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Abstract The possibilities for detecting defects in multilayer carbon composites occurring as a combination of layers of carbon fiber fabric with thin layers of epoxy resin (CFRP) are presented in this paper. The most common defect in this type of structure is delamination. For non-destructive testing of CFRP samples with deliberately introduced defects, infrared thermography with laser thermal stimulation was used and both computer numerical simulations (using the ThermoCalc program) and experimental tests were carried out. In the experimental testing, a semiconductor laser with a wavelength of 808 nm and a maximum power of 32 W was used for thermal stimulation. Changes of temperature field on the surface of the test sample were recorded with a thermal camera, FLIR 7600 SC. Both pulsed and lock-in thermography methods were used, and this paper presents selected results, together with their analysis. The results presented in this paper show that it is possible to use a laser as a heating source in thermographic non-destructive testing of CFRP composite. [ABSTRACT FROM AUTHOR]

Highlights • This paper deals with the mechanical response of glued in rods. • It takes into account the diameter of rods, the material of rods and the direction of the grain of the timber. • It gives an overview and presents known design models and technical approvals comparing the different approaches. • Comparison between the experimental results and the main equations of the literature review. • The authors propose one equation for the case where the bars are parallel to the timbers' grain. • In addition, another one for the case where the bars are perpendicular to the timbers' grain. Abstract We address the mechanical response of glued in rods, taking into account parameters such as rod diameter, the material properties of the rods (carbon or glass fibre-reinforced polymer), and the grain orientation of the timber (rods parallel and perpendicular to the timber grain). All the experimental setups employ pull-out tests, where rods are glued in only on one side of the specimen, and these tests are assessed at the premises of the Laboratory for Composite Materials and Composite Structures at the University Claude Bernard Lyon I. In addition, we give an overview and present known design models, technical approvals and regulations, national standards, and guidance papers, comparing the different approaches among them and with the experimental results. Good agreement was shown and further useful results were observed. [ABSTRACT FROM AUTHOR]

Thermal expansion properties of textile reinforced composites with certain structure symmetries can be efficiently calculated by a size-limited unit cell. In this paper, a general approach is developed for the establishment of such a unit cell model. For the derivation of unit cell boundary conditions, three rules are summarized according to the displacement fields in translational, reflectional and 180° rotational symmetric structures under a uniform temperature change loading. The application scope of present unit cell method is clarified from the thermal and mechanical point of views. Three typical composites, i.e., unidirectional fiber reinforced composite, plain woven composite and multi-harness (4HS, 5HS, 6HS, 7HS and 8HS) satin woven composites are then studied, and four, three and two size-reducing unit cells are formulated, respectively. The thermal expansion behaviors of each composite are analyzed, and the effective thermal expansion coefficients are predicted. The influence of structure symmetries on the deformation pattern of unit cell models is clarified. The numerical models are validated by the identical results obtained from unit cells of different sizes and also by the results available in literatures. The approach developed in this paper can be applied to thermal expansion studies of any other composites with relevant structure symmetries. [ABSTRACT FROM AUTHOR]

In this paper, free vibration behavior of carbon nanotube (CNT) reinforced functionally graded thick laminated composite plates utilizing Reddy’s higher-order shear deformation theory (HSDT) is studied. To the best of authors’ knowledge, this paper is the first to incorporate HSDT with one of the element-free approaches to investigate this issue. The element-free IMLS-Ritz method is employed and four types of CNT distributions are considered. The resulting effective material properties of the CNT-reinforced composite are estimated by a detailed and straightforward Mori–Tanaka approach. The numerical results have been compared with the literature showing excellent agreement. Considering various CNT orientation angles, a parametric study showing the effects of CNT volume fraction, plate aspect ratio, plate width-to-thickness ratio and number of plate’s layers on the non-dimensional natural frequencies is investigated. Finally, the influence of boundary conditions on the sequence of the first six mode shapes for various lamination arrangements is presented. [ABSTRACT FROM AUTHOR]

In this paper, a piecewise shear deformation theory for laminated composite and sandwich plates is presented by integrating the advantages of the layerwise theory and equivalent single layer theory. A C 0 continuous four-noded quadrilateral isoparametric plate element based on this theory is developed for free and forced vibration analysis of laminated composite and sandwich plates in thermal environments. The accuracy and effectiveness of piecewise shear deformation theory and finite element formulation are validated by comparing the numerical results obtained by the present finite element formulation with the analytical and exact results available in literatures as well as the numerical results computed by MSC.Nastran software. It is demonstrated that the piecewise shear deformation theory and finite element formulation are suitable for the vibration analysis of thin and thick laminated composite and sandwich plates. As compared with the layerwise theory, high-order zigzag theory and high-order equivalent single layer theory, the piecewise shear deformation theory is able to produce a sufficiently accurate result at a very low computational cost. The work reported in this paper provides an efficient modeling approach for thermal vibration analysis of laminated composite and sandwich plates in practical engineering. [ABSTRACT FROM AUTHOR]

This paper presents the free vibration behavior of carbon nanotube (CNT) reinforced functionally graded composite plates in a thermal environment based on Reddy’s higher-order shear deformation theory (HSDT). The element-free kp-Ritz method is used in this study. Four different types of CNT distributions are considered. The literature reveals that there is a research gap in investigating the mechanical behaviors of CNT reinforced functionally graded composite plates using Reddy’s HSDT in association with any of the mesh-free methods. To the authors’ knowledge, this paper is the first to use this approach to investigate the vibration behavior of CNT reinforced functionally graded composite plates in a thermal environment. The rule of mixture is used to estimate the resulting effective material properties. To verify the reliability of the present model, the obtained numerical results based on a conversion study have been compared with those found in the literature with evident agreement. Moreover, parametric studies have been conducted on the effects of CNT distribution, boundary conditions, plate aspect ratio, plate thickness-to-width ratio and CNT volume fraction on the non-dimensional natural frequencies. Furthermore, the effects of plate aspect ratio and plate thickness-to-width ratio on the sequence of the first six mode shapes have been investigated. [ABSTRACT FROM AUTHOR]

