Composite Plies Research Articles

In Situ Investigation of the Kinematics of Ply Interfaces During Composite Manufacturing

Abstract The kinematics of composite ply interfaces critically affects both the manufacturing processes and deformation mechanisms and is often responsible for the formation of defects such as wrinkles and delamination. In the present work, processing-induced defects in a carbon fiber prepreg composite are evaluated by devising a novel in situ experimental approach. Carbon fiber prepreg laminates are cured in a specially designed autoclave with viewports with plies laid up on a mold with cylindrical tooling set up to maximize the ply-movement. Four-ply layup orientations of [90/90]s, [90/0]s, [90/45]s, and [90/−45]s and three-mold configurations with cylindrical tools of diameter 9.5 mm (3/8 in.), 12.7 mm (1/2 in.), and 15.9 mm (5/8 in.) are used for the parametric study. Strains, ply-movement, and formation of defects are observed in situ using digital image correlation (DIC) during the autoclave cure cycle, through the viewports. The processing-induced defects in the composite are further characterized by X-ray micro-computed tomography (micro-CT). We observed that the mold with the larger radius of curvature (15.9 mm cylinder) leads to higher strains in both in-plane directions and higher displacement in out of plane directions. The maximum average out-of-plane ply movement, as well as the largest wrinkle, are observed for [90/−45]s layup on the mold with the highest radius of curvature.

Influence of the glass non-crimp fabric intrinsic undulation on the stiffness of the composite ply: A micromechanical approach

Load carrying components of modern wind turbine blades are manufactured from composites, consisting of non-crimp fabrics infused with polymer resins. The effective stiffness of the resulting laminate is a combination of the properties of its building blocks i.e. fibers, and matrix as well as from the fabric texture imperfections e.g. fiber undulations. Moreover, ply inherent boundary conditions, e.g. the restriction of the Poisson deformation of the matrix imposed from the adjacent fibers, are determining the in-situ orthotropic performance. Towards modelling the in-plane stiffness of a unidirectional (UD) infused non-crimp fabric, a two-step modular procedure is proposed, accounting for the aforementioned parameters, based only on experimental data and analytical formulations. Initially, a micromechanical model is predicting the stiffness of the ideal UD ply i.e. disregarding fiber undulations. Subsequently, a plate model is generated based on the classical lamination theory, approximating the UD laminate as a multiaxial configuration of ideal UD sub-plies. Each sub-ply thickness and orientation is based on the fiber angle density distribution of dry fabrics and cured laminates. These are derived experimentally with an integrated optical camera system and Computer Tomography scans respectively. The theoretical laminate stiffness is correlating very well with standard and thick UD laminate quasi-static tests.

Specific Heat and Thermal Expansion Coefficient of Hybrid Epoxy Composites

Thermal behavior of hybrid epoxy composites reinforced with different types of plain weave fabrics and ply orientation at various angles was investigated in this research. It was analyzed their thermal linear expansion coefficient and specific heat measured with Thermomechanical Analyzer (TMA) and Differential Scanning Calorimeter (DSC) respectively. Also, in this paper was studied the influence of carbon black - aramid powder and carbon black - barium ferrite mixtures added into epoxy matrix between certain plies of the hybrid composites. The experimental results showed that the addition of filler mixtures led to a significant decreasing of thermal expansion coefficient and specific heat of the hybrid epoxy composite with carbon outer plies. It was recorded a good structural stability in case of hybrid carbon-glass composite in the temperature range of 40-60�C.

Single and multi-layer core designs for Pseudo-Ductile failure in honeycomb sandwich structures

This paper investigates the possibility of triggering ductile modes of failure in a high-performance sandwich structure by employing a multi-core design with dispersed composite plies. Here, two sandwich configurations, based on a carbon fiber reinforced epoxy face sheet and a NomexTM honeycomb core, have been manufactured and their properties evaluated in flexure and compression. The mechanical performance of these multi-layer configurations was then compared with the properties offered by a conventional sandwich panel of similar thickness, manufactured using the same number of carbon/epoxy prepreg plies. The effect of span length on the flexural and compressive properties of the sandwich panels was investigated by performing tests on differently-sized specimens. Theoretical models have been developed to predict the mechanical properties of the sandwich panels. In both theoretical and experimental approaches, superior mechanical properties were obtained in the shorter span length samples. The deviation between the predicted and experimental results was reasonably low.

