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Highlights This work aims to study the effects of bionic spiral stacking sequence, impact energy and impactor shape on the impact resistance of laminates. The finite element model is established based on the stress failure criterion, progressive damage evolution, and the triangle traction‐separation law. The reliability of the finite element model is validated through rigorous comparison with experimental data. The study investigates the influence of laminate layup sequence, impact energy, and impactor shape on the impact resistance of laminates. The results show that during low‐speed impacts, laminate damage is primarily characterized by fiber breakage, matrix cracking, and delamination. Matrix cracking and delamination become more pronounced as the impact energy increases. The design of linear spiral ply and power function spiral ply has a positive effect on the impact resistance of laminates. The impact resistance of laminates is sensitive to the sharpness of the impactor and the level of impact energy. Higher impact energy and sharper impactor shapes lead to increased energy absorption in the laminate, resulting in more pronounced damage failure. The impact resistance of bionic spiral composite laminates is studied. Three biologically inspired stacking sequences were designed. A numerical simulation method is proposed and verified. The low‐velocity impact characteristics of bionic laminates are revealed. [ABSTRACT FROM AUTHOR]

Ultrasonic testing (UT) is a commonly used method to detect internal damage in composite materials, and the test data are commonly analyzed by manual determination, relying on a priori knowledge to assess the status of the specimen. In this work, A method for the automatic detection of delamination defects based on improved EfficientDet was proposed. The Swin Transformer block was adopted in the Backbone part of the network to capture the global information of the feature map and improve the feature extraction capability of the whole model. Meanwhile, a custom block was added to prompt the model to extract object features from different receptive fields, which enriches the feature information. In the Neck part of the network, the adaptive weighting was used to keep the features that were more conductive to the prediction object, and desert or give smaller weights to those features that were not desirable for the prediction object. Two kinds of specimens were prepared with embedded artificial delamination defects and delamination damage caused by low-velocity impacts. Ultrasonic phased array technology was employed to investigate the specimens and the amount of data was increased by the sliding window approach. The object detection model proposed in this work was evaluated on the obtained dataset and delamination in the composites was effectively detected. The proposed model achieved 98.97% of mean average precision, which is more accurate compared to ultrasonic testing methods. [ABSTRACT FROM AUTHOR]

Sizing is the surface treatment of glass fibres (GFs) to protect them from wear, handle with ease and to create a good interface with the polymer matrix. This paper reports on the discernible procedure for the synthesis of epoxy-compatible amino silane sizing formulation for E-GF and its efficacy based on inter-laminar shear strength (ILSS) and flexural strength of in-house sized GF (SGF) woven roven mat (WRM) reinforced epoxy composites and compared to commercially available sized WRM [0°/90°]. The formulations of sizing were prepared by varying the proportions of aminopropyltriethoxy silane (APS, coupling agent) and polydimethylsiloxane (PDMS, anti-foam agent), for a fixed proportion of film former, surfactant and lubricant. Commercially available sized GF WRM were heat cleaned prior to sizing with prepared formulations. Interestingly, sizing with formulation of 0.65 wt% APS and 0.35 wt% of PDMS, coded as S6, showed homogeneous coating morphology and the loss on ignition (LOI) is comparable to commercially available SGF. Also, Fourier transform-infrared spectroscopy of SGF6 indicated the formation of silanol interphase by the presence of bands corresponding to Si–O–Si and Si–OH functional groups. The contact angle with epoxy hardener solvent by sessile drop method demonstrated good wetting favourable for better interphase with epoxy matrix. Thermogravimetric analysis indicated that sizing pick up and thermal stability were best for S6 formulation SGF. The mechanical testing shows that S6-sized WRM composite (SWC) yielded the best ILSS and flexural strength due to the effect of better wetting and good interfacial adhesion between SGF and matrix as revealed from fractography. The discernible recipe formulation and the better mechanical properties exhibited by SWC-reinforced SGF WRM sized with S6 comparable to that of commercial GF WRM epoxy composite indicate plausible industrial applications. [ABSTRACT FROM AUTHOR]

In this study, the effect of fiber angle on the tensile load-bearing performance and damage failure characteristics of glass composite laminates was investigated experimentally, analytically, and numerically. The glass fabric in the laminate was perfectly aligned along the load direction (i.e., at 0°), offset at angles of 30° and 45°, or mixed in different directions (i.e., 0°/30° or 0°/45°). The composite laminates were fabricated using vacuum-assisted resin molding. The influence of fiber orientation angle on the mechanical properties and stiffness degradation of the laminates was studied via cyclic tensile strength tests. Furthermore, simulations have been conducted using finite element analysis and analytical approaches to evaluate the influence of fiber orientation on the mechanical performance of glass laminates. Experimental testing revealed that, although the composite laminates laid along the 0° direction exhibited the highest stiffness and strength, their structural performance deteriorated rapidly. We also determined that increasing the fiber offset angle (i.e., 30°) could optimize the mechanical properties and damage failure characteristics of glass laminates. The results of the numerical and analytical approaches demonstrated their ability to capture the mechanical behavior and damage failure modes of composite laminates with different fiber orientations, which may be used to prevent the catastrophic failures that occur in composite laminates. [ABSTRACT FROM AUTHOR]

