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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·cm 3 ·g −1 ) and flax-paper/PLA (217 MPa·cm 3 ·g −1 ) composites are similar to those made using woven glass fabrics impregnated with epoxy (227–278 MPa·cm 3 ·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. [ABSTRACT FROM AUTHOR]

Folding of coated papers is examined numerically using the finite element method. The analysis is focused on the influence from dynamic effects on the folding process. In particular, the behaviour of field variables relevant for cracking of the coating layers are studied in some detail. The results presented indicate that dynamic effects are of little importance as regards maximum strain levels in the coating but will influence the stress and strain distributions. Accordingly, a quasi-static analysis of the problem will be sufficient in order to describe many of the important features related to cracking. [Copyright &y& Elsevier]

Abstract: Natural continuous bast fibres (hemp, flax, etc.) are gaining popularity in composite materials because they can advantageously replace glass fibres. It is found that intrinsic properties of unidirectional (UD) composites are almost equivalent to those of unidirectional glass fibre composites. Unfortunately it is difficult to get repeatable results because of the inherent variability in the properties of bast fibres compared to glass fibres. Their quality is largely affected by the weather conditions, the extraction location along the plant and the techniques used to extract them (retting, bleaching, etc.). In this work, unidirectional hemp/paper/epoxy and flax/paper/epoxy composites are manufactured by adding one or two sheets of paper at the surface of a unidirectional layer of hemp or flax fibres before molding. The composites are tested under tensile loads and the results compared with those obtained for UD composites made without paper. The results show a significant increase in strength and modulus repeatability when the paper layer is present. A significant increase in strength and modulus was also obtained compared to the base epoxy properties. The resin permeability of the flax–paper reinforcement has also been evaluated and compared to that of a standard glass fabric. Globally, the results suggest that adding thin sheets of paper at the surface of reinforcement layers stabilizes and increases the composite strength but slightly reduces the Young’s modulus. However the permeability of the flax–paper reinforcement was much lower than that of the glass fabric. Modifications to the paper architecture will be required to improve this aspect. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Space debris is a major threat to the safety of spacecraft in orbit, and advanced protective materials with lightweight and high impact resistance are effective passive protection solutions. In order to investigate the hypervelocity impact protection performance of aluminum matrix ceramic ball composite materials plate, this paper has carried out the hypervelocity impact test of aluminum matrix ceramic ball composite materials plate, and verified the accuracy of the numerical model based on the FE-SPH adaptive method, and then carried out a comparative study on the hypervelocity impact protection performance of aluminum plate and aluminum matrix ceramic ball composite materials plate with the same area density. The results show that: compared with the aluminum plate conditions, the composite materials plate has a better ability to broken the projectile. Compared with the aluminum plate, the kinetic energy absorbed by the composite materials plate is increased by 37.04%, and the kinetic energy of the generated debris cloud decreased by 69.57%. The proportion of the internal energy dissipation of the composite materials impact system reaches up to 72.03%. The secondary pollution generated by the composite materials plate is less and the hazardous debris have less threat to the rear wall. The research in this paper can provide reference for the selection of hypervelocity impact protection materials. [ABSTRACT FROM AUTHOR]

To address the cumbersome nature of mesoscale models for concrete as well as other similar materials, this paper presents an efficient and high-volume fraction mesoscale modeling framework with a novel aggregate surface tracking algorithm as well as an aggregate-cement interface interaction model to replace the interface transition zone (ITZ) in the three-dimensional space. This model could also be used in composite materials such as Fiber-reinforced polymer concrete. In this study, results are compared with the traditional 3 phases concrete models, uniaxial tension and compression models, and triaxial compression models for validation. The results show that the three-dimensional mesoscopic model can capture the detailed process of mesoscopic damage and the macroscopic behavior under different stress states while showing good agreement with the experimental results. It is demonstrated that compared with traditional methods, the new model can save up to 84 % of the preprocessing and computation time and provides a more convenient and efficient scheme for the three-dimensional mesoscopic modeling of concrete alike composite materials with good accuracy. This paper also presents several 3D mesoscale models of Fiber-reinforced polymer concrete. [ABSTRACT FROM AUTHOR]