Structural health monitoring of composite structures is essential for timely detection and localization of low velocity impacts. In this paper, a novel method for localizing impacts using an error outlier based impact localization algorithm is proposed. The outlier method for impact localization was developed and implemented by localizing several impact points on a quasi-isotropic composite structure. Multiplexed FBG sensors were attached to the composite structure to monitor low velocity impacts, sampled at a frequency of 100 kHz using a high speed FBG interrogator. Localization results for the error outlier based localization method were compared with the RMS and correlation based impact localization results. The novel localization technique presented in this paper demonstrates an effective means of estimating impact locations. Furthermore, overall improvement in the localization performance was obtained using the outlier based impact localization algorithm: impact points on the composite plate were localized with an average error of 10.7 mm. [ABSTRACT FROM AUTHOR]

The shear capacity of reinforced concrete beams strengthened with cement based composite materials (FRCM, Fabric Reinforced Cementitious Matrix) was investigated in the paper. The analysis refers to a FRCM system made by PBO (short of Polypara-phenylene-benzo-bisthiazole) fabric meshes with dry fiber strands disposed along two orthogonal directions bonded to concrete surfaces with a cement based mortar. A total of 9 reinforced concrete beams strengthened both in flexure and shear with PBO-FRCM have been tested varying both the shear strengthening configuration (continuous and discontinuous U-wrapped strips) and the PBO reinforcement ratio. The main issues focussed in the paper are: (i) the effectiveness of the PBO-FRCM system as shear strengthening system, (ii) the shear performance, including failure modes, of reinforced concrete beams strengthened in terms of the PBO, the internal transversal steel reinforcement (steel stirrups), the internal longitudinal steel reinforcements and the concrete strength, (iii) the interaction between transversal internal steel and external PBO-FRCM shear reinforcements and, by comparison with experimental results, (iv) the reliability of models found based on the Ritter–Morsch criteria failure to predict the shear capacity of PBO-FRCM strengthened reinforced concrete beams. [ABSTRACT FROM AUTHOR]

This paper presents a multi-scale procedure for the study of flat composite structures with discontinuities. In this procedure, the structure is solved using shell elements while the laminate performance and the structural discontinuities (e.g. connections or change in the laminate thickness) are analysed with a subscale model made with solid 3D elements. The kinematics of both models are coupled following the Kirchhoff–Love theory. This coupling is used during the homogenization procedure where the characteristic behaviour of the different micro-models is obtained. Periodical boundary conditions are used for the laminates whereas a combination between periodical and linear boundary conditions are used for the discontinuities. The proposed procedure allows to reproduce accurately the structure elastic behaviour, as well as the stress and strain states in regions with discontinuities, which until now could only be accurately simulated by means of expensive numerical models using volumetric solid elements. [ABSTRACT FROM AUTHOR]

Timely and effective health evaluation of carbon fiber reinforced polymer (CFRP) structures under dynamic fatigue loading is one of the key issues of CFRP in practical engineering applications. This paper uses a combination of acoustic emission (AE) and digital image correlation (DIC) to study the fatigue behavior of CFRP open-hole laminated beams with different stacking sequences under pure tensile loading, and analyze the influence of different damage mechanisms on the ultimate fatigue failure of the specimens. Two parameters, AE hit rate and absolute AE energy, are used to characterize the stiffness degradation behavior of CFRP structures. The strain field distribution characteristics during the damage failure of specimens with different stacking sequences are discussed in association with DIC. Based on the data obtained by AE and DIC, the damage modes of two types of layered CFRP laminated beams are identified by combining principal component analysis (PCA) and the K-means++ algorithm. The clustering results are consistent with the microscopic image results of the fracture surface. The identified modes are employed to characterize the dynamic evolution of fatigue damage in CFRP, and the contribution of different damage modes to the ultimate fatigue failure of the specimens is analyzed using cumulative damage index (CDI) values. [ABSTRACT FROM AUTHOR]

Variable-stiffness (VS) laminated shells with curved fibers have gradually become a promising lightweight structural concept in the aerospace field. However, as the complexity of the shell geometry and the fiber angle increases, the traditional finite element method (FEM) requires mesh refinement to guarantee accurate discretization, which significantly increases the computational cost, especially in the post-buckling analysis with multiple solving requirements. In this study, a non-uniform rational B-Splines (NURBS)-based degenerated solid shell formulation in the total Lagrangian framework is proposed for the post-buckling analysis of VS laminated shells. Since the nonlinear function related to the rotational degrees of freedom (DOFs) is additionally introduced into the displacement field expression, the restriction on the magnitude of the nodal rotations is effectively eliminated. In addition, the paper adopts the NURBS surface reconstruction technology based on the point-line-surface features to build B-spline CAD models from the point cloud with geometric imperfect features. Then, an integrated modeling-analysis framework for predicting the post-buckling behavior of VS structures with geometric imperfections is proposed. A series of numerical examples systematically demonstrate that the framework can effectively conduct the post-buckling analysis of shell structures with geometric imperfections. Meanwhile, the accuracy and robustness of the solutions by IGA are proved to be better than FEM. [ABSTRACT FROM AUTHOR]

In this paper, a multi-scale uncertainty analysis framework to deal with hybrid random and interval uncertainties is presented to analyse dynamic characteristics of FRP composite structures. The proposed method consists of a perturbation-based probabilistic analysis, particle swarm optimization interval analysis to address the hybrid uncertainties, and the Mori–Tanaka method based homogenization theory to propagate uncertainties from microscale to macroscale. The proposed numerical approach provides estimates of the natural frequencies of composite plates, with mean and standard deviation lower and upper bounds, at a low computational cost. Numerical examples are presented to demonstrate the feasibility of the proposed method for different forms of composite structures, and also to verify its effectiveness and accuracy in predicting natural frequencies. Results show that the proposed method is highly accurate in calculating the upper and lower frequency bounds for advanced composite laminates under complicated uncertainties. [ABSTRACT FROM AUTHOR]