Highlights In this study, interleaving electrospun Nylon 6.6 nanofibers in the laminate was utilized to improve interlaminar properties of glass/epoxy composite during punching and as a solution to this issue. In order to do so, virgin and nano modified samples are produced and punched and the results are compared to determine the effect of nanofibers on the damaged area as well as the three‐point bending residual strength (carried out on both penetration and exit sides) of the punched laminate. Furthermore, punch—die clearance was also varied to investigate the range in which the nano modification improves the outcome of the process on the laminated composite. According to results of this study, using of Nylon 6.6 nanofibers between composite layers causes significant decrease in the damaged area up to 50%. Also, in both virgin and nano modified laminates, increasing the punch—die clearance increases the damaged area up to 50% and reduces the residual strength of the laminates up to 17%. In addition, nano modified laminates, the residual strength increased up to 28% and 22% for three‐point bending tests carried out on both punch penetration‐ and exit‐side samples, respectively. This is due to the fact that the punch exit‐side contains more damage compared to the penetration‐side. Toughening glass/epoxy laminates using electrospoon Nylon 6.6 nanofibers. Decreasing the damage area occur in the punching of composite laminates. Decreasing the damage area in punching by decreasing the punch‐die clearance. Increasing the strength of punched laminates by applying nanofibers. [ABSTRACT FROM AUTHOR]

This study presents a meshless modeling approach to design variable-stiffness laminates considering manufacturing constraints. The governing equations are derived using lamination parameters and first-order shear deformation theory. The solution approach uses Chebyshev polynomials and Galerkin's method to obtain the discretized equations of motion. The developed framework was used to maximize the fundamental frequency of composite plates. The variable-stiffness designs provided up to 28.4% higher frequencies compared to optimum constant-stiffness laminates, although the actual level of improvement depends on the number of layers. Finally, manufacturable fiber paths were obtained considering the allowed fiber curvature, which can also reduce the frequency values. [ABSTRACT FROM AUTHOR]

Mode II (in-plane-shear) delamination tests are more complex than mode I (opening) due to the presence of a contact force between the two arms. This force is essential for the calculation of the energy release rate (ERR) and is closely linked to friction effects. A novel formulation is presented in this article to estimate the contact force analytically. Specifically, the contact force is derived within the context of the rigid, semi-rigid, and flexible joint models. The analytical solutions consider the case of a generally layered composite laminate with residual hygrothermal stresses and are used to evaluate the ERR. The new formulation is compared with numerical models created using the Virtual Crack Closure Technique (VCCT) and the Cohesive Zone Method (CZM) for a fiber–metal laminate. The results show that the new formulation provides nearly identical ERR predictions to those of the VCCT and CZM models. Additionally, it is demonstrated that the effect of friction on the ERR is less than 1%. [ABSTRACT FROM AUTHOR]

Owing to the importance of fibre architectures in the design of textile composite materials, understanding their effect on the failure mechanisms of these composites have taken more considerations. In this regards, non-crimp preform and 2/2 twill fabric have been manufactured from glass fibre by using pin-board and power loom machines respectively, and subsequently composites laminates from both preforms are manufactured via vacuum assisted resin infusion method. In addition, quasi-static tensile and compressive strength tests have been conducted for the composite laminates that have same volume fraction. Damage failure mechanisms that occurred in compressive strength in both composites have been examined by using scanning electronic machine (SEM). Findings show that the mechanical properties are primly determined by fibre architectures and the presence of fibre crimp can further weaken tensile and compressive strength values. It was noticed that the tensile strength of non-crimp composite was 610 MPa whereas that of twill fabrics composite laminates it was found to be 350 MPa and 440 MPa in warp and weft directions respectively. Moreover, in case of twill fabric composites, fibre crimp has a considerable effect on the mode and characteristics of damage in the compressive loading; this leads to fibre fracture and kinks that primarily happened in the intersection point of warp and weft yarns. Results also showed improved ductility, strain to failure and absorbing energy in the twill fabric composites compared to non-crimp composites under in-plane shear strength test. [ABSTRACT FROM AUTHOR]

The problem of obtaining anisotropic auxetic composite laminates, i.e. having a negative Poisson's ratio for at least some directions, is examined in this paper. In particular, the possibility of obtaining auxeticity stacking uni-directional identical plies is considered. It is shown that if the ply is composed by isotropic matrix and fibers, then it is impossible to obtain totally auxetic orthotropic laminates, i.e. auxeticity for each direction, unless at least one among matrix and fibers is auxetic itself. Moreover, it is shown what are the conditions, in terms of the mechanical properties of the constituents and of the volume fraction of the fibers, to fabricate uni-directional plies with which to realize laminates having a negative Poisson's ratio for some directions. Several existing materials are also examined. All the analysis is done using the polar formalism, very effective for the study of plane anisotropic problems. [ABSTRACT FROM AUTHOR]

Interfacial imperfection usually exists in composite laminates, which are widely used in the aerospace and shipbuilding industries. Such laminates are usually impacted by external loads in practical applications, and there are very few impact studies considering the interfacial imperfection. Therefore, based on Reddy's shear deformation theory, zigzag displacement theory, Hertz's contact law and layer spring model, a 2-dimensional numerical model is proposed to more effectively study the effects of interfacial imperfection on the low- velocity impact behavior of composite laminates. In addition, the rationality of the numerical model is verified by combining numerical examples from other references. It can be clearly seen from the numerical results that due to the interfacial imperfection, the normal stresses in the fiber direction of each layer increase while the maximum contact force decreases simultaneously. As the dimensionless interface parameter R increases, the transverse shear stress σ xz of the middle local layer decreases, while the transverse shear stress σ xz in both side layers increases. Although the transverse shear stress σ yz along the entire plate thickness decreases as R increases from 0 to 0.2, σ yz shows a similar pattern when R ranges between 0.2 and 0.3. Furthermore, based on the principle of energy balance, we further propose a semi-theoretical formula to predict the damage degree of composite laminates when the interfacial bonding gradually weakens under low-velocity impact. Research results show that when R increases from 0 to 1.0, relative sliding between layers will cause the damage of composite laminates to increase sharply. But when R is greater than 1.0, the damage degree is no longer sensitive to the increase in R . [ABSTRACT FROM AUTHOR]

