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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.

Paper-like electronics with thin multi-layer structures, which can be repeatedly bent and twisted like paper, are promising for diverse innovation application. Employing surface-mounted flexible strain sensors is a general way to monitor the bending characteristics of each layer inside paper-like electronics, however, the deviation of measured strain will reach up to 15% on account of the layer-to-layer interface sliding, neutral axes offset resulted from the extra effect of the flexible strain sensor. Here, we proposed a precise measurement method to evaluate the surface bending strain of paper-like electronics by using flexible sensors. In combination with the effects of the thickness, length, etc, a strain compensation model has been established to compensate the measurement deviation. Meanwhile, a slip failure model has been developed to guide the design of the sensitivity unit within the non-sliding area of the flexible sensor. Validated by finite element analysis and experiment, the bending strain of paper-like electronics (eg., ∼50 μm) can be evaluated much more accurate compared to the frequent access one. The newly-proposed method provides a powerful basis for further research of paper-like electronics.

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.

A finite element procedure, developed in order to account for the effect of high anisotropy at folding of coated papers, is presented. The anisotropic behaviour (with very low stiffness in the thic ...

Folding of coated paper is examined numerically using the finite element method. Particular emphasis is put on the behaviour of field variables relevant for cracking of the coating layers. In the n ...

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.

Aramid paper honeycomb cores have been widely used in lightweight structures. However, shear properties of aramid paper can hardly be obtained accurately due to the shear buckling of the paper. Therefore, we make the paper into a laminate to prevent buckling and apply the v-notched beam test method to the laminate to obtain the in-plane shear properties of aramid paper. The shear strength and failure strain of aramid paper are characterized experimentally for the first time. The properties are then used in finite element simulations to study the shear behaviors of paper honeycombs and the good agreement with the experimental results indicates the validity of the shear properties. Furthermore, the failure mode map of the paper honeycomb subjected to combined shear and compressive loads is obtained, which presents the importance of the accurate characterization on the shear properties of the paper. This characterization method can also be applied to other types of paper.

This paper presents sound absorbing structure constructed using multiple layers of fibrous papers. Sound absorbing materials converts sound wave into thermal energy due to the porosity. The proposed sound absorbing structure has helical-like shapes in layered format with cyclic winding of the mulberry papers and coconut fibers. The acoustic properties changes depending on the thickness of the air cavity between the thin layers and porosity of the layers themselves. Sound absorption coefficients were measured using two-microphone impedance tube in order to investigate the acoustic property variations with porosity. The measured results were compared to predictions utilizing wave propagations through a slit for verification of the sound absorbing mechanism. A simple method for improving the sound absorption with less material by composite structure is proposed and verified through the experiment. Dynamic stiffness and loss factor of the helical-shaped sound absorbers were measured and compared by analyzing supporting properties to the vibrating beam. Depending on the construction method, the frame dynamic properties showed variation even when the sound absorption was identical. This information is required for optimal design of absorbing layers for specific noise control applications.

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.

ispartof: COMPOSITE STRUCTURES vol:226 status: published

This paper begins by briefly outlining the behaviour of paper structures under mechanical and hygrothermal loading. The results of an experimental test program are then presented and it is shown that the strain to failure is not a suitable parameter for adequately differentiating between the virgin and recycled paper structures. Indeed, the present paper postulates that the accumulated inelastic work may be a better tool for assessing the effect of furnish. The results of a creep test program are also presented and a possible extension of the master curve concept, which was developed for the visco-elastic response of amorphous polymers to the visco-plastic regime is discussed.

Paper is an orthotropic, rate sensitive material which makes modelling its stress-strain behaviour a difficult task. Classical methods are inadequate behaviour of paper and comparisons are made with previous experimental work for a particular paper. Comparisons between the two models are also made.

This work presents a complete test methodology for the testing of paper and other thin materials with the special requirements for constitutive modelling in mind. It also presents results obtained using these test regimes and speculates about what they reveal about paper. Specifically that paper seems to exhibit ‘over stress’ phenomenon.

This communication is to correct the sensitivity analysis formula of frequencies for composite structures in the paper [1]. It is further pointed out that even though the sensitivity analysis formula of frequencies has been corrected, the first derivatives of frequencies with respect to design variables cannot be achieved by direct differentiating the equations of eigenvalues and eigenvectors because the first derivatives of vibration modes with respect to design variables are still unknown. The readers are referred to the analytical solution of the sensitivity analysis of frequencies and modes for composite structures in the literature [2], which is derived using linear space theory.

A major disadvantage of polymer-based composite materials is their poor performance when exposed to relatively high heat flux. At temperatures typically above 300–400 °C, the organic matrix decomposes releasing heat, smoke, and toxic volatiles, jeopardising the composite mechanical performances. The heat flux resistance of composites can be improved by integrating protective coatings on their surface. In the present work, graphene nanoplatelet (GNP) films are employed as thermal barrier coatings (TBCs) to mitigate the thermal degradation of Carbon Fibres Reinforced Plastic (CFRP). The films were bonded onto the heat-exposed surface of CFRP laminates. Laser spot heating tests, with different laser power densities (25–150 kW/m2), were adopted to assess the effectiveness of the coating. The effect of the thickness of the protective layer has been tested too, by changing the number of layers (from 1 to 2 layers). From the results, a significant reduction in the temperatures and damages were observed. Moreover, the damage extension and residual mechanical performances of exposed samples have been assessed as a function of the power density and the protection level. A significant improvement of the post-heat flexural moduli and a significant reduction of the damaged areas have been obtained in laminates protected with high-loaded graphene nanoplatelet films.

