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The quality of transformer insulation is related to the safe and stability operation of the power grid, and the life of the transformer oil-paper insulation system mainly depends on the state of paper insulation. With the continuous improvement of the voltage level, the requirements for insulating paper are becoming higher and higher. Based on the molecular dynamics simulation technology, the effects of carbon nanowires with three different structures on the thermodynamic properties of cellulose insulating paper were analyzed from the aspects of mechanical parameters, MSD (mean square displacement), CED (cohesive energy density) and hydrogen bond. The simulation results showed that the carbon nanowires with three different structures have improved the thermodynamic properties of cellulose. Among them, the hard chiral carbon nanowires have the most significant improvement in the thermodynamic properties of cellulose. The elastic modulus, bulk modulus, and shear modulus have been increased by 30%, 25.6%, and 32.9%, respectively. It was found that the thermal stability of cellulose insulation paper modified by three different structure carbon nanowires was improved. The increase of thermal stability was mainly due to the increase of the cohesive energy density and the number of intermolecular hydrogen bonds.

The mechanical response of three different structural core sandwich panels in out-of-plane compression and shear has been analyzed. Specific core shapes examined are arc-tangent, wavy trapezoidal and hemispherical. Unit cells consisting of representative elements of the core attached to face sheets were selected for analysis. Both face sheets and core were assumed made from paper. Finite element analysis employing large deformation and rotations and orthotropic elastic–plastic behavior was used. The results show that the arc-tangent and trapezoidal cores are prone to collapse by extensive bending and buckling, whereas the hemispherical core behaved more stably in compression and shear. Core sheets with a hemispherical shape were prepared from copy paper sheets in a specially designed forming machine. Sandwich test specimens were prepared from this core and tested in out-of-plane compression, and the load-displacement response was compared to predictions from finite element simulations. The experimental and finite element results were consistent.

This review paper provides a key information, allowing readers to comprehensively understand the direct tensile behavior and test methods for textile reinforced cementitious composites (TRCCs). The tensile parameters of TRCCs corresponding to bilinear or trilinear tensile stress–strain behavior were summarized and the influencing factors on the tensile parameters were discussed. Various direct tensile test methods for TRCCs were evaluated according to the geometry of specimens, load transmitting mechanism, measurement and loading methods. Conclusive requirements for a standard test method for assessing the tensile stress–strain behavior of TRCCs were proposed.

Natural fibers are gaining much interest in composite materials because of their potential to replace conventional glass fibers. In particular, unidirectional flax fiber composites have shown excellent mechanical properties with different thermoset matrices. However, few data are available when they are made with a biodegradable thermoplastic matrix. This paper compares the mechanical properties of two types of unidirectional flax composites using polylactic acid as the matrix: one made with layers of aligned flax rovings alone and the other containing an additional paper layer fabricated using paper making techniques. The results show that specific tensile properties of the flax/PLA (252 MPa·cm3·g−1) and flax-paper/PLA (217 MPa·cm3·g−1 ) composites are similar to those made using woven glass fabrics impregnated with epoxy (227–278 MPa·cm3·g−1). Very high impact strength (600 J/m) was also obtained for UD flax-paper composites compared to unreinforced resin (15 J/m). Together these mechanical properties are promising for industrial applications of this new reinforcement.

The dynamic compression behaviors of aramid paper honeycombs, i.e. samples with heights 2.10 mm, 3.10 mm, 5.14 mm, and 10.11 mm, were investigated by the split Hopkinson pressure bar devices (SHPB). The results showed abnormal size effect in the peak stress for samples with height 2.10 mm. The high speed camera was used to capture the deformation process. Two deformation modes, i.e. the in-plane shear wrinkles (characterized as larger deformation and lower strength in the stress-strain curves) and the out-of-plane wall buckling (characterized as smaller deformation and higher strength in the stress-strain curves), were distinguished from the complicated deformation processes. The abnormal size effect was resulted from the dominance of in-plane shear failure for samples 2.10 mm, rather than the out-of-plane wall buckling for other samples. Further investigations were conducted in details on the effects of cell width, wall thickness, and ends constraint condition (i.e. ends bonded with face sheets) to the deformation process based on the experiments, theoretical analyses of buckling, and Finite Element method (FEM). The transition of two deformation modes was analyzed, and the abnormal size effect would disappear for samples with larger wall thickness. It might be helpful for design purpose of honeycombs to some extent.

This paper deals with the effect of various thermal modification temperatures (160 °C, 180 °C and 210 °C) on changes in the basic chemical components of wood, and the effect of these changes in wood on the impact bending strength (IBS) of European oak ( Quercus robur L.) and Norway spruce (Picea abies (L.) H. Karst.) wood in comparison with unmodified wood. The IBS was determined according to ISO 13061-10 (2017), and the chemical composition (extractives, lignin, holocellulose , cellulose and hemicelluloses) by NREL methods (wet chemical methods). During the thermal modification the IBS increased in both wood species at first (temperature of 160 °C), but at higher temperatures these values gradually decreased. The highest drop in IBS values, found at a temperature of 210 °C, was 32.2% for oak and 39.8% for spruce. Thermal modification caused an increase in the relative content of extractives , lignin and cellulose, but the relative content of hemicelluloses in the wood was lower due to the increase in the thermal modification temperature. Changes in IBS correlate with chemical changes in both wood species (especially in the case of cellulose); closer correlations were found in spruce.