Thin-plies are generally defined as composites with ply thicknesses below 100 μm. These materials are rapidly gaining interest for high-performance applications, for example, the aerospace sector. Many practical techniques have been proposed to prevent delamination and improve the strength of composite laminates. A recent study has shown that the delamination could be postponed by replacing layers of CFRP with thin-ply in a unidirectional composite laminate, a configuration known as hybrid laminates reinforced with thin-plies. Since fiber orientation is known to be one of the most important parameters in composite laminate design, this study investigates the effect of oriented layers of thin-ply or both thin-ply and conventional CFRP in a hybrid laminate under out-of-plane tensile loading. A numerical Representative Volume Element (RVE) model for CFRP and thin-ply was generated, considering the unidirectional [0], cross-ply [45/−45], and [0/90] in order to better understand the effect of angle-plied hybrid composite laminates. Experimental results show that angle-plied composite laminates present higher failure load under out-of-plane tensile loading compared to the unidirectional ones. This can be attributed to the fact that an initiated crack is faced with a significantly more complex crack path in an angle-plied laminate to advance in the through-the-thickness direction. [ABSTRACT FROM AUTHOR]

Although the abrasive water jet (AWJ) has proven to be a suitable process for machining composite materials, it has some limitations related to dimensional inaccuracy and surface defects. As the performance of the AWJ process mainly depends on the machining parameters, an optimal selection of them is crucial to achieving an improved quality of cut. In this context, the present study reports an experimental investigation to assess the influence of AWJ machining parameters on kerf taper angle (θ) and surface roughness (Ra) of E glass/Vinylester 411 resin laminates. The experiments are carried out using a full factorial design by varying the water pressure, traverse speed, abrasive flow rate, and standoff distance. A first-ever attempt is made in this paper to optimize the AWJ process using a hybrid approach combining artificial neural networks (ANNs) with a recently proposed metaheuristic algorithm known as multi-objective bonobo optimizer (MOBO). The results show that standoff distance and abrasive flow rate were the most significant control factors in influencing θ and Ra, respectively. The developed ANN models are capable to predict the output responses with high accuracy and the solutions from the Pareto front provide a sufficient performance with a trade-off between θ and Ra. The corresponding levels of the optimal process parameters are 430 g/min for the abrasive flow rate, the range of 140–180 mm/min for the traverse speed, 280 MPa for the pressure, and 1.5 mm for the standoff distance. [ABSTRACT FROM AUTHOR]

The time difference of arrival (TDOA) method has traditionally proven effective for locating acoustic emission (AE) sources and detecting structural defects. Nevertheless, its applicability is constrained when applied to anisotropic materials, particularly in the context of fiber-reinforced composite structures. In response, this paper introduces a novel COmposite LOcalization using Response Surface (COLORS) algorithm based on a two-step approach for precise AE source localization suitable for laminated composite structures. Leveraging a response surface developed from critical parameters, including AE velocity profiles, attenuation rates, distances, and orientations, the proposed method offers precise AE source predictions. The incorporation of updated velocity data into the algorithm yields superior localization accuracy compared to the conventional TDOA approach relying on the theoretical AE propagation velocity. The mean absolute error (MAE) for COLORS and TDOA were found to be 6.97 mm and 8.69 mm, respectively. Similarly, the root mean square error (RMSE) for COLORS and TODA methods were found to be 9.24 mm and 12.06 mm, respectively, indicating better performance of the COLORS algorithm in the context of source location accuracy. The finding underscores the significance of AE signal attenuation in minimizing AE wave velocity discrepancies and enhancing AE localization precision. The outcome of this investigation represents a substantial advancement in AE localization within laminated composite structures, holding potential implications for improved damage detection and structural health monitoring of composite structures. [ABSTRACT FROM AUTHOR]

A novel damage tolerance design method of discrete interleaved toughening for laminated composite was proposed. In this paper, discrete thermoplastic Polyamide-6,6 (PA 66) films were deployed at between the selected adjected layers, and three toughening dimensions were considered. Besides, three impact positions of specimens were tested under 5 J, 10 J, and 15 J, respectively. The impact behavior and compression after impact (CAI) of specimens were discussed and compared. Experimental results showed that delamination damage was suppressed in the process of propagation outwards across the toughened region, and the path of delamination crack propagation was changed and swerved. Therefore, the delamination damage projected area (DDPA) of all specimens were reduced by 21.09%–62.85%, compared with the non-toughened plate (base plate). In addition, the finding demonstrated that CAI strength is influenced by the delamination position and DDPA. The proposed method could improve the CAI strength by suppressing delamination and swerving the propagation path of delamination. Moreover, the CAI strength of all toughened plates were improved by 16.35%–61.94% in contrast to the related base plates. [ABSTRACT FROM AUTHOR]

We propose a large combined moving component composed of carbon fiber reinforced polymer (CFRP) laminates for making lightweight machine tools with high dynamic performance. The accurate dynamic prediction of composite machine tools is essential for the new generation machine tool. This paper aims to address two challenges in numerical dynamic modeling and the design of composite machine tools to enhance development efficiency. (1) Anisotropic composite laminates, which form the composite machine tool, exhibit coupling in various directions. We propose the generalized continuity condition of the boundary to tackle this dynamic modeling challenge. (2) Composite machine tools feature numerous composite-metal coupled structures. The mechanical model correction of isotropic metals is performed to address their dynamics. We take the example of a five-axis gantry machine tool with composite moving parts, establish a dynamic model for efficient prediction, and verify it through simulation and experimentation. The proposed method yields remarkable results, with an average relative error of only 3.85% in modal frequency prediction and a staggering 99.7% reduction in solution time compared to finite element analysis. We further discuss the dynamic performance of the machine tool under varied stacking angles and layer numbers of the composite machine tool. We propose general design criteria for composite machine tools to consider the modal frequency and manufacturing cost of machine tools. [ABSTRACT FROM AUTHOR]