Lamb waves are ultrasonic guided waves that propagate between two parallel free surfaces and their use for damage detection has been widely explored and demonstrated. Damage in materials/structures can be detected by analyzing the difference between the phase/group velocity and the loss of amplitude of Lamb waves on damaged and un-damaged specimens. The propagation characteristics of Lamb waves are described in the form of dispersion curves, which are plots of phase/group velocities versus the product of frequency-thickness generated by solving the Lamb wave equations. Lamb waves’ dispersion behaviors for isotropic materials are well established in the literature; however, such is not the case for the laminated composites. The most common methods for solving the Lamb wave equations in composites consist of using laminated plate theory or 3D linear elasticity by assuming an orthotropic and/or higher symmetry. This assumption may not be true, if the actuators and sensors in an orthotropic or transversely isotropic laminates are installed in a non-principle direction or the layup is symmetric but not balanced. This paper presents a full derivation of Lamb wave equations for n-layered monoclinic composite laminates based on linear 3D elasticity by considering the displacement fields in all three directions using the partial wave technique in combination with the Global Matrix (GM) approach. In the partial wave technique, the principle of superposition of three upward and three downward travelling plane waves are assumed in order to satisfy the associated boundary conditions. The bounded upper and lower surfaces reflect the waves and the combination of these reflections going towards the upper or lower interfaces results in the propagating guided waves. The GM approach is used to assemble all the equations from each layer to form a global, unified matrix that describes the displacement and stress fields along the entire laminate associated with the wave propagation. A robust method for numerically solving the Lamb wave equations is also presented. The presented method was verified experimentally by analyzing the propagation of Lamb waves in two different composite panels constructed out of unidirectional carbon-fiber epoxy prepreg and fiber-metal laminate (GLARE 3-3/4). The panels were instrumented with lead zirconate titanate (PZT) piezoelectric sensors, which were excited at different frequencies ranging from 20kHz to 500kHz to generate and acquire the waves. The waves were excited and gathered at three different propagation angles of 0°, 45°, and 90° for the carbon-fiber epoxy laminate panel and at six different angles of 0°, 20°, 45°, 70° and 90° for the fiber-metal laminates (GLARE). The phase and group velocities of the fundamental symmetric (So ) and anti-symmetric (Ao ) Lamb waves were extracted by tracking the peaks of each individual wave phase and the wave envelope respectively using an in-house code developed in MATLAB. It was found that the presented 3D linear elasticity model followed the experimental data closely for both symmetric and anti-symmetric Lamb modes. The analytical method presented in this paper was able to predict the Lamb wave dispersion for both the carbon-fiber epoxy laminate and the hybrid fiber-metal laminate proving the robustness and versatility of the solution method. [ABSTRACT FROM AUTHOR]

Abstract The application prospect of a permanently installed structural health monitoring (SHM) system for composite structural prognosis may be determined by the reliability, cost and the added weight of the system. In this paper, we aim at exploiting information encoded in multipath scattering Lamb wave signals, so that damage imaging could be achieved with as few as possible sensors. Firstly, dispersion compensation is applied to the residual signal between two measurement sequences, and thus the propagation distance of each wave packet could be identified. Assuming that acoustic rays propagate according to Fermat's principle, the multipath scattering between the known features (e.g. edges, stiffeners, lap joints, and rivets) and the damage is equivalent to adding a virtual transducer distributed in a symmetrical position. Thus enlarges the element-number and the aperture of the sensor array. On this basis, a modified elliptical method is established to accommodate for these multipath scattering signals for damage imaging. An experimental example is introduced, where the specimen is a carbon fiber-reinforced polymer (CFRP) laminate and the damage is simulated by an added mass. The results show that the damage could be correctly detected and accurately localized even with a single transmitter-receiver pair. [ABSTRACT FROM AUTHOR]

Abstract This paper provides an efficient method for performing global layup optimization of composite laminates with buckling and manufacturing constraints. The optimization problem is divided into two stages and is based on the use of lamination parameters. During the first stage, exact finite strip analysis and continuous optimum design are employed for buckling optimization of the lamination parameters and laminate thickness. In the second stage, a logic-based procedure combining the branch and bound method with a global layerwise technique is employed to find the optimal stacking sequences to match the optimized lamination parameters obtained in the first stage. In order to ensure the optimized layup can be used in practice, four manufacturing constraints are added into the logical search process, and the feasible region for the lamination parameters with a manufacturing constraint which requires at least 10% of each of four possible ply orientations is studied. By comparing the logic-based method with the use of a genetic algorithm for searching stacking sequences under different requirements, the high efficiency and ability to achieve a global optimal result of the logic-based method are demonstrated. [ABSTRACT FROM AUTHOR]

Abstract The paper deals with numerical and experimental investigations aimed to develop a Finite Element (FE) model for predicting wave propagation in a blended composite winglet. Material anisotropy, inhomogeneity, multi-layered configuration and complex geometries tend to increase the complexity of the wave propagation phenomena and consequently the development of established FE models. Moreover, even if 2D finite elements seem to be not appropriate for modelling guided waves propagation, especially for complex anisotropic structural components, they are more attractive than 3D ones, because of the computational cost saving. For this reason, part of the presented research activity is addressed to investigate the efficiency of shell finite elements in modelling guided waves propagation in a such complex structure as a winglet. The development of an efficient model depends also on the numerical characterization of the medium within which guided waves propagate through. As a consequence, preliminary simple experimental bending and modal tests have been carried out to support the material properties modelling. Subsequently, guided wave propagation FE analyses were performed and the results compared with provided experimental data. A good agreement between numerical and experimental results of the different analyses has been achieved in terms of both signal time of flight and amplitudes. [ABSTRACT FROM AUTHOR]

Abstract Recycling of construction and demolition waste (CDW) which contains materials with high resource value such as plastics, metals, wood, glass and concrete has been investigated in recent years due to tightening environmental legislation. Wood-plastic composites as an environmentally friendly material provide a possibility to enhance the recycling of material. The effect of the addition of CDW on the mechanical and moisture properties of WPC is studied in this paper. Three types of wood-plastic composites were manufactured with different amounts and composition of a CDW filler. Their properties were compared to the reference composite manufactured with virgin wood fibers and polypropylene as the polymer. It was found that the addition of the CDW filler weakened the mechanical properties of the composite generally, except for the fact that the Charpy impact strength was improved significantly. Minor variations in the composition of the CDW filler did not have an impact on the mechanical properties, but an increase in the amount of filler increased the deterioration of mechanical properties. For moisture properties, the addition of the CDW filler decreased the water absorption and thickness swelling of the composites. An increase in the filler content improved the thickness swelling more significantly than water absorption. [ABSTRACT FROM AUTHOR]

