies exhibit very good agreement with the analytical results. Moreover, ply-level approach based on shell elements in combination with finite thickness cohesive zone elements offers a numerically efficient simulation tool to predict delamination behavior in laminates. [ABSTRACT FROM AUTHOR]

Plastics reinforced with glass fiber have a significant likelihood of being replaced by natural fiber hybrid composites (NFHCs). Making holes helps in part assembly, which is a crucial activity in the machining of composite constructions. As a result, choosing the right drill bit and cutting parameters is crucial to creating a precise and high-quality hole in composite materials. The present study employs the Taguchi approach to examine the delamination behavior and hole quality of ramie–bamboo composite laminates consisting of epoxy and nano-fillers (SiC, Al2O3) with feed, spindle speed, and three distinct drill bit types. Surface roughness and delamination are significantly influenced by feed and spindle speed, as indicated by the results of the analysis of variance. It was found that the spindle speed had a major impact on the delamination factor and surface roughness, while the feed and drill bit type had a minor influence. The surface roughness (76.5%) and delamination factor (66.7%) are significantly affected by the spindle speed. [ABSTRACT FROM AUTHOR]

This study utilises several methods centred on acoustic emission (AE) waveform characteristics to conduct in-depth analysis of the damage mechanism of composite materials under mode I. Combined with the peak frequency statistics and continuous wavelet transform (CWT) of AE signals, three damage modes of laminated composites during loading are identified. The cumulative energy of three damage modes separated by variational modal decomposition (VMD) is used to characterize their damage evolution process. Based on the findings, delamination and fiber–matrix debonding are proven to be the dominant damage mechanisms for mode I. The comprehensive analysis results from AE's accumulated energy, peak frequency distribution, CWT, VMD, and sentry function can accurately detect the onset time of delamination in laminates. The equivalent crack length is calculated by the load value determined at the initial time of delamination, and the strain energy release rate calculated by the load value and crack length at this time is used as the interlaminar fracture toughness value of mode I. This research method will provide a new idea for the damage tolerance analysis of mode I delamination in laminated structures. [ABSTRACT FROM AUTHOR]

A thermo-mechanical progressive analysis model is proposed for predicting fatigue-driven delamination in composite laminated plates. The delamination is simulated based on extended layerwise method (XLWM). The traction-separation law is employed to the heat flux transfer and mechanical load transfer across the delamination front. A thermo-mechanical cohesive zone model (TM-CZM) is developed by Peerlings damage law to simulate the fatigue characteristic of delamination front. In the numerical examples, the effects of mesh lengths, acceleration multipliers, interface strengths, and cyclic temperature load magnitudes on the delamination expansion process are investigated, and the temperature-life curves of composite laminates are determined as well. [ABSTRACT FROM AUTHOR]

Highlights It is more and more difficult to identify defects in carbon fiber composite materials due to the difficulty in making defect samples and the single signal analysis method. In order to better solve the problem of defect identification in carbon fiber composite materials, this study uses ultrasonic phased array equipment to quantitatively locate and detect carbon fiber composite laminates with embedded delamination defects, so as to more intuitively and effectively display the appearance of different delamination defects. The time domain analysis of the collected ultrasonic original signal and the time‐frequency domain analysis using wavelet packet are carried out. A total of 6 eigenvalues were extracted to reflect the ultrasonic signals of different delamination defects. By using genetic algorithm to optimize BP neural network, the recognition accuracy of delamination defects of different sizes is more than 95%, and the recognition accuracy of delamination defects of different depths is 100%, so as to realize the effective intelligent recognition of delamination defects of different sizes and depths of carbon fiber composites. This study is of great significance to improve the accuracy and reliability of defect identification of carbon fiber composite materials. The ultrasonic phased array equipment is used to quantitatively locate the carbon fiber composite laminates with embedded delamination defects, so that the appearance of different defects can be displayed more intuitively and effectively. Using time domain analysis and time‐frequency domain analysis based on wavelet packet, the combination of the two can more comprehensively extract the effective features of the defect signal. The BP neural network is optimized by genetic algorithm, and the results can effectively and automatically identify different layered defects, which lays a good foundation for the rapid and accurate identification of more defects in the future. [ABSTRACT FROM AUTHOR]

Developing constitutive models that can be used to model composite structural systems subjected to impact loads where modeling damage, failure, and delamination are critical is a challenging task. Often processing experimental data to develop the composite material model and, especially, the delamination model can be tedious, subjective, and error prone. In this paper, procedures for characterizing the behavior of laminated unidirectional composites (UDCs) are explained. First, laboratory tests are conducted to obtain the orthotropic material properties of a commonly used composite, IM7-8552. The obtained data are used in the MAT_213 model in LS-DYNA. Second, fracture-related tests, i.e., double-cantilever beam (DCB) and end-notched flexure (ENF) tests, are conducted to obtain the load-displacement responses. Finally, an inverse analysis approach is used by combining response surface methodology (RSM) and gradient-based numerical optimization to find the optimal values of the traction-separation law (TSL) parameters to build the constitutive model for modeling delamination. The developed cohesive zone element (CZE) model is then validated using the experimental results of a quasi-isotropic tension test. It is shown that the overall process of analyzing the experimental data and building a finite-element model is simpler, and the results from the validation test indicate that the developed methodology can yield accurate predictions. [ABSTRACT FROM AUTHOR]