Composite materials have been widely used in the field of pressure shells for underwater vehicles due to their great potential. This paper presents an experimental study of composite cylindrical shells with and without the circular hole subjected to external hydrostatic pressure. The mechanical response of the composite cylindrical shells under external hydrostatic pressure is recorded by the strain acquisition system, and the buckling deformation process of the shells is also captured by the high-speed camera system. According to the experimental results, the entire compression process of the shell can be divided into three stages based on variations in strain. Based on the findings that buckling starts very early and accounts for about 35 % of the entire process, the concept of initial buckling pressure is proposed to distinguish the critical point of the shell transition from a stable state to an unstable state. It is found that the damage positions of the three shells all correspond to the troughs in the buckling waveform, and the final fiber breakage of the perforated shells with the reinforcements both occur near the reinforcement component. Finally, a comparative analysis of the strain response of the shell with the hole and the complete shell is carried out to explore their similarities and differences. The experimental results in this paper provide valuable information for the design of perforated composite pressure shells for underwater vehicles. [ABSTRACT FROM AUTHOR]

In this paper, the process deformation of 2D SiC/SiC composite materials in the pyrolysis stage of precusor infiltration and pyrolysis process (PIP) is investigated experimentally and computationally through a multiscale study. The pyrolysis process of PCS was described by a kinetic model. The intrinsic properties and behaviors of pyrolysed PCS were characterized experimentally. The macroscopic properties and behaviors of 2D-SiC/SiC composite materials were obtained through the homogenization method. The proposed methodology can successfully predict the process deformation of an L-shaped 2D SiC/SiC composite structure in cyclic PIP process. It is revealed that the pyrolysis of precusor can generate significant chemical shrinkage and stiffness variation, leading to the process deformation of the entire composite structure. Unlike the precusor, the variation of properties and behaviors of composite materials are anisotropic. Furthermore, the cyclic process and the evolution of matrix composition complicate the process deformation. The behavior of composite material is dominated by pyrolysing PCS at first several cycles whereas by pyrolysed SiC at last several cycles. The fundamental understating on process deformation in this paper may benefit the design and the manufacture of SiC/SiC composite materials. [ABSTRACT FROM AUTHOR]

This paper presents the Green's Functions Stiffness Method (GFSM) for solving linear elastic static problems in arbitrary axially non-uniform Timoshenko beams and frames subjected to general external loads and bending moments. The GFSM is a mesh reduction method that seamlessly integrates elements from the Stiffness Method (SM), Finite Element Method (FEM), and Green's Functions (GFs), resulting in a highly versatile methodology for structural analysis. It incorporates fundamental concepts such as stiffness matrices, shape functions, and fixed-end forces, in line with SM and FEM frameworks. Leveraging the capabilities of GFs, the method facilitates the derivation of closed-form solutions, addressing a gap in existing methods for analyzing non-uniform reticular structures which are typically limited to simple cases like single-span beams with specific axial variations and loading scenarios. The effectiveness of the GFSM is demonstrated through three practical examples, showcasing its applicability in analyzing non-uniform beams and plane frames, thereby broadening the scope of closed-form solutions for axially non-uniform Timoshenko structures. [ABSTRACT FROM AUTHOR]

This work presents a novel numerical methodology to model the degradation and failure of composite materials like GFRP submitted to monotonic and high cycle fatigue loads. This is done by using the Serial–Parallel Rule of Mixtures homogenisation technique together with a proper mechanical characterisation of the constituent materials of the composite. This paper also proposes an efficient way of estimating the fatigue properties of each of the material constituents (fibre or matrix) to comply with the experimental results obtained at composite level; this enables to estimate the fatigue strength of any stacking/orientation of fibres with only one mechanical characterisation of the material properties. A comparison of the results obtained analytically and experimentally for GFRP is presented. The results show the applicability and accuracy of the proposed methodology in this field. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

