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Abstract Composites reinforced with braided textiles exhibit high structural stability and excellent damage tolerance, making them very attractive for defence, aerospace, automotive and energy industries. Considering the real-life service environment, it is crucial to study a dynamic response of a composite structure and its energy-dissipation ability, especially under repeated low-velocity impacts. So, a series of drop-weight tests were carried out followed by X-ray computed micro-tomography to characterize damage morphology of braided composite specimens. Meanwhile, a multi-scale computational approach was explored and implemented as a user-defined-material subroutine (VUMAT) for ABAQUS/Explicit to capture main damage modes of a braided textile composite, while its delamination was modelled by employing cohesive-zone elements. Load- and energy-time curves were obtained both experimentally and numerically. The predicted levels of peak forces and absorbed energy were found to agree with the experimental data. An extent of delamination and damage accumulation in the braided composite was predicted numerically and analysed; it was found that material responses to repeated impacts had two types depending on the level of normalised impact energy. The presented modelling capability could contribute to design of braided composite structures for various applications. [ABSTRACT FROM AUTHOR]

Porosity is a major manufacturing defect which affects the matrix dominate properties of continuous fibre composites, in particular the transverse strength. The simulation of porosity allows for predictions on the reduction of strength to be made, however, there is a trade-off between accuracy and computational efficiency. The multi-scale modelling approach presented here allows for accurate 3-dimensional geometry data of voids to be used. This is accomplished by first evaluating the effect the porosity has on degrading the matrix then subsequently, by using a representative unit cell, ply level strength can be predicted. The model is validated against empirical tensile and compressive testing of unidirectional autoclave cured prepreg with strong correlation. The approach allows for a reduction in overengineered structures by predicting accurate material properties for a given porosity generation. [ABSTRACT FROM AUTHOR]

Traditional carbon-steel armour wires pose limitations ( e.g. long spans, weight reduction, corrosion and fatigue) for flexible risers to operate in demanding and deeper water environments. In this context, an alternative to carbon-steel tensile armour wires is proposed recently by the authors (Gautam et al ., 2016), comprising of hexagonally packed polymer composite rods with uni-directional fibres and an over-braided ( i.e. bi-axial braid with high performance fibres) sleeve. These hybrid composite wires offer opportunities to tailor their mechanical properties by varying the geometrical ( e.g. rod diameter, packing) and processing parameters ( e.g. material selection, braid pattern) involved. In order to understand the mechanical behaviour of these hybrid composite armour wires, this paper presents a multi-scale model developed by using a combined analytical-computational approach. The multi-scale model is developed to predict the torsional and flexural behaviour of the hybrid composite wires; and the role of over-braid structural parameters, pre-tension and internal friction are investigated. The behaviour of the multiscale model is found to be in good agreement with the experimentally observed behaviour. After validating the multi-scale model with the experimental data available for specific configurations, parametric studies are conducted on the torsional and flexural behaviour of the hybrid composite wires to study the role of internal friction between un-bonded components and the braid tow tension in the over-braided sleeves. [ABSTRACT FROM AUTHOR]

In this paper we investigate micro–macro mechanical relations in Palmetto wood, a natural composite, by means of a computational homogenisation-based multi-scale modeling approach. The homogenisation procedure is solved sequentially at multiple length scales by following a bottom-up approach. The proposed strategy consists of homogenising the three length-scales by means of representative volume elements of material. The resulting effective macroscopic properties allow us to compute the mechanical response of individual macro-fibres and bulk wood specimens by means of a standard finite element procedure. The numerical predictions are compared successfully with experimental measurements. Following the validation, a parametric study is performed to investigate the micro–macro relations in Palmetto wood. The influence of several micro-structural features present at different length scales are investigated, such as the cellulose content and its crystallinity, the microfibril angle, the cell-wall thickness of micro-fibres, and the porosity of macro-fibres and their surrounding matrix, among others. The effects of such microscopic parameters are investigated on the Young’s modulus and density of Palmetto bulk wood. These results show that the present strategy can pave the way for further studies on complex microstructure–property relationships existing in different wood species, and on the possible development of more advanced bio-inspired composite materials. [ABSTRACT FROM AUTHOR]

This paper presents a detailed numerical analysis of two types of 3D woven composite architecture. a unit cell, full finite element meso model, and a mosaic macro model are developed to simulate the elastic and damage progression behaviour of the two weave types. Both models predicted the tensile modulus and strength within 20% of the experimentally measured values, and the predicted failure sequence was similar to the experimental observation. [ABSTRACT FROM AUTHOR]