Laminated composite materials are susceptible to damage, such as delamination, due to poor out-of-plane properties and inadequate adhesion at the layer interface. Using the interlayering method is an effective way to enhance resistance against delamination in these materials. However, interlayer synthesis methods have been limited and sometimes very expensive. Therefore, this study investigates the impact of novel 3D-printed polyvinyl alcohol (PVA) interlayers on the mode II interlaminar fracture toughness in glass/epoxy laminated composites. Notably, the interlayers feature a geometrical structure consisting of square cell shapes, allowing the filament to have an equal volume percentage to the resin in the interlayer. To achieve this, end-notch flexure specimens (ENF) were prepared, and the mode II interlaminar fracture toughness was calculated using the compliance-based beam method (CBBM). The specimens' crack growth resistance curve (R-curve) analysis revealed that using the desired 3D-printed interlayer increases the mode II interlaminar fracture toughness by 73%. Furthermore, the behavior of crack growth resistance in laminated composites reinforced with 3D-printed interlayer shows an increasing trend. The mode II fracture surface analysis of reinforced specimens demonstrates the influence of square cell shapes in creating shear hackles, increasing surface friction at the interface of laminated composite layers, crack pinning mechanism, and changing the direction of crack growth, ultimately leading to increased resistance to crack growth. [ABSTRACT FROM AUTHOR]

Composite materials are utilized in various industries due to their advantageous properties. Drilling is a crucial process for joining these materials to construct the structures. During the drilling of composite materials, several types of defect can occur, with delamination being the most prevalent. Delamination adversely effects the properties of the drilled hole and diminishes the quality of the final structure. Thrust force is a key parameter used to monitor the drilling process; a higher thrust force increases the likelihood of defects, particularly delamination, in the drilled area. In this article, a central composite design is applied to the drilling process of carbon fiber-reinforced polymer (CFRP) composites, focusing on parameters such as rotational speed, feed rate, and the angle between the composite layer sequences. The objective is to minimize delamination factors and thrust force. The effect of drilling parameters on the responses is analyzed independently. The results indicate that the derived models can predict the thrust force and delamination factors in the drilling of CFRP composites. [ABSTRACT FROM AUTHOR]

The paper presents a study on the stresses and deformations induced by the transversal loading of some flat plates made of E Glass EWR Fiberglass Woven Roving type, having delamination type defects. The parametric analysis of the delamination influence on the stresses and displacements occurring in the plate material is performed. The results of the static calculation are compared with the experimental tests of the flat plates subjected to bending, highlighting the concordance of the variation of the stresses and displacements versus to the delamination position. [ABSTRACT FROM AUTHOR]

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The radial displacement of hydrogen storage composite overwrapped pressure vessel (COPV) is a key factor related to the collapse failure and burst pressure. However, there is still lack of effective method to monitor the radial displacement during the hydrostatic test and service process of the vessels. This paper proposed a method to measure the radial displacement based on fringe projection profilometry (FPP). Hydrostatic test with a composite overwrapped pressure vessel verifies the proposed method. The variation of displacement changing with inner pressures observed in the experiments is in agreement with that obtained from finite element analysis. Also Computed Tomography (CT) scanning is conducted and a widely spread micro defects is found in the composite layer of the cylinder. 78.9% of the total observed defects are small (<1000 mm2). In regions where large-sized defects (>1000 mm2) are concentrated, the total area of defects reaches 16,200 mm2. According to the statistical analysis, the strong correlation is found between the defects area and radial displacement. • Presented a novel method based on fringe projection profilometry (FPP) for monitoring the deformation of composite overwrapped pressure vessels (COPVs) during hydrostatic testing. • Employing 3D point clouds from FPP to reconstruct the cylinder and measure radial displacement. • The variation trend of radial displacement with pressure obtained by experiments is consistent with the finite element results. • Utilized Computed Tomography (CT) to detect and quantify micro delamination defects in the cylinders. • A significant correlation was found between the area of defects and radial displacement, while the relationship between the number of defects and radial displacement was less pronounced. [ABSTRACT FROM AUTHOR]

The article provides information on Insight's current awareness service, which includes a database covering various non-destructive testing (NDT) techniques and related fields, and details how users can access literature, technical reports, and standards through various institutions.

The biophysical heterogeneity of the bone–cartilage interface requires complex materials to mimic differences in bone density, extracellular matrix composition, and mineralization. Biomaterial approaches to repair osteochondral tissue typically use multilayer scaffolds, which require multistep fabrication and may undergo delamination at the construct interface. This work describes the development of functionalized microgels for the repair of osteochondral tissues using an N‐cadherin peptide, bone morphogenetic protein‐2 (BMP‐2) peptide, and changes in stiffness to create pro‐osteogenic and prochondrogenic microgels. Microgels, when annealed into a scaffold, outperforms bulk hydrogel controls evidenced by upregulation of osteogenic and chondrogenic markers in mesenchymal stromal cells (MSCs). The macroporous void space present in microgel anneals scaffolds enabled robust cell proliferation and extracellular matrix (ECM) deposition throughout the entire scaffold. A bilayer functionalized annealed microgel scaffold is then created and the ability to spatially control the differentiation of MSCs is assessed. Osteochondral bilayer scaffolds exhibit distinct regions of osteogenic and chondrogenic protein expression as a function of microgel population upon immunostaining for osteocalcin and aggrecan, respectively. Spatial transcriptomics confirm osteogenic and chondrogenic genes are upregulated in their respective microgel regions. These studies highlight the tunable and functionalizable nature of microgels and the importance of macroporous void space. [ABSTRACT FROM AUTHOR]