This paper reports an experimental and numerical investigation of the effects of gaps and overlaps on the buckling behavior of variable-stiffness composite laminates. In the experimental study, variable-stiffness composite laminates with a constant-curvature fiber path were manufactured and tested under uniaxial compression to failure with simply supported edges. The tested panels were optimized to simultaneously maximize the in-plane stiffness and the buckling load. Two manufacturing strategies – complete overlaps and complete gaps – were adopted to allow the independent effect of each type of defect to be investigated in isolation. In the numerical study, a two-dimensional finite element model was built using the commercial software Abaqus through a Python input script. A MATLAB routine was also implemented to localize the gaps and overlaps within the studied variable-stiffness laminates. A linear buckling analysis was performed to calculate the pre-buckling strength and the critical buckling load for each tested composite laminate. Thereafter, a nonlinear analysis using the Riks method was performed to predict the load–displacement relationship, considering the geometric imperfections of cured composite laminates. A good correlation was observed between the results obtained from the finite element simulations and from the experiments.

Pultruded glass fibre reinforced polymer (GFRP) profiles have low weight, high strength and corrosion resistance, but their brittle failure raises concerns about their use in seismic regions. Moreover, although their static monotonic response is reasonably well understood, the cyclic and hysteretic behaviour of GFRP frame structures and their beam-to-column connections have not yet been comprehensively investigated. This paper presents experimental and numerical investigations on the cyclic behaviour of a novel tubular GFRP beam-to-column sleeve connection system, comprising internal metallic parts. Four series of the connection system were tested, with varying number and position of the beam connection bolts, namely with: (i) one bolt in the webs (W1); (ii) two bolts in the flanges (F2); (iii) four bolts in the flanges (F4); and (iv) two bolts in the flanges with larger edge distance (F2S). The results show that series W1 presents the worst overall cyclic behaviour. On the other hand, the addition of bolt rows (F4 vs. F2) did not improve the cyclic response of the connection system. Conversely, increasing the edge distance (F2S vs. F2) led to significant improvements of the hysteretic behaviour, namely in the capacity to dissipate energy. Using the Pivot hysteresis model in the numerical study, a design-oriented model comprising frame and link elements was developed to simulate the response of the best performing series (F2S). In spite of its simplicity, the numerical simulation provided good agreement with the experimental results. In a companion paper, the behaviour of full-scale frames comprising F2S connections under monotonic and cyclic quasi-static sway tests was experimentally assessed. Numerical models of these tests were also developed to simulate the cyclic behaviour of the frames, using the parameters of the Pivot hysteresis model calibrated herein.

Composite frames in aerospace applications often show a lack of dimensional fidelity, which creates the necessity of costly shimming efforts within final assembly lines. A numerical study on C-profile frames has been performed to examine the mechanisms inducing these distortions. The paper shows that distortions are directly related to the composite’s material architecture, the part geometry and interestingly also to the state of the art zone-based frame design philosophy applied today. The paper shows that frame distortions arise from the 3D orthotropic nature of composite laminates , in-plane CTE inhomogeneity of laminate zones and coupling of different part areas. The main mechanisms inducing frame distortions are deduced from the numerical models. The paper also shows that trimming operations, which are necessary to bring frames from the manufactured to the engineered shape, significantly affect the frames’ final distortion state, the internal residual stress level as well as corresponding assembly forces during assembly.

This paper aims to highlight the effect of SiO2 substrate on the buckling behavior of Zinc Oxide (ZnO) nanotube . ZnO nanotube is modeled in various length and diameter to show the effect of size on buckling loads. Silicon Dioxide (SiO 2) is the material on which Zinc Oxide (ZnO) is grown and used as gas/chemical sensor. In present paper, SiO2 substrate is modeled as two parameters elastic layer. In order to consider the elastic substrate, Winkler and Pasternak foundation models are used. Different values of elastic foundation parameters are taken into consideration to see the effect of these parameters on buckling loads of Zinc Oxide (ZnO) nanowire . Additionally, to take the size effect into consideration on calculating buckling loads, nonlocal elasticity, surface elasticity, modified couple stress , modified strain gradient theories are used. Some benchmark results have been depicted. Some detailed results are also given and compared in figures and tables.

Current challenges in the automotive industry are the reduction of fuel consumption and the CO2 emissions of future car generations. These aims can be achieved by reducing the weight of the car, which further improves the driving dynamics. In most currently mass-produced cars, the body accounts for one of the largest parts by weight, and hence designing a lightweight car body assumes great importance for reducing fuel consumption and CO2 emissions. Extremely lightweight designs can be achieved by using purely composite materials, which are very light but also highly cost intensive and not yet suitable for large scale production due to the necessity of manual processing. A promising approach for the automated, large-scale production of lightweight car structures with a high stiffness to weight ratio is the combination of high strength steel alloys and CFRP prepregs in a special hybrid material/fiber metal laminate (FML) – which can be further processed by forming technologies such as deep drawing. In current research work at the Chair of Forming and Machining Technology (LUF) at the University of Paderborn, innovative manufacturing processes are being developed for the production of high strength automotive structural components made of fiber metal laminates. This paper presents the results of technological and numerical research that is currently being performed at the LUF into the forming of hybrid fiber metal laminates. This paper focuses on the results of basic research and the individual measures (tool, process and material design) necessary for achieving the desired part quality.

The paper analyses the possibilities and effectiveness of using infrared thermography with optical thermal excitation by reflection approach in the case of multilayer composites reinforced fibers . The advantages and limitations of this technique have been demonstrated via composites reinforced with three types of fibers: glass, aramid, and carbon. The most common defects in such structures are: delaminations , fiber cracks, matrix cracks , and separation of fibers from the matrix. The paper describes some results of the numerical simulations of defects detection using optical IR thermography with the emphasis made on the inspection of multi-layer composite materials. The simulations were conducted in order to determine the detection of defects depending on the depth of their location under the front surface of composite material as well as their geometrical dimensions.