Laminated composite structures operating in a marine environment are subject to moisture ingress. Due to the slow diffusion process of moisture, the distribution of moisture is not uniform so that the laminates can develop hygrothermal stresses. An accurate prediction of the moisture concentration and the associated hygrothermal stress is vital to the understanding of the effect of marine environment on failure initiation. The present paper investigates the time-dependent moisture diffusion and the stress distribution in carbon fibre reinforced polymeric (CFRP) composites by means of experimental study and Finite Element Analysis (FEA). Samples were made from CFRP pre-preg autoclave-cured, and then immersed in fresh water and sea water at a constant 50 °C for accelerated moisture diffusion. Laminates with [0] 16 , [90] 16 , [±45] 4s lay-up sequences were investigated. A multiscale 3D FEA model was developed to evaluate the interfacial stresses between polymer matrix and carbon fibre and the stress distribution in the composite laminates. The analysis revealed that both the stress distribution and stress level are time-dependent due to moisture diffusion, and the interphase between fibres and matrix plays an important role in both the process of moisture diffusion and the stress/strain transfer. The interlaminar shear stresses of the laminates induced by hygrothermal expansion exhibited a significant specimen edge effect. This is correlated with the experimental observations of the flexural failure of laminates. [ABSTRACT FROM AUTHOR]

• A new multi-scale elasticity model with translational and rotational degrees of freedom was presented to study the effects of heterogeneity in three-dimensional particulate composite structures. • The lattice structure was determined using redundant terms from the open formula of the motion equation. • The phase velocity behavior of primary ultrasonic waves was analyzed in geopolymer products with various schemes. • The theoretical model presented parameters to capture the characteristics of geopolymer products. Research on the characteristics of composites material has received enormous interest in recent years. The multi-scale nature of composite material leads to employing advanced techniques. Moreover, the presence of a wave with the high-frequency source adds complexity to the analysis. In this paper, a novel multi-scale elasticity model was developed to predict the wave dispersion property of particulate composites. The methodology was based on the simultaneous participation of translational and rotational degrees of freedom in motion equations. The method scheme of gaining motion equations was accomplished by using Taylor's expansion as a continualization method. The framework of the motion equations and restrictions determined the layout of the lattice. The obtained lattice was a network of polydispersive particles in three-dimensional as a unit cell. The model proposed the inertia coefficient and dimension coefficient, which reflected the heterogeneity on the particulate composite. The theoretical results were then validated with experimental results. For this aim, concrete and mortar with the various classes of heterogeneity were nominated as a sample of particulate composite material. The nominated composites were prepared with different mixture schemes from geopolymer paste. The wave dispersion behavior of geopolymer specimens was established by measuring phase velocity in various frequencies. In addition to geopolymer products, experimental results from independent literature were scrutinized for concrete and mortar with cement binder. In the end, the capability of the presented parameters was indicated for realizing the state of concrete curing and features of the binder. [ABSTRACT FROM AUTHOR]

In-plane centroids of textile composites are simulated as cross-correlated random fields. Each tow position is defined as an average trend quantified from experimental data, added with zero-mean deviations produced as a stochastic field. Realisations of these fields are generated using a framework based on the Karhunen–Loève series expansion that is calibrated with experimental information from prior work. Positional deviations are obtained that are correlated along the tow and between neighbouring tows. The application is a 2/2 twill woven carbon fibre reinforced epoxy consisting of multiple unit cells. Generated in-plane deviations of the warp and weft tows resemble the experimental fluctuations with similar wavelengths. Simulation of thousand specimens demonstrates that the virtual in-plane positions possess the experimental standard deviation and correlation lengths on average. [ABSTRACT FROM AUTHOR]

Abstract: A significant challenge in the numerical modelling of composite structures with a multi-axis fibre architecture is the reproducibility of the textile mechanics . A numerical analysis procedure for woven composite structures using a multi-scale finite element approach has been developed, and is presented in this paper. The approach is demonstrated for a flat two-dimensional woven glass/epoxy laminate. Digital microscopy is used to estimate tow cross-section and path, and quantify the amount of variation of these parameters. This data is used to generate both a meso-scale model of a single unit cell as well as a macro-scale model of the complete structure. Numerical results from the proposed approach are compared to experimental stress–strain data, which show good agreement in the lower strain range. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to present a multi-scale method for the mechanical modelling of sandwich structures. Low- and high-order sandwich elements are formulated on the basis of Carrera’s Unified Formulation (CUF) and bridged within the Arlequin framework. According to CUF, an N-order polynomials approximation is assumed on the beam cross-section for the unknown displacements, being N a free parameter of the formulation. Low-order, computationally cheap elements are used to describe the global mechanical response. High-order, computationally demanding elements are used to capture the local effects in the boundary layers. CUF framework is here enhanced by the assumption of the Constrained Variational Principle (CVP) in order to derive a new class of layered beam finite elements with an independent kinematic field for each lamina. Results are assessed towards two- and three-dimensional finite element solutions. The numerical results show that, in the context of CUF, the Arlequin method effectively couples sub-domains modelled via variable order finite elements. The proposed coupled models yield accurate results, being able to predict both the global solution and the local effects, with a reduced computational cost. [Copyright &y& Elsevier]