Abstract In order to benefit from the expected high luminosity performance provided by the Phase-I upgraded LHC at CERN, the New Small Wheel (NSW) will be installed in the ATLAS detector during the Long Shutdown 2. The ATLAS NSW will be equipped with a new technology for the detection of muons: the MicroMegas (MM) detectors. They consist of different panels made of composite materials and when charged particles traverse the drift space, the gas is ionized and electrons are liberated; the avalanche of electrons takes place in the amplification region after the mesh and they are detected by read-out strips to reconstruct the trajectory of muons produced after the collision. Very tight mechanical tolerances are given in the design phase and they must be preserved from the panel construction to the final operation in the ATLAS cavern. In this paper the construction procedure of these very precise particle detectors is described and the mechanical modelling to predict their mechanical behaviour is presented. Finally, the experimental tests done to validate the numerical models are discussed. [ABSTRACT FROM AUTHOR]

Abstract Elastic guided wave based evaluation is useful to detect delamination in composites. This paper reports a detailed account of guided wave interaction with delamination in laminated composites modeled using time domain spectral finite element (TSFE) method. Wave scattering due to different delamination positions is studied. Wave field near the delamination in different layer-wise positions is investigated, which is useful to further characterize and correlate the far field wave packet carrying the parametric signature of delamination. Off-axis delamination in composite laminate creates an asymmetry in the elastodynamic stress transfer. It leads to wave mode conversions. Sensitivity of the delamination to the wavelengths is studied. The outcome of this study indicates potential possibilities to use frequency-wavelength information to discriminate various sizes of delamination. Energy of the scattered waves and dissipation/conversion of the wave energy due to the defects depend on the resonance characteristics of the sub-laminates. Wave scattering effect due to length-wise multiple delaminations and edge delamination is also analyzed. The resonance patterns in the signals are analyzed with reference to the defect quantification problem. [ABSTRACT FROM AUTHOR]

Abstract This paper presents comparisons between experimental and numerical studies of low-velocity impact damage for thermoplastic (IM7/PEEK) and thermoset (IMS65/MTM) carbon fibre reinforced composites. The experiments were conducted at two key impact energies (8 and 30 J) under identical conditions allowing a systematic comparison to be made. Three LS-DYNA Finite Element Analysis (FEA) models (standard, continuum damage mechanics (CDM) and discrete) were implemented, all using cohesive interface elements for delamination. The role of Mode II fracture toughness is highlighted. The predictive capabilities of different modelling techniques are compared and discussed and the CDM model gave better correlation with experiments. Fibre failure was predicted by the numerical approaches. The thermoplastic materials did not show noticeably superior behaviour to the thermoset materials and were governed by unstable delamination damage propagation for the same impact energy. [ABSTRACT FROM AUTHOR]

Abstract This paper presents an investigation on flutter speed of cracked composite plates. This work is divided into two sections: (a) variation of crack length at a fixed location on the plate, and (b) variation of crack location on the plate with a fixed crack length, modelled as a unidirectional composite for 0 0 , 90 0 and 135 0 orientations. Mori-Tanaka homogenization model is applied to obtain the effective composite constitutive properties as the function of fiber and matrix volume fraction. Doublet Lattice Method (DLM) is used to calculate the unsteady aerodynamic forces, i.e., lift distributions. It is found that the existence of small crack ratio on the composite plate (less than 0.4) has triggered an increment of the flutter speed. To support this statement, flutter response modes for each crack ratio are plotted, where the structure appears to be more stiffened than the undamaged plate. However, the crack results in the reduction of flutter speed when the crack ratio reaches 0.5. For the crack location assessment, the flutter speed increases as the crack location moves from the root to the tip due to the reduction of flutter frequency. The results show a good agreement with the validation using Strip Theory considering unsteady aerodynamics. [ABSTRACT FROM AUTHOR]

A research project has been carried out at the University of Manitoba, Canada, to develop fibre-reinforced polymer (FRP) meteorological guyed towers. Both theoretical and experimental results are presented in this paper. The theoretical work involved the development of finite element models to analyze the structural behavior of an 81 m tower. The experimental work involved the testing of an 8.6 m long segment, representing the bottom segment of the 81 m guyed tower used in the analysis, under static and dynamic loading. This segment was constructed from three cells bonded together with epoxy resin to form an equilateral triangle shape. Each cell was fabricated using four layers of glass fibre matting for a total thickness of 5 mm with a sequence of [90°/0°/0°/90°] impregnated in epoxy resin. An extensive material testing program was also carried out to define the material properties for finite element analysis. The numerical results are compared with the experimental results to confirm the validity of the finite element models. [ABSTRACT FROM AUTHOR]

Abstract: The composite bistable structures discussed in this paper consist of a material-based mechanism with a saw-tooth strain/stress curve. They have a more damage-tolerant behavior, and considerably higher strain-to-failure than the corresponding monolithic material. Past work showed successful results with bistable composites made of ultra high molecular weight polyethylene (UHMWPE). This paper investigates a number of hybrid composite configurations, with geometry and manufacturing process inspired by the previous work. Bistable designs are based on the synergy of two parts, main links and waiting links, with a user-configurable balance of strain, strength and stiffness. In this paper, carbon and UHMWPE (Spectra®) hybrids are studied in terms of their maximum strain and specific strain energy. Considerably higher ductility was experienced for some configurations, while increased strength may be obtained in principle with a higher number of layers than those of the current work. [ABSTRACT FROM AUTHOR]

Abstract: Adhesively bonded joints are widely used due to their ease of fabrication, maintenance and controllable stress distribution in the overlap region. There are extensive investigations in literature that characterize the behavior of bonded joints with different geometries and materials (i.e. isotropic and anisotropic) and subject to various loading conditions. Although much work has been done on this subject, investigation into the effect of the material non-linearity on joint behavior is scarce. Our earlier experimental and numerical works revealed interesting, yet unpredictable results. This paper aims at addressing some of the ambiguities experienced in our previous work. The paper presents the development of numerical models of tubular joints made of composite/metallic adherends, subject to high strain fields as a result of torsional loading, with the consideration of both material and geometrical non-linearity. The results showed that the non-linearity of the adherends’ materials at high strain magnitudes played an important role in governing the bond efficiency in joints whose overlap length surpassed a certain limit. It was also realized that joints performance was also significantly affected by fabrication related issues. [Copyright &y& Elsevier]