This paper presents an experimental testing of a pultruded glass fiber reinforced polymer (FRP) structure used for emergency applications continuing the discussion presented in a previous paper (part 1) where the study of the characteristics of material and elements are presented. First, the design of the composite structure and components and the evaluation of the structural behavior by means of numerical and analytical approach according to current regulatory codes are described. In this frame, the global and local response was observed according to load paths deriving from the design loads at limit states. Then, the experimental test on a full-scale 2D model is presented at different load configurations up to the failure of the composite structure, comparing the experimental results with the load capacity predicted by the numerical model. The results indicate that the high performances of the structure provide a very good capacity in view of facing service loads and also in extreme conditions. Moreover, it was observed that the actual capacity obtained by the experimental test is much higher compared to the bearing capacity provided by the design according to current technical codes.

This paper presents an investigation of the free vibration of annular sector sandwich plates with carbon nanotube reinforced face-sheets and various edges boundary conditions. It is assumed that carbon nanotubes are radially aligned and distributed uniformly (UD) or functionally graded (FG) in the thickness direction. The effective material properties of CNT reinforced face-sheets are estimated using the extended rule of mixture, which contains efficiency parameters to consider the size-dependent material properties. A variable kinematic model which has been utilized for FGM cases is extended to FG-CNTs to describe the continuous variation of properties through the thickness. In this paper, the Carrera’s Unified Formulation (CUF) is developed for the analysis of asymmetric sector plates for the first time. The governing equations and associated boundary conditions are obtained employing the Principle of Virtual Displacements (PVDs) based on the CUF and solved using the generalized differential quadrature (GDQ) method. Numerical results for some special cases are presented and validated by comparison with available results in the literature. Some numerical results are tabulated to show the effects of the volume fraction of carbon nanotubes, boundary conditions and geometrical parameters on the free vibration behavior of the annular sector sandwich structures with CNTRC face-sheets.

Terrestrial laser scanning is extensively adopted in the area of high-precision monitoring and three-dimensional measurement. Architectural structures today are increasingly complex and health monitoring plays an important role in guaranteeing their safety. Therefore, how reliability deformation monitoring can be improved is one of the key problems in the field of engineering. This paper combines the three-dimensional laser scanning technology and finite element method (FEM) to investigate the deformation mechanism of arched structures. Within this paper, we simulated arched structures using the FEM, which is consistent with the result of terrestrial laser scanner (TLS) measurement. We aimed at constructing an intelligent and efficient FEM model which can be extensively applied in the monitoring of many constructs, such as bridges and ancient architecture. The focus in this research lies mainly on deformation analysis, which is based on FEM model simulation with the calibration of TLS measurement.

Extensive studies have been conducted on the use of fibre-reinforced polymer (FRP) as a confining material in hybrid tubular columns for civil construction, where the design of columns is often controlled by the stiffness and/or strength requirements. By contrast, the capacity of sustaining large deformation without losing structural integrity can be critical in some applications such as the standing supports for underground mines. This paper presents the conceptual development of a novel column form with large deformation capacity. The novel column consists of an outer FRP tube, and an infill made of coarse lumps/aggregates, which can be from coal rejects or other waste/recycled materials, as well as calcium sulfoaluminate (CSA)-based cementitious material with high water content. In addition to its large deformation capacity, the new column allows the extensive, direct and easy use of waste materials and eliminates the need for mixing concrete on site or transporting commercial concrete. This paper also presents the results from a series of compression tests on the new columns as well as two similar column forms. These tests demonstrate the very large deformation capacity of the new column and show that an existing stress-strain model for FRP-confined normal concrete can be used to provide reasonable predictions of the behaviour of the confined infill material in the new column. The potential applications of the new column and the needs for future research are also discussed.

Skin antenna structure is a kind of multifunctional composite structure in which printed antennas are integrated. This paper proposes a new smart skin antenna structure with embedded fiber Bragg grating (FBG) strain sensors. Utilizing the measured strains from limited FBG strain sensors, the deformation shapes are reconstructed by using a strain–displacement transformation, and the deteriorated radiation patterns caused by the deformations are compensated by using a strain-electromagnetic coupling mode which is derived and validated for the first time in the paper. Moreover, a two-step sensor placement method considering the information redundancy is proposed to determine optimal FBG strain sensor locations. The comparisons show that the two-step sensor placement method is advantageous in terms of the sensor distribution and computational cost. The fabrication process of the smart skin antenna structure was presented, and an experimental system with optimal sensor placements was developed. The measured results demonstrate that the reconstructed shapes agree well with the real ones, and that the deteriorated radiating patterns were almost completely restored to the desired ones. The proposed smart skin antenna structure which can provide the electromagnetic receiving-sending, structural deformation monitoring and electrical correction functions has a potential application prospect in the future mobile vehicle.

In this paper, a general vibration analysis of functionally graded porous (FGP) structure elements of revolution with general elastic restraints is performed for the first time. Under the basic framework of the first order shear theory, the moderately thick and thin FGP structure elements of revolution are considered. The specific FGP structure elements include the cylindrical shell, conical shell, spherical shell, cylindrical panel, conical panel and spherical panel. In order to simulate general elastic restraints, the artificial spring boundary technique is adopted this paper. And the modified series solution composed of a standard cosine Fourier series and two auxiliary terms is introduced in the Rayleigh-Ritz method to express the admissible function. A series of numerical examples are conducted, which show that the current model has superior convergence characteristics, computational accuracy and stability. On this basis, a series of innovative results are also given in the paper, which may provide basic data for other algorithm research in the future.

Failure prediction for composite materials is still a major area of interest within both the academic literature and the composite industry. This paper presents a modelling and experimental framework which has been developed for the application of Strain Invariant Failure Theory (SIFT), but which can be adapted to other failure prediction methods if required. SIFT is set of physics-based failure criteria developed to predict the onset of irreversible deformation of glassy polymers. The paper provides an overview of the available evidence for SIFT and provides a complete methodology for applying SIFT to predict the onset of failure in a composite component. Fundamental to the method presented is a modular and hierarchical dehomogenisation procedure; allowing for the evaluation of stress and strain state variables within the constituent components of the composite. Due to the constant evolution of composite characterisation techniques, the method presented in this paper is designed to be extensible (i.e. accommodating of new materials and product forms) and modular (i.e. subsections of the method can be easily substituted or refined). Known limitations of the current method are clearly presented and discussed. A companion paper will present validation of the approach for simple uniaxial and biaxial coupon experiments and will discuss how the method can be applied to a range of existing composite characterisation testing standards.