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

Abstract Consolidation of a prepreg layup to a target thickness is critical in order to achieve the required fibre volume fraction and dimensions in a composite part. Experiments show that different processing conditions lead to different levels of compaction and variability in the thickness. This paper presents an analysis of processing conditions and their effects on consolidation of thick composite components. A model that accounts for both percolation and squeezing flow is employed to study two toughened prepreg systems – IM7/8552 and IMA/M21. This paper analyses the significance of the process parameters on the thickness of prepregs and its variability. The analysis of different layups and processing conditions suggests several strategies to control target thickness and its variability. The IMA/M21 prepreg system was found to have lower variability due to its toughening mechanism. The presented results provide a better understanding of the composite manufacturing and can be used to provide an informed choice in design for manufacture of composite structures. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract This paper presents the determination of the crack resistance curve of the unidirectional carbon-epoxy composite material IM7-8552 for intralaminar fiber tensile failure under dynamic loading. The methodology, proposed by Catalanotti et al. (2014) for quasi-static loading conditions, was enhanced to high rate loading in the order of about 60 s−1. Dynamic tests were performed using a split-Hopkinson tension bar, while quasi-static reference tests were conducted on a standard electromechanical testing machine. Double-edge notched tension specimens of different sizes were tested to obtain the size effect law, which in combination with the concepts of the energy release rate is used to measure the entire crack resistance curve for the fiber tensile failure mode. Digital image correlation is applied to further verify the validity of the experiments performed at both static and dynamic loading. The data reduction methodology applied in this paper is suitable for intralaminar fiber failure modes without significant delamination. Sufficient proof is given that quasi-static fracture mechanics theory can also be used for the data reduction of the dynamic tests. It is shown, that the intralaminar fracture toughness for fiber tensile failure of UD IM7-8552 increases with increasing rate of loading. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract Requiring probability density distribution functions of uncertain variables, it is difficult for conventional structural reliability analysis methods to calculate the reliability of composite laminates with both random and interval variables. To evaluate the safety of composite laminated structures under this circumstance, this paper is aimed at developing a precise and efficient univariate method for the reliability assessment of composite laminates taking use of Legendre orthogonal polynomials and Monte Carlo Simulation. In this paper, based on the Tsai-Wu failure criterion and first-ply failure assumption, the failure criterion of composite laminate is constructed firstly. Then the performance function of each lamina is approximately expressed as the sum of univariate contributions of all uncertain variables on the basis of the univariate method. Taking use of Legendre orthogonal polynomials, the univariate functions of random and interval variables are constructed by the least square fitting method and the univariate contribution bounds of interval variables are derived afterwards. Furthermore, the interval of failure probability can be calculated by substituting contribution bounds of interval variables and sample points of random variables into univariate function. At last, two examples and an engineering application are provided to demonstrate the efficiency and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper proposes a new framework of defining an overall constitutive model of the fibrous composite material with separated constitutive models of the fiber and the matrix phases. The framework stems from the first-order reduced-order-homogenization (ROH) method. The ROH method requires a set of influence functions to describe the interaction between material phases, which can be computed by solving a number of finite element problems of the fibrous meso-structure with periodic boundary conditions. This paper proposes an analytical methodology to evaluate the influence tensors. In this way, the overall constitutive model of the fibrous composite material is constructed by analytical influence tensors and stand-alone material models of the fiber and matrix phases, which can represent comprehensive material degradation mechanism at meso-scale. Finally, an example of overall constitutive model is defined and tested, with both fiber and matrix phases are assumed as the isotropic continuum damage model. [ABSTRACT FROM AUTHOR]

Fabric-reinforced cementitious matrix (FRCM) are composite materials more and more used for the reinforcement of masonry structures. The combination of high tensile strength fabrics (or meshes) with cementitious matrices, having good thixotropic capabilities and vapour permeability, makes such composites suitable for reinforcing a large number of masonry structures, including the one belonging to the historic heritage. FRCMs are bonded to the outer surfaces of structural masonry elements and, thanks to their adhesive capacity, bear much of the tensile stresses that unreinforced masonry cannot withstand. The effectiveness of such reinforcements, which is highly dependent on their ability to adhere to the masonry substrate, is generally investigated throughout specific experimental investigations (shear tests). Almost all the papers in the literature devoted to bond-slip analysis refer to the case of flat bonding surfaces, although these reinforcements are also widely used on curved structural elements such as arches and vaults. Therefore, this paper reports and examines the results of an extensive experimental program concerning the behavior of FRCM systems applied on curved masonry specimens. The results point out the influence of both curvature and reinforcement position (intrados or extrados) on the response of specimens in terms of bearing capacity, failure mode and post-peak response. [ABSTRACT FROM AUTHOR]