In this paper, an enhanced VARTM process is proposed and its pressure effect on resin infusion behavior and composite material performance is studied to reveal the control mechanism of the fiber volume fraction and void content. The molding is vacuumized during the resin injection stage while it is pressurized during the mold filling and curing stages via a VARTM pressure control system designed in this paper. Theoretical calculations and simulation methods are used to reveal the resin's in-plane, transverse, and three-dimensional flow patterns in multi-layer media. For typical thin-walled components, the infiltration behavior of resin in isotropic porous media is studied, elucidating the control mechanisms of fiber volume fraction and void content. The experiments demonstrate that the enhanced VARTM process significantly improves mold filling efficiency and composite's performance. Compared to the regular VARTM process, the panel thickness is reduced by 4% from 1.7 mm, the average tensile strength is increased by 7.3% to 760 MPa, the average flexural strength remains at approximately 720 MPa, porosity is decreased from 1.5% to below 1%, and the fiber volume fraction is increased from 55% to 62%. [ABSTRACT FROM AUTHOR]

This research conducts a comprehensive experimental and numerical analysis of carbon fiber composite laminates under low‐velocity impact conditions. Split Hopkinson Pressure Bar (SHPB) was employed to conduct impact testing on laminates with two layup sequences (03/+45/−45s and 03/903s) under impacts ranging from 2 to 6 J. For assessing damage in laminates, a comprehensive progressive damage model and a simplified model were developed. The primary distinction between these models lies in the fact that the former incorporates the strengthening effect of compression on the delamination behavior, while the latter does not. Both models produce a good agreement with experimental results in terms of force–time, force–displacement curves, and absorbed energies. However, notable differences were noted in the predicted internal damage; the simplified model was underpredicted in terms of damage size and distribution compared to the comprehensive progressive damage model. Highlights: Impact tests were conducted on 03/+45/−45s and 03/903s composites.In order to simulate damage caused by impacts in laminates, an energy‐based damage model was created, and cohesive elements were utilized to model interlaminar delamination.The strengthening effect of compression was explored.Distinct damage criteria were established for two cases.Damage detection of composites by x‐rays. [ABSTRACT FROM AUTHOR]

A computational framework combined with the commercial finite element software Abaqus and Bayesian optimization algorithm is proposed. The proposed computational framework leverages the Gaussian process based-probabilistic capability in Bayesian optimization as a surrogate model for minimizing the computational cost of objective function evaluations in the finite element analysis. The optimization problem in this work is to enhance the maximum fundamental frequency of the composite laminates, which is one of the critical parameters in the design for composite structures. The optimization of stacking sequence selection is investigated and validated by the results obtained from literatures. The effectiveness and efficiency of the proposed Bayesian optimization computational framework in maximizing the fundamental frequency of composite laminates are demonstrated by comparing the optimized results from different optimization techniques through a series of cases including the 8-layer rectangular plates with 11 different boundary conditions and the 10 to 20-layer trapezoidal plates with the same boundary condition. The proposed framework developed in this work has highly potential as an engineering tool to address a broader range of structural optimization in vibration problems.

Rubber containing nanofiber is investigated as structural modifiers of carbon fiber reinforced polymers (CFRPs) to improve their delamination resistance. The ability to obtain long lasting nanofibrous shape from liquid nitrile butadiene rubber (NBR) has been demonstrated even without requiring instant cross‐linking when a second component is used together with NBR in the process. The second component can be a thermoplastic which either melts, such as poly(e‐caprolactone) (PCL), or holds steady, as poly(m‐phenylene isophthalamide) (PMIA) or nylon 66, during CFRP curing process. Assuming that NBR is not crosslinked, during curing of low Tm thermoplastic‐based rubber nanofibers, their morphology is lost, as for NBR/PCL, and both polymers blend with the surrounding epoxy resin thus leading to an outstanding toughening effect, but at the cost of the overall composite mechanical performance. When a second component is used which is preserved in nanofibrous fashion during curing (NBR/PMIA, NBR/nylon 66), instead, a good compromise is achieved, with still outstanding delamination hindering ability, together with almost fully preserved thermo‐mechanical performances. [ABSTRACT FROM AUTHOR]

AbstractAdvancements in technology and the compulsion to use environment-friendly materials have been challenging tasks for researchers for the past two decades. Researchers have been focusing on the utilization of plant fibers to produce good quality fiber-reinforced polymer/polyester composites for automobile, structural, and building applications. Researchers have been looking for high-quality and cost-effective drilling processes. The primary goal of this study is to identify optimal drilling conditions for CCFRP composite laminates, affecting thrust force and torque. This is achieved by manipulating drilling process variables using Taguchi’s Design of Experiments (TDOE), Analysis of variance (ANOVA), Response Surface Methodology (RSM), Desirability Function Analysis (DFA) and Artificial Neural Network (ANN). From the results, it was observed that the spindle speed of 2000 rpm, feed of 15 mm/min, point angle of 90°, fiber length of 6 mm, fiber volume of 30%, and fiber diameter of 7 microns gave the optimum results for obtaining minimum thrust force and torque. Further RSM revealed that an increase in fiber vol % and a decrease in spindle speed resulted in an increase in thrust force and torque. From DFA optimization results, the minimum thrust force of 24.0042 N and minimum torque of 0.8001 N-m was obtained. Finally, the experimental values of thrust force and torque were compared with the corresponding values predicted by the MLP-ANN model. The average error percentage for thrust force and torque was 1.75% and 6.56% respectively.