Thrust force is a remarkable intermediate variable in the drilling-countersinking process of carbon fiber reinforced plastic (CFRP) and aluminum (Al) stacks. Because of the poor machinability and extreme abrasiveness of CFRP, tool wear cannot be neglected. Focusing on the CFRP drilling and countersinking thrust force of stacks, this paper provided a novel insight into the modeling of the influence of tool wear on thrust force through both experiments and theoretical analysis. The experimental exploration is first presented in this study, and the flank wear was observed and it is the primary abrasion type. Then a method of infinitesimal elements with substitution has been used to develop the theoretical model of thrust force based on the mathematical abstraction of flank wear size and morphology, and the forces of the deformation region under the wear land in the element are the increscent component which contributes a lot to the variation of the total thrust force. Finally the theoretical model is verified by the multivariate experiments, and the results show that proposed model can work well. The work in this paper enables us to understand the influence of flank wear on thrust forces through the fundamental insight of CFRP cutting process.

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.

Increasing awareness of environmental concerns is leading a drive towards more sustainable structural materials for the built environment. Natural fibres such as flax and jute have increasingly been considered for fibre-resin composites, with a major motivation for their implementation being their notable sustainability attributes. This paper is part of an ongoing effort by the author to demonstrate the structural properties of primary structural elements and members fabricated from natural fibre composites of flax and jute. Previously the structural properties of flat plates, plain channel sections and channel sections with complex stiffeners were investigated under pure compression. This paper presents investigations of channel sections with complex stiffeners under pure bending. A series of sixteen channels with varying geometries, complex stiffener arrangements and composite thicknesses were tested in pure flexure. Material tests indicated that the mean tensile elastic stiffness and strength values were 6386 MPa and 55.1 MPa for flax, and 6941 MPa and 62.1 MPa for jute. The experimental results indicated that flexural failure of the channel sections was governed by tensile fracturing. The ultimate moment capacities varied from 1.043 to 1.501 kNm for four-layered composites, and 2.184 to 2.511 kNm for six-layered composites. The analytical models predicted the experimental ultimate moment capacities well, with a mean and coefficient of variation of the test to predicted ratio of 0.97 and 0.06, respectively. Finite element models used progressive damage analysis via stress-based damage initiation models and damage evolution laws, to replicate the tension fracture failure mode of the channels. The numerical models predicted the experimental ultimate moment capacities well, with a mean and coefficient of variation of the test to predicted ratio of 0.99 and 0.06, respectively.

Structural Health Monitoring (SHM) systems allow early detection of damage which allows maintenance planning and reduces the maintenance cost.Several researchers have proposed use of Neutral Axis (NA) location as a damage sensitive feature. It has been shown that through proper signal processing, NA location is insensitive to measurement noise and ambient temperature changes. This paper presents a methodology for successful NA tracking under changing axial loading conditions. The novel methodology is validated using numerical data as well as experimental results. This paper demonstrates the development of a Kalman Filter (KF) based NA tracking strategy under different loading conditions. The methodology was employed on a composite beam instrumented with fiber optic strain sensors. The stable NA tracking under changing loading conditions allows the application of the NA as a damage sensitive feature. The change in NA location is used to detect delamination in the beam. The method has been validated through a numerical model and experiments. The results show that the novel formulation of the KF for NA tracking allows more robust use of NA location as a damage sensitive feature in a wider range of applications.

The bending strength and the tensile strength are the important mechanical properties of T-shaped hooking composite structures , which are one of the connection structures in aero-engine . In this paper, a kind of 2.5D woven T-shaped hooking composite structure, which consists of a groove structure and a T-shaped structure, was designed and used for bending test and tensile test , respectively. Experimental results showed that the initial damages both occur at the root-edge of web, and then the damages extend to the root-middle in bending test or extend perpendicular to the root-edge to the edge of the flange in tensile test. Then, a strength prediction method of progressive damage for 2.5D woven T-shaped hooking composite structure based on three-unit-cell model was proposed and used to describe the mechanical behaviors and damage processes of the connecting structure under bending/tensile loads, respectively. The comparisons between the simulation results and tests showed that the maximum error of the bending/tensile strengths is less than 15% and the damage modes between tests and simulations are similar. Therefore, the predicted strengths are accurate and the strength prediction method of progressive damage proposed in this paper is effective.

The use of Glass Fiber Reinforced Polymers (GFRP) can delay or prevent tension failure in timber beams under loading and highly reduce tensile stresses . In this paper, the problem of reversibility , compatibility, and poor performance at high temperatures, of “traditional” organic adhesives was mitigated through the use of metal connectors . The flexural behaviour of hardwood and softwood beams reinforced by mechanically connected GFRP plates, has been studied through series of experimental investigations and numerical modelling . The experimental program included strengthening and testing of a total of 91 beams (50 hardwood and 41 softwood). Each beam was loaded above its service load until complete failure. Different strengthening layouts and quantity of metal connectors were used. The increment in capacity and stiffness is the main focus of this paper and effects of strengthening on deflection, failure load and failure mode, strain, and beam ductility were discussed in details. The combination of different GFRP configurations with appropriate amount of metal connectors, led to the doubling of the maximum load carrying capacity of the beams.