Combining interlocking structures and gradient design, this paper proposes a 2D biomimetic nacreous composite material with a gradient interlocked structure. Experimental and numerical simulation methods are employed to investigate the initiation and propagation behavior of cracks in the regular interlocking and gradient interlocked biomimetic nacreous composite materials. Samples of biomimetic nacreous composite materials with pre-existing cracks are manufactured using 3D printing technology, and uniaxial tensile tests are conducted to explore their crack propagation behavior. For biomimetic nacreous composite materials with periodic interlocking structures, the interlocking angle is found to play an important role in the toughening mechanism of composites. In biomimetic composite materials with large interlocking angles, crack inhibition effects are achieved through structural gradient design, effectively preventing catastrophic failure. Within the numerical analysis framework, a cohesive zone modeling approach is employed to represent the softer phase of the material, and the numerical simulation are validated by direct comparison with experimental results. In addition, a comprehensive numerical analysis is carried out to comprehend how the structural gradient affects the spread of cracks in these nacreous composites. This research not only illuminate the underlying deformation and toughening mechanisms intrinsic to interlocking nacreous composites but also pave the way for innovative strategies in the design of biomimetic materials through the incorporation of structural gradients. [ABSTRACT FROM AUTHOR]

This paper proposes a multiphase material topology optimization (MMTO) method for thermal–mechanical structures that takes into account the effect of the temperature field under stress and frequency constraints. Thermoelastic coupling occurs when an engineering structure is concurrently subjected to thermal and mechanical loads. This work focuses on the following: (1) the derivation of three interpolation schemes that use stress, dynamic, and thermal loads based on the extended solid isotropic material penalization (SIMP) method; (2) the incorporation of stress and dynamic constraints of multi-material thermal-elastic structural designs; and (3) the effective control of the impact of thermal loads on structures based on stress and frequency levels. Numerical experiments show that a clear distinction exists in the final topologies for mechanical and thermal loads among materials. Coupled and uncoupled mechanical and thermal behaviors can be predicted using finite element models based on materials' coefficients of thermal expansion. We see this method is capable of optimizing a structure's strength and stability while solving the issue of a thermal environment. In addition, it allows for the precise management of stress and vibration levels when structures are not exposed to a thermal environment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Composite materials made with textile fibers both with polymeric and cementitious matrices are often adopted for the retrofitting of masonry arches and vaults. A specific project that analyzes the performance of ancient masonry arches and vaults strengthened with composite systems has been recently concluded at Politecnico of Milan. The project involves the experimental evaluation and the development of numerical and analytical simulations. In this paper the experimental campaign is described, whereas the numerical validation is provided in an accompanying paper (Bertolesi et al., 2017). The tests were performed in situ on ancient masonry arches and vault elements. In particular, three barrel vaults and two arches either unreinforced or reinforced with Steel Reinforced Grout (SRG), Textile Reinforced Mortar (TRM) and Fiber Reinforced Polymer (FRP) were tested. The arches had a span equal to 3.30 m, a rise equal to 0.83 m and were built with common Italian bricks regularly spacing out two bricks laid edge on (thickness of the arch 12 cm) with two bricks (one over the other) disposed in single leaf. Barrel vaults had the same geometry of the arches but were made with a single leaf. In all cases, an eccentric vertical load was applied at 1/4 of the span and was increased up to failure. The experimental results on unreinforced structures are compared with those obtained on the strengthened ones in terms of failure mode, maximum load, stiffness and ductility. [ABSTRACT FROM AUTHOR]

This paper proposes a particle filter-based Bayesian framework for damage prognosis of composite laminates exhibiting concurrent matrix cracks and delamination. Literature shows a number of applications of particle filtering for real-time prognosis of metallic structures and, recently, matrix crack density evolution in composites. The work presented here enhances the methodology proposed in previous papers by extending the Bayesian framework to multiple damage mechanisms, and validates the approach using damage progression data from notched cross-ply CFRP coupons subject to tension-tension fatigue. A multiple damage-mode model for the estimation of the strain energy release rate and the remaining stiffness of damaged laminates constitutes the core of the particle filtering algorithm, thus allowing the prognostic framework to extend for monitoring of simultaneous, coexisting damages. Also, the damage state can be evolved into the future enabling simulation of damage progression and prediction of remaining useful life of the composite material. The proposed prognostic unit successfully predicts damage growth and fatigue life of the laminate, and the results are critically discussed with respect to filtered estimation of damage progression and remaining life prediction. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The paper presents a refined two-dimensional formulation for the thermo-mechanical analysis of laminated doubly-curved shell structures under thermodynamic equilibrium conditions. Both the kinematic configuration variables and the temperature variation with respect to the natural equilibrium state are described with a generalized formulation, following the Equivalent Layer-Wise (ELW) approach employing higher order polynomials, along with some proper zigzag functions. The governing equations are derived from the stationary configuration of the Helmholtz free energy of the system, and a semi-analytical solution is found. In the post-processing phase, the Fourier-based Generalized Differential Quadrature (F-GDQ) is applied to recover the actual three-dimensional response of the doubly-curved shell solid, and very accurate results are obtained for the quantities of both mechanical and heat conduction problems. In addition, the integrals occurring in the theory are performed numerically with the Taylor-based Generalized Integral Quadrature (GTIQ), showing a high level of accuracy with a reduced number of sample points. The model is validated in some case studies, where the accuracy of the model is shown, and the numerical predictions are successfully compared with those ones from refined three-dimensional numerical simulations. After that, an extensive set of parametric investigations is reported, pointing out the effects of the panel curvature, lamination schemes and different levels of coupling on the thermo-mechanical structural behavior of the investigated panels. [ABSTRACT FROM AUTHOR]