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.

In this paper, for the first time, we present a simple and straightforward method to improve not only electrical conductivity and complex permittivity but also electromagnetic interference (EMI) shielding effectiveness of reduced graphene oxide (rGO)/polystyrene (PS) nanocomposites through sulfur doping. Sulfur-doped reduced graphene oxide with thiophene-like structure (2.6 at.% S), synthesized through a simple heating process of a mixture of graphene oxide and sulfur powder, revealed almost three times larger electrical conductivity (1095 S m −1 ) than undoped rGO (395 S m −1 ). The SrGO/PS nanocomposite showed not only 150% larger electrical conductivity and 50% larger complex permittivity, but also improved EMI shielding effectiveness (24.5 dB) at a frequency of 18 GHz than rGO/PS nanocomposite (21.4 dB) at the same loading level of 7.5 vol.%. Considering the simplicity and effectiveness of process, sulfur doping of graphene is expected to be used as a versatile method to improve EMI shielding efficiency of graphene/polymer nanocomposites.

In this paper, we study the buckling characteristics of curvilinear fibre composite laminates exposed to hygrothermal environment. The formulation is based on the transverse shear deformation theory and it accounts for the lamina material properties at elevated moisture concentrations and thermal gradients. A 4-noded enriched shear flexible quadrilateral plate element is employed for the spatial discretization. The effect of a centrally located cut-out, modelled within the framework of the extended finite element method, is also studied. A detailed parametric investigation by varying the curvilinear fibre angles at the centre and at the edge of the laminate, the plate geometry, the geometry of the cut-out, the moisture concentration, the thermal gradient and the boundary conditions on the buckling characteristics is numerically studied. � 2015 Elsevier Ltd.

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.

A meshfree radial point interpolation method is presented for static and buckling analysis of thick laminated plates, taking into account the continuity of interlaminar transverse shearing stresses and zigzag variation of the displacement field through the plate’s thickness. The kinematics of the deformation field is based on the refined theory of Wang and Shi (2015). The corresponding weak-form for static bending and buckling analyses of plates based on the Wang and Shi’s theory are derived through minimum potential energy principle. The discretized systems of equations for static bending and buckling analyses are derived and the associated numerical integration is performed by the novel CTM quadrature. This integration method can exactly model the problem geometry and thus is very fast and accurate. Several numerical examples are solved to demonstrate the accuracy and efficiency of the plate theory adopted in this paper. Additionally, the examples are also analyzed using the FSDT, TSDT and Shi’s plate theory (2007). Because the four theories considered in this paper use five field variables and they also have some other similarities, the influence of higher order terms of the displacement field, transverse shearing strain energy consistency, and imposition of transverse shearing stress continuity on results is investigated.

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.

In this paper, compressive material tests on unconfined/confined resin and steel reinforced resin were experimentally evaluated in order to validate the numerical results. The uniaxial model which combines damage mechanics and Ramberg-Osgood relationship is proposed in this paper to describe the uniaxial compressive behaviour of resin and steel reinforced resin. Numerical homogenization is conducted to predict the tensile and shear behaviour of steel reinforced resin after validated by compressive material test results. The friction angle β , the ratio of the yield stress in triaxial tension to the yield stress in triaxial compression K, and the dilation angle ψ of the linear drucker-prager plastic model are obtained based on experiments and numerical homogenization simulation. The confinement effects on resin and steel reinforced resin could be effectively simulated by combing above parameters and uniaxial compressive model. Finite element simulations on unconfined/ confined resin and steel reinforced resin material tests were conducted to validate the material parameters proposed in this paper. A good agreement is observed, indicating the model and parameters proposed in this paper could be effectively used in the finite element simulation of injected bolts.

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.

This paper presents the results of an experimental study on the flexural behavior of carbon fibre reinforced polymer (CFRP) anchorage system strengthened reinforced concrete (RC) beams under the coupled effects of corrosion damage and sustained loading. The test beams were subjected to combined accelerated corrosion in 5% NaCl solution and sustained loads for 25, 50 and 100 days at 0% and 50% load levels of the virgin beam ultimate load capacity. The failure modes, load carrying capacity, deflection, ductility and strain response of the beams were investigated in detail. The results indicated that CFRP anchorage systems enhanced the yield and ultimate load of the corrosion-damaged beams. The use of CFRP anchorage system restored the ultimate load of corroded beams between 87.6% and 104.8% and the yield load between 81.9% and 92.7% with respect to those of the virgin beam. In contrast, the ductility and energy absorption index suffered a decline. CFRP-strengthened beams showed a reduction of 4.5%–28.9% for the ductility index compared with their counterparts without CFRP anchorage system. Sustained loads resulted in more considerable reductions in load-bearing capacity, greater loss of rebars mass, wider width of corrosion cracks, indicating a significant coupling effect between sustained loading and corrosion damage. Three typical failure modes of the CFRP-strengthened beams were observed and explained in the paper, thus revealing the failure mechanism of CFRP-strengthened beams. In the engineering practice of CFRP anchorage system, the coupled effects of corrosion damage and sustained loading on the strengthened systems should be taken into accountant comprehensively.