Adhesive bonding is one of the most suitable joining technologies in terms of weight and mechanical performance for current carbon fiber reinforced polymer aircraft fuselage structures. However, traditional joint topologies such as single overlap joints induce high peel stresses, resulting in sudden failure and low joint strength when compared to metal adherends. This drawback in using carbon fiber reinforced polymer is hindering their performance and efficiency in full-scale structures where joints are essential. The goal of this paper is to review how the joint design can help to improve the lap shear strength of composite bonded joints, to recognize the challenges that still need to be understood and to give insight into new opportunities. The focus is thereby on means to increase the matrix-dominated out-of-plane strength of the adherend in order to postpone delamination failure, as it is known to be the most prone type of failure of composite bonded joints. The paper is divided in two main parts: firstly, a review of topology-related and material-related design parameters is given and secondly, future opportunities to improve out-of-plane strength of CFRP bonded joints yet to be explored are discussed.

International audience; The cohesive zone modelling (CZM) is extensively used for the simulation of delamination propagation of composite laminated materials. The Finite Element (FE) method is able to support the CZM. Nevertheless, a refined mesh in the cohesive zone is required to describe accurately the energy dissipation. A 1D-beam simplified analysis based on the macro-element (ME) technique has been developed for the stress analysis of bonded joints, supporting damage evolution adhesive material law. The objective of this paper is to provide a validation of the ability of this macro-element technique for the simulation of delamination propagation in pure mode I of composite laminates. This validation is led through a comparative study between experimental test results, 3D FE model predictions and 1D-beam ME predictions. The experimental test campaign allows in particular for the assessment of the interlaminar strength and critical energy release rate in pure mode I. The thermoset unidirectional (UD) prepreg composite material IMA/M21E is used for this paper.

This paper address the challenges in using traditional film inserts specific to hot gas torch (HGT) assisted and in-situ consolidated automated fibre placement (AFP) manufactured double cantilever beam (DCB) and end notched flexure (ENF) specimens using carbon fibre (CF) reinforced polyetheretherketone (PEEK) prepregs . Traditional films suffer from thermal degradation , wrinkling and distortion under the consolidation roller. In this study, steel shim inserts with different thicknesses (25 and 50 µm) were used to manufacture the DCB and ENF specimens for fracture toughness evaluation. The processing parameters for in-situ consolidation were selected based on prior optimisation studies on short beam tests. In-situ consolidated specimens suffer from high void content which can affect the mechanical properties. Hence, optical micrography was used to investigate on the void contents in the manufactured specimens. Microhardness measurement is proposed as an AFP manufacturing quality assessment tool for DCB and ENF specimens which were taken from different regions of composite laminates . A manufacturing methodology for producing high quality DCB and ENF specimens by hot gas torch assisted in-situ consolidation using AFP is proposed in this paper that provide comparable results with those specimens manufactured using traditional methods.

Micromechanics based models are used for predicting the effective mechanical properties of reinforced polymer matrix composites. This paper reviews micromechanics based models for fiber reinforced polymer composites starting with the bounds established by Voigt and Reuss models, Hashin-Shtrikman model and then on to well-known micromechanics based models like Mori-Tanaka model, Self-consistent model and Differential scheme based models. The main objective is to critically review the areas in which these micromechanics based models hold good and analyse the limitations of these models. One of the limitations of the above mentioned models is the assumption of dilute dispersion and this is overcome in this paper by revising the Mori-Tanaka model by combining the differential scheme with Eshelby's model to take into account the non-dilute dispersion effect. Numerical results are verified by finite element based simulation of the representative volume element (RVE). Experiments were carried out to estimate the effective elastic constants for different fiber volume fractions. Theoretical results are reviewed with reference to experimental measurements.

In this paper, a shell based finite element (FE) model previously introduced for capturing the face-on impact and compressive strength after impact (CSAI) response of composite laminated structures is evaluated for predicting the dynamic response and damage due to edge-on impact and the resulting CSAI. The model utilizes an in-plane progressive damage and failure material model, Enhanced Schapery Theory (EST), for modeling the full field damage and failure of a single lamina in the 1–2 plane. The material model captures the pre-peak matrix non-linearity due to micro cracking using Schapery Theory (ST). The Bažant-Oh Crack Band (CB) model is utilized for matrix cracking and fiber rupture at the lamina level. Discrete cohesive elements are used for delamination initiation and propagation. Edge-on impact data for a range of impact energies is used for evaluating the models capability. The predicted impact damage state of the laminate is used for acquiring predictions of the CSAI of the laminate. The model predictions show promising results for the impact and CSAI response of an edge-on impacted coupon. The paper discusses the strengths of the model, the issues encountered, as well as topics for future study.

In this paper, exact solutions for the static bending response of laminated composite beams have been derived considering the effect of strain gradient elasticity. Separate solutions for the laminate comprising of isotropic and orthotropic laminae have been presented here. The sixth-order tensor for the higher order elastic coefficients has been evaluated using the generalized first strain gradient elasticity model with three independent material length constants. The significance of the gradient elasticity in low-dimensional structures has been established employing numerical examples of micro and nano-beams, and comparing the current results with the classical elasticity results. A parametric study over the variation of the axial, transverse Cauchy and physical stresses across the thickness of the beam has been conducted. Also, the discontinuity of the inter-laminar transverse Cauchy stresses and higher order stresses across the thickness has been studied in the current framework. The exact solutions developed in this paper may be used as benchmark results for validating further research involving the strain gradient elastic response of low-dimensional laminated composite beams.