This paper comprehensively investigated the effect of the tool geometries on heat transfer and special crystallization characteristics in the drilling of carbon fiber‐reinforced polyetheretherketone (CF/PEEK) composite for the first time. Three tools with different geometries (twist, brad, and dagger drill) are carried out on CF/PEEK drilling experiments, and temperature, as well as crystallinity and surface roughness, are introduced to make a comparative evaluation on the effect of the tool geometries. The results show that the drilling temperature is determined by the contact length of the profile between the drill and the material. Correspondingly, better surface quality with lower surface roughness and smoother surface is obtained by using a twist drill due to the heat‐induced PEEK smearing effect. Further, in situ XRD and differential scanning calorimeter (DSC) were used to characterize the crystallization characteristics where crystal phase transformation at different temperatures was measured. The occurring temperature of the strongest crystallization characteristics for CF/PEEK is basically not affected by tool geometries, all at 300°C, and the limiting crystallization temperature is 344.5 ± 0.7°C. The crystallinity is found to be affected by drilling temperature and cooling rate and gradually decreases along the heat transfer direction. This work provides a new sight into the improvement of CF/PEEK drilling. Highlights: The effect of tool geometries on thermal and crystallization for CF/PEEK is provided.In situ XRD and DSC are used to characterize crystallization characteristics.The drilling temperature and cooling rate are both determinants of crystallinity.The limiting crystallization temperature of the CF/PEEK composite is 344.5 ± 0.7°C.Heat‐induced PEEK smearing effect improves the surface quality of CF/PEEK drilling. [ABSTRACT FROM AUTHOR]

This article presents high‐fidelity observations of the time evolution of unstable delamination growth in tapered composite beams under tensile loading via ultra‐high‐speed camera in conjunction with digital image correlation. Asymmetrically tapered GFRP laminates, with a thickness drop from 18 plies to 12 plies, are manufactured in three different layup configurations with grouped drop‐offs: [06/(02)3/06], [06/06/06], and [06/(±45)3/06]. Beam specimens are subjected to quasi‐static tensile loading in a servo‐hydraulic axial testing machine, and the damage evolution on one edge of the tapered beam is monitored in situ at 124,000 fps with a high‐speed camera (HSC). In selected tests, digital image correlation (DIC) analyses are conducted either on the edge or on the tapered surface to obtain the full‐field strain and elucidate failure mechanisms. Real‐time HSC observations of the damage sequence consistently affirm that the initial failure mechanism is a transverse crack at the nearest drop‐off to the thin section, which is followed by delaminations at interfaces of sublaminates. Unstable delamination growth is characterized by measurements of lower bounds of crack tip speeds. The unique crack speed data provide evidence for the existing effect of through‐the‐thickness stresses on delamination growth. The detailed sequence and patterns of dynamic failure presented in this paper can serve as a benchmark to validate damage models used in simulations of tapered laminates. Highlights: The sequence of unstable delamination growth in tapered GFRP beams is investigated experimentally.Characterized unstable delamination growth based on crack tip speed measurements.Strain fields are evaluated along beam edges using DIC in real‐time.In‐situ observations of dynamic failure can serve as a benchmark to validate damage models. [ABSTRACT FROM AUTHOR]

The main, well-known scientific results were obtained by outstanding scientist Yu. N. Rabotnov in the field of creep theory, hereditary elasticity, damage mechanics, technical theory of shells. However, in the last years of his life, the academician Yu. N. Rabotnov was keen on some problems in the field of mechanics of fiber-reinforced composite materials. He proposed new, original strength criteria, simplified methods for ply-by-ply calculation of composite laminates. One of Yu. N. Rabotnov's scientific passions were connected with description of specific fracture modes of layered fibrous composites having the types of delamination and splitting. On the base of energy fracture criterion, it's possible to estimate the scale effect of strength – the dependence of critical stresses on the absolute sizes of composite members. Such models of specific fracture modes make it possible to estimate the danger of such technological defects as thin polymeric film between composite layers. [ABSTRACT FROM AUTHOR]

The drilling of composite structure leads to its damage, near the peripheral areas of the drilled hole, due to the laminar structure of the composites. The goodness of a drilling process can be evaluated by measuring the extent of the damage caused by it. In the present study, lock-in thermography non-destructive inspection method is proposed to characterise the drilling damages in glass fiber–reinforced polymer panel. A semi-automated image processing methodology is proposed to calculate damage parameters, namely delamination area and size, and delamination factor. The effect of excitation frequency on the damage characterisation is studied to decide the optimum frequency range to measure the damage parameters through signal-to-noise ratio and damage visibility. The damage parameters are measured at optimum frequency. The study showed that the lock-in thermography technique has the potential to characterise drilling damage. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to assess the hydrogen embrittlement sensitivity of carbon gradient heterostructure materials and to verify the reliability of the scratch method. Design/methodology/approach: The surface-modified layer of 18CrNiMo7-6 alloy steel was delaminated to study its hydrogen embrittlement characteristics via hydrogen permeation, electrochemical hydrogen charging and scratch experiments. Findings: The results showed that the diffusion coefficients of hydrogen in the surface and matrix layers are 3.28 × 10−7 and 16.67 × 10−7 cm2/s, respectively. The diffusible-hydrogen concentration of the material increases with increasing hydrogen-charging current density. For a given hydrogen-charging current density, the diffusible-hydrogen concentration gradually decreases with increasing depth in the surface-modified layer. Fracture toughness decreases with increasing diffusible-hydrogen concentration, so the susceptibility to hydrogen embrittlement decreases with increasing depth in the surface-modified layer. Originality/value: The reliability of the scratch method in evaluating the fracture toughness of the surface-modified layer material is verified. An empirical formula is given for fracture toughness as a function of diffused-hydrogen concentration. [ABSTRACT FROM AUTHOR]