High fidelity geometry models of the preforms are essential in mechanical property analysis for composite materials. The paper presents a virtual braiding method to generate digital material twins of 3D braided preforms. The four-step 3D braiding process is simulated based on the concept of virtual fiber and according to the rule of yarn movement. Fiber-level models of the preform are constructed which display the micro-scale architectures explicitly, including compressive deformations of yarn cross-sections and variations of yarn paths. The Micro-CT test is employed to investigate the internal fiber structures of 3D braided preform and quantitatively verify the accuracy of the proposed modeling method. Influence of the braiding parameters, such as yarn tension and arrangement angle in the initial state on geometric parameters of the preform is then investigated. [ABSTRACT FROM AUTHOR]

The property measurement and structure optimization of composite materials are difficult topics due to the diversity of combinations of composite constituents and complexity of their application environments. The distribution of composite constituents and heat sources can cause heat aggregation phenomenon which may lead to failure of materials. In this paper, we first propose a deep learning-based surrogate model (DLBSM) which can quickly and accurately achieve the end-to-end prediction between the layout of composite materials under multiple heat sources and its temperature field. The prediction result depicts that the coefficient of determination for the maximum and average temperature of all cases exceeds 0.996. Then, the layout optimization is transformed into a combinatorial optimization problem, and the DLBSM is combined with optimization algorithm to optimize the maximum temperature, temperature gradient, and uniformity of the temperature field. The optimized maximum temperature and temperature gradient are significantly reduced, while the temperature uniformity is improved. These enhancements effectively reduce the probability of failure in composites. This approach can significantly improve the efficiency of thermal behavior prediction of composite and its layout optimization compared with finite element method (FEM). [ABSTRACT FROM AUTHOR]

The application of the sampling surfaces (SaS) method to the three-dimensional (3D) stress analysis of laminated composite plates is presented in a companion paper (Kulikov GM, Plotnikova SV. Strong sampling surfaces formulation for laminated composite plates. Compos Struct 2017;172:73–82). In this paper, we extend the strong SaS formulation to the 3D vibration analysis of laminated plates. The SaS formulation is based on choosing the arbitrary number of SaS parallel to the middle surface in order to introduce the displacements of these surfaces as basic plate unknowns. Such choice of unknowns with the use of the Lagrange polynomials in spatial approximations of displacements through the layer thicknesses leads to a robust laminated plate formulation. The feature of the proposed approach is that all SaS are located inside the layers at Chebyshev polynomial nodes. The use of outer surfaces and interfaces is avoided that makes it possible to minimize uniformly the error due to the Lagrange interpolation. Therefore, the strong SaS formulation based on direct integration of equations of motion of the elastic body can be applied efficiently for the 3D free and forced vibration analyses of laminated composite plates. [ABSTRACT FROM AUTHOR]

The application of composite materials in marine propellers is a relatively recent innovation. Methods have been presented to analyse the hydro-elastic behaviour of these type of propellers and in some studies these methods have been validated as well. Differences between measured and predicted responses are typically explained from inaccuracies in structural or fluid modelling. It is beyond all doubt that for an accurate finite element (FE) model a correct modelling of the fibre orientations and material properties is required. Both subjects are addressed in this work. An approach is presented in order to accurately define the element dependent fibre orientations in doubly curved geometries like (marine) propeller blades. In order to improve the structural response prediction this paper presents an inverse method based on experimental and numerical results which can be used for structural identification and FE model updating. In the developed approach the residual between measurement results obtained with static experiments and results obtained with an FE model is minimized by adapting the stiffness properties in the FE calculation. This method has been successfully applied to two small scale composite propellers. The obtained material properties have been determined with a relatively high confidence level. A verification by means of measured and calculated eigenfrequencies show also that accurate results are obtained with the inverse method. Therefore, this paper gives a positive answer on the research question whether it is possible to determine the stiffness properties of small scale composite marine propeller blades from a static experimental data. [ABSTRACT FROM AUTHOR]