A finite deformation Cosserat continuum model for uncured carbon fibre composites

A new three-dimensional, finite deformation Cosserat continuum model for the elastic response of uncured carbon fibre composites is presented. The new composite process model captures the bending contribution of bundles of fibres at the microscale within a mesoscale continuum description of a composite ply. This is achieved by introducing higher-order, independent rotational degrees of freedom into the continuum formulation. This paper demonstrates the inclusion of such mechanics is essential to accurately model various bending responses induced during typical composite manufacturing processes. This includes large deformation forming, finite strain consolidation and wrinkling (the formation of an unwanted defect). If such mechanics are not included, the literature demonstrates the resulting finite element solutions have a pathological dependence on the mesh size. As a result, simulations require users to fit mesh-dependent material parameters, which limits confidence in their predictive capabilities. The Cosserat continuum, which can be seen as a form of the regularised continuum model, overcomes these challenges. In particular, this paper presents details of the finite element formulation of the new continuum model within a nonlinear Taylor–Hood Cosserat Element. Implementation details of embedding this new element within the commercial code Abaqus are given, alongside a series of increasingly complex validation simulations. Notably, the examples include modelling the formation of internal fibre wrinkles and large deformation forming, which involves complex ply-to-ply and tool-to-ply contact. The paper concludes by describing: (1) how the elastic Cosserat model can be integrated into existing large deformation process models in the literature. The approach set out readily allows researchers to include the important effects of resin flow, cure kinetics and temperature distribution, not considered in this contribution, and (2) how it is envisaged that the ply scale model can be naturally scaled up to large laminate scale simulation using mathematical upscaling techniques.

Free vibration analysis of hybrid laminated plates containing multilayer functionally graded carbon nanotube-reinforced composite plies using a layer-wise formulation

This paper studies the free vibration behavior of hybrid laminated plates using a layer-wise formulation. The laminated structures are made of graphite fiber-reinforced composite (GFRC) and multilayer functionally graded carbon nanotube-reinforced composite (FG-CNTRC) plies. The internal single-walled carbon nanotubes (SWCNTs) are distributed in layer-wise form along the ply thickness direction either uniformly or functionally graded according to four separate patterns. Both the linear and nonlinear of the CNTs distributions form are considered. Theatrical formulations are based on the first-order shear deformation theory (FSDT), and finite element method is employed to obtain the nondimensional natural frequency. The elastic modulus of both GFRC and CNTRC plies composed the hybrid laminated plate is obtained by a micromechanics model. Initially, the provided results are validated by comparing it with the literature; thereafter, a parametric study is carried out the influences of laminates configuration, number of CNTRC plies layers, CNT volume fraction, CNT distribution patterns, linear and nonlinear distribution of CNTs, plate width-to-thickness ratio, plate aspect ratio, boundary conditions, stacking sequences, and number of plies on the free vibration behavior of hybrid laminated plates. Some final remarks considering the laminated plate configuration and the distribution form of the CNT fillers are presented in order to provide a useful observation on the design criteria of the laminated composite plate structures.

Progressive damage analysis of composite laminates subjected to low-velocity impact using 2D layer-wise structural models

The present work deals with the progressive damage analysis of composite laminates subjected to low-velocity impact. We develop a numerical model using higher-order structural theories based on the Carrera Unified Formulation (CUF) with Lagrange polynomials and resulting in a 2D refined layer-wise model. To model damage, we use a combination of the continuum damage-based CODAM2 intralaminar damage model to account for fibre and matrix damage within the ply, and cohesive elements to account for delamination between successive composite plies. We carry out numerical assessments for the case of a linear elastic composite plate subjected to impact, to compare the current framework with standard approaches based on 3D finite element (FE) analysis. We, then, consider the elastoplastic analysis of a bimetallic laminated plate to compare the performance of the proposed layer-wise model and 3D-FE approaches, for the case of nonlinear impact analysis. The final assessment considers progressive damage due to low-velocity impact, and the results are compared with available literature data. The numerical predictions show a good correlation with reference experimental and simulation results, thus validating the current framework for impact analysis of composite structures. Comparisons of the proposed layer-wise structural models with those based on 3D finite elements demonstrate the improved computational efficiency of the CUF models in terms of model size and analysis time.