This study presents a novel approach to conducting the electromechanical impedance (EMI) technique for delamination detection in composite structures without the need for permanently attaching PZT (Lead Zirconate Titanate) transducers to the surface. Instead, a device is created that can be simply placed on top of the composite structure, enabling one to perform the EMI technique for detecting damage. The primary objective is to investigate the effectiveness of this device in detecting delamination within composite materials. Additionally, this study explores the impact of placing additional weight on top of the transducer to investigate the performance of the device subjected to higher pressure. Experimental results and analysis will be presented to evaluate the feasibility and reliability of this approach for non-destructive testing and structural health monitoring of composite components. This research is significant as it lays the groundwork for developing automated damage detection systems using robotics in the near future. By demonstrating the proposed concept that can be easily integrated into robotic platforms, this study contributes to the advancement of automation in structural health monitoring. Implementing this technique in robotic systems has the potential to revolutionize maintenance practices by enabling continuous, real-time monitoring of composite structures, enhancing safety, and minimizing downtime due to structural defects. Moreover, the investigation into the impact of additional weight on the transducer's performance is crucial for setting minimum weight limits in robotic systems, ensuring optimal functionality and accuracy during automated damage detection tasks. [ABSTRACT FROM AUTHOR]

Fiber‐reinforced composites acquire widespread use in the technologically advanced sectors on account of their lightweight and outstanding strength. Considering the significant connection between component quality and durability, bearing strength is a critical factor in the mechanically assembled parts during the manufacturing process. Therefore, in this paper, an experimental study was performed on hybrid glass fiber/aluminum‐mesh composites to assess the impact of abrasive water jet (AWJ) drilling on delamination and bearing strength. The samples were drilled, and the delamination damage was assessed. Finally, the bearing test was conducted on the sample in conformity with ASTM D5961M. The impact of stress distribution on the specimen's failure patterns was studied. Experimental outcomes reveal that the GFA specimen had a 2.9% higher average bearing strength than the AGF specimen. The drilling‐induced damage was determined to be the primary factor responsible for a 27% reduction in bearing strength. The optimization of operational parameters yields improved bearing load and lower damage. A significant level of conformity was detected by the regression accuracy and validation test results with a residual error of less than 10%. Highlights: A comparison of two hybrid composites utilizing various layouts for damage and load bearing.Delamination damage is a key factor in predicting the efficiency of bolted connections.Using optimized operation parameters enhances drilling output response significantly. [ABSTRACT FROM AUTHOR]

The growing use of thermoplastic composites (TPCs) like low-melting polyaryletherketone (LM-PAEK) matrices reinforced with unidirectional carbon fiber (CF) in aircraft structures presents a significant challenge in terms of lightning strikes and electromagnetic interference shielding during aircraft operations. This is due to the weak electrical conductivity of TPC structures, which results in widespread damage when struck by lightning. The repair and maintenance of these extended damaged sites can increase operational costs and loss of flights. Several lightning strike protection (LSP) systems have been developed and implemented to address these concerns. This study evaluated a highly conductive coating with a low filler rate for its effectiveness as an LSP solution for TPCs on exterior aircraft surfaces. The TPC panel without any coatings was first studied. Subsequently, the level of conductivity was increased by incorporating the nanoscale conductive fillers, silver-coated copper (Ag/Cu) nanoflakes, with a silver content of 20 wt.% (Ag20/Cu) and 30 wt.% (Ag30/Cu), correspondingly, into the coating at two loadings of 55 wt.% and 70 wt.% in an epoxy carrier for the surface coatings. The behavior of electrical and surface conductivity was thoroughly examined to understand the impact of Ag/Cu with a high aspect ratio and the effectiveness of the LSP solution. In addition, the spray-coated TPC panels underwent rigorous Zone 2A lightning strike testing using simulated lightning current, in agreement with the industry standard of Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 5412B. Despite the higher resistance due to the lower conductive coating weight, the TPC panels with Ag30/Cu at loading of 70 wt.% achieved better results than those with Ag30/Cu at loading of 55 wt.%. This is evidenced by the minor structural delamination and CF breakage on the front surface, which proposes a new economic route for a sustainable post-processed LSP system in the aviation industry. [ABSTRACT FROM AUTHOR]

The paper focuses on the cutting behavior of Glass Fiber Reinforced Polymer (GFRP) composites and GNP–GFRP composites that contain varying amounts of Graphene Nano Platelets (GNP). GFRP composites are increasingly being used in a variety of industrial applications due to their excellent mechanical properties, such as high strength, stiffness, and low weight. However, their machining and cutting behavior can be challenging due to the presence of the reinforcing fibers. Therefore, the study aims to investigate the machining behavior of GFRP composites and the effect of adding GNP on their cutting behavior. The effect of different parameters such as cutting speed, feed rate and reinforcement rate on cutting forces and delamination factor is investigated. In addition, the tensile strength and fatigue behavior of the composite materials with the best and worst delamination factors were also determined. Addition of up to 0.2 wt.% of GNP to GFRP composites resulted in an increase in cutting forces and delamination factor when drilling GFRP composites. While the cutting force and delamination factor decreased with the increase in cutting speed, the cutting force and delamination factor increased with the increase in the feed rate. Analysis of variance (ANOVA) was performed to determine the effects of drilling parameters and reinforcement ratio on cutting force and delamination factor according to full factorial experimental design. The most efficient factor on the cutting forces is found to be feed rate (84.97%), followed by the reinforced rate (6.48%) and cutting speed (6.13%). The most efficient factor on the delamination factor is determined to be feed rate for (44.49%), followed by the reinforced rate (29.20%) and cutting speed (21.73%). [ABSTRACT FROM AUTHOR]