The object of this paper is to present a novel semi-analytical method and its associated applications for linear vibration analyses of functionally graded carbon nanotube reinforced composite (FG-CNTRC) doubly-curved panels and shells of revolution on with arbitrary boundary conditions. Distribution of the carbon nanotubes through the thickness of the structures may be uniform or functionally graded and four types of the CNTs distribution are considered in this paper. Properties of the composite media are determined by a refined rule of mixtures approach which contains the efficiency parameters. The translation and rotation displacements of the doubly-curved structures are uniformly expressed as the superposition of a standard cosine Fourier series and several auxiliary functions introduced to eliminate all potential discontinuities of the original displacement function and its derivatives at the edges. Based on that, the energy expression of the FG-CNTRC doubly-curved panels and shells of revolution is examined where the first-order shear deformation elasticity theory is considered. Lastly, to solve the natural frequencies as well as the associated mode shapes by means of the Ritz-variational energy method. Unlike other existing methods, the proposed method is capable of handling various combinations of boundary constraints in a unified fashion, including free, simply-supported, clamped and elastic-supported boundary conditions. Comprehensive studies on the convergence, accuracy, stability and efficiency of the method are derived via the comparison with existing results reported in publications. The parametric studies concerning the influence of the geometrical parameters, CNTs distributions, volume fraction of CNTs as well as boundary restraint parameters on free vibration of FG-CNTRC doubly-curved panels and shells of revolution is also investigated in detail. [ABSTRACT FROM AUTHOR]

A comprehensive asymptotic homogenization model for the analysis of composite and reinforced magnetoelectric plates is developed. The model culminates in a set of unit cell problems via which the relevant expressions for the effective coefficients are derived. The micromechanical and geometrical platform pertinent to the desired model is set up in this paper which also obtains the effective elastic, piezoelectric and piezomagnetic coefficients; the remaining properties, including product properties, are calculated in another work. Examples of structures that can be examined include diagonally- and triangularly-reinforced plates, hexagonal honeycomb sandwich plates and others. The developed model can be used to customize the effective properties of a reinforced plate to the requirements of a particular engineering application by changing some geometric, structural or material parameter of interest. It is shown that in the limiting case of a thin composite plate of uniform thickness whereby electrical conductivity is ignored and all pertinent quantities are time-averaged the presented model converges to the familiar classical plate model. Overall, this paper represents an important addition to the existing literature in terms of the complex geometries that can be designed and analyzed and at the same time constitutes an important refinement over previously published work. [ABSTRACT FROM AUTHOR]

Textile-reinforced concrete (TRC) as a novel composite material offers a wide range of capabilities and flexibility in the manufacturing of thin-walled, lightweight structures. The application of textile reinforcement in fine aggregate high-performance concrete has enabled the dimensioning of structural concrete in very small thicknesses. This possibility allows for the fabrication of thin-walled TRC shell structures with complex geometries. On the other hand, structural planning and construction require new modeling approaches to comprehend the structural behavior of such forms. In this paper, we present the fabrication procedure of a large-scale TRC vault shell, together with the performed experimental study. The shell structure was tested under a two-step loading scenario to study the load-bearing behavior. The particular focus of the paper is on the analysis of the structural behavior by means of an anisotropic strain-hardening material model specifically developed for the simulation of TRC shells. The prediction obtained using the nonlinear finite element simulation has been compared with the test results to validate the modeling approach. The performed studies are used to evaluate and discuss the structural redundancy included in the applied linear ultimate limit state assessment procedure. [ABSTRACT FROM AUTHOR]

This paper reports on the mechanism of rate-dependent off-axis compression of a unique unidirectional composite with unusually high matrix volume fraction of 65%. The test material is an E-glass fibre reinforced polypropylene composite and was subjected to quasi-static, medium and high strain rates (with strain rates from 10 - 3 s −1 to 10 3 s −1 ). This paper has shown experimental evidence of significant rate-dependence of yielding, strain softening and fracture strain of the test composite. Also, the study reports on the effect of strain rates on evolution of different failure modes of the composite. The observed rate-dependence was shown to result from the influence of the pure matrix on the constitutive behaviour of the composite. The work has used a two-process Ree-Eyring yield model of the matrix to demonstrate the origin of the observed rate-dependent yielding of the composite. The data derived in this study will be significant for further micro-mechanical modelling of finite deforming composites used in especially damage tolerant applications. Composite design engineers and stress analysis experts should benefit also from the findings in this work. [ABSTRACT FROM AUTHOR]