• This paper presents the superior energy absorption capacity of hollow-cylindrical-joint hierarchical honeycomb composite tubes under axial quasi-static crushing. • The fiber orientation (α) has a significant effect on the energy-absorbing characteristics of these structures and specific energy absorption (SEA) tend to initially decrease and then increase with increasing α. • The SEA of the biomimetic multi-cell tubes can be significantly enhanced by employing different fiber orientations. • The highest SEA value measured on these structures, 77.3 J/g, is more than double measured on a traditional circular tube (34.9 J/g). This paper investigates the superior energy-absorbing characteristics of hollow-cylindrical-joint hierarchical honeycomb composite tubes under axial quasi-static crushing. Biomimetic multi-cell tubes (BMCTs), inspired by the forewing of a beetle, were manufactured using the vacuum bag moulding method. Quasi-static compression tests were conducted to characterize the force–displacement response, the progression of failure, the crashworthiness characteristics, and the corresponding energy absorbing mechanisms. The experimental results show that the fiber orientation (α) has a significant effect on the energy-absorbing characteristics of these structures. The crush load efficiency (CLE) and specific energy absorption (SEA) tend to initially decrease and then increase with increasing α. It has also shown that the SEA of the BMCTs can be significantly affected by employing different fiber orientations. The highest SEA value measured on these structures, 77.3 J/g, is more than double of 34.9 J/g measured on the single circular tube counterpart. The energy-absorbing capacity of the composite structures is devisable and can be enhanced by integrated biomimetic design. [ABSTRACT FROM AUTHOR]

In the present study, a novel theoretical model is developed, based on the energy method, to predict the equivalent mechanical properties of a new morphing structure with zero Poisson's ratio, which is composed of continuous fiber reinforced composite struts. Due to the employing glass fiber in fabricating the proposed cruciform honeycomb, higher strength than the structures made of pure isotropic materials is obtained. The use of cells with a zero Poisson's ratio also increases the flexural strength of the structure. In the continuation of the paper, by examining the geometric effects on the equivalent properties, a parametric study is performed. Then, using the appropriate failure criterion, the tensile and compressive strength of this network is obtained. The extracted properties are used in nonlinear analysis of the response of morphing structures and to improve the performance of these structures under different loads. It should be noted that in order to validate the relationships developed in this paper, the experimental tests and finite element methods are employed. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional finite element models have been developed to study the effects of bolt-hole clearance on the mechanical behaviour of bolted composite (graphite/epoxy) joints. In Part I of this two-part paper, the model was developed in the finite element code MSC.Marc and validated against experiments and other finite element solutions. In this second part of the paper, results from the model showing the effects of clearance in single-lap, single-bolt joints are presented and compared with experimental results. Both quasi-isotropic and zero-dominated lay-ups are studied. It is shown that increased clearance leads to increased bolt rotation, decreased bolt-hole contact area, and decreased joint stiffness. In single-lap joints, clearance is shown to cause three-dimensional variations in the stress distribution in the laminate, and these variations are also dependent on the lay-up; the radial and tangential stresses in each ply are shown to illustrate this point. Finally, a prediction of the failure onset load using Hashin’s failure criteria is given and the results are compared with experimental results. [Copyright &y& Elsevier]

In structures, high vibration levels can destroy components; therefore, the reduction of the vibration levels represents a fundamental target in the industrial field. The structural intensity (STI) field provides information about the energy flow that propagates within a structure. It shows the position of dampers in structures, thus, it can be used for solving structure-borne noise problems. The analysis of the energy distribution on a structure allows to understand where to implement locally concentrated modifications introducing damping patches in order to reduce structure-borne noise. The aim of this paper is to investigate the structural intensity in composite structures and to evaluate the influence of some modeling parameters, such as mesh density, structural damping, etc., on the accuracy of the results. In this paper, numerical investigations on the STI in a fiber composite plate are carried out. Two different numerical models are considered: the first one with solid elements, the second one with shell elements. The two-dimensional model does not allow to analyze and study the composite plate on micro level. The first law of thermodynamics allows to define the accuracy of the STI evaluations. The investigation on two-dimensional and three-dimensional composite structure modeling presents the limits and the advantages of the two different types of approach. The different characteristics of the STI fields provided by these models are analyzed in order to understand the difference between local and global approach. [ABSTRACT FROM AUTHOR]

Fiber reinforced polymer (FRP) composites, exhibit tremendous potential to supersede conventional materials in various manufacturing sectors due to their extra-ordinary properties. Nevertheless, a few obstacles hinder the application and promotion of FRP, among which poor connection performance is reviewed in this present paper. In order to overcome the shortcomings of traditional joining technology, hybrid (bonded/bolted) joining technology emerges prominently and becomes a hot research area with immerse engineering importance. Moreover, with the complexity of the design for hybrid joints (HJs), the finite element modeling (FEM) has become the main research method, which assists applications of HJs by systematic selection of parameters for efficient manufacturing. This review paper summarizes an up-to-date progress in FEM analysis for composite HJs, and it covers: modeling about numerical damage of HJs(consists of FRPs modeling, adhesive modeling and bolt modeling), simulation for manufacturing of HJs (consists of bonding technology, drilling process and installing fasteners) and simulation for the mechanical performance of HJs (including substrates geometry, adhesive parameters, multi-fasteners configuration, fastener-hole fit conditions and clamping force). Additionally, the existing numerical methods in literature are analyzed in detail followed by remarks on the current scope in the designing of HJs. This general review of simulation analysis methods for composite HJs not only helps to discover future research possibilities, but also provides an extensive guideline for new researchers to implement the simulation of HJs successfully and accurately. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer (CFRP) components can be generally prepared near-net-shape, however, they still need machining after manufacturing to meet the geometrical accuracy with excellent surface quality required for assembly. Due to the cutting temperature is prone to exceed the glass-transition temperature of the resin, the irreversible chemical and mechanical degradation are therefore difficult to avoid. It becomes a crucial challenge to eliminate the particular thermal effect on the composite machining process. In comparison with the temperature measurement, the analytical model not only can reveal the physical essence of thermal effect, but also can predict the temperature field distribution to provide the reasonable cutting parameters. In particular, a key parameter for calculating the cutting temperatures is the heat partition ratio. However, the research work on this issue was rarely found. In this paper, a fiber orientation-based analytical model was developed to predict the heat partition ratio based on the classical Hertz contact theory. The finite element model was also built with the validation of the experimental measurement from the thermal imaging tests. The results suggest the heat partition ratio is mainly determined by the cutting parameters. Moreover, the fiber orientations have a remarkable impact on this ratio. Due to the heat partition ratio is considerably larger in a CFRP workpiece than the cutting tool, more heat energy was transferred to the CFRP during machining. Therefore, a small depth of cut leads to a reduced tendency for thermal effect on the CFRP composites. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on low-velocity impact response of bi-material structures (BMS). BMS is a structural composite consisting of a 3D-printed lattice structure filled with a reinforcement material. In this study, BMS is made of polylactide (PLA) lattice structure and polyurethane (PU) foam for an enhanced impact resistance. Three different PU foams, including one rigid foam and two flexible foams, are selected for BMS. The impact tests are conducted to compare the impact attenuation, stiffness, toughness, and strength using the measured rate of change of acceleration (known as “jerk”), displacement, energy absorption, and the maximum acceleration, respectively. The results show that flexible foams have more positive effects on the impact properties than the rigid foam. The maximum reduction of jerk is about 9% compared to the baseline structure without foam reinforcement. The maximum displacement is increased by 17%; the maximum energy absorption is increased by 23%. The maximum acceleration remains similar for all samples. In conclusion, a proper selection of foam filling can significantly improve the impact properties. [ABSTRACT FROM AUTHOR]