The impact-echo method (IE) is a non-destructive testing method commonly used in civil engineering. We propose a completely new approach for air-coupled actuation based on supersonic jet flow. The impinging jet sound generates continuously high sound pressures with a broad frequency bandwidth. This novel concept of utilising aeroacoustic sound for air-coupled IE was evaluated on two concrete specimens and validated using a classical IE device with physical contact. The results show a high agreement with the expected frequencies. Delaminations are correctly detected in depth and size. This proves the high reliability of air-coupled IE based on supersonic jet flow. [ABSTRACT FROM AUTHOR]

In order to address the problem of poor imaging detection of internal defects caused by the multi-layer curved surface structure and strongly attenuating material of composite insulators, the total focusing method (TFM) with two-layer curved surface correction was investigated, Virtual source (VS) technology was introduced to improve the transmitted energy of the single-element transmission, combined with the delay multiply and sum (DMAS) algorithm to improve the signal-to-noise ratio (SNR) and ultrasonic imaging detection of defects of different sizes of side-drilled holes (SDHs) in the sheath layer and core rod layer. An examination ofthe delamination defects at the core rod layer of the composite insulators was also carried out. The results show that the double-layer surface correction method is able to calibrate the propagation time of the ultrasonic wave to accurately localise the defects. Moreover, the VS technique is able to effectively detect SDI-15 and delamination defects in the core rod layer that cannot be detected using the conventional TFM. After incorporating the DMAS technique, the average SNRs of the TFM and virtual source total focusing method (VSTFM) are improved by 12.75 dB and 13.77 dB, respectively. This shows that the DMAStechnique can significantly improve the SNR of detection. [ABSTRACT FROM AUTHOR]

Purpose: This study aims to examine how the thickness of layers and printing speed impact the energy absorption capacity of honeycomb structures through drop-weight experiments. In addition, the effect of printing orientation on the resulting microstructure and mechanical performance was targeted to be examined. Design/methodology/approach: In this paper, after manufacturing test specimens using fused deposition modeling technique with three distinct layer thicknesses (0.16 mm, 0.20 mm and 0.28 mm) and printing speeds (40 mm/min, 50 mm/min and 70 mm/min), drop weight tests were carried out. Then to see the effect of printing orientation on mechanical performance, three-point-bending tests were performed and damage mechanisms were comparatively examined through scanning electron microscopy. Findings: An increase in layer thickness from 0.16 mm to 0.28 mm resulted in a notable 37% decrease in the impact resistance of the printed part. In addition, increasing the printing speed from 50 to 70 mm/min reduced the energy absorption capacity of the printed part by approximately 36.5%. Moreover, in terms of printing direction, transversely printed specimens showed 10% lower flexural strength than longitudinally printed specimens. Finally, scanning electron microscopy (SEM) observation showed that internal defects were more prominent in transversely printed specimens, resulting in premature failure. Furthermore, delamination was also detected in transversely printed specimens as another damage mechanism accelerating material failure. Originality/value: It is seen that the effect of printing parameters on the fundamental mechanical properties including tensile strength, strain at break, ductility and elastic modulus were studied by various researchers. However, to the best of authors' knowledge, the effect of printing speed and layer thickness on the energy absorption of polylactic acid based hexagonal honeycomb was not encountered. In addition, in-depth SEM analysis to discover the influence of printing direction significantly contributes to the literature. [ABSTRACT FROM AUTHOR]

Interleaving is a well-recognized method for enhancing the interlaminar fracture toughness of composite materials. However, theoretical toughness is often not realized since cracks tend to propagate through regions of lower toughness. While most studies have focused on the effect of interleaf properties, e.g., thickness and interleaf resin properties, on composite interlaminar fracture toughness, the effect of matrix resin properties on interleaved composite toughness has been overlooked. Recently, we hypothesized that there is a relationship between toughness translation and the ratio of matrix to interleaf resin toughness. In this work, we use additive manufacturing to test this hypothesis with a range of resins in interleaved composite. Toughness is quantified via mode I delamination resistance. Our results confirm our hypothesis that the ratio of matrix to interleaf resin fracture toughness, i.e., the ratio of K I C , is directly correlated to the degree of toughness translation. More specifically, a ratio of approximately 0.5 is required for effective interleaving; below 0.5, the toughness translation is significantly below the theoretical value. This approach has important implications for the design and fabrication of tough composites via choice of resins. [ABSTRACT FROM AUTHOR]

In recent years, composite structures have been used in a large number of applications in aerospace, machinery, marine, and civil engineering. However, there are inevitably many uncertainties in the whole life cycle of composite structures, which can easily lead to structural damage and failure. Therefore, it is important to analyze the reliability and sensitivity of composite structures. At present, most of the contributions use the first-order reliability method (FORM) and the second-order reliability method (SORM) to study the reliability of composite structures and compare them with the results of the Monte Carlo simulation. However, both methods have their limitations. FORM cannot guarantee the calculation accuracy for the highly nonlinear limit state equation, and the calculation efficiency of SORM is too low. Therefore, this paper proposes to use importance sampling (IS) and backpropagation neural network-based Monte Carlo (MC-BPNN) to study the reliability, sensitivity, and dispersion of delamination growth of composite laminates. The results show that compared with FORM and SORM, IS and MC-BPNN have higher calculation accuracy and efficiency and can more accurately evaluate the failure degree of composite structures and ensure their safe operation in the field of aerospace equipment. The universality of this method can make it being widely used in the reliability and sensitivity analysis of different composite materials as well as dispersion analysis. [ABSTRACT FROM AUTHOR]