Auxetic composite laminates, i.e. laminates with a NPR (Negative Poisson’s Ratio), are regarded as a promising solution to combat LVI (Low-velocity impact) delamination BVID (Barely visible internal damage) and ensuing property degradation, a cause for concern in aerospace components, mainly inflicted by fortuitous accidents during handling operations. In order to potentiate the auxetic effect through the minimization of the Poisson’s ratio, a thorough analysis of material properties and stacking sequences is required, as only a restricted domain of combinations can generate the desired effect, either in an IP (In-plane) or TTT (Through-the-thickness) configuration. This paper focuses on a MATLAB program developed for IP and TTT auxetic laminate design, based on the CLT (Classical Lamination Theory). Cases studies on NPR domain definition of C/E (Carbon/epoxy), G/E (Glass/epoxy) and hybrid C-G/E (Carbon-Glass/epoxy) laminates are presented. Moreover, the influence of fibre volume fraction on C/E and G/E laminates is analysed.

The low-velocity impact (LVI) characteristics and compression after impact (CAI) properties of aircraft composite laminates under the influence of ultraviolet (UV) radiation were studied. Firstly, 0, 30, 60, 90 days of UV radiation experiments were carried out on the specimens to analyze the characteristics of the surface topography. Secondly, LVI experiments, the impact energies of 10, 17, and 25 J, were carried out on specimens to analyze the impact response laws and the characteristics of the impact instantaneous deformation. Finally, through CAI experiments and scanning electron microscope (SEM) techniques to clarify the influence mechanism of UV radiation on the residual compressive strength of specimens. The results showed that with the increase of UV radiation time, the shrinkage and erosion phenomena of the surface matrix and the exposure degree of the surface fibers were gradually increased, and the surface fibers were completely exposed after 90 days of radiation. At the same impact energy, with longer radiation time, the peak force gradually decreased (from 8673.86 to 7851.68 N at 17 J), the absorbed impact energy (from 9.48 to 11.13 J at 17 J) and maximum displacement (from 4.1 to 4.73 mm at 17 J) increased, the depth of pit on the impact surface decreased (from 0.213 to 0.14 mm at 17 J), and the internal damage projection area increased (from 906.68 to 1158.66 mm2 at 17 J). The compression damage of the specimens not exposed to/exposed to UV radiation was mainly fiber cracking/interlayer delamination, respectively. Highlights • Ultraviolet (UV) radiation makes the matrix of surface shrink, the fibers exposure increase. • The Stage II has obvious oscillation phenomenon in four stages of the lowvelocity impact process. • With prolonged UV radiation, the projected area of internal damage expands. • The residual compressive load increases with the increase of UV radiation time. [ABSTRACT FROM AUTHOR]

Lamb wave-based damage quantification in large-scale composites has always been one of the concerning and intractable problems in aircraft structural health monitoring. In recent years, machine learning (ML) algorithms have been utilized to deeply explore the damage feature of Lamb wave signals, which aims to enhance the precision and accuracy of damage quantification. However, multi-damage quantification becomes one of the bottleneck problems because ML algorithms critically depend on the dataset. In this paper, a prioritizing selection and orderly permutation method is proposed to construct multi-damage dataset based on Born approximation principle, which shows the interaction between wave signals under multi- and single-damage conditions. Based on the multi-damage dataset, a multi-task deep learning algorithm is introduced to identify multiple damage, including the damage number, location, and size, in composite laminates. In the algorithm, a multi-branch 1D-convolution neural network framework, which includes a trunk network and branch networks is established to explore the damage features in Lamb wave scattering signals. Compared with single-task models, it has the ability to learn shared features for multiple tasks, effectively boosting the task results. The results show that the proposed multi-task learning (MTL) method saves 23.03% training time compared with the single-task learning method. In the task of quantifying multiple damage of composite laminate, the results of MTL are good for both the constructed test set and the measured test set, especially in the quantification of damage size, which shows the feasibility and reliability of this method. [ABSTRACT FROM AUTHOR]

The tensile creep tests for two kinds of epoxy resin matrix composite laminates were performed under 25°C and 50°C. Results indicate that these materials deform in a time‐dependent manner, that is, the tensile strain increases within a short time after the tests began, and then decreases during most of the loading period. Such increase and decrease trends reflect the creep characteristic and effect of physical aging, respectively. A linear viscoelastic model is developed to include both the creep temperature effect and physical aging effect, starting from the Boltzmann superposition principle. In modeling, these two effects both contribute to the transient compliance and result in the increase and decrease of the transient compliance. Time–temperature/aging time superposition principle is adopted to deal with the influences of temperature and physical aging. This theoretical model is further implemented numerically within a finite element framework. To verify the newly proposed model, not only the experimental and numerical results of the tensile deformations of laminates are compared, but also the bending deformations of structural components made from the above laminates are analyzed. Highlights: Tensile creep tests were done for carbon fiber/epoxy laminates.The strain shows creep temperature and aging effect (TE and AE).New constitutive relation was proposed to describe the TE‐ and AE‐caused strains.Structural‐level flexure creep tests and simulation analyses were conducted. [ABSTRACT FROM AUTHOR]