Strength improvement of bolted joints in composite materials by use of patched metal inserts

Metal inserts are occasionally used to improve bearing load capacity of bolted joints in laminated composite materials. This paper investigates a new reinforcement concept where inserts are built by locally replacing composite plies with metal patches of various diameters, surrounding the holes. The inserts are built during composite manufacturing by alternately placing the metal patches through the thickness of the laminate at locations where holes are to be drilled after consolidation. An extensive experimental study including pin-loaded, open–hole tensile, and single-shear testing of bolted specimens is presented. Considerable improvement of the bearing strength – 50-60% – is attained for pin-loaded specimens with inserts, demonstrating the potential of the reinforcement concept. The open–hole tensile tests show that the by-pass strength can be maintained or even improved with up to 20% if the inserts are properly designed. Finally, the results from the single-shear tests of bolted joints show more than 25% improvement in strength for reinforced single- and double-bolt specimens. It is possible that the inserts would maintain clamping pressure over time, which could then almost double the imrovement (47%) for bolted joints.

Effect of ply angle on nonlinear static aeroelasticity of high-aspect-ratio composite wing

In order to reduce the weight and improve aerodynamic characteristics, the new aircraft generally adopts lightweight composite materials and high-aspect-ratio layout. Such the structural layout aircraft will produce large nonlinear aeroelastic deformation under the action of aerodynamic loads. Due to the anisotropy of the composite, the composite ply angle of wing skin has a great influence on the elastic deformation of the high-aspect-ratio wing. In order to study the influence of the ply angle on the nonlinear static aeroelastic wing deformation, based on CFD/CSD unidirectional fluid-solid coupling, the structural deformation and stress of high-aspect-ratio composite wing were numerically solved. The wing deformation along the lift direction was taken as the optimization target. The structure strength was taken as the constraint. The ply angle for the composite skin of the high-aspect-ratio composite wing was optimized by the Screening method. The optimization results show the nonlinear static aeroelastic deformation of the wing in the lift direction is reduced by 39.1 %. The maximum stress of the wing beam and rib is reduced by 39.0 %. The maximum Tsai-Wu failure factor of the wing skin is reduced by 47.1 %.

Lay-up optimisation of fibre–metal laminates panels for maximum impact absorption

This paper introduces a methodology utilising a ply-ply damage Finite Element models with Genetic algorithm optimisation procedure to investigate the effect of lay-up configuration on the impact absorption properties of fibre metal laminates (FMLs). The methodology was carried out in two steps. In the first step, a pseudo-2D model was used to explore the vast design space to identify potential optimised layup-configurations. In the second step, the optimised configurations were studied in full 3 D, with high fidelity simulations, verifying the results obtained from the optimisation process. The design variables used include thickness and material (including fibre orientation) of each ply. The results produced an optimised configuration consisting of a metallic ply on the impacted side followed by a cross-ply composite lay-up. The results also suggest that the first composite ply (second ply of the FML) should be about 3 times thicker than the other plies.

Modeling of damage behavior of carbon fiber reinforced plastic composites interference bolting with sleeve

Interference-fit is getting increasing attentions in the aerospace field owing to its excellent enhancement on the sealing and fatigue life of composites assembly, but the severe damage around the hole caused by installation is still a huge challenge. In this paper, a three-dimensional anisotropic non-linear progressive damage model based on continuum damage mechanics was developed to reveal the damage mechanism of carbon fiber reinforced plastic (CFRP) composites interference bolting with sleeve. The proposed model adopted strain-based failure criteria and non-linear damage evolution law to predict damage initiation and propagation of composite laminates. A cohesive model taking into account the traction-separation constitutive response was employed to capture the delamination behavior between the composite plies. The elastoplastic model was utilized to characterize the sleeve's deformation behavior. Numerical simulations of different installation modes are performed and compared with experiments. The inserting load, damage modes and damage positions predicted by the proposed model agree well with the experimental results. Moreover, it is also verified that the interference installation of sleeved fasteners can remarkably reduce the intra-laminar damage and avoid the inter-laminar delamination of composite laminates compared to the conventional bolt.