Wood Textile Composites (WTCs) represent a new and innovative class of materials in the field of natural fiber composites. Consisting of fabrics made from willow wood strips (Salix americana) and polypropylene (PP), this material appears to be particularly suitable for structural applications in lightweight construction. Since the threads of the fabric are significantly oversized compared to classic carbon or glass rovings, fundamental knowledge of the mechanical properties of the material is required. The aim of this study was to investigate whether WTCs exhibit classic behavior in terms of fiber composite theory and to classify them in relation to comparable composite materials. It was shown that WTCs meet all the necessary conditions for fiber-reinforced composites in tensile, bending, and compression tests and can be classified as natural-fiber-reinforced polypropylene composites. In addition, it was investigated whether delamination between the fiber and matrix can be simulated by using experimentally determined mechanical data as input. Using finite element analysis (FEA), it was shown that the shear stress components of a stress tensor in the area of the interface between the fiber and matrix are responsible for delamination in the composite material. It was also shown that the resistance to shear stress depends on the geometric conditions of the reinforcing fabric. [ABSTRACT FROM AUTHOR]

The demand for non-destructive testing of carbon fiber-reinforced polymer (CFRP) is becoming increasingly pressing to ensure its safety and reliability across different fields of use. However, the complex structural characteristics and anisotropic bulk conductivity of CFRP make achieving high sensitivity in detecting internal defects such as delamination extremely challenging. To address this issue, a novel triple rectangular coil probe with high sensitivity developed for detecting delamination in CFRP is presented in this paper. A finite element model using COMSOL Multiphysics was developed for CFRP delamination eddy current testing with the designed probe. Based on this model, the probe parameters were determined through orthogonal experiments. By analyzing the eddy current distribution in CFRP samples, the scanning mode was defined. Following this, the detection voltage was evaluated for various delamination parameters, and the sensitivity of different probes was compared. Results indicate that, under the same excitation coil parameters, for a 5 mm delamination lateral dimension change, the single pancake and single rectangular coil probes exhibit sensitivities of 88.24% and 72.55%, respectively, compared with the designed probe. For a 0.5 mm delamination thickness change, their sensitivities are 49.04% and 56.69% of those of the designed probe. The designed probe meets the demand for high-sensitivity detection. [ABSTRACT FROM AUTHOR]

The primary goal of this study is to develop cost-effective shield materials that offer effective protection against high-velocity ballistic impact and electromagnetic interference (EMI) shielding capabilities through absorption. Six fiber-reinforced epoxy composite panels, each with a different fabric material and stacking sequence, have been fabricated using a hand-layup vacuum bagging process. Two panels made of Kevlar and glass fibers, referred to as (K-NIJ) and (G-NIJ), have been tested according to the National Institute of Justice ballistic resistance protective materials test NIJ 0108.01 Standard-Level IIIA (9 mm x 19 mm FMJ 124 g) test. Three panels, namely, a hybrid of Kevlar and glass (HeS), glass with ceramic particles (CeS), and glass with recycled rubber (ReS) have been impacted by the bullet at the center, while the fourth panel made of glass fiber (G-S) has been impacted at the side. EMI shielding properties have been measured in the X-band frequency range via the reflection-transmission method. Results indicate that four panels (K-NIJ, G-NIJ, HeS, and G-S) are capable of withstanding high-velocity impact by stopping the bullet from penetrating through the panels while maintaining their structural integrity. However, under such conditions, these panels may experience localized delamination with variable severity. The EMI measurements reveal that the highest absorptivity observed is 88% for the KNIJ panel at 10.8 GHz, while all panels maintain an average absorptivity above 65%. All panels act as a lossy medium with a peak absorptivity at different frequencies, with K-NIJ and HeS panels demonstrating the highest absorptivity. In summary, the study results in the development of a novel, costeffective, multifunctional glass fiber epoxy composite that combines ballistic and electromagnetic interference shielding properties. The material has been developed using a simple manufacturing method and exhibits remarkable ballistic protection that outperforms Kevlar in terms of shielding effi- ciency; no bullet penetration or back face signature is observed, and it also demonstrates high EMI shielding absorption. Overall, the materials developed show great promise for various applications, including the military and defense. [ABSTRACT FROM AUTHOR]

In this paper, a novel statistical vibration-based damage detection method is developed considering uncertainties in measured resonance frequencies. The proposed method is based on the application of resonance frequencies as the most accurate and easiest measurable vibration feature. For proof of efficiency of the proposed method, case studies were undertaken using two identical composite plates, one delaminated and the other pristine. In this respect, the frequency response functions (FRFs) were measured and used as the main input to the Resonance Detection Algorithm as the proposed method. Applying these FRFs to a Resonance Detector Function can determine the resonant frequencies and their statistical distribution. Through the statistical distributions of the corresponding resonant frequencies, their reliability of detecting damage has been obtained via the beta distribution. By observing the damage detection reliability of the two sets of corresponding resonant frequencies, it has been determined that the changes in natural frequencies are due to structural changes and not random errors through measurement. [ABSTRACT FROM AUTHOR]

Delamination is one of the most feared failure modes in laminated composites and yet there is a lack of well established procedures to find the ply configuration with the highest delamination resistance for a given geometry of the component and a set of elastic and strength constraints. This paper presents a methodology for determining the quasi-isotropic ply sequence more resistant to delamination under off-axis uniaxial tension. Delamination is considered to be triggered by interlaminar stresses at free-edges or by matrix cracks. Two different ply thicknesses (standard and thin) and two sets of ply orientations (multiples of π /4 or π /8) are considered. The results show that the use of thin plies enlarges the design domain by up to 70% and generates a practically isotropic safe space. The methodology presented is also suitable for optimizing the delamination resistance of other load cases with stress singularities. [ABSTRACT FROM AUTHOR]

Strain energy release rate calculations for various cases of delaminations emanating from matrix cracks are developed and used to predict the onset of delaminations and their growth size as a function of applied tension and shear loads in composite laminates. The method determines the matrix crack spacing, the delamination onset load, the delamination size at onset and, through the use of a newly proposed delamination resistance curve, the size of delaminations as they grow under load. The method can be applied to any symmetric laminate. Comparisons to test results in the literature for a variety of layups and materials shows very good agreement with the exception of cases where significant edge delaminations appear before delaminations caused by matrix cracks. [ABSTRACT FROM AUTHOR]

Due to their anisotropic behavior, composite structures are weak in transverse direction loading. produces transverse cracks, which for a laminated composite, may lead to delamination and total failure. The transition from transverse crack to delamination failure is important and the subject of recent studies. In this paper, a simulation of transverse crack and its transition to delamination on cross-ply laminate was studied extensively using a cohesive element Finite Element Method (FEM). A pre-cracked [0/90] composite laminate made of bamboo was modeled using ABAQUS/CAE. The specimen was in a three-point bending configuration. Cohesive elements were inserted in the middle of the 90° layer and in the interface between the 0° and 90° layer to simulate transverse crack propagation and its transition to delamination. A load–displacement graph was extracted from the simulation and analyzed. As the loading was given to the specimen, stress occurred in the laminates, concentrating near the pre-cracked region. When the stress reached the tensile transverse strength of the bamboo, transverse crack propagation initiated, indicated by the failure of transverse cohesive elements. The crack then propagated towards the interface of the [0/90] laminates. Soon after the crack reached the interface, delamination propagated along the interface, represented by the failure of the longitudinal cohesive elements. The result of the numerical study in the form of load–displacement graph shows a consistent pattern compared with the data found in the literature. The graph showed a linear path as the load increased and the crack propagated until a point where there was a load-drop in the graph, which showed that the crack was unstable and propagated quickly before it turned into delamination between the 0o and 90° plies. [ABSTRACT FROM AUTHOR]

Composite pressure vessels can be exposed to extreme loadings, for instance, impact loading, during manufacturing, maintenance, or their service lifetime. These kinds of loadings may provoke both visible and invisible levels of damage, e.g., fiber breakage matrix cracks and delamination and eventually may lead to catastrophic failures. Thus, the quantification and evaluation of such damages are of great importance. Considering the cost of relevant full-scale experiments, a numerical model can be a powerful tool for such a kind of study. This paper aims to provide a numerical study to investigate the capability of different modeling methods to predict delamination in composite vessels. In this study, various numerical modeling aspects, such as element types (solid and shell elements) and material parameters (such as interface properties), were considered to investigate delamination in a composite pressure vessel under low-velocity impact loading. Specifically, solid elements were used to model each layer of the composite pressure vessel, while, in another model, shell elements with composite layup were considered. Compared with the available experimental data from low-velocity impact tests described in the literature, the capability of these two models to predict both mechanical responses and failure phenomena is shown. [ABSTRACT FROM AUTHOR]

Finite element analyses of the propagation of damage such as fiber compressive failure and delamination have greatly contributed to the understanding of failure mechanisms of fiber-reinforced plastics owing to extensive studies on methodologies using Continuum Damage Mechanics and Fracture Mechanics. Problems without the need for consideration of inertia, such as Double-Cantilever Beam tests, are usually solved by implicit FE solvers, and explicit FE solvers are appropriate for phenomena that progress with very high velocity such as impact problems. However, quasi-static problems with unstable damage propagation observed in experiments such as Open-Hole Compression tests are still not easy to solve for both types of solvers. We propose a method to enable the static FE solver to solve problems with unstable propagation of damage. In the present method, an additional process of convergence checks on the averaged energy release rate of damaged elements is incorporated in a conventional Newton–Raphson scheme. The feasibility of the present method was validated by two numerical examples consisting of analyses of Open-Hole Compression tests and Double-Cantilever Beam tests. The results of the analyses of OHC tests showed that the present method was applicable to problems with unstable damage propagation. In addition, the results from the analyses of DCB tests with the present method indicated that mesh density and loading history are not significantly influential to the solution. [ABSTRACT FROM AUTHOR]

This paper deals with an innovative nondestructive technique for composites (local-IET), which is based on the Impulse Excitation Technique (IET) and, in the presence of damage, assesses the degradation of the elastic properties of a local region of the laminate by reversibly clamping its boundaries. In this paper, a numerical analysis of the sensitivity of the local-IET to the delamination damage mechanism is conducted. Firstly, a Finite Element (FE) model of the local-IET test is determined through experimental investigations on undamaged composite laminates, which cover a wide range and are made of glass or carbon fibers, through resin infusion or pre-preg consolidation and with unidirectional or fabric textures. The vibrational response of a glass fiber composite with local delamination is then assessed with the local-IET. By modeling the delamination in the simulation environment, the effectiveness of the FE model in replicating the vibrational response, even in the presence of delamination, is shown through a comparison with the experimental results. Finally, the FE model is exploited to perform a sensitivity analysis, showing that the technique is able to detect the presence of delamination. [ABSTRACT FROM AUTHOR]

Delamination is one of the most common types of damage in fiber-reinforced composites. In this paper, borrowing from the concept of defect modes in imperfect phononic crystals, we propose a method for identifying delamination damages in composite materials. Specifically, by periodically arranging concentrated masses on the surface of a finite carbon fiber beam, the carbon fiber beam becomes a phononic crystal, and defect modes in Bragg bandgaps can be observed when there is delamination. With the spectral element method formulation, we show that the frequencies of defect modes are directly related to the length and location of the delamination. By performing algebraic addition and subtraction on the generated defect mode frequencies when applying excitations at the two ends of a defected carbon fiber composite beam, an approximately linear correlation with the length and location of delamination can be obtained. Our analytical and experimental results indicate that, by introducing periodicity on the carbon fiber composite beam and applying appropriate post-processing algorithm to the generated defect modes, rapid and accurate identification of the delamination length and location can be achieved based on the bandgap characteristics of the imperfect phononic crystals. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the R-curve effect in delamination propagation in composite materials. The R-curve effect refers to the phenomenon whereby resistance to crack propagation increases with the advancement of the delamination, due to toughening mechanisms, such as fiber bridging. Numerical models often neglect this effect assuming a constant value of the fracture toughness. A numerical approach based on cohesive elements and on the superposition of two bilinear traction-separation laws is adopted here to accurately predict the R-curve effect in skin-doubler composite specimens subjected to three-point bending tests. The carbon-epoxy material presents two different sensitivities to the fiber bridging phenomenon resulting in two different R-curves. Comparisons with literature experimental data, in terms of load and delaminated area vs. applied displacement, and ultrasonic C-scan images show the effectiveness of the adopted approach in simulating the R-curve effect. The predicted numerical stiffness aligns with the experimental scatter, although the maximum load is slightly underestimated by approximately 15% compared with the average experimental results. The numerical model accurately predict the R-curve effect observed in the experimental data, demonstrating a 31% increase in the maximum load for the material configuration exhibiting greater sensitivity to fiber bridging. [ABSTRACT FROM AUTHOR]

To investigate the failure mechanism of the composite T-joints subjected to low-velocity-impact (LVI) loading varying impact locations, a methodology composed of improved conventional vacuum-assisted resin infusion (VARI) and LVI testing based on ASTM D 7136 is developed and employed. Moreover, the simulation is conducted to mainly reveal the damage evolution of interlaminar delamination between web and web/skin laminate based on surfacebased cohesive behavior in the commercial software of Abaqus 2020. Further, the effect of LVI loading with various distances from the web laminate to the impact location on the mechanical property and failure mode of the composite T-joints are presented and discussed as well. By comparison, larger vertical cracks and higher residual tensile strength are found in the T-joints impacting the corner. However, the T-joints with the impact point near the corner have lower residual tensile strength and suffer more interlaminar delamination in the interface between web and skin, which is the most critical position of failure. Highlights • GFRP T-joints are manufactured by the vacuum-assisted resin infusion process with a newly designed mold. • The damage evolution in low-velocity-impact tests is presented via the quasi-static tests and finite element simulation. • Larger vertical cracks and higher residual tensile strength are found in the T-joints impacting the corner. • The T-joints with the impact point near the corner have lower residual tensile strength. • Interlaminar delamination in the interface between web and skin is the most critical factor of tensile failure. [ABSTRACT FROM AUTHOR]

Nondestructive evaluation (NDE) techniques that use nonlinear wave-damage interactions have gained significant attention recently due to their improved sensitivity in detecting incipient damage. This study presents the use of finite element (FE) simulation with the experimental investigation to quantify the effects of guided waves' propagation through multiple delaminations in unidirectional glass fiber-reinforced polymer (GFRP) composites. Further, it utilizes the outcomes of nonlinear interactions between guided waves and delaminations to locate the latter. This is achieved through probabilistic Bayesian updating with a structural reliability approach. Guided waves interacting with delaminations induce nonlinear acoustic signatures that can be quantified by the nonlinearity index (NLI). The study found that the NLI changes with the interrogation frequency, as confirmed by numerical and experimental observations. By using the numerical outcomes obtained from the nonlinear responses, a Bayesian model-based approach with subset simulation is proposed and subsequently used to locate multiple delaminations. The results indicate that both the log-likelihood and log-evidence are key factors in determining the localization phenomenon. The proposed method successfully localizes multiple delaminations and evaluates their number, interlaminar position, width, and type. [ABSTRACT FROM AUTHOR]

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Localization of the delamination is an essential task that is conducted by various destructive and non-destructive approaches, which may require time, experts, and cost. Various intelligent non-destructive techniques are utilized to reduce time consumption, the need for expertise, and expenditures for localizing compositing delaminations. Yet, developing an accurate, robust, and low-cost intelligent delamination identification technique becomes a challenging task due to the anisotropy and the variation in the fiber orientation of the composites. Based on those issues, it is aimed to develop an effective intelligent model to localize delaminations in composite plates. This study measures the performance of the Bagging and Boosting techniques on delamination localization in thin composite plates. To validate the effectiveness of the proposed approaches; cross-ply, angle-ply, and quasi-isotropic composite plates having 2400 different delamination cases are considered. The bagging and boosting models are trained with the vibrational characteristics of the healthy and delaminated composite structures. The free vibration analysis is conducted for those structures to obtain the first five natural frequencies and the corresponding mode shapes. For this purpose, Classical Plate Theory is employed by using Finite Element Analysis. It is concluded that both bagging and boosting techniques are robust, precise, and accurate in localizing delamination. [ABSTRACT FROM AUTHOR]