The paper describes a numerical and experimental study investigating the load carrying capacity of thin-walled composite columns with a top-hat cross section under axial compression . The tested columns were made of carbon-epoxy laminate with symmetrical lay-up and ply orientation [0/90/0/90]s. The experiments were performed on a universal testing machine, Zwick Z100, under full load conditions until total failure of the structure. In the experiments, post-critical equilibrium paths of the structure were determined, defining the relationship between compressive force and deflection and enabling the validation of the numerical models. Based on the obtained post-critical equilibrium paths, the critical load of the structure was determined by approximation methods. Simultaneously, a numerical analysis was performed by the finite element method using the Abaqus® software. The critical state was determined via a linear eigenvalue analysis , and the critical load and a corresponding buckling mode were estimated. The next stage of the numerical analysis involved solving a nonlinear stability problem for a structure with initialized geometric imperfection reflecting the lowest buckling mode. The geometrically non-linear problem was solved by the Newton-Raphson method . The load carrying capacity of the composite structure in the post-buckling state was determined by the progressive failure criterion which estimates damage initiation in the composite material based on the Hashin criterion, while the progression of damage is described with the energy criterion describing the degradation of stiffness of finite elements. The numerical and experimental results show high agreement, which confirms the adequacy of the developed numerical models of composite structures. Results presented in the paper are part of a study conducted under the project no. UMO-2015/19/B/ST8/02800 financed by the National Science Centre Poland.

Nowadays, with the development of demand for construction, the use of composite structures in architectures become more and more popularity. How to improve the intelligent level of deformation monitoring has become one of the key problems. A detailed understanding about deformation behavior is significant for better monitoring of structures, especially in terms of accuracy and detail. The innovation of this paper focuses on that terrestrial laser scanning (TLS) measurement is adopted to investigate deformation of the composite masonry structures. In this paper, deformation segmentation and analysis of the masonry structures are investigated and the deformation tendency is compared and analyzed, based on the intelligent data extraction by window selection method, where high precision 3D laser technology provides reliable experimental data for this research. The deformation of different surfaces of a composite arch is considered and the maximum displacement distribution is analyzed through partially comparing the deformation of different epoch data.

This paper contributes to the understanding of a stress – strain diagram (SSD) in the region beyond the elastic limit (Viscoelastic and Plastic region). According to our results, the SSD in this region can be approximated by a second degree parabola. The advantage of such an approach is the possibility to simply calculate the value of stress if the strain is known and vice versa. Another advantage is in the possibility to simply calculate the tangent modulus. In the experimental part we determined the tangent and chord moduli for beech ( Fagus sylvatica L.) and aspen ( Populus tremula L.) “ WS ”. We also investigated the influence of the following factors on their value: material thickness “ MT ” (4, 6, 10, and 18 mm), degree of densification “ DOD ” (10% and 20% of the original thickness), and the number of loading cycles “ NC ” (0 versus 10,000). The paper describes the behavior of the tangent modulus in the whole force – deflection diagram.

This paper investigates buckling behavior of functionally graded graphene-reinforced composite (FG-GRC) laminated plates. The extended Halpin-Tsai model is used to estimate the effected material properties of the GRCs. The effects of rotary inertia and transverse shear deformation are incorporated by the first-order shear deformation theory (FSDT). Using the meshless kp-Ritz method, the buckling solutions are obtained. In order to derive the discretized eigenvalue equation, the kernel particle approximation is employed in the field variables and the energy functional is minimized through the Ritz procedure. The stability and accuracy of the meshless kp-Ritz method is verified through comparison and convergence studies by considering the effect of support size and number of nodes. The effects of boundary temperature, distribution of graphene, plate width-to-thickness ratio, plate aspect ratio and number of layers are investigated by detail parametric studies. Besides, the influences of boundary conditions, the types of buckling load and lamination angle are considered in this paper.

Terrestrial laser scanning (TLS) technology plays an important role in the field of deformation monitoring nowadays, and a lot of the architectures are arched composite structures, which are required to be further understood for their mechanical and physical properties. Only a theoretical numerical analysis is sometimes difficult and defective to achieve, due to varying loads and other factors influencing the stability and reliability of such structures. Therefore, measurements can improve the theoretical analysis in several ways. One of the most important issues is the deformation analysis within load experiments. This paper focuses on the presentation of new possibilities for surface-based measurement techniques and their applicability in the deformation analysis of arched structures. The goal of this paper is to investigate the difference of load-displacement relationships between arched and beam structures though a static load experiment based on high accuracy TLS measurement techniques and the surface analysis method. In this paper, we try to reveal the influencing mechanism of ‘prestressing-like’ force and interpret why arched structures can withstand a surprisingly greater load than beam structures under the same conditions.

The Taylor-Hopkinson test is widely used in predicting the dynamic strength of materials. For low-density foam materials, the output bar in the Taylor-Hopkinson test is often made from much weaker materials like polymethylmethacrylate (PMMA) to reduce the wave impedance mismatch between the foam sample and the output bar. These low impedance materials exhibit viscoelastic properties and may affect the dynamic stress prediction of foam samples. In this paper, the shock wave theory for the foam sample in conjunction with the viscoelastic wave propagation in the output bar is employed to illustrate the influence of the viscoelastic output bar on the dynamic response of foam samples in the Taylor-Hopkinson impact. The theoretical model presented in this paper can be degenerated into previous elastic target bar model and rigid target bar model. Finite element simulations are further performed to verify the proposed theory. The influence of material parameters on the final deformation of the foam sample, impact duration and residual velocity of the foam sample are investigated. The results in this paper not only help in understanding the effect of the viscoelastic bar on the dynamic response of foam samples, but also set up guidelines to perform the Taylor-Hopkinson test more precisely.

In this contribution, the impact dynamic response and energy dissipation mechanism of composite multilayer array structures are investigated and presented. The composite multilayer array structures in this paper comprise of three layers of composite solid ball elements between four layers of hybrid composite laminates . A fairly extensive experimental investigation are conducted in conjunction with a detailed finite element analysis . The experimental tests are conducted and the finite element analysis is carried out using a commercially available finite element software. The results of maximum contact force, contact duration and corresponding failure modes are presented, compared and discussed in this technical paper. Simulation and test results show that this new structure has good energy absorption efficiency and provides a new way for the design of underwater protective structure.

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.