External impacts have serious harm to fiber reinforced composite laminates, because Barely Visible Impact Damage (BVID) may happen, which reduces strength and stiffness of laminates. In this paper, an impact monitoring method is proposed to locate impacts for composite laminates based on a parameterized laminate model. The method is divided into two steps: (1) inverse material parameters by two impacts with known locations, which is called the calibration step. This step can be used to calibrate material parameters before actual applications of structures, or regularly as required to ensure the accuracy of impact localization. (2) Inverse impact locations when the material parameters are known, which is called the monitoring step. The arrival time of flexural waves extracted by continuous wavelet transforms is used as input features during the inverse process. A multi-frequency localization technique is adopted to overcome the inaccuracy of measurements of arrival time at certain frequencies. Finally, the proposed method with sufficient accuracy of impact localization is verified by experiments on a unidirectional laminate. [ABSTRACT FROM AUTHOR]

This paper deals with how the use of Polybenzimidazole (PBI) as a film coating over conventional composite designs could help improve the hypervelocity impact ballistic performance of the system. PBI coated composite samples were studied for resistance to Low Earth Orbit environment conditions like high vacuum, thermal cycling, Atomic Oxygen and Ultraviolet Radiation in a simulation facility. It was observed that the PBI coated composites reduced mass loss and surface erosion compared to the non-coated samples after LEO exposure. Hypervelocity impact experiments were conducted on the PBI coated composites for impact velocities between 2.5 to 3 km/s. The experiments showed that the PBI film coating significantly increased the energy absorption of the composite system. The effect of thickness increase as a result of the film application on energy absorption was also found to be negligible confirming the effectiveness of PBI coating as a hypervelocity shield. [ABSTRACT FROM AUTHOR]

Discontinuous unidirectional fiber-reinforced composites are shown to possibly exhibit pseudo-ductile failure that is lacking in continuous fiber composites. The aim of this paper is to use a discontinuous and interdigitated design strategy which mimicks the nacre structure to improve the specific energy absorption (SEA) of a carbon/expoxy composite tube. Quasi-static axial compressive experiment is combined with a digital image correlation system to analyze the failure process of the specimens. Four kinds of tubular specimens which are based on different ply cut intervals and distributions are fabricated and crushed. The load-crushed displacement curves and the SEA values are obtained showing that circular shaped continuous ply cuts result is the highest fluctuation of the compressive force. Moreover, the tubes with helical and nacre mimicking ply cut structures result in a flatter load-crushed displacement curve. This work demonstrates that in a crush process, unidirectional composites with a well-designed discontinuity at the ply level can improve the SEA over 51% as compared to the unidirectional continuous tubes. [ABSTRACT FROM AUTHOR]

This paper is a continuation of the investigations reported in the Part I. Based on the asymptotic approach and the lubrication theory, the composite with rhombic inclusions are studied. Models of composites with curvilinear rhombic inclusions and thin interfaces on phase boundaries are constructed with the help of non-smooth argument substitution and asymptotically equivalent functions. The effective conductivity is derived for absolutely conducting and non-conducting rhombic inclusions, taking into account thin interface effects. Generalisation of the classical Dykhne formula is proposed. Finally, asymptotic solutions are obtained for the effective conductivity of a hexagonal lattice with circular large-size and absolutely conducting/non-conducting inclusions. [ABSTRACT FROM AUTHOR]

In this paper, an analysis method for the determination of displacement, strain and stress fields in cylindrically curved cross-ply laminates under bending load is presented. The considered cross-ply laminates may be either symmetrically or unsymmetrically laminated and are clamped at one end while the other end is loaded by an evenly distributed bending moment. The analysis method employs a layerwise plane strain approach in the inner regions of the laminate in which the stresses in each layer are represented by adequate formulations for Airy’s stress function. In the regions of the free laminate edges where significant three-dimensional and possibly singular interlaminar stress fields are to be expected, the plane-strain approach is upgraded by a layerwise displacement-based formulation wherein the physical laminate layers are discretized into a number of mathematical layers with respect to the thickness direction. The governing differential equations for the unknown additional displacement functions with respect to the width coordinate in the form of the Euler-Lagrange equations stemming from the underlying variational statement can be solved exactly and eventually lead to an eigenvalue problem that needs to be solved numerically. Usage of continuity conditions between the individual laminate layers and formulation of adequate boundary conditions at the free edges in an integral sense then lead to complete representations for displacements, strains and stresses at every location in the considered laminate. While the analysis approach relies on a discretization of the laminate into a number of mathematical layers with respect to the thickness direction and further requires a numerical solution of a quadratic eigenvalue problem, it provides closed-form analytical solutions concerning the width direction and can thus be classified as being a semi-analytical solution. The presented analysis method is compared to the results of comparative finite element simulations and is shown to be in good agreement, however with only a fraction of the computational effort that is required for according finite element simulations. [ABSTRACT FROM AUTHOR]