On the prediction of the bi-axial failure envelope of a UD CFRP composite lamina using computational micromechanics: Effect of microscale parameters on macroscale stress–strain behavior

A computational micromechanics based Finite Element (FE) analysis methodology is presented to predict the bi-axial failure envelope of a unidirectional (UD) carbon-epoxy composite ply. In order to estimate the effect of various microscale parameters that are influencing the macroscopic stress–strain behavior, under individual load cases, detailed numerical studies are conducted using a 3D RVE (Representative Volume Element) model. The constituent epoxy matrix plastic deformation in the RVE is captured using the linear Drucker-Prager plasticity model.The effect of the fiber–matrix interface damage, followed by frictional sliding of the constituent materials on the computed interface tractions is captured using the cohesive zone damage model combined with the Coulomb friction law, which is implemented into Abaqus using VUMAT. From the detailed FE analysis of the RVE under individual load cases, it is observed that the predicted macroscopic stress–strain behavior is sensitive to the fiber–matrix interface properties as well as the in-situ epoxy stress–strain behavior. Hence, using a coupled experimental-computational micromechanics approach the interface and the in-situ epoxy material properties are calibrated and validated. Using the calibrated interface and in-situ epoxy material properties, the bi-axial (transverse tension/transverse compression – in-plane shear) failure envelope of a UD composite ply is estimated. Comparing the predicted damage profiles and the failure envelope with the experimental results leads to good agreement and validates the proposed numerical methodology.

Fibre-direction strain measurement in a composite ply under quasi-static tensile loading using Digital Volume Correlation and in situ Synchrotron Radiation Computed Tomography

Digital Volume Correlation (DVC), in concert with in situ Synchrotron Radiation Computed Tomography (SRCT), has been applied to Carbon-Fibre Reinforced Polymers (CFRPs) under quasi-static tensile loading. DVC represents a relatively novel tool for quantifying full-field volumetric displacements and implicit strain fields. The highly anisotropic and somewhat regular/self-similar microstructures found in well-aligned unidirectional (UD) materials at high volume fractions are shown to be intrinsically challenging for DVC, especially along the fibre direction. To permit the application of DVC to displacement and/or strain measurements parallel to the fibre orientation, the matrix was doped with a sparse population of sub-micrometre barium titanate particles to act as displacement trackers (i.e. fiducial markers). For the novel materials systems we have developed, measurement noise is considered along with the spatial filtering intrinsic to DVC data processing. Compared to volume images acquired through Micro-focus Computed Tomography (µCT), hold-at-load artefacts are mitigated through scan times on the order of seconds using SRCT, as opposed to hours. Instances of individually fractured fibres evolving into clusters of breaks are presented, together with the associated strain redistribution (imaged at a voxel resolution of 0.65 µm). It is shown that the distance over which strain is recovered in the broken fibres not only increases with the applied force, but also with the number of broken fibres, delineating aspects of the load shedding phenomenon. The study demonstrates that unprecedented, mechanistically-consistent three-dimensional (3D) strain measurements may be made in relation to fibre failure events, that can be used to validate micromechanical models for predicting UD tensile failure. We believe this work presents the first application of DVC to the SRCT imaging of failure in CFRPs, achieving significantly higher resolution than reported previously within the literature.