Today, the Structural Health Monitorin (SHM) methodology is the main way to deal with the detection and identification of damages in a range of engineering sectors, mainly in the military and civil aerospace industry. The need to monitor damages and failures in increasingly complex structures has led to the development of various detection techniques. Identification of damages through intelligent signal processing and optimization algorithms are particularly emphasized. The methods discussed here are mainly elaborated by the evaluation of modal data due to the great potential of their application. Moreover, the optimization of damage identification is approached through methods of optimal positioning of sensors for the acquisition of the data that must be evaluated so that conclusions can be made. This article discusses the use of computational and intelligent techniques for structural monitoring in the form of a review with emphasis on composite materials. Despite the excellent mechanical performance already known about composite materials, they have a weak point. While damages, in a metallic material are easily visible (in some cases), composite materials often have the superficial appearance as if in perfect condition, when, inside, there are serious damages. This paper can be seen as a guideline or a starting point for developing and improving SHM systems. The contents of this paper aim to help engineers and researchers find a starting point in developing a better solution to their specific structural monitoring problems, either by inverse methods, pattern recognition, and intelligent signal processing.

The intent of this paper is to firstly perform the vibration analysis of the functionally graded carbon nanotube reinforced composites (FG-CNTRC) circular, annular and sector plates with arbitrary boundary conditions by means of the semi-analytical method which is proposed by the author’s team. In the material model, the distribution of the carbon nanotubes is uniform or functionally graded along with the thickness direction of structures and four types of the CNTs distribution are studied in this paper. The refined rule of mixtures approach containing the efficiency parameters is adopted to determine the properties of the composite media. The admissible displacement functions of the FG-CNTRC circular, annular and sector plates are uniformly expanded as the modified Fourier series which embodies a standard cosine Fourier series and several auxiliary functions which are introduced to eliminate the limit of the boundary conditions. On this foundation, the first-order shear deformation elasticity theory is employed to construct the energy expression of the FG-CNTRC circular, annular and sector plates. Then the Ritz-variational energy method is used to decide the natural frequencies and the associated mode shapes. To examine the convergence, accuracy, stability and efficiency of the computational model, the comprehensive studies are conducted. Based on that, some crucial parametric studies covering the influence of the geometrical parameters, CNTs distributions, volume fraction of CNTs and boundary conditions are also investigated in detail.

In this paper, free vibration behavior of coupled functionally graded (FG) doubly-curved revolution shell structures with general boundary conditions is studied by the using unified Jacobi-Ritz method. The first-order shear deformation theory in conjunction with a multilevel partition technique is adopted to establish the theoretical model. The substructure of the theoretical model mainly includes the FG elliptical, hyperbolical, paraboloidal and cylindrical shells and three kinds of coupled FG shell structures containing paraboloidal-cylindrical shells, elliptical-cylindrical shells and hyperbolical-cylindrical shells are also considered in actual calculation. To obtain the continuous conditions at the interface and satisfy the arbitrary boundary conditions, the boundary and coupling spring techniques are adopted in this paper. In despite of the shell components and the boundary conditions, a mix function which is with the Jacobi polynomials along the meridional direction and the standard Fourier series along the circumferential direction is used as the admissible displacements of each shell segment. The convergence and comparison studies for the FG doubly-curved revolution shell structures with different boundary conditions, coupling parameters and Jacobi parameters are carried out to verify the reliability and accuracy of the present solutions. To enhance the understanding of the titled problem, some mode shapes of coupled FG shell structure are provided. Through comparative analysis, including the experimental and numerical comparison, it is obvious that the current method has good stability and rapid convergence properties and the present results agree closely with those reference results no matter what the frequency parameters or mode shapes are. The influence of the geometric dimensions and material constants on the vibration behavior of coupled FG shell structure is also reported.

This paper presents a methodology to evaluate the effects of microscopic manufacturing defects, namely fibre misalignment , waviness and volume fraction, on the mechanical performance. Influences of these defects on the effective elastic properties of composites are quantified by a dual homogenization method. For estimating stochastic characteristics of the properties induced by the variations in these defects, a probabilistic extension of the dual homogenization method is developed and numerically implemented through a perturbation-based stochastic finite element method . It is further incorporated in a multiscale finite element based reliability method to measure the influences of these manufacturing defects on structural performance in terms of reliability. The effectiveness of the proposed method in capturing defects is illustrated initially by investigating the effective elastic properties of a unidirectional fibre composite based yarn and then a plain woven textile composite . The capability of the proposed method in quantifying the variations in these defects is further demonstrated through statistical analysis of the effective elastic properties and a woven textile composite and structural reliability analysis of a textile composite laminate. This paper represents a significant advancement in the probabilistic prediction of the behaviour of woven and non-woven composites.

This paper investigates the vibro-acoustic characteristics of the moderately thick laminated rectangular plate-cavity coupling system by using a modified Fourier series method. In the established model, two boundary conditions of moderately thick laminated rectangular plate are set as uniform elastic supports and non-uniform elastic supports including multi-points elastic supports or partial elastic supports. Besides, the three-dimensional rectangular cavity with rigid walls or impedance walls is discussed in the studied model. The admissible function of the displacements and sound pressure of the coupling system is set as the superposition function combined with Fourier cosine series and Fourier sin series. The Lagrangian energy principle is used to express the coupling system. The vibro-acoustic coupling characteristics can be obtained based on Rayleigh-Ritz technique. It shows good convergence and agreement of the present method by being compared with the finite element method (FEM) and the related literatures. This paper focuses on the study of free and forced vibro-acoustic analysis for the moderately thick laminated rectangular plate-cavity coupling system. Moreover, some results are achieved by discussing the effects of various plate thicknesses, different types of elastic supports, lamination schemes and various impedance boundary conditions on the natural characteristics and responses of the coupling system.