The present article investigates the failure mechanism of the composite laminates with respect to a coupled thermo-mechanical effect. To this end, a novel coupled thermo-mechanical modeling scheme is presented. To accurately capture the nonlinear deformation at macro- and micro-scale affected by the temperature field, the finite volume method is employed to compute the thermal stress, which is introduced into the representative volume element to evaluate the whole microscopic stress field and the macroscopic mechanical behaviors on basis of the finite volume direct averaging micromechanics. In addition, a visco-plastic constitutive model with full consideration of the temperature effect is implanted into the presented multi-scale model to investigate their nonlinear responses. The thermo-mechanical coupling test scheme is subtly designed, and the experimental data shows a high consistency with the theoretical results. On this basis, the microscopic morphology of the fracture surface is further characterized. It is revealed that an elevate temperature improve the matrix toughness to some extent. [ABSTRACT FROM AUTHOR]

In this paper, the repeated low velocity impact responses and compression‐after‐impact (CAI) behaviors of woven glass fiber‐reinforced composite laminates were studied at the same total impact energy. Three types of impact energies, 30, 40, and 60 J, were chosen for the repeated impact tests relative to the single 120 J impact event. The impact behaviors including impact contact force–displacement and energy–time curves during the impact testing were recorded. The variations of the impact mechanical characteristics such as peak impact force, maximum displacement, and energy absorption were evaluated. The methods of visual inspection and stereo microscope were applied to identify the damage morphology of the impacted laminates. The damage accumulation was evaluated employing the absorbed energy fraction and normalized maximum displacement. It was found that the influences of impact energy on the repeated impact mechanical response are more remarkable than that of the impact number, and the impact events with fewer impact numbers cause more damage to the laminates for the same accumulative impact energy. Furthermore, the load–displacement and the CAI strength versus impact number curves were compared for the CAI testing. The ultimate load and the resulted CAI strength decrease with the increase of impact number owing to the influence of damage accumulation in repeated impacts. Highlights: Repeated impact responses were studied at the same total impact energy.The impact energy makes significant influence on the impact dynamic responses.The impact events with fewer impact numbers cause more damage to the composites.The CAI behavior was explored considering the effect of damage accumulation.The CAI strength of damaged laminates decreases with impact number increasing. [ABSTRACT FROM AUTHOR]

This study exhibits the effect of different damage mechanisms on the mechanical behavior of composite laminates subjected to low‐velocity impact load. To achieve this, seven low‐velocity impact energies were applied to laminates with five different thicknesses and four unique stacking sequences. The extent of damage was quantitatively evaluated through contact measurements and ultrasonic C‐scan techniques. Numerical simulations, showing good agreement with experimental results, characterized the distribution and evolution of damage within the laminates. The findings reveal that when matrix and delamination damage are predominant, the dent depth increases slowly and is visually invisible, while the delamination area expands almost linearly. Conversely, When fiber damage prevails, the laminate absorbs more energy, leading to rapid increases in dent depth, the appearance of visible cracks, and the occurrence of sharp fluctuations with multiple peaks in contact force and displacement curves. Nonetheless, the delamination area and projection tend to be stable and unchanged. Furthermore, alterations in thickness and stacking sequence directly affect the damage characteristics and energy absorption properties. This research offers valuable theoretical insights and serves as a reference for enhancing the comprehension of the failure mechanism of laminates and optimizing their performance. Highlights: Demonstrate the utility of dent depth in identifying changes in damage mechanisms.Evaluate the delamination damage of laminates after impacts by ultrasonic C‐scan techniques.Reveal the regulation of impact response parameters for different damage mechanisms.Investigate the effect of variation in thickness on damage characteristics and energy absorption.Evaluate the impact resistance of laminates with different stacking sequences. [ABSTRACT FROM AUTHOR]

Advancements in technology and the compulsion to use environment-friendly materials have been challenging tasks for researchers for the past two decades. Researchers have been focusing on the utilization of plant fibers to produce good quality fiber-reinforced polymer/polyester composites for automobile, structural, and building applications. Researchers have been looking for high-quality and cost-effective drilling processes. The primary goal of this study is to identify optimal drilling conditions for CCFRP composite laminates, affecting thrust force and torque. This is achieved by manipulating drilling process variables using Taguchi's Design of Experiments (TDOE), Analysis of variance (ANOVA), Response Surface Methodology (RSM), Desirability Function Analysis (DFA) and Artificial Neural Network (ANN). From the results, it was observed that the spindle speed of 2000 rpm, feed of 15 mm/min, point angle of 90°, fiber length of 6 mm, fiber volume of 30%, and fiber diameter of 7 microns gave the optimum results for obtaining minimum thrust force and torque. Further RSM revealed that an increase in fiber vol % and a decrease in spindle speed resulted in an increase in thrust force and torque. From DFA optimization results, the minimum thrust force of 24.0042N and minimum torque of 0.8001N-m was obtained. Finally, the experimental values of thrust force and torque were compared with the corresponding values predicted by the MLP-ANN model. The average error percentage for thrust force and torque was 1.75% and 6.56% respectively. [ABSTRACT FROM AUTHOR]

Sandwich beams, comprising two stiff facesheets bonded to a core, are light and strong and see widespread application. However, debonded regions between the facesheets and the core can significantly reduce the strength and have an impact on the vibration behaviour. In this study, the previous literature was referred to, and finite element simulations were conducted using ANSYS on debonded sandwich beams with [0°]4, [+45°/−45°]S and [90°]4 carbon fibre reinforced polymer facesheets. The effects of the configurations of the three debonded regions and facesheet stacking orientations were studied. The most important finding is that if debonds cover vibration nodes, the natural frequencies drop more significantly because the stiffness decreases more. It was determined that when there were debonded regions, the natural frequencies of the bending modes of the beams using [0°]4 facesheets decreased more compared to those of the beams using [+45°/−45°]S and [90°]4 facesheets; while the trends are opposite for the natural frequencies of torsion modes of the beams using [+45°/−45°]S facesheets. Reducing the natural frequency can cause a structure to vibrate at the resonance frequency, leading to structural failure. This study contributes to furthering our understanding of how different debonds and facesheet stacking orientations affect the vibrations of sandwich beams. [ABSTRACT FROM AUTHOR]