Experimental characterization of monotonic and cyclic behavior of steel-to-CLT nailed joints strengthened with composite plies

Connections play a key role in timber structures because they are the parts devoted to energy dissipation during an earthquake or, when specific hysteretic devices are introduced, their hyper-resistant design allows for a rigid fastening of those devices, thus assuring an elastic behavior of the structure under seismic loads. A promising technique to improve both the load-bearing capacity and the stiffness of joints with dowel-type fasteners, consists in increasing the embedment strength of the wood-based materials, usually by applying a superficial reinforcing layer to the timber-connection shear plane interface. In this framework, results of an experimental campaign carried out on steel-to-CLT panel nailed joints strengthened with a carbon fiber ply glued to the shear plane interface are presented and discussed in this paper. Two different load configurations (perpendicular and parallel to grain) and two different loading protocols (monotonic and cyclic) have been considered, analyzing the results in order to evaluate the effectiveness of the reinforcement technique in terms of load-bearing capacity and stiffness, with respect to the reference unreinforced configuration. Finally, the applicability of the current design method is verified together with the proposal of a simplified design procedure.

A combined experimental/numerical study on the scaling of impact strength and toughness in composite laminates for ballistic applications

In this paper, the impact behaviour of composite laminates is investigated, and their potential for ballistic protection assessed, as a function of the reinforcing materials and structures for three representative fibre-reinforced epoxy systems involving carbon, glass, or para-aramid fibre reinforcements, respectively. A multiscale coupled experimental/numerical study on the composite material properties is performed, starting from single fibre, to fibre bundles (yarns), to single composite ply, and finally at laminate level. Uniaxial tensile tests on single fibres and fibre bundles are performed, and the results are used as input for non-linear Finite Element Method (FEM) models for tensile and impact simulation on the composite laminates. Mechanical properties and energy dissipation of the single ply and multilayer laminates under quasi-static loading are preliminarily assessed starting from the mechanical properties of the constituents and subsequently verified numerically. FEM simulations of ballistic impact on multilayer armours are then performed, assessing the three different composites, showing good agreement with experimental tests in terms of impact energy absorption capabilities and deformation/failure behaviour. As result, a generalized multiscale version of the well-known Cuniff criterion is provided as a scaling law, which allows to assess the ballistic performance of laminated composites, starting from the tensile mechanical properties of the fibres and fibre bundles and their volume fraction. The presented multiscale coupled experimental-numerical characterization confirms the reliability of the predictions for full-scale laminate properties starting from the individual constituents at the single fibre scale.

Fibre direction strain measurement in a composite ply under pure bending using Digital Volume Correlation and Micro-focus Computed Tomography

This paper presents an experimental demonstration and validation of high-resolution three-dimensional experimental strain measurement using Digital Volume Correlation (DVC) on Carbon Fibre-Reinforced Polymers, via through-thickness strain analysis under a state of pure bending. To permit the application of DVC to displacements and/or strain measurements parallel to the fibre direction in well-aligned unidirectional materials at high volume fractions, a methodology was developed for the insertion of sparse populations of 400 nm BaTiO3 particles within the matrix to act as displacement trackers (i.e. fiducial markers). For this novel material system, measurement sensitivity and noise are considered, along with the spatial filtering intrinsic to established DVC data processing. In conjunction with Micro-focus Computed Tomography, the technique was applied to a simple standard specimen subjected to a four-point flexural test, which resulted in a linear strain distribution through the beam thickness. The high-resolution, fibre-level strain distributions (imaged at a voxel resolution of ∼0.64 µm) were compared against the classical beam theory (Euler–Bernoulli) in incrementally decreasing averaging schemes and different sub-set sizes. Different sampling and averaging strategies are reported, showing that DVC outputs can be obtained that are in very good agreement with the analytical solution. A practical lower limit for the spatial resolution of strain is discerned for the present materials and methods. This study demonstrates the effectiveness of DVC in measuring local strains parallel to the fibre direction, with corresponding potential for calibration and validation of micromechanical models predicting various fibre-dominated damage mechanisms.

Mechanical properties of glass/carbon inter-ply hybrid polymer composites at different in-situ temperatures

An inter-ply hybrid composite consists of at least two types of reinforcements along different plies in fiber-reinforced polymer composites. The tensile behaviour is deteriorated with increasing temperature, which possesses a severe concern that needs to be tackled before incorporating it into engineering applications. The tensile testing was done on the specimens at different temperatures (30 °C, 70 °C and 110 °C) by changing the hybrid ratio with different stacking sequence. The results reveal that by incorporating two layers of carbon fiber at the top end in glass fibers epoxy (GE) composite at 30 °C enhances the tensile modulus by 76.8% more than the control GE composite. Also, the strain to failure increases by 20.7% than carbon fiber reinforced epoxy (CE) composite by substituting one glass fiber by one carbon fiber at the top end. Although at 110 °C temperature, strain to failure amplifies by 9.3% than that of CE composite. The dominant modes of failure were observed under scanning electron microscopy of the failed specimens.