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.

Thick composite materials are commonly used as load-bearing structures in marine applications. Developing a suitable and sophisticated non-destructive testing (NDT) method for thick composites is an urgent challenge to improve the safety, reliability and maintenance of these structures. Digital shearography has become an important NDT technique for detecting defects in thin composite materials because of the advantages of high sensitivity to deformation change, and whole-field measurement. So far, the efficacy of shearography for thick composite inspection (e.g. thickness as more than 50mm) has not been fully characterised. This paper combines finite element methods (FEM) and experimental tests to investigate the defect detection capabilities of shearography for inspecting thick glass fiber-reinforced polymer laminates. A thermal-mechanical model was established by computing equivalent thermal and mechanical properties and was evaluated by experimental shearography testing. In order to reliably simulate major defects in thick composite, flat bottom holes were manufactured in the specimen. Both simulations and experiments show that shearography is a promising technique to inspect thick composites. The thresholds for defect-induced phase change and the corresponding defect-induced deformation are determined for shearography testing of thick composites in this paper. Afterwards, the effect of mechanical boundary conditions on defect-induced deformation is studied by FEM.

Concrete confined using fiber-reinforced polymer (FRP) composites experience significant enhancements in strength and strain. For the seismic retrofitting of existing reinforced concrete (RC) structures, a large rupture strain (LRS) FRP (i.e., polyethylene terephthalate and naphthalate, denoted as PET and PEN respectively), with a larger rupture strain of more than 5%, is a promising alternative to conventional FRPs with a rupture strain of less than 3%. The majority of analytical models on the stress–strain behavior of FRP-confined concrete under axial compression have focused largely on concrete confined with the traditional FRP material. Analytical research on LRS FRP-confined concrete is, however, limited. Moreover, all existed stress–strain models were determined based on theoretical analysis and test data fitting. In this paper, the artificial neural networks (ANN) method is employed to build a confinement model directly from experimental data to predict the different components of the stress–strain response. A test database consisting of 226 axial compression tests on LRS FRP-confined concrete specimens is used. The test results, in terms of full confined stress–strain response, strength, strain, FRP rupture strain, and dilation response were investigated. Predictive expressions and practical ANN models for the strength, strain, and shape of an axial stress–strain response are provided. Existing models for LRS FRP-confined concrete were also evaluated. The results of the existing and proposed models report that the proposed methods achieve significantly better results. Concrete confined using fiber-reinforced polymer (FRP) composites experience significant enhancements in strength and strain. For the seismic retrofitting of existing reinforced concrete (RC) structures, a large rupture strain (LRS) FRP (i.e., polyethylene terephthalate and naphthalate, denoted as PET and PEN respectively), with a larger rupture strain of more than 5%, is a promising alternative to conventional FRPs with a rupture strain of less than 3%. The majority of analytical models on the stress–strain behavior of FRP-confined concrete under axial compression have focused largely on concrete confined with the traditional FRP material. Analytical research on LRS FRP-confined concrete is, however, limited. Moreover, all existed stress–strain models were determined based on theoretical analysis and test data fitting. In this paper, the artificial neural networks (ANN) method is employed to build a confinement model directly from experimental data to predict the different components of the stress–strain response. A test database consisting of 226 axial compression tests on LRS FRP-confined concrete specimens is used. The test results, in terms of full confined stress–strain response, strength, strain, FRP rupture strain, and dilation response were investigated. Predictive expressions and practical ANN models for the strength, strain, and shape of an axial stress–strain response are provided. Existing models for LRS FRP-confined concrete were also evaluated. The results of the existing and proposed models report that the proposed methods achieve significantly better results.

Rectangular concrete-filled thin-walled steel tube (CFT) columns using high-strength steel and concrete are susceptible to local buckling, which will weaken the constraint effect and reduce the advantages of high-strength materials. Fiber reinforced polymer (FRP) has favorable mechanical properties, which can be used to delay or even prevent the outward buckling of steel tube. This paper presents the experimental and theoretical study on axial compressive behavior of FRP confined concrete-filled thin-walled steel tube (FRP-CFT) stub columns with high-strength materials. The test variables included the aspect ratio of section, concrete strength and the layers of FRP, etc. Results show that the confinement of FRP delays the local buckling of steel tubes and increases the restraint strength of core concrete. The ductility of high-strength concrete is also improved. Based on the superposition theory and the effective width theory, this paper proposed the design approach to calculate the ultimate compressive resistance, considering the effect of local buckling of thin-walled steel tube and the confinement of the FRP. The design approach was verified with experimental results and can be used in practice.

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.

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.