Scarf repair is a widely used repair method for damaged composite structures, which can be divided into two categories: dry (precured-patch) repair and wet (wet-layup) repair. The ultimate strength, failure mode and patch debonding of scarf repaired composite laminates were studied by static tensile test. The specimens with different damage sizes were repaired by dry repair and wet repair method. The experimental results show that the higher recovery of failure strength and the lower probability of patch debonding can be obtained by using precured-patch repair method. Based on the experimental investigation, finite element analysis was carried out to accurately predict the ultimate strength, stress-strain distribution and damage evolution process of scarf repaired laminates. The simulation results show that the use of precured-patch repair can improve the mechanical performance of the repaired laminates compared to that using wet-layup repair. The stress distribution of the bondline is more uniform, the stress level and stress concentration are lower, and the load of interface failure and cohesive failure are higher.

This paper aims at numerical finite element (FEM) research of guided Lamb waves propagation in multidirectional composite plates. All simulations were conducted in the Abaqus/CAE software by using the dynamic/explicit solver. The material considered in this work was carbon/epoxy composite laminate with [90⁰/θ/θ/θ/-θ/-θ/-θ/90⁰] stacking sequence where θ set was equal 0⁰, 30⁰, 45⁰, 60⁰ and 90⁰. The main goal of the analysis was to evaluate the influence of fiber orientation angles θ on propagation behavior of the separate symmetric S0 and asymmetric A0 Lamb wave modes. Numerical model was created by using the C3D8R brick element. The Lamb waves were generated by using concentrated force with 200 kHz frequency. The acoustic signal generated by travelling wave was registered at two nodes that represent the acoustic emission sensors. Obtained results were presented in tabular form where separate mode velocities were collected and on the normalized displacement versus time plots depicted registered wave signals. In addition, the contour diagrams and through-thickness deformations plots were created to present behavior of the extensional and the flexural modes. The greatest value of the S0 mode velocity was obtained for unidirectional laminates whereas the lowest for composite plate with 45⁰ fiber orientation angle. The asymmetric mode found to generate slightly greater deformation of plate in XZ plane than the symmetric. Recognition of the Lamb wave behavior in multidirectional laminates will allow to better planning the experimental acoustic emission tests.

Mode II (in-plane-shear) delamination tests are more complex than mode I (opening) due to the presence of a contact force between the two arms. This force is essential for the calculation of the energy release rate (ERR) and is closely linked to friction effects. A novel formulation is presented in this article to estimate the contact force analytically. Specifically, the contact force is derived within the context of the rigid, semi-rigid, and flexible joint models. The analytical solutions consider the case of a generally layered composite laminate with residual hygrothermal stresses and are used to evaluate the ERR. The new formulation is compared with numerical models created using the Virtual Crack Closure Technique (VCCT) and the Cohesive Zone Method (CZM) for a fiber–metal laminate. The results show that the new formulation provides nearly identical ERR predictions to those of the VCCT and CZM models. Additionally, it is demonstrated that the effect of friction on the ERR is less than 1%.

Composite materials have been widely used in civil aviation aircraft, but due to low electrical conductivity and thermal conductivity, a wide range of ablation damage occurs after lightning strikes, threatening the operation safety of civil aviation aircraft. In order to make composite materials more safely used in civil aviation aircraft, the lightning strike protection characteristics of commonly used curved composite materials in civil aviation aircraft are studied. The electrothermal coupled finite element analysis model of lightning strike damage of curved composite materials was established, and the influence of different factors on the lightning strike damage performance of curved laminates was studied. It is found that the ablation damage area of the curved composite laminate is increased by 1. 51 times under the action of the peak lightning current of 50 kA due to the influence of thermal conductivity, and the depth expands to the sixth layer, and the damage area of the smallest layer increases by 10. 67 times. In addition, the peak value of the lightning strike current and the curved angle have a greater impact on the lightning strike damage results, while the aspect ratio, the direction of fiber, and the thickness are less affected by the potential and thermal conductivity of the curved laminate. It is quite different from the influence characteristics of lightning strike factors of uncurved laminates. [ABSTRACT FROM AUTHOR]

The nonlinear Lamb waves are sensitive to closed micro cracks and have a great potential for the detection of damage to prevent the catastrophic failure of entire structure. A five-stage closed micro crack localisation method based on time of flight (ToF) of the second harmonic Lamb waves is proposed in this research. The first step is frequency analysis of the received signals to ensure the generation of second harmonic wave. The second step is to extract second harmonic wave component in frequency range. The third step is to obtain wave signals in time domain. The fourth step is to extract ToF of second harmonic wave. The fifth step is to locate damage with ToFs. The method provides an effective strategy based on wavelet transform to extract the second harmonic Lamb waves from the dynamic response signals for the detection of closed micro crack. The numerical and experimental results indicate that the proposed method can accurately locate the closed micro crack in plate-like metallic and composite structures. [ABSTRACT FROM AUTHOR]

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The dynamic mechanical behaviors of the [ 0 2 ° / 90 2 ° ] 4 s fiber-reinforced composite laminates subjected to double-position low-velocity impacts are investigated by finite element method. Two impact positions symmetrical about the center of the laminates are impacted sequentially with three impact distances (10 mm, 20 mm, and 40 mm) under three impact energies (5 J, 10 J, and 20 J) to study the interference effect of impact damage. For comparison, plastic damage model (PDM) and elastic damage model (EDM) are established to describe the intra-laminar constitutive, respectively. Compared with available experimental data, the mechanical responses calculated by PDM are more accurate, especially at high energies. Affected by the impact interference, the oscillation of force-time curve for the second impact rather than the first impact is relatively weaker, while the severity of impact damage is reversed. The results show that the maximum displacement is more suitable for characterizing the degree of damage interference than bending stiffness, peak force, and energy dissipation. [ABSTRACT FROM AUTHOR]