Characterisation of Hybrid Carbon Glass Fiber Reinforced Polymer (C/GFRP) of Balanced Cross Ply and Quasi Isotropic under Tensile and Flexural Loading

This study is performed to characterize composite material of hybrid carbon glass reinforced polymer (C/GFRP) of two (2) types; namely balanced cross ply and quasi isotropic subjected to tensile and flexural loading. The mechanical testing performed on the hybrid composite as per ASTM standard and aimed to extract the mechanical properties related to tensile and flexural. The failure modes associated with the rupture of the composites/hybrid composites samples under tensile and three-point bending were assessed via JEOL 6010 Plus Scanning Electron Microscopy (SEM) instrument. The combination of GFRP lay-up at 0° at tensile side, GFRP lay-up 90° at compression side and ±45° lay-up of CFRP at shear/compression region enable the hybrid composite cross ply 8 to record the highest flexural strength. The substitution of 0° GFRP with 0° layup CFRP together with layup of 90° GFRP in hybrid composite cross ply 4, matrix/resin dominated layup acts as stress reliever in compression region during flexural loading taking place. This has induced to the increase of flexural strength, which observed to improve its original constituent of cross ply 2 (balanced cross ply GFRP). The factor of layup GFRP at transverse direction has enabled hybrid composite to possess a higher tensile modulus and to record considerably high tensile strength. The role of GFRP layup in enhancing the strain to failure in tensile and its role as a reliever in improving flexural strength during flexural loading has been tested and justified in this experiment.

Effective bending modulus of thin ply fibre composites with uniform fibre spacing

Cauchy homogenisation methods are based on the assumption that materials show size independent behaviour. For example, the effective bending modulus of a beam is constant for all cross-sectional areas and is equal to the tensile modulus. However, as demonstrated by numerous authors, the stiffness of heterogenous materials under non uniform loading (e.g. bending) can be influenced by size effects, when the microstructural size scale is comparable to the macroscopic one. In other words, an effective elastic modulus can underestimate or overestimate the bending stiffness depending on the specimen size. In this work, the effect of the thickness dimension on the effective bending modulus of composite plies is investigated. The composite is represented by cylindrical fibres uniformly distributed in a uniform matrix. The cases of fibres stiffer and more compliant than the matrix are considered. The results show that the effective bending modulus depends on fibre volume fraction ratio, mismatch of fibre/matrix material properties and the number of fibres through the thickness. The implications for the analysis of bending of thin ply composites are discussed.

Transverse impact behavior and residual axial compression characteristics of braided composite tubes: Experimental and numerical study

In this study, transverse low-velocity impact response and residual axial compression behavior of braided composite tube with different ply number was investigated by experimental and numerical methods. The transverse low-velocity impact tests with 5.6 J energy were conducted on the composite tubes. The quasi-static axial compression performance of intact and pre-impacted tubes was compared to evaluate the effect of impact damage. A two-step finite element (FE) model was also established to reveal damage mechanisms of braided tube under impact loading and following axial compression. It was found that the wall thickness had a significant influence on the impact response. Obvious structural deformation occurred in 2-ply tube when subjected to impact loading, resulting in a large projected delamination area. In following axial compression process, the delamination failure significantly reduced the local compression stiffness and caused a structural instability, which led to a buckling failure mode. In contrast, increasing bending stiffness of 3 or 4-ply tube suppressed its structural deformation during impact, leading to a confined projected delamination area and therefore the buckling of tube wall was prevented effectively when subjected to axial compression. For all impacted tubes, the transition of failure mode from progressive folding mode in intact tube led to a lower energy absorption capacity per composite ply, especially for 2-ply tube.