This paper describes a phenomenon that significantly affects the stress distribution in notched laminate composite structures. Ply-to-ply interactions under non-uniform laminate stress fields induce interlaminar and in-plane stresses that are not described in classical laminate theory. The theory is consist of a power series expansion along the thickness-wise coordinate. The most prominent influence of this effect is a substantial reduction in stress concentrations around notches. This may explain various experimental facts that conflict with classical laminate theory. Although this phenomenon appears to be a derivative of the so-called free-edge effect, it is not based on free-edge boundary conditions. For verification of the theory, this paper also presents finite element analysis cases for 5 mm diameter open hole [90/0] s cross-ply laminate plates. Reductions in stress concentration around hole edges were up to 10% of the far field stress. Insertion of low elasticity layers between 90° and 0° plies, the use of thick plies, or the adoption of ply materials with low transverse shear moduli were identified as effective means of reducing stress concentration around a hole. These findings may enable the application of new design principles to deal with stress concentrations for notched composite structures. [ABSTRACT FROM AUTHOR]

In this paper, a novel multiscale tri-level finite element (FE) numerical analytical method is proposed to study the mechanical performance of SiCp/CNT dual-scale hybrid reinforced aluminum matrix composite materials. The analysis is ranging from nano-scale CNT reinforcement to micro-scale SiCp particles to macro-scale composite systems. The microstructures of the CNTs and 6061Al matrix at the nano-RVE and the SiCp particles, interfaces, and 6061Al matrix at the micro-RVE are established. At the macro-scale, the structural model is established, whose elastic–plastic material properties and damage state are derived from the homogenization calculation of the representative volume element (RVE) at the micro-scale. Multi-scale progressive damage simulation of the macro-scale model is achieved by invoking the micro-scale and nano-scale RVE. The stress–strain curve of one finite element in the macro-structure model is randomly extracted and comparing it with the calculated micro-material-properties of SiCp (CNT)/ 6061Al, the good consistency verifies the effectiveness and accuracy of the multi-scale tri-level FE calculation method proposed in this paper. Using this multi-scale tri-level FE model, we can see the load-bearing and fracture process of the macrostructure, and the damage evolution process of the microstructure, which enables us to further carry out the research work of composite microstructure design, interface regulation, etc. [ABSTRACT FROM AUTHOR]

This paper presents analytical solutions for the effective viscoelastic properties of composite materials based on a homogenization approach. The cases of spherical inclusions and cracks were recently developed. The objective of this paper is to use the same technique to deal with the case of periodic media containing cuboidal inhomogeneities. The viscoelastic behavior of both the matrix and the homogeneous equivalent medium is modeled by the Zener rheological model while inclusions are assumed to be linearly elastic. The viscoelastic Hill tensor required for the calculation of the effective viscoelastic tensors is obtained explicitly in the Laplace-Carson space in terms of Fourier series. The final expressions show that overall behavior depends on the viscoelastic properties of the matrix, the 3D dimensions of the inclusions and the thickness of the matrix layer between two nearby inclusions. Applications to masonry structures are presented to illustrate the theoretical results. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on epoxy resin-carbon fibers composites interleaved with Nylon 6,6 nanofibers. In particular, the paper focuses on the effect of the thickness of the nanoreinforce into two types of laminae: unidirectional (UD) and plain wave (PW). The effectiveness of the nanoreinforce has been addressed by comparing critical and propagation energy release rates, calculated by testing samples under Mode I and Mode II fracture mechanic loads. Experiments show a general improvement in delamination resistance when the nanofibers are interleaved. Nevertheless slightly different behaviour has been found between the two types of lamina: micrographs of crack paths have been used to explain the reinforce mechanisms and such differences, suggesting a strong interaction between the nature of the fabric and the thickness of the nanointerlayer. [ABSTRACT FROM AUTHOR]