Auxetic laminates are part of a fascinating branch of materials denominated as auxetics, which display a NPR (Negative Poisson's Ratio), an uncommon property in the engineering world. Auxeticity is at the root of enhancements in shear and indentation resistance, fracture toughness and energy absorption, making the design of NPR a desirable feat in structural engineering. In composite laminates made of conventional, i.e. positive Poissons ratio, materials, auxeticity is a result of the combination between particular angular configurations and anisotropic materials. Such laminates can be used in a wide array of engineering applications, especially those which require high energy absorption capacity, including aerospace, personal defense and sports industries. This review focuses on particular property enhancements reported in the scientific world brought about by the design and application of IP (In-plane) and TTT (Through-the-thickness) NPR fibre-reinforced polymer laminates under QSI (Quasi-static indentation), LVI (Low-velocity impact) and fatigue solicitations. Furthermore, some insight is given on some possible future paths for further investigation of this topic. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Experimental methodologies for fatigue lifetime prediction are time-intensive and susceptible to environmental variables. Although the cohesive zone model is popular for predicting adhesive fatigue lifetime, entropy-based methods have also displayed potential. This study aims to (1) provide an understanding of the durability characteristics of carbon fiber-reinforced plastic (CFRP) adhesive joints by incorporating an entropy damage model within the context of the finite element method and (2) examine the effects of different adhesive layer thicknesses on single-lap shear models. As the thickness of the adhesive layer increases, damage variables initially increase and then decrease. These peak at 0.3 mm. This observation provides a crucial understanding of the stress behavior at the resin–CFRP interface and the fatigue mechanisms of the resin. [ABSTRACT FROM AUTHOR]

In this paper, the free-edge effects which play an important role in the failure analysis of composite laminates are studied. Pure temperature change and its combination with biaxial tension are investigated for a composite plate with a centric circular hole that is adequately small in comparison to the dimension of the plate. Moreover, the laminate is assumed to be thin, the temperature change of all points of the plate is identical and the biaxial tensile load is applied far from the center of the hole. For this purpose, a semi-analytical method based on a two-dimensional thermoelastic boundary layer theory and a minimum potential energy principle is proposed. It is shown that the present method predicts the interlaminar peeling and shear stresses surrounding the hole with good agreement compared to the results obtained by FEM analysis. [ABSTRACT FROM AUTHOR]

The effect of low-velocity impact loading and seawater aging on the residual bending properties of glass fiber reinforced polymers (GFRPs) was evaluated. The self-sensing capability of the composite laminates provided by a hybrid combination of multiwall carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) was also examined. The composite laminates were fabricated by RTM with the incorporation of different contents of MWCNTs and GNPs onto the glass fiber surface using the spray coating technique. Impact tests were performed on specimens with and without seawater (SW) aging and then bending specimens were taken to evaluate the after-impact bending behavior. An important reduction in mechanical properties of composite laminates produced by SW aging, caused by plasticization and swelling effects of the polymer matrix, was confirmed. Despite this physical degradation in the mechanical behavior of GFRP composites, a positive synergistic effect of the carbon nanostructures (CNSs) in the composite laminates was observed. Carbon nanofillers cause the maximum force during the impact test increase in comparison with neat specimens; this effect was exhibited by both samples, with and without seawater aging. Previous impact damage reduced flexural strength and flexural modulus of dry (14% and 43%, respectively) and wet samples (15% and 26%, respectively); however, the reduction in flexural strength is slightly smaller in a certain hybrid combination of CNSs. Another important finding was that the self-sensing capability of GFRPs with CNSs was preserved, even after the impact loading and even after the seawater aging, making this technique suitable for structural health monitoring of marine components. [ABSTRACT FROM AUTHOR]

Background: Interlaminar delamination is a common damage mechanism in composite laminates that can lead to structural failure. Assessment using contemporary numerical modeling techniques requires delamination behavior as a traction–separation response. However, existing experimental characterization approaches are not well suited to support these modeling techniques as specimens were developed to assess single delamination parameters, not a full traction–separation response, or utilize analysis schemes that require knowledge of material properties. Objective: To develop a test specimen and data analysis methodology to directly measure the traction–separation response of Mode I delamination in a laminated fiber-reinforced polymer (FRP) composite, including strength, toughness, and damage response. Methods: The proposed composite Rigid Double Cantilever Beam (cRDCB) specimen is comprised of a [0]_4 unidirectional E-glass/epoxy laminate co-cured to rigid metallic adherends. Traction–separation response was assessed directly from measured force and displacement behavior using a closed-form analysis scheme that does not require a priori knowledge of composite material properties. Standard double cantilever beam (DCB) tests were performed for comparison. Results: The cRDCB specimen captured early damage initiation and progression in greater detail than the DCB, with measured strain energy release rates agreeing well between the two approaches. The cRDCB also captured the effects of large-scale damage mechanisms such as fiber bridging. The measured traction–separation responses are suitable for scenarios where prediction of the initiation and early damage response of delamination is important. Conclusions: Combined with a data processing technique, a single cRDCB test enabled measurement of the full Mode I traction–separation response. In addition, the cRDCB provided high-resolution and could detect early-stage Mode I delamination damage in FRP laminates. The measured traction–separation responses can be directly inputted into cohesive zone models to predict the initiation and progression of Mode I delamination. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)