This paper presents a novel parallel optimization method for obtaining blended layups of composite laminates which closely match target lamination parameters. Firstly, a guide-based adaptive genetic algorithm (GAGA) which stochastically searches the layups is developed. Then the parallel optimization method DLBB-GAGA is developed by combining GAGA and a dummy layerwise branch and bound method (DLBB) which performs a logic-based search in a parallel computation, during which optimization information is shared between the two methods. The combination of these two different methods gives the parallel DLBB-GAGA method the advantages of both, enhancing the searching ability for the blended layup optimization. The superiority of the parallel DLBB-GAGA method is demonstrated through comparisons between the three methods, and it is concluded that the method is particularly effective for practical design where more layup design constraints are considered. [ABSTRACT FROM AUTHOR]

The paper focuses on the calculation of the effective elastic properties of a laminated composite shell with imperfect contact between the layers. To achieve this goal, first the two-scale asymptotic homogenization method (AHM) is applied to derive the solutions for the local problems and to obtain the effective elastic properties of a two-layer spherical shell with imperfect contact between the layers. The results are compared with the numerical solution obtained by finite elements method (FEM). The limit case of a laminate shell composite with perfect contact at the interface is recovered. Second, the elastic properties of a spherical heterogeneous structure with isotropic periodic microstructure and imperfect contact is analyzed with the spherical assemblage model (SAM). The homogenized equilibrium equation for a spherical composite is solved using AHM and the results are compared with the exact analytical solution obtained with SAM. [ABSTRACT FROM AUTHOR]

This paper offers a new insight into the computational modelling of crack and delamination of carbon fiber composite. Both transversal cracks (intralaminar) and delamination (interlaminar) are modelled with Extended Finite Element Method (XFEM). Constitutive and fracture laws are integrated to model the initiation of crack or delamination, and their subsequent evolution. The study includes the size effect assessment of composite due to the increment of composite thickness. The results are in close agreement between the experimental and analytical data of each specimen modelled based on the size of the carbon fiber composite volume. [ABSTRACT FROM AUTHOR]

The present paper investigates the effectiveness of two-steps clinching for joining aluminum and Carbon Fiber Reinforced Polymer (CFRP). To this end, different reshaping tools were involved after joining by clinching with split dies. The reshaping force was also varied. Single lap shear tests of simple clinched and reshaped specimens were performed to determine the influence of the process conditions on the mechanical behavior of the joints. The morphology and geometry of joints were analyzed to understand how the reshaping step influences the main quality criteria of the joints and the damage produced on the CFRP sheet. According to the experimental results, simple clinched connection failed by pullout; thus, the critical parameter was the undercut. The reshaping method, using optimized tools enabled to increase the strength of the joints by 32%. Such an improvement was due to an increase in the undercut that resulted from a higher and improved material flow. The main defects that may appear in these joints have been identified as well as their causes. [ABSTRACT FROM AUTHOR]

This paper presents a first know study of the effect of orientation angle of carbon nanotubes (CNTs), embedded in a single-ply polymer-based CNT-reinforced composite, on buckling behavior of skew plates using an isogeometric analysis. The plate model is based on Reddy’s higher-order shear deformation theory (HSDT). Four in-plane loading conditions together with simply supported and cantilevered boundary conditions have been considered. The optimum CNT fiber orientations for CNT-reinforced composite rhombic plates with varying skew angles and boundary conditions are presented. Moreover, the effect of width-to-thickness ratio on optimum CNT orientation is explored. The results show that the efficiency of the skew plate structures can be significantly improved by simply placing the CNTs in the correct orientation. [ABSTRACT FROM AUTHOR]

The effects of the hydroelastic coupling on structures in contact with fluid are pronounced when composite materials are used due to the low composite density. This paper presents an analytical solution for the free vibration analysis of thick rectangular composite plates in contact with a bounded fluid. The plate displacement field is analyzed using Carrera Unified Formulation in order to consider displacement theories of arbitrary order. The displacement variables are approximated using the Ritz method. Analysis considering any of the classical boundary conditions is performed using suitable functions in the Ritz series. The fluid kinetic energy is obtained by considering an inviscid and incompressible fluid, and thus the velocity potential is applicable. The kinetic and potential energy of the plate are also obtained, and the energy functional is used to derive the eigenvalue problem. Validation is performed with results in the open literature and by using 3D finite element software. The influence of the fiber orientations on the natural frequencies is discussed, as well as the influence of the plate and fluid domain geometry. Certain natural frequencies are seen to be independent of the fluid domain depth, depending on the symmetric or antisymmetric nature of the normal mode. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the thermomechanical behavior of hollow square columns made of pultruded glass fiber reinforced polymer (GFRP). The specimens are subjected to monotonic and cyclic loadings until compression failure occurs in conjunction with elevated temperatures ranging from 25 to 175 °C. The twofold investigation emphasizes material- and structure-level responses, including dynamic mechanical analysis, constitutive behavior, load-carrying capacity, and failure mode. GFRP phase transitions (from elastic to plastic) due to thermal distress influence displacement compatibility at the boundary between the fibers and resin. Consequently, the stress-strain behavior changes from linear to bilinear ascending, accompanied by discrete composite-rupture and scattered fiber-tearing failure modes, depending upon temperature exposure. With respect to the capacity of the columns, the effect of the monotonic and cyclic loadings is marginal regardless of temperature. By contrast, the specimens subjected to cyclic loading show a higher coefficient of variation in comparison with those tested monotonically. With an increase in temperature, the location of primary failure shifts from the bolted connection in the vicinity of the support toward the middle of the column, which is attributable to minor lateral translation of the column induced by the thermally weakened resin. Using a probability-based model, a temperature-dependent capacity reduction factor is proposed. The alteration of functional probability-domains in thermomechanically-loaded columns is examined based on an uncertainty quantification method (i.e., metric distance measure). [ABSTRACT FROM AUTHOR]

• Compressive buckling behavior of damaged composite stiffened panel is investigated. • Skin between two ribs is the first location where the buckling failure occurs. • Stiffened panel still with a strong load capacity at post-buckling stage is verified. • Good agreement among theory, simulation and experiment has been achieved. • Advice for improving structural anti-buckling and material utilization rate is given. The compressive buckling and post-buckling behaviors of J-type composite stiffened panels before and after impact load were investigated through theoretical, numerical and experimental methods. In this paper, the load-bearing characteristics, including the buckling strength, ultimate strength and failure process of the intact composite stiffened panels were theoretically predicted and tested. Next, the impact tests were carried out on both the center of the skin and the back of the rib of the J-type composite stiffened panel, and then the compression test was performed on the damaged stiffened panel. The residual strength and failure process of the damaged J-type stiffened panel were obtained. By means of numerical simulation, the effects of different degrees of impact damage at the center of the skin on the buckling and post-buckling behavior of stiffened panels were revealed. Good agreement among theoretical, numerical and experimental results has been achieved. Finally, the influence of the adhesive layer, lay-up method of both the stiffened rib and skins, and geometric dimensions of the J-type ribs on structural buckling and post-buckling behaviors was systematically studied. The outcome from this research provides reasonable suggestions for improving the buckling load, ultimate load and material utilization rate of stiffened panels. [ABSTRACT FROM AUTHOR]

• Compressive buckling behavior of damaged composite stiffened panel is investigated. • Skin between two ribs is the first location where the buckling failure occurs. • Stiffened panel still with a strong load capacity at post-buckling stage is verified. • Good agreement among theory, simulation and experiment has been achieved. • Advice for improving structural anti-buckling and material utilization rate is given. The compressive buckling and post-buckling behaviors of J-type composite stiffened panels before and after impact load were investigated through theoretical, numerical and experimental methods. In this paper, the load-bearing characteristics, including the buckling strength, ultimate strength and failure process of the intact composite stiffened panels were theoretically predicted and tested. Next, the impact tests were carried out on both the center of the skin and the back of the rib of the J-type composite stiffened panel, and then the compression test was performed on the damaged stiffened panel. The residual strength and failure process of the damaged J-type stiffened panel were obtained. By means of numerical simulation, the effects of different degrees of impact damage at the center of the skin on the buckling and post-buckling behavior of stiffened panels were revealed. Good agreement among theoretical, numerical and experimental results has been achieved. Finally, the influence of the adhesive layer, lay-up method of both the stiffened rib and skins, and geometric dimensions of the J-type ribs on structural buckling and post-buckling behaviors was systematically studied. The outcome from this research provides reasonable suggestions for improving the buckling load, ultimate load and material utilization rate of stiffened panels. [ABSTRACT FROM AUTHOR]

This paper presents an analytical solution for the buckling and free vibration analysis of laminated beams by using a refined and generalized shear deformation theory which includes the thickness expansion. The eigenvalue equation is derived by employing the Rayleigh quotient, and the Ritz method is used to approximate the displacement field. The functions used in the Ritz method are chosen as either a pure polynomial series or a hybrid polynomial-trigonometric series. The hybrid series is used due to the superior convergence and accuracy compared to conventional pure polynomial series for certain boundary conditions. The boundary conditions are taken into account using the penalty method. Convergence of the results is analyzed, and numerical results of the present theory are compared with other theories for validation. Nondimensional natural frequencies and critical buckling loads are obtained for a variety of stacking sequences. The effect of the normal deformation on the fundamental frequencies and critical buckling loads is also analyzed. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of CAI (compression after impact) of tapered composite laminate. Two types of layup with thickness changing from 4 mm to 6 mm are considered. A new CAI testing rig, suitable for the specimens and a brief description of impact damage are presented first. Then, the behavior of impacted specimens under compressive loading is discussed along with the effects of ply drop-off parameters (taper angle, ply drop-off disposition and configuration) and impact point location on the failure mechanism. Finally, the residual strength in CAI of tapered laminates is compared with equivalent flat laminates. The results show that compressive behavior of the specimens is mostly governed by a coupling between compression and bending, generated by the discontinuity of the neutral axis in the tapered region. Despite this difference of behavior with flat laminates, the presence of ply drop-off has little effect on the residual strength in CAI. [ABSTRACT FROM AUTHOR]

This paper considers the conduction of heat in composite media in the steady-state. The original domain is subdivided into part domains and it is investigated how a part domain influences the complementary structure and the temperature field. This can be represented by two interfacial effects. Firstly, a part domain acts as a perfect thermal sink which effect is described by a homogeneous interface condition of the first kind on its interface(s), which enables to separate the part domains and to establish a temperature part solution within them independently. Secondly, the temperature field is corrected for the finite conductivity of the part domains which effect is described by re-forming the original structure and feeding back the opposite of the fluxes, which had been “sunk” previously, at the interfaces. At this step, all the other external and internal excitations are homogenized. A general, non-iterative procedure for obtaining the steady temperature field in composite media is thus also established. Practical applications of the developed procedure are discussed, e.g., improvements and extension of analytical and coupled (numerical combined with analytical) solution techniques for the heat equation. An illustrative example is also presented. [ABSTRACT FROM AUTHOR]

Fish scale structures have been known to resist penetration when subject to localized impact, while cellular materials such as cork are lightweight and have good energy absorption capacity. Combining the fish scale structure with cellular material can lead to a composite structure with improved performance against low-velocity impact. In this paper, the feasibility of such composite system is explored as a protective layer against low-velocity impact. Finite element simulations are conducted to investigate the mechanical behavior of the fish scale-cellular composite system, followed by experimental validation of finite element models using drop-weight impact test. Results from simulation indicated that specimens with curved scales produce the lowest peak stress transferred to a protected object or surface, and out-perform sandwich specimens with the same volume of materials. Most importantly, there exists an optimum range for the stiffness and strength of the scales relative to the underlying layer for effective dissipation of the impact energy and minimal stresses transferred to the protected object. [ABSTRACT FROM AUTHOR]

Fibre kinking is one of the main failure modes of composite laminates under compression loading. In this paper, the role of kinking in the failure of quasi-isotropic composites subjected to a bearing load is investigated. High-resolution CT scans show that kinking is largely involved in the events leading to laminate collapse, notably by triggering other damage modes such as delamination. Kink bands develop extremely progressively, leading to the formation of a wide localization zone (or FPZ, failure process zone). Such behaviour calls for a non-local modelling approach. Local damage models would lead to overly conservative sizing. A simple model, based on Hashin failure criteria and non-local effective stresses is confronted to experiments, and its limits are highlighted. It will be shown that proper modelling of the bearing failure requires the characteristic behaviour of kink bands to be taken into account. [ABSTRACT FROM AUTHOR]