Understanding how the load bearing capacity of particulate reinforced metal matrix composite (PRMMC) is related to the underlying microstructure is essential for developing new PRMMC materials. In a recent study, we presented a numerical technique which combines the homogenization technique, direct method, and statistical analyses. By using this approach, ultimate strength and endurance limit of the PRMMCs can be predicted from the composite microstructure in the mesoscale . Because PRMMCs are typical random heterogeneous materials , the size of RVE models, as well as boundary conditions, have been acknowledged to have nontrivial influence over the predicted effective behavior. In this paper, the objective is to understand how the strength predicted from the direct method is influenced by the RVE size and boundary condition. Because in our previous study it has already been shown that homogenization results are insensitive to boundary conditions applied on the RVE, the focus of the present paper is laid on the influence of the embedded cell model. In order to capture the influence of RVE size and embedded cell configuration, an exemplary PRMMC material, WC-20 Wt% Co, was selected. A great number of representative volume elements (RVE) models were built as artificial microstructures of this material. By analyzing the effective behaviors, especially the strength, predicted from these samples from a statistical point of view, the influence of the boundary condition and the size effect were revealed. On that basis, this study proposes a feasible solution to these issues.

In this paper, the state space method is used to analyze the static and dynamic behaviors of laminated magneto-electro-elastic rectangular plates with simply supported boundary conditions . Multilayers are considered as well as viscoelastic interfaces. The Kelvin-Voigt model is used to take into accounted the viscoelastic interface effects. The mathematical formulation is elaborated in a general form and an arbitrary number of layers as well as the orthotropic behavior can be considered. The coupling time and thickness variables formalism is presented in an elegant matrix way allowing a versatility and capability of the formalism to handle more general multilayered plates. The solution procedure is based on the transfer relationships coupled with the Lagrange polynomials in discretised time intervals. The static behavior of multilayered magneto-electro-elastic rectangular plates is analyzed and the main results are the viscoelastic interface effect on the dynamic behavior of multifunctional structures. New results have been obtained by this coupling state space-Lagrange polynomial methodology. The mathematical procedure, elaborated in this paper, is original and allows the numerical study of the effects of the viscoelastic interfaces. The effectiveness of the proposed methods has been demonstrated by performing variant numerical tests.

This two-part paper presents a numerical study on the fire resistance behaviour of pultruded GFRP profiles with square tubular cross-section simultaneously subjected to four-point bending and the standard fire of ISO 834. The first part [1] presented the numerical models and results focused on the most relevant kinematic issues (deflections and strains), while this second part (present paper) reports numerical results associated to the static issues, particularly the evolution of stress distributions and failure initiation with fire exposure time. The paper first presents and discusses the evolution of stress distributions in both longitudinal and transversal directions of an unprotected GFRP beam under one-side fire exposure (reference beam). Next, the influence of using a fire protection system and exposing the beams to fire in three sides is assessed by comparison with the case of reference beam. Additionally, the Tsai-Hill failure (initiation) criterion was used to identify the zones (sections and points) of GFRP beams that are most sensitive to failure due to high temperatures. The numerical results obtained show that (i) longitudinal stresses across the section become highly nonlinear as a result of temperature increase; (ii) shear stress diagram is severely affected by fire exposure in three sides; (iii) transversal stresses are negligible compared to the longitudinal and shear ones; and (iv) the collapse of beams (considering Tsai-Hill failure analysis) occurs due to top flange and web (top part) failure, which generally agrees with experimental observations.

This two-part paper presents a numerical study about the fire resistance behaviour of pultruded GFRP profiles with square tubular cross-section subjected to bending and the ISO 834 time-temperature curve. The first paper deals with the development of numerical and analytical models and the discussion of relevant kinematic issues, including the evolution of beam deflection and position of neutral axis with the fire exposure time. The second part [1] reports an in-depth investigation on the side of static issues, which include the evolution of stress distributions and failure initiation with the fire exposure time. In the present paper, three–dimensional finite element models were developed to simulate fire resistance tests previously conducted by the authors on GFRP beams, in which different degradation curves were considered for compressive, tensile and shear moduli , based on experimental data. In these numerical simulations, both effects of varying the assignment of material properties (depending on the position of neutral axis) and of considering different thermal expansion coefficients were taken into account, and some conclusions were drawn on their influence on mid-span deflection evolutions. Since no failure criterion was implemented, both models were not able to reproduce the failure of the beams, but the overall tendency of the numerical results was consistent with the experimental data. Alongside the numerical study, analytical models based on Timoshenko beam theory were also developed and allowed obtaining accurate predictions of the mid-span deflection evolution of the GFRP beams; the analytical results were in close agreement with the numerical ones and also with the experimental data.

Two formulas which can predict the tensile strength of hybrid composite filled with short fibers and spherical particles are derived in this paper. One of them is suitable for predicting the tensile strength of hybrid composites in which there is no cluster. And another formula is suitable for predicting the tensile strength of hybrid composites in which clusters exist. There are two vital points in this paper. One is that the influence of fiber end on the tensile strength has been quantitatively studied, while most predictions of the tensile strength of the composite from other authors neglect it. Another one is that the influence of reinforcement aggregation on the tensile strength of the composite has been quantitatively studied.

In this paper, an equivalent damage model was proposed to predict the compressive behavior of three T-stiffened composite panels subjected to low velocity stiffener impact. A softening inclusion model combined with the reduction of material properties was used to describe the impacted stiffener damage. The progressive damage of the impacted stiffener and the failure of the panels under compression were simulated within the finite element software package ABAQUS. Compression-After-Impact (CAI) numerical results show that the compressive buckling modes of the stiffened panels are influenced by the damage degree of the impacted stiffener, and the fracture of the impacted stiffener triggers the global buckling of the panel. The buckling and failure loads of the panels predicted by this paper are in good agreement with the experimental results, which proves the validity of the model.