In the conventional methods, the failure load and failure mode of composite pre-tightened tooth connections are determined by experiment. To determine the failure load and failure mode of the composite pre-tightened tooth connection without performing experiments, a new method based on the characteristic lengths is presented in this paper. This method involves three steps: first, based on the concept of the method to determine the shear characteristic length, the compressive characteristic length is defined and the characteristic length of composite tooth can be determined; second, the stress distribution of the composite joint is analyzed by the use of a finite element method; finally, combined with failure criterion and the stress distribution of the composite joint, the failure load and failure mode of this joint can be determined by the compression factor f c and shear factor f s . The prediction methods based on the characteristic lengths are validated by the tests for composite joints with different parameters, and the results indicate that the methodology proposed in this paper can accurately and effectively predict failure loads as well as failure modes in composite pre-tightened tooth. [ABSTRACT FROM AUTHOR]

A wind turbine blade generally has complex structures including several layers of composite materials with shear webs, making its structure design very challenging. In this paper, a structural optimisation model for wind turbine composite blades has been developed based on a parametric FEA (finite element analysis) model and a GA (genetic algorithm) model. The optimisation model minimises the mass of composite blades with multi-criteria constraints. The number of unidirectional plies, the locations of the spar cap and the thicknesses of shear webs are taken as design variables. The optimisation model takes account of five constraints, i.e. stress constraint, deformation constraint, vibration constraint, buckling constraint, and manufacturing manoeuvrability and continuity of laminate layups constraint. The model has been applied to the blade structural optimisation of ELECTRA 30 kW wind turbine, which is a novel VAWT (vertical-axis wind turbine) combining sails and V-shape arm. The mass of the optimised blade is 228 kg, which is 17.4% lower than the initial design, indicating the blade mass can be significantly reduced by using the present optimisation model. It is demonstrated that the structural optimisation model presented in this paper is capable of effectively and accurately determining the optimal structural layups of composite blades. [ABSTRACT FROM AUTHOR]

• Bertrand's paradox for chords on a circle can be extended to fibers on a square, with many methods of fiber placement. • A combination approach of fiber placement methods can arrive at an acceptable level of uniformity. • Homogeneous fiber layout requires invariance to Euclidean transformations: rotation, scale, and translation. • Fiber "line density" and "fragment size" are defined and used to evaluate methods of uniform random fiber placement. • The proposed exact solution creates homogeneous uniform random fiber placement for the creation of digital twin geometry. This paper considers the random placement of fibers on a square to achieve a desired homogeneous uniform distribution, as required in production of digital twin geometry for computational analysis of composite materials. Six methods of random fiber placement were demonstrated. Bertrand's paradox for placement of random chords on a circle provided the framework to compare, contrast, and evaluate similar methods for placing fibers on a square. Simulations were performed with chord placement on a circle and fiber placement on a square. Results were evaluated using uniformity of areal line density and fragment size distributions. A combined approach for placement of fibers on a square was developed that achieved homogeneous uniform distribution. A proposed exact solution to achieve the desired uniform distribution was demonstrated. A statistical analysis and data regression demonstrated the homogeneous results achieved by the combined approach and the proposed exact solution. The objectives of this paper were achieved through demonstration of both a combined approach and a proposed exact solution to random fiber placement on a square to achieve homogeneous, uniform distribution of fibers for use in digital twin geometry for computational testing of composite materials. [ABSTRACT FROM AUTHOR]

Safety margins of aerospace structures can be improved through altering the laminate parameters of composite materials to increase flutter and divergence velocities. Existing work demonstrates the impact of material uncertainties on low-fidelity structural models that are not sufficient to represent realistic aircraft designs. A gap exists in quantifying laminate parameter uncertainties on aeroelasticity for high-fidelity three-dimensional composite structures in realistic tailored designs. This paper puts forward an efficient methodology for uncertainty quantification on the aeroelastic characteristics of three-dimensional composite structures using FE-based parametric composite models and advanced Kriging surrogate models. The methodology is tested on both low and high fidelity case studies to represent the composite wing structure. Similarities between the case studies are observed in the coefficient of variance of all hard flutter modes being within 0.15-1.4% of each other. The difference was found for divergence and soft flutter velocities where the coefficient of variance could be over ten times higher in the high fidelity case. Global sensitivity results revealed similar physical behavior cases can be produced from both studies at early design stages. • Proposed method for UQ on aeroelastic qualities of 3D composite structures. • Effectively demonstrated methodology on low-fidelity plate composite model. • Effectively demonstrated methodology on high-fidelity Finite Element model. • Found strong agreement in uncertainty for hard flutter behaviour between studies. • Found low agreement in uncertainty for soft flutter & divergence between studies. • Most influential uncertain laminate parameters identified via sensitivity analysis. [ABSTRACT FROM AUTHOR]