Composite Prepreg Research Articles

Modelling the development of defects during composite reinforcements and prepreg forming.

Defects in composite materials are created during manufacture to a large extent. To avoid them as much as possible, it is important that process simulations model the onset and the development of these defects. It is then possible to determine the manufacturing conditions that lead to the absence or to the controlled presence of such defects. Three types of defects that may appear during textile composite reinforcement or prepreg forming are analysed and modelled in this paper. Wrinkling is one of the most common flaws that occur during textile composite reinforcement forming processes. The influence of the different rigidities of the textile reinforcement is studied. The concept of 'locking angle' is questioned. A second type of unusual behaviour of fibrous composite reinforcements that can be seen as a flaw during their forming process is the onset of peculiar 'transition zones' that are directly related to the bending stiffness of the fibres. The 'transition zones' are due to the bending stiffness of fibres. The standard continuum mechanics of Cauchy is not sufficient to model these defects. A second gradient approach is presented that allows one to account for such unusual behaviours and to master their onset and development during forming process simulations. Finally, the large slippages that may occur during a preform forming are discussed and simulated with meso finite-element models used for macroscopic forming. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Thermo-viscoelastic analysis of GLARE

Abstract In this paper, the study of residual stresses in Fibre Metal Laminates (FMLs) is improved using a thermo-viscoelastic model developed for composites which have an orthotropic viscoelastic response. Prepreg layers of GLARE consist of epoxy adhesive that has temperature dependent and viscoelastic response in thermal environments such as the curing cycle. Thermal and viscoelastic response of the adhesive is already characterised using thermo-mechanical methods. In order to get the response for unidirectional fibre-epoxy composite (prepreg) layers, the Correspondence principle is utilised with self-consistent micromechanics equations in the Laplace domain. By reverting, the thermo-viscoelastic response of the composite layer is obtained in the time domain. A user defined material model is added to ANSYS able to analyse the orthotropic thermo-viscoelastic behaviour. Using the developed model, residual stresses of GLARE are calculated. Comparison to the curvature measurements of manufactured non-symmetric laminates, shows the accuracy of the modelling and the improvements with respect to the thermo-elastic analyses.

Experimental modal analysis of masonry arches strengthened with graphene nanoplatelets reinforced prepreg composites

Strengthening of masonry structures with different engineering materials has become a challenge for civil engineers with the advancements in engineering technology. This study mainly focused on dynamic parameters of masonry arches, which were strengthened with nanoparticle reinforced prepreg composites. For this purpose, the prepreg composites were produced with carbon fiber and epoxy system which was reinforced with graphene nanoplatelets particles that was 1wt% of the epoxy system. Afterwards, four different strengthening alternatives were performed at the extrados and intrados of the masonry arches. Finally, dynamic characteristics such as mode shapes and modal frequencies of unstrengthened and strengthened arches were determined through experimental modal analysis (EMA) and numerical analysis.

Effects of hygrothermal and ambient humidity conditioning on shear strength of metal–GFRP single lap joints co-cured in and out of water

Installing and repairing of oil and gas infrastructure is required to be conducted out of water (OW) and in water (IW). In service, this infrastructure is subjected to various environmental ageing which may affect its mechanical performance. In this study the shear strength of metal–glass epoxy prepreg composites was investigated using single lap joint (SLJ) testing. For this, three different surface preparation methods (grit blasting, wire brushing and needle gunning) and two different manufacturing methods (OW and IW) were employed. Results showed that the specimens prepared by grit blasting had superior shear strength followed by these by needle gunning and wire brushed for both OW and IW methods. Manufactured specimens were also subjected to hygrothermal conditioning (WC) and ambient humidity conditioning (HC) to assess the durability of the produced SLJ specimens. The WC specimens absorbed more moisture than the HC specimens and the shear strength of the WC specimens were found to have deteriorated more than the HC specimens. For the conditioned SLJ specimens, the lower loss of shear strength was found with specimens manufactured IW when compared to specimens manufactured OW for grit blasted and needle gunned specimens and an opposite trend was seen for wire brushed specimens. However, the loss of shear strength was found to be varied with the moisture uptake for individual surface preparation technique and the variation was considered to be due to a combination of many factors other than the type of adhesive used such as metal surface profile, failure mode, rust formation and presence of black coloured metallic remnants.

Improved surface quality and hygrothermal performance of epoxy based prepreg by liquefied dicyandiamide with enhanced solubility and dispersibility

Liquefied dicyandiamide (L-DICY) was fabricated by pretreatment of DICY with polyetheramine (PEA)/diglycidyl ether of bisphenol A (DGEBA) adduct, and physicochemical properties of L-DICY, mechanical, thermal and optical properties of L-DICY cured epoxy resins and hygrothermal performance of L-DICY cured epoxy based prepregs were evaluated to compare with those of DICY and PEA/DICY blends. Amine hydrogen equivalent weight of L-DICY was increased with inferior reactivity to PEA/DICY. The solubility of PEA/DICY and L-DICY in epoxy resin was increased due to good dissolving ability of PEA and PEA/DGEBA adduct. The dispersibility of PEA/DICY and L-DICY in epoxy resins was improved, while dispersion stability of L-DICY was superior to that of PEA/DICY. The mechanical and thermal properties of three kinds of resin matrix changed little, while transparency of L-DICY cured resin films was higher than those of DICY and PEA/DICY. The surface qualