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Highlights The application of carbon fiber reinforced polymer (CFRP) in aerospace, automotive and other fields is expanding rapidly due to its high strength‐to‐weight ratio and excellent mechanical properties. However, the susceptibility to delamination damage in the thickness direction still needs to be addressed urgently. In this study, carbon fiber prepregs were combined with short‐cut aramid fiber (SAFs) veils through hot compression molding to produce SAFs‐interlaminar‐toughened carbon fiber reinforced polymer (SAFs‐CFRP). The effect of the areal density of the SAFs on mechanical properties and model II interlaminar fracture toughness (IFT) was investigated. The results demonstrate that at a fiber areal density of 15 g/m2, tensile and flexural strengths increased by 33.96% and 34.39%, respectively, compared to the untoughened specimens. Additionally, model II IFT improved by 108.05%, reaching 9.05 KJ/m2, a significant enhancement over the untoughened specimens. Cross‐sectional fracture analysis revealed that the toughening mechanism involved fiber bridging, fiber pull‐out, fiber breakage, and fiber debonding of the SAFs within the resin‐rich layers. These processes effectively hinder crack propagation and absorb energy, contributing to the enhancements in toughness and mechanical strength. This study provides a comprehensive framework for interlaminar toughening using SAFs, addressing the critical challenge of delamination and paving the way for CFRP materials with enhanced performance and reliability in advanced engineering applications. Effective CFRP interlayer toughening was achieved using wet veil‐formed SAFs. The model II IFT of CFRP was significantly increased by 108.05%. The tensile and flexural properties of CFRP were also enhanced. [ABSTRACT FROM AUTHOR]

CFRP laminates are commonly used in aircraft structures due to their high structural efficiency. The phenomenon-based failures of CFRP laminates have poor predictability for the complex failure process with buckling and matrix fractures. This work carried out seven shear tests of CFRP laminates. It also attempts to reveal its failure characteristic points by directly modeling the measured strain data based on the relative deformation distribution catastrophe. The failure characteristic points are revealed by constructing the stressing state modes/parameters and applying the clustering analysis criterion, whose accuracy can be verified by comparing the shear failure characteristic points of nondestructive/impacted CFRP laminates. HIGHLIGHTS: Conduct 7 nondestructive/impacted CFRP laminates shear tests. Construct the modes and characteristic parameters characterizing the shear-stressing state of CFRP laminates. Applying clustering analysis criterion to reveal the characteristic loads of CFRP laminates. Verify the accuracy and robustness of the characteristic loads. [ABSTRACT FROM AUTHOR]

The evaluation of carbon fiber reinforced polymers (CFRP) laminates during and after the Mode I test was successfully conducted, integrating non-destructive testing of acoustic emission (AE) and ultrasound scanning. Two different specimens [+45°/-45°/0°]2S and [0°/+45°/-45°]2S were used to detect the effects of stacking sequences of the laminates. Results indicated that applying AE sensors to the specimens slightly affect to the laminate performance. Thus, the laminates are validated and showed that for [+45°/-45°/0°]2S laminates, the system can withstand load and increase the displacement at break more than twice of [0°/+45°/-45°]2S laminates. Moreover, the ultrasound scanning showed that the crack trace is visible. [+45°/-45°/0°]2S laminates have smaller crack around 24 mm compared to [0°/+45°/-45°]2S laminates with 30 mm. Image analysis revealed that after specimen are forced to open, the [+45°/-45°/0°]2S laminates can prevent long crack compared to [0°/+45°/-45°]2S laminates. The double cantilever beam (DCB) test, employing various stacking sequences, demonstrated excellent examination results using non-destructive testing. Theoretical calculations regarding residual thermal expansion due to different coefficients of thermal expansion also revealed a slight impact of varying manufacturing temperatures on the laminates. These findings offer valuable insights for detecting, predicting, and preventing specimen failures in aircraft and aerospace structures without resorting to destructive examinations, facilitating appropriate preventive maintenance.

The verification of fiber orientation and layup in carbon fiber-reinforced polymer (CFRP) composite laminates is essential to guarantee the performance of the material. This work reports the background, methodology, and results of a rotational eddy current testing (ECT) system for determining fiber orientation and ply layup of unidirectional CFRP components. The system presented mechanically rotates a 15 MHz directional transmit-receive ECT probe, which poses several speed and consistency advantages. The paper presents a theoretical discussion of how these scans are produced, a unique preprocessing normalization method, and an optimization process for determining the layup from the captured datasets. Results show a high level of accuracy for identifying the unique directions present in a laminate, and the theory-supported model enables the extraction of layup order information of multilayer parts, even where repeated lamina are present within the laminate. [ABSTRACT FROM AUTHOR]

Highlights An interlaminar ultrafine spraying method was proposed for monolayer graphene oxide (GO) modified CFRP nanocomposite laminates. The well‐dispersed monolayer GO nano‐solutions were prepared by multi‐level dispersion and then sprayed on carbon fiber/epoxy prepreg by the ultrafine atomizing technique. A series of GO/CFRP laminate specimens with different GO loadings were fabricated for our fracture toughness tests and SEM characterization. The test results indicated that the Mode I fracture toughness (GIC$$ {G}_{\mathrm{IC}} $$) was enhanced by 140% with quite a low fraction of monolayer GO nanosheets. The distinct toughening effect at such a low level of nano‐contents was attributed to the sufficient quantity of monolayer GO nanosheets and also a uniform distribution, which was found to be more important than the volume/weight fraction as the principal structural parameter. Thus, the proposed interlaminar toughening approach owns the virtues of good effectiveness and especially low cost owing to the largely reduced weight percentage of nanographene, showing a promising potential of industrial scale‐up. Proposed an interlaminar ultrafine spraying method for monolayer GO nanosheets. Found that the quantity of GO nanosheets is vital in polymer toughening. Realized significant toughening with extremely low loadings of monolayer GO. Provided a way with high toughening effect and low cost for industrial scale‐up. [ABSTRACT FROM AUTHOR]

In instances of impact, the preeminent load-bearing framework for on-board hydrogen storage cylinders manifests in the carbon fiber reinforced polymer (CFRP) composite layer. Investigating the morphology of impact and elucidating the mechanism governing damage evolution in CFRP is crucial for optimizing the impact resistance of the cylinder. In this work, impact tests were performed on CFRP laminates with six types of interlaminar mismatch angles, under varying impact energies. The impact damage of laminates was characterized using an extended depth-of-field 3D microscopy. A numerical model, implemented in ABAQUS/Explicit, was devised to scrutinize the mechanical properties and damage mechanisms in laminates. A subroutine (VUMAT) was crafted to effectively predict intralaminar damage, while the application of a bilinear cohesive model served to capture interlaminar damage. In addition, this study delves into the ramifications of impact energy and interlaminar mismatch angles. The results reveal that the increase of impact energy leads to more irreversible damage and energy dissipation within laminates. Compared to damage depth (D) and cross-sectional area (A c), the assessment of damage volume (V) and surface area (A s) are more reflective of energy dissipation in laminates. The laminates with mismatch angle of 0° and "helicoidal" lay-up sequence exhibit the worst impact resistance. Within the range of 24°–90°, laminates with a smaller interlaminar mismatch angle demonstrate better impact resistance. These findings lay the groundwork for optimizing the composite layer to enhance the impact resistance of hydrogen storage cylinders. [Display omitted] • The damage surface area and volume were used as new characterization parameters. • A user-defined subroutine VUMAT involving intralaminar damage model was developed. • The mechanical response and damage mechanism of CFRP laminates were studied. • Effects of impact energy and interlaminar mismatch angle were investigated. [ABSTRACT FROM AUTHOR]

The study considers the delamination resistance of carbon/epoxy laminates modified with Thermoplastic Nanoparticles of Polysulfone (TNPs). A new electrospinning nanofiber technique was utilized to convert polysulfone polymer into nanoparticles and uniformly disperse them within the resin. Fracture toughness was evaluated under loading modes I and II. In mode I, the toughness (GIC) increased significantly from 170 to 328 J/m² with TNPs incorporation. However, mode II showed minimal change, with GIIC values of 955 J/m² for virgin and 950 J/m² for TNPs-modified specimens. Scanning Electron Microscopy (SEM) was employed to depict the influence of TNPs on damage characteristics and crack propagation patterns. In mode I, crack deviation enhanced toughness as TNPs bypassed the PSU, while in mode II, cracks propagated through TNPs, resulting in particle smearing on the epoxy surface. This highlights TNPs' potential to modify the fracture toughness in mode I loading, but their effect is constrained in mode II loading scenarios.

Experimental and simulation studies of the response of bonded‐repaired and scarf‐repaired carbon fiber‐reinforced polymer laminates under the low‐velocity impact (LVI) were carried out. The center‐ and eccentric‐repaired laminates were prepared for experiments. The experimental results verify the validity of the finite element model in terms of the force response and different modes of damage. The effect of impact energy and the variation of repair position on the impact response of the laminate was investigated based on the benchmark model. The bonded repaired laminates have higher peak impact forces and lower energy absorption than the scarf‐repaired laminates; the damage of the bonded repaired laminate is mainly intralaminar damage and delamination damage of the patch; the adhesive of the scarf repair is prone to failure, leading to poor impact resistance. The relative positions of the impact point and the patch determine the impact response of a laminate repaired at different locations. The impact point at the edge of the patch has a detrimental effect on the repaired laminate. The impact point vertical position near the bottom hole edge of the parent plate in the scarf‐repaired case harms the impact resistance of the laminate. Highlights: The low‐velocity impact response of bonded versus scarf‐repaired laminates was compared.Bonding repairs provide superior impact resistance to damaged laminates than scarf repairs.The risk of adhesive failure in scarf repair is significantly higher than in bonded repair.The relative position of the damage (repair) to the impact point critically influences the impact response.Impacts near the edge of the initial damage hole can cause severe damage to scarf‐repaired laminates. [ABSTRACT FROM AUTHOR]

In the process of preparation and service, the carbon fiber reinforced polymer (CFRP) material is prone to defects such as delamination and inclusions, which seriously impact the normal use. In this paper, non-linear frequency-modulated (the logarithmic sweep) thermal excitation is used to carry out finite element simulation and experimental research. In order to investigate how different detection process parameters affect defects, the impact of different detection parameters on the surface temperature difference and contrast is analyzed, and the detection parameters that can quickly identify defects and have better recognition effect are obtained. [ABSTRACT FROM AUTHOR]

There is limited research on the fracture toughness of carbon-fiber-reinforced polymer (CFRP) materials under accelerated UV aging conditions. In this study, the primary focus was on investigating the influence of varying durations of ultraviolet (UV) irradiation at different temperatures on the Mode I, Mode II, and mixed-mode fracture toughness of CFRP laminates. The results indicate that with increasing UV aging duration, the material's Mode I fracture toughness increases, while Mode II fracture toughness significantly decreases. The mixed-mode fracture toughness exhibits an initial increase followed by a subsequent decrease. Furthermore, as the aging temperature increases, the change in the fracture toughness of the material is more obvious and the rate of change is faster. In addition, the crack expansion of the composite layer of crack-containing Type IV hydrogen storage cylinders was analyzed based on the extended finite element method in conjunction with the performance data after UV aging. The results reveal that cracks in the aged composite material winding layers become more sensitive, with lower initiation loads and longer crack propagation lengths under the same load. UV aging diminishes the overall load-bearing capacity and crack resistance of the hydrogen storage cylinder, posing increased safety risks during its operational service. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) composites offer a corrosion-resistant, lightweight, and durable alternative to traditional steel material in concrete structures. However, the lack of established inspection methods for assessing reinforced concrete elements with externally bonded FRP (EB-FRP) composites hinders industry-wide confidence in their adoption. This study addresses this gap by investigating non-destructive testing (NDT) techniques for detecting damage and defects in EB-FRP concrete elements. As such, this study first identified and categorized potential damage in EB-FRP concrete elements considering where and why they occur. The most promising NDT methods for detecting this damage were then analyzed. And lastly, experiments were carried out to assess the feasibility of the selected NDT methods for detecting these defects. The result of this study introduces infrared thermography (IR) as a proper method for identifying defects underneath the FRP system (wet lay-up). The IR was capable of highlighting defects as small as 625 mm2 (1 in.2) whether between layers (debonding) or between the substrate and FRP (delamination). It also indicates the inability of GPR to detect damage below the FRP laminates, while indicating the capability of PAU to detect concrete delamination and qualitatively identify bond damage in the FRP system. The outcome of this research can be used to provide guidance for choosing effective on-site NDT techniques, saving considerable time and cost for inspection. Importantly, this study also paves the way for further innovation in damage detection techniques addressing the current limitations. [ABSTRACT FROM AUTHOR]

Carbon-fiber-reinforced polymer (CFRP) composites have many advantages, and have been widely used in aerospace structures, buildings, bridges, etc. The analysis of dynamic response characteristics of CFRP composite structures is of great significance for promoting the development of smart composite structures. For this reason, vibration experiments of CFRP laminates with surface-attached fiber Bragg grating (FBG) sensors under various dynamic loading conditions were carried out. Time- and frequency-domain analyses were conducted on the FBG testing signals to check the dynamic characteristics of the CFRP structure and the sensing performance of the installed sensors. The results show that the FBG sensors attached to the surface of the CFRP laminates can accurately measure the dynamic response and determine the excited position of the CFRP laminates, as well as invert the strain distribution of the CFRP laminates through the FBG sensors at different positions. By performing Fourier transform, short-time Fourier transform, and frequency domain decomposition (FDD) on the FBG sensing signals, the time–frequency information and the first eight modal frequencies of the excited CFRP structure can be obtained. The modal frequencies obtained by different excitation types are similar, which can be used for structural damage identification. The research in this paper clarifies the effectiveness and accuracy of FBG sensors in sensing the dynamic characteristics of CFRP structures, which can be used for performance evaluation of CFRP structures and will effectively promote the design and development of intelligent composite material structures. [ABSTRACT FROM AUTHOR]

Strengthening of composite beams is highly needed to upgrade the capacities of existing beams. The strengthening methods can be classified as active or passive techniques. Therefore, the main purpose of this study is to provide detailed FE simulations for strengthened and unstrengthened steel–concrete composite beams at the sagging and hogging moment regions with and without profiled steel sheeting. The developed models were verified against experimental results from the literature. The verified models were used to present comparisons between the effect of using external post-tensioning and CFRP laminates as strengthening techniques. Applying external post-tensioning at the sagging moment regions is more effective because of the exhibited larger eccentricity. In the form of an initial camber and compressive stresses in the bottom flange prior to loading, this reasonable eccentricity induces reverse loading on the reinforced beams, reducing the net tensile stress induced during loading. Using CFRP laminates on the concrete slab for continuous composite beams is more effective in enhancing the beam capacity in comparison with using the external post-tension. However, reductions in the beam ductility were obtained.

To investigate the delamination characteristics of CFRP laminates in barely visible impact damage (BVID) state, low-velocity impact tests (LVIT) and nondestructive 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. [ABSTRACT FROM AUTHOR]

The feedstock recycling of carbon fiber reinforced plastics (CFRPs) is typically conducted by either thermal decomposition or dissolution methods for resin separation. A common issue in the recycling processes is that neither oxygen nor solvents can easily penetrate dense CFRP; therefore, penetrating diffusion paths such as voids is expected to improve the separation efficiency. This study presents voids formation characteristics by low-temperature annealing as pre-treatment in typical epoxy-based cross-ply CFRP laminates. The void characteristics were evaluated by electrical treatment and observations after the annealing in the air or N2 atmosphere. Annealing in air resulted primarily in ply delamination and matrix cracking through slight oxidative reactions. By contrast, annealing in the N2 atmosphere denatured the epoxy resin, resulting in the formation of voids and swelling that reached the interior of the specimens. The void characteristics could be controlled by the atmosphere in the low-temperature annealing, leading to the penetrating diffusion paths. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study about the use of innovative bio-based pultruded carbon-fiber-reinforced polymer (CFRP) laminates for structural strengthening. The bio-based laminates were produced in the framework of an applied research project (BioLam) using a resin system with 50% (wt.%) bio-based content, obtained from renewable resources. In the first part of the study, their tensile and interlaminar shear properties were characterized and compared with those of conventional oil-based CFRP laminates. In the second part of the study, the bond behavior to concrete of both types of CFRP laminates applied according to the externally bonded reinforcement (EBR) technique was assessed by means of single-lap shear tests performed on CFRP-strengthened concrete blocks; the experimental results obtained from these tests were then used in a numerical procedure to calibrate local bond vs. slip laws for both types of laminates. The final part of this study comprised four-point bending tests on full-scale EBR-CFRP-strengthened reinforced concrete (RC) beams to assess the structural efficacy of the bio-based laminates; these were benchmarked with tests performed on similar RC beams strengthened with conventional CFRP laminates. The results obtained in this study show that the (i) material properties, (ii) the bond behavior to concrete, and (iii) the structural efficacy of the developed bio-based CFRP laminates are comparable to those of their conventional counterparts, confirming their potential to be used in the strengthening of RC structures. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer (CFRP) laminates are modified to enhance their suitability for various thermal applications. A synergistic approach utilizing the effect of various conductive and insulative modifiers with diglycidyl ethers of bisphenol A (DGEBA) epoxy resin and/carbon fiber (CF) is explored. In CFRP laminates developed after modifications made in epoxy resin using a thermoplastic material, such as polycarbonate (PC) and/or acrylonitrile butadiene styrene (ABS), exhibit high thermal resistance (TR) of 77.1% compared to unmodified CFRP. In contrast, modifications made using conductive mediums like phosphonium (P), imidazolium (I), or silanized‐graphene oxide (SGO) have lower TR of 25.7%, 30.5%, and 32.4%, respectively. A temperature gradient (TG) enhancement of 75% is reported for the 1.5 wt% PC/ABS modified CFRP laminates. On the contrary, modifications using 0.5 parts per hundred (phr)P, 0.5 phr I, and 1 g L−1 SGO in epoxy reduce the TG by 25%, 30%, and 32%, respectively. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses are done to explore the thermal characteristics of each case of modification. Finally, scanning electron microscopy images confirm the distribution profile of the modifiers used. Based on the types of modifications performed, the current study can offer insightful information on the thermal performances of modified CFRP laminates. [ABSTRACT FROM AUTHOR]

Strengthening of composite beams is highly needed to upgrade the capacities of existing beams. The strengthening methods can be classified as active or passive techniques. Therefore, the main purpose of this study is to provide detailed FE simulations for strengthened and unstrengthened steel–concrete composite beams at the sagging and hogging moment regions with and without profiled steel sheeting. The developed models were verified against experimental results from the literature. The verified models were used to present comparisons between the effect of using external post-tensioning and CFRP laminates as strengthening techniques. Applying external post-tensioning at the sagging moment regions is more effective because of the exhibited larger eccentricity. In the form of an initial camber and compressive stresses in the bottom flange prior to loading, this reasonable eccentricity induces reverse loading on the reinforced beams, reducing the net tensile stress induced during loading. Using CFRP laminates on the concrete slab for continuous composite beams is more effective in enhancing the beam capacity in comparison with using the external post-tension. However, reductions in the beam ductility were obtained. [ABSTRACT FROM AUTHOR]

The impact responses of a patch-repaired carbon-fiber-reinforced polymer (CFRP) specimen under double impacts were compared to study the damage interference mechanism through the combination of experiment and numerical analysis. A three-dimensional finite element model (FEM) with iterative loading based on continuous damage mechanics (CDM) and a cohesive zone model (CZM) was employed to simulate the double-impacts testing with an improved movable fixture at an impact distance of 0 mm–50 mm. The influence of impact distance and impact energy on the damage interference was explored by mechanical curves and delamination damage diagrams of the repaired laminates. When impactors fell within the range of the patch with an impact distance of 0 mm–25 mm at a low level of impact energy, delamination damage of the parent plate caused by the two impacts overlapped, resulting in damage interference. With the continuing increase in impact distance, the damage interference gradually disappeared. When impactors fell on the edge of the patch, the damage area caused by the first impact on the left half of the adhesive film gradually enlarged, and as the impact energy increased from 5 J to 12.5 J, the damage interference caused by the first impact on the second impact was gradually enhanced. [ABSTRACT FROM AUTHOR]

Image-based methods have been applied to support structural monitoring, product and material testing, and quality control. Lately, deep learning for compute vision is the trend, requiring large and labelled datasets for training and validation, which is often difficult to obtain. The use of synthetic datasets is often applying for data augmentation in different fields. An architecture based on computer vision was proposed to measure strain during prestressing in CFRP laminates. The contact-free architecture was fed by synthetic image datasets and benchmarked for machine learning and deep learning algorithms. The use of these data for monitoring real applications will contribute towards spreading the new monitoring approach, increasing the quality control of the material and application procedure, as well as structural safety. In this paper, the best architecture was validated during experimental tests, to evaluate the performance in real applications from pre-trained synthetic data. The results demonstrate that the architecture implemented enables estimating intermediate strain values, i.e., within the range of training dataset values, but it does not allow for estimating strain values outside those range. The architecture allowed for estimating the strain in real images with an error ∼0.5%, higher than that obtained with synthetic images. Finally, it was not possible to estimate the strain in real cases from the training performed with the synthetic dataset. [ABSTRACT FROM AUTHOR]

Delamination, a prevalent failure mode observed in laminated composites, exerts a significant impact on structural integrity and performance. The occurrence of fiber bridging during the fracture process adds complexity and elevates the research challenges associated with this phenomenon. Existing models exhibit limitations in accurately capturing bridging behavior and discerning its underlying mechanical mechanisms. This study addresses these limitations by analyzing experimental results, employing the J -integral, and analyzing R -curve behavior, proposing a mechanism-based four-linear cohesive zone model along with a new finite element implementation method. Comprising three overlapping bi-linear CZMs, this model effectively simulates mode I fracture behavior in laminates with different stacking sequences. Moreover, it intuitively illustrates the mechanical mechanisms during crack propagation and offers simplicity in implementation. This research contributes to a deeper understanding of composite fracture mechanics and provides a practical model for predicting delamination behavior in laminated structures. [Display omitted] • Ply angle variation affects initial and propagation fracture toughness. • A mechanics-based four-linear constitutive model is proposed. • Overlapping cohesive elements simulate various mechanisms in fracture processes. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) composites offer a corrosion-resistant, lightweight, and durable alternative to traditional steel material in concrete structures. However, the lack of established inspection methods for assessing reinforced concrete elements with externally bonded FRP (EB-FRP) composites hinders industry-wide confidence in their adoption. This study addresses this gap by investigating non-destructive testing (NDT) techniques for detecting damage and defects in EB-FRP concrete elements. As such, this study first identified and categorized potential damage in EB-FRP concrete elements considering where and why they occur. The most promising NDT methods for detecting this damage were then analyzed. And lastly, experiments were carried out to assess the feasibility of the selected NDT methods for detecting these defects. The result of this study introduces infrared thermography (IR) as a proper method for identifying defects underneath the FRP system (wet lay-up). The IR was capable of highlighting defects as small as 625 mm2 (1 in.2) whether between layers (debonding) or between the substrate and FRP (delamination). It also indicates the inability of GPR to detect damage below the FRP laminates, while indicating the capability of PAU to detect concrete delamination and qualitatively identify bond damage in the FRP system. The outcome of this research can be used to provide guidance for choosing effective on-site NDT techniques, saving considerable time and cost for inspection. Importantly, this study also paves the way for further innovation in damage detection techniques addressing the current limitations.

Specific nano- and micro-scale morphologies of composites can affect the resulting optical and UV reflectivity of the materials. One example is “Vantablack®” made from aligned carbon nanotubes (CNTs) with 99.96% absorption. A similar material with CNTs grown on surface-activated aluminum (CNTs/sa-Al) even yielded 99.995% absorption, one order of magnitude higher than Vantablack®. On the other hand, fresh snow reflects 90% or more of the incident electromagnetic radiation with wavelengths between 400 and 1,000 nm. The reflectivity of snow originates from multiple scattering in the porous morphology made of snow grains. Taking these complex morphologies as inspiration, CFRP epoxy composites with different types, sizes, shapes, and amount of nanoparticles are prepared and compared regarding their optical and ultraviolet (UV) reflectivity. Increasing the reflectivity in the near and far UV may be beneficial for the durability of the epoxy composites, but selective higher or lower reflectivity in certain wavelength ranges may also yield tailored visual effects. Results from different processing approaches with selected nanoparticles are presented and discussed.

Hail ice impact and the induced damage are a real threat to the exposed composite laminates in the aircraft structures. In the present work, a nonlinear finite element (FE) model is proposed to investigate the mechanical behavior of carbon fiber reinforced plastic (CFRP) laminates under hail ice impact. Lagrangian FE and smooth particle hydrodynamics method (FE-SPH) is comprehensively applied to reproduce the impact behavior of hail ice and the equation of state (EoS) for liquid is introduced to describe the flow characteristics of hail failure. A rate-dependent constitutive model is adopted to identify the intra-laminar response of laminates and a bilinear cohesive zone model is used for modeling delamination of interface. A subroutine VUMAT is coded and executed to attain the numerical solution based on ABAQUS/Explicit solver. The transient dynamic process of CFRP laminates subjected to hail impact is replicated in detail. The failure threshold velocity (FTV) model and damage characteristics of CFRP laminates under different impact conditions are investigated thoroughly. The numerical results are mostly consistent with the available experimental data thus validates the effectiveness of the proposed hail ice impact model. [ABSTRACT FROM AUTHOR]

In this study, hybrid nonlinear finite element models (FEM) were developed to examine the flexural performance and the ultimate load capacity of bonded and unbonded two-way reinforced concrete post-tensioned (PT) slabs that were pre-strengthened with external carbon-fiber reinforcement polymer (CFRP) laminates. Full 3D simulations, using ANSYS models, have been created for five different slab samples that were selected from a previously available experimental study. The model results were assessed to enable further numerical analysis. The result calibration included measurements of first crack loads, ultimate loads, deflections, strains in the extreme fiber of concrete, strains in CFRP laminates, and failure modes. The results proved a good correlation between FEM output and experimental ones. Based on this, the influencing parameters that affect plate stiffness, as well as the bending capacity of PT slabs, were examined by performing a detailed parametric study. The parameters included real-life load simulation, cable-to-CFRP strength contribution, and CFRP laminate location selection. The results demonstrated that strengthening using CFRP laminates have significantly increased the ductility index of both bonded and unbonded PT concrete slabs by 62.18% and 59.87%, respectively. In addition, strip strengthening locations near supports are much more effective than in the middle of slabs. Additionally, the CFRP strengthening contribution is very considerable in slabs with low PT ratios. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced plastic (CFRP) structures are vulnerable to low-speed impacts, which will lead to almost invisible impact damage. Therefore, the timely localization of impact is of great significance to damage detection and maintenance of the structure. In this article, a low velocity impact supervisory and testing system based on fiber Bragg grating (FBG) sensors was built up for CFRP laminates to obtain the low velocity impact strain sensitivity model. Meanwhile, genetic algorithm was applied to optimize the configuration of the FBG sensing network. The eigenvectors of the impact signals were extracted by applying fast Fourier transform (FFT) transform and principal component analysis (PCA) technology used as the input of the back propagation (BP) neural network model, while the corresponding impact coordinates were used as the output, to train the model. After training, the impact position prediction model based on BP neural network was obtained, thereby achieving the impact localization for CFRP laminates successfully with an average localization error of 2.1 cm. [ABSTRACT FROM AUTHOR]

The huge amounts of old and damaged tires spread worldwide has caused many complex environmental risks. The old tires have been converted to crumb rubber (CR) and tire recycled steel fiber (RSF) to facilitate their use. This study used CR to partially replace natural sand in reinforced (RC) columns. Externally bonded (EB) carbon-fiber-reinforced polymer (CFRP) laminates, welded wire mesh (WWM), and RSF were used to enhance the axial behavior of the tested columns to overcome the concrete deficiencies resulting from the inclusion of the CR instead of natural sand. Eighteen columns were prepared and tested to discuss the effects of strengthening type, CR content, RSF, and strengthening area on the axial behavior of the RC columns. Certain columns were internally reinforced with WWM, while others were externally strengthened with EB CFRP laminates. Partially or fully EB CFRP laminates were used to strengthen the columns. Moreover, one column was cast with NC and 0.2% RSF to investigate the role of RSF in confining the column. The results demonstrated a concrete strength reduction for the rubberized concrete (CRC) as the CR content increased. Conversely, the strengthened columns experienced higher load capacities than the corresponding un-strengthened ones cast with the same concrete mix. Moreover, adding 2% RSF to the NC mix could enhance the column capacity, although it decreased the concrete strength. Furthermore, using two CFRP layers increased the load capacity and ductility of the strengthened columns. The strengthened column cast with 50% CR showed the highest load efficiency (334.3% compared to the un-strengthened one). [ABSTRACT FROM AUTHOR]

In order to understand the ablation behaviors of CFRP laminates in an atmospheric environment irradiated by continuous wave laser, CFRP laminates were subjected to a 1080-nm continuous wave laser (6-mm laser spot diameter), with different laser power densities carried out in this paper. The internal delamination damage in CFRP laminates was investigated by C-Scan. The rear- and front-face temperature of CFRP laminates were monitored using the FLIR A 655 sc infrared camera, and the rear-face temperature was monitored by K type thermocouples. The morphology of ablation damage, the area size of the damaged heat affected zone (HAZ), crater depth, thermal ablation rate, mass ablation rate, line ablation rate, etc., of CFRP laminates were determined and correlated to the irradiation parameters. It is found that the area size of the damage HAZ, mass ablation rate, line ablation rate, etc., increased as the laser power densities. The dimensionless area size of the damaged HAZ decreased gradually along the thickness direction of the laser irradiation surface. [ABSTRACT FROM AUTHOR]

Carbon-fiber-reinforced polymer (CFRP) composites have many advantages, and have been widely used in aerospace structures, buildings, bridges, etc. The analysis of dynamic response characteristics of CFRP composite structures is of great significance for promoting the development of smart composite structures. For this reason, vibration experiments of CFRP laminates with surface-attached fiber Bragg grating (FBG) sensors under various dynamic loading conditions were carried out. Time- and frequency-domain analyses were conducted on the FBG testing signals to check the dynamic characteristics of the CFRP structure and the sensing performance of the installed sensors. The results show that the FBG sensors attached to the surface of the CFRP laminates can accurately measure the dynamic response and determine the excited position of the CFRP laminates, as well as invert the strain distribution of the CFRP laminates through the FBG sensors at different positions. By performing Fourier transform, short-time Fourier transform, and frequency domain decomposition (FDD) on the FBG sensing signals, the time–frequency information and the first eight modal frequencies of the excited CFRP structure can be obtained. The modal frequencies obtained by different excitation types are similar, which can be used for structural damage identification. The research in this paper clarifies the effectiveness and accuracy of FBG sensors in sensing the dynamic characteristics of CFRP structures, which can be used for performance evaluation of CFRP structures and will effectively promote the design and development of intelligent composite material structures.

This paper aims at experimental and numerical research of delamination process in carbon/epoxy composite laminates with different fiber orientation angles in stacking sequence exhibiting the bending–twisting elastic couplings. Experimental specimens were subjected to the double cantilever beam (DCB) tests according to the ASTM D5528 regulations. Values of the mode I strain energy release rates were calculated by using three different data reduction schemes: the modified beam theory, the compliance calibration method and the modified compliance calibration. Determination of delamination initiation point was conducted in twofold way: by visual observation of crack tip using high resolution camera and by utilization of the acoustic emission technique. Numerical analyss were prepared in Abaqus/CAE Software environment by using the virtual crack closure technique (VCCT). The numerical beam model consisted of SC8R continuum shell elements. Obtained outcomes revealed that extensive fiber bridging phenomenon occurring during delamination process pronouncedly affected propagation values of strain energy release rate (GIprop) and numerically obtained load–displacement curves. Nevertheless, in initial stage of delamination, results obtained by using the VCCT were in agreement with experimental data. The greatest value of the mode I fracture toughness equal 0.56 N/mm was obtained for the BT45 laminate. [ABSTRACT FROM AUTHOR]

This work mainly focuses on the mode II interlaminar fracture behavior of carbon fiber composite that was interlaminar toughened by short aramid fiber modified epoxy resin. The short fibers were first dispersed in the epoxy matrix and then in the laminate interlayer area during the lamination process. The ENF test evaluated the Mode II interlaminar fracture toughness of laminates. The results show that mode II interlaminar fracture toughness increases with the addition of short fibers. When the fiber content is 10 g/m2, the toughening effect is the best at 2437.72 J/m2, which is about 61.8% higher than the untoughened sample. The toughening mechanism of CFRP laminates by adding short fibers was observed by mode II fracture surface SEM images. In addition, flexural properties and Charpy impact properties of short fiber modified resin interlayer toughened laminates were also studied. The results showed that with this method, flexural properties and impact strength were improved to a certain extent, proving that this method is an effective interlaminar toughening method. [ABSTRACT FROM AUTHOR]

This study investigates assessment of the barely visible impact damage (BVID) caused by low‐velocity impact (LVI) in single‐sided patch‐repaired orthotropic carbon fiber‐reinforced polymer (CFRP) laminates using experimental and numerical nonlinear Lamb wave‐mixing techniques. An accurate finite element method (FEM) approach consisting of three analysis steps of LVI, vibration damping, and nonlinear ultrasonic mixing detection is proposed to predict the relationship between LVI damage and nonlinear Lamb wave‐mixing propagation. The LVI step in the FEM simulation is verified by drop‐weight impact tests, and the nonlinear Lamb wave‐mixing detection step in FEM is verified by comparing with the RITEC RAM‐5000 SNAP nonlinear mixing detection system. The BVID of patch‐repaired specimens is assessed with mixing relative acoustic nonlinearity parameters (MRANPs). CFRP laminates repaired with various external patch parameters of different sizes, numbers of layers and stacking sequences are evaluated by the FEM approach. Finally, an optimized patch design in which the MRANPs are the smallest is determined. Therefore, the CFRP laminate containing a hole is recommended to be repaired with a circular three‐layer patch with a radius of 2.5 r and a stacking sequence of [90/90/90]. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted to determine the form and characteristics of damage that temporary fasteners may cause to the composite hole in the aviation temporary fastening process. According to the shapes of the temporary fastener clamping feet, copying indenters were designed. The similarity between the quasi-static indentation (QSI) experiment and the installation of temporary fasteners was discussed. The QSI experiment was used to investigate the influence of shapes of clamping feet, prepreg category, installation direction, and other factors on the failure of the composite hole, and the displacement-load curves were obtained. The damage morphology was observed with the optical microscope and ultrasonic microscope. The results show that the QSI experiment can simulate the temporary fastening damage well. Due to the expansion of delamination, and the final occurrence of fiber breakage in the composite hole, the bearing capacity is reduced. The temporary fastening damage process can be divided into three stages: in the elastic stage, the laminate does not suffer damage, and the surface resin appears slight dents; in the delamination stage, the layers delaminate at the edge of the contact surface, and expand in the direction of fiber; in the failure stage, sudden damage occurs suddenly under the indenter, and the fiber matrix is crushed into powder. The propagation range of the damage depends on the material properties and installation direction. Compared with T700/3234, T800/X850 has greater energy absorption before the damage. Laminates have the worst anti-intrusion ability out-of-plane in the surface fiber direction. These results increase the understanding of the out-of-plane compression damage of the composite material and provide a theoretical basis and method guidance for the control of the temporary fastening damage of the composite structures. The copying indenter designed in this paper can be used for further research on temporary fastening damage. [ABSTRACT FROM AUTHOR]

The strengthening of concrete structures with laminates of Carbon-Fiber-Reinforced Polymers (CFRP) is a widely adopted technique. retained The application is more effective if pre-stressed CFRP laminates are adopted. The measurement of the strain level during the pre-stress application usually involves laborious and time-consuming applications of instrumentation. Thus, the development of expedited approaches to accurately measure the pre-stressed application in the laminates represents an important contribution to the field. This paper proposes and benchmarks contact-free architecture for measuring the strain level of CFRP laminate based on computer vision. The main objective is to provide a solution that might be economically feasible, automated, easy to use, and accurate. The architecture is fed by digitally deformed synthetic images, generated based on a low-resolution camera. The adopted methods range from traditional machine learning to deep learning. Furthermore, dropout and cross-validation methods for quantifying traditional machine learning algorithms and neural networks are used to efficiently provide uncertainty estimates. ResNet34 deep learning architecture provided the most accurate results, reaching a root mean square error (RMSE) of 0.057‰ for strain prediction. Finally, it is important to highlight that the architecture presented is contact-free, automatic, cost-effective, and measures directly on the laminate surfaces, which allows them to be widely used in the application of pre-stressed laminates. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer (CFRP) laminates are featured by superior mechanical properties and excellent structural functions, which have been a promising alternative to conventional alloys and steels in diverse industrial fields. Mechanical drilling is a compulsory operation to shape fibrous composites to desired workpiece quality and target dimensional accuracy. Progressive tool wear is an extremely crucial issue when drilling CFRPs as it may result in undesired machining consequences such as increased force generation, elevated temperatures, and deteriorated surface quality. Despite great endeavors already made to address the wear issues, a complete review of the wear phenomena for CFRP drilling is still significantly lacking. The present paper aims to report the key research advances in tool wear aspects when drilling CFRPs. The wear phenomena, wear modes, and tool failure mechanisms are carefully reviewed. The effects of wear progression on various drilling outputs of CFRP laminates, such as drilling forces, cutting temperatures, and surface quality attributes, are discussed. A particular focus is placed on the quantification and assessment of tool wear during drilling CFRPs. Some research prospects for both academia and industry are outlined. The paper intends to offer a comprehensive understanding of wear mechanisms controlling the drilling of CFRP laminates.

Fracture fatigue entropy (FFE) is considered to be independent of the loading amplitude in fatigue experiments, and this conclusion has been applied to fatigue life prediction of composites and metals with satisfactory prediction accuracy. However, the existing research work mainly focuses on high-cycle fatigue, and it is not clear whether the FFE is independent of the loading amplitude in low-cycle fatigue, which requires more in-depth research. Fatigue experiments were conducted on three CFRP laminates with different ply orientations, and the FFE values corresponding to low-cycle fatigue were obtained by thermographic analysis. The results show that the FFE value is no longer independent of the loading amplitude under low-cycle fatigue conditions. On the contrary, it decreases with the increase of fatigue loading amplitude and also varies with different ply orientations. The relationship between low-cycle fracture fatigue entropy and loading amplitude of composites with different ply orientations was analyzed and modeled, and a novel low-cycle fatigue life prediction method based on FFE was developed. • FFE of CFRP under the LCF loading is no longer independent of loading amplitude. • The value of FFE for LCF decreases with the increase of ply angle. • The relationship between FFE, loading amplitude and ply angle is modeled. • A novel method is developed to predict LCF life based on the concept of FFE. [ABSTRACT FROM AUTHOR]

This paper examines the impact of flexural strengthening on the percentage of damaged strands in internally unbonded tendons in partially prestressed concrete beams (0, 14.28%, and 28.57%) and the recovering conditions using CFRP composite longitudinal laminates at the soffit, and end anchorage U-wrap sheets to restore the original flexural capacity and mitigate the delamination of the soffit of longitudinal Carbon Fiber Reinforced Polymer (CFRP) laminates. The composition of the laminates and anchors affected the stress of the CFRP, the failure mode, and thus the behavior of the beam. The experimental results revealed that the usage of CFRP laminates has a considerable impact on strand strain, particularly when anchors are employed. The EB-CFRP laminates increased the flexural capacity by approximately 13%, which corresponds to strand damage of 14.28%, while flexural capacity increased by 9.3%, strand damage increased by 28.57% for members strengthened with laminates only, and around 21.58% and 16.85% for members reinforced with laminates and end anchorings. Quasi-experimental equations have been proposed to estimate the actual stress of untethered tendons considering the effect of CFRP laminates and final fixation winding. [ABSTRACT FROM AUTHOR]

This study is devoted to test the structural response of reinforced concrete horizontally curved box beams in presence of vertical and transverse opening restored by CFRP Laminates or Reactive Powder Concrete (RPC) technique and compared to that having opening but without restoring. Six horizontally circular curved box beams were casted and tested in this experimental work, three specimens with opening in vertical direction and the other three specimens including opening in transverse direction. Accordingly, each direction has three types of beams, first one including opening without restoring and used as a reference specimen, second one with opening restored by CFRP Laminates technique and third one with opening restored by Reactive Powder Concrete (RPC) technique. The test program includes the main variables; direction of opening. The beams were tested as a two-span continuous beam, each span represents a quarter circle, under the effect of two concentrated loads each load positioned at top face of midspan of the beam. The findings of the experiments showed that the use of Reactive Powder Concrete (RPC) and CFRP Laminates and techniques around opening notably increased the ultimate load capacity of specimens (CB2.V60.S1, CB3.V60.S2, CB5.T60.S1 and CB6.T60.S2) by about (18.5% and 15.3%,135% and 38.3%) respectively, when compared with specimens without restoring. Service mid-span deflection response not give clear effect, while service mid-span twisting was improved by about (55.28%, 13%, 10.73% and 2.37%) for all restored specimens (CB2.V60.S1, CB3.V60.S2, CB5.T60.S1 and CB6.T60.S2) respectively. Genially, significant increase in stiffness can be observed for restored specimens as a comparison with those without restoring. [ABSTRACT FROM AUTHOR]

This work studies the improvement in the inelastic mechanical property of steel-concrete composite beams strengthened by carbon-fiber-reinforced polymer (CFRP) laminates under a negative moment. First, the monotonic loading test was designed and performed on an inverted simply supported beam to simulate the negative moment regions of continuous composite beams. Second, the finite element analysis (FEA) on flexural capacity and cracking resistance in negative moment regions of the composite beams was carried out with ANSYS software. A series of mechanical indexes including the load-deflection curve, component strain, and crack propagation were obtained from the FEA model and verified through the test data. The results showed that the FEA model can accurately predict the general mechanical behavior of composite beams under negative moments, and the calculation results considering the interface slip effect were closer to the experimental values. Finally, based on the FEA model, the parametric study on static behavior in negative moment regions of composite beams strengthened with/without CFRP laminates was conducted. The effects of the CFRP layout width, layout position, layer number, longitudinal reinforcement ratio, and shear connection degree on the bearing capacity of the composite beams were considered. The CFRP laminates can effectively improve the bending resistance of composite beams under negative moments. At a low reinforcement ratio, the bearing capacity was greatly affected by the layout width and layer number of the CFRP laminates, but the rate of increase was not significant when the layer number was more than 3. The reinforcement ratio had a great influence on the bearing capacity of composite beams under negative moments, but the influence of the shear connection degree was not remarkable.

This study undertakes a comprehensive investigation of the shear behavior of reinforced concrete (RC) beams strengthened by near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminates. Different strengthening configurations were employed by varying the length and inclination angle of the CFRP laminates. Results indicated that NSM‐CFRP strengthening increased the load‐carrying capacity, ductility, stiffness, and toughness from 8% to 41%, 9% to 78%, 24% to 159%, and 22% to 254%, respectively. Results also confirmed that as the CFRP laminate length decreases, the efficacy of the strengthening process increases, where the load‐carrying capacity, ductility, stiffness, and toughness improved from 8% to 19%, 10% to 21%, 8% to 68%, and 26% to 119%, respectively. Also, the comparative results revealed that specimens strengthened with 45°‐inclined CFRP laminates versus those strengthened with vertical laminates had higher load‐carrying capacity (2%–10%), ductility (1%–35%), stiffness (24%–40%), and toughness (13%–32%). Two analytical formulations to predict the contribution of the possible distinct NSM‐CFRP shear strengthening configurations for the shear resistance of RC beams were considered. Results indicated an agreement between the experimental and the analytical results for both formulas, where the average values for the safety factor, k, k=Vtana/Vtexp were 0.80 and 0.73, with corresponding values of standard deviation of 0.195 and 0.125, respectively. [ABSTRACT FROM AUTHOR]

In this study, hybrid nonlinear finite element models (FEM) were developed to examine the flexural performance and the ultimate load capacity of bonded and unbonded two-way reinforced concrete post-tensioned (PT) slabs that were pre-strengthened with external carbon-fiber reinforcement polymer (CFRP) laminates. Full 3D simulations, using ANSYS models, have been created for five different slab samples that were selected from a previously available experimental study. The model results were assessed to enable further numerical analysis. The result calibration included measurements of first crack loads, ultimate loads, deflections, strains in the extreme fiber of concrete, strains in CFRP laminates, and failure modes. The results proved a good correlation between FEM output and experimental ones. Based on this, the influencing parameters that affect plate stiffness, as well as the bending capacity of PT slabs, were examined by performing a detailed parametric study. The parameters included real-life load simulation, cable-to-CFRP strength contribution, and CFRP laminate location selection. The results demonstrated that strengthening using CFRP laminates have significantly increased the ductility index of both bonded and unbonded PT concrete slabs by 62.18% and 59.87%, respectively. In addition, strip strengthening locations near supports are much more effective than in the middle of slabs. Additionally, the CFRP strengthening contribution is very considerable in slabs with low PT ratios.