A 3D finite element model is presented to simulate the thermal transport properties of a polymer composite made of GradeM25 graphene nanoplatelets (GnP) in a low-viscosity bisphenol-A epoxy resin. This is a multi-scale approach that consists of a unit cell, a representative volume element (RVE) and a macroscopic specimen model. There is a hierarchy of structural scales, where the response at one size level is described by one (or more) parameters and passed to the next level up. At the unit cell level, the material nano-characteristics are used to calculate the local thermal response, while the material architecture is captured by the RVE. A statistical sample is studied and the average thermal response is compared to the macroscopic experimental data. Afterwards, the experimental reinforcement distribution is applied to the macro specimen model. The thermal diffusivity mapping was obtained numerically and compared to the flash method measurements following the CEN/CWA 16799 standard, through the decomposition and reconstruction of the fields with orthogonal Zernike polynomials and their comparison on a linear correlation plot. Finally, the simulation of the transient thermal response was successfully validated under both generic and full-field comparison. This research work is, therefore, proposing a validated simulation tool to estimate the thermal response of polymer nanocomposites, while being engineer-friendly to promote advanced material design that eliminates the trial and error approach. [ABSTRACT FROM AUTHOR]

The failure modes of laminated composites are complex and varied, being strongly dependant on layup and loading conditions. In this paper, a novel representation of damage initiation in laminated composites on the structural scale is presented. The proposed approach combines the knowledge of the composite design space with detailed meso-scale models to generate a complete representation of damage initiation under all possible combinations of bi-axial loading and for any layup in the composite design space. The 2D bi-axial failure envelopes are then combined with the knowledge of the composite's physical behaviour and the surface compatibility conditions to generate 3D failure manifolds using Gauss surface equations. The resulting system is a complete parametric description of damage initiation in laminated composite for any layup under general loading conditions. The proposed manifolds are accurate and computationally efficient tools for structural (macro) scale analyses, design and optimisation. [ABSTRACT FROM AUTHOR]

In this study, the impact behavior and damage mechanism of novel carbon-Dyneema hybrid fabric reinforced plastic composites are investigated using drop-weight and steel ball impact tests. The force-displacement response, energy absorption and damage mechanisms of the hybrid composite were analyzed and compared with those of woven carbon laminates. The experimental results suggest that the impact resistance of the hybrid composites has been improved when benchmarked with carbon fiber composites. The Dyneema fibers suppress the splitting of bottom surfaces during impact but are susceptible to buckling due to the low compressive strength of Dyneema fibers. A multi-scale approach is proposed for modelling impact damage of hybrid woven composite accounting for the interweave patterns. A meso-scale representative volume element of the interwoven hybrid structure is analyzed and discretized into several sub-sections, each regarded as a type of laminate. Then, a macro-scale impact model is established based on the homogenized effective properties of the sub-sections using continuum shell elements. The failure of the material is modelled using energy-based continuum damage mechanics. Overall, the numerical simulations show good agreement with experimental data in terms of impact force history and damage features, providing insight into the impact damage behavior of hybrid composites. [ABSTRACT FROM AUTHOR]

This paper investigates the feasibility of using porous pyrolytic carbon (PyC) to produce low thermal expansion (CTE) carbon/carbon composites by using a multi-scale representative volume element (RVE) based on a finite element (FE) homogenisation strategy. The distribution of pores is characterized by mercury intrusion test and multi-peak Gaussian function. The RVEs of porous structures at micro- and macro-scale are generated using a modified random sequential adsorption (RSA) algorithm. The effective CTEs of porous PyC composites are obtained by implementing the periodic boundary conditions and the numerical implementation is detailed herein. The predicted effective macroscopic CTEs are validated by experimental measures. The results show that the absolute values of effective macroscopic CTEs of PyC composites nonlinearly increase with increasing pore volume fraction. For the high-texture (HT) chopped C/C composites, the effective CTEs reach a minimum at fibre yarn volume fraction of V fm ≈ 7%, whereas the effective CTEs of low-texture (LT) PyC composites increase with increase of the fibre volume fraction. In addition, the effect of aspect ratio of the fibre yarn on the effective CTEs of both HT and LT composites are not significant. [ABSTRACT FROM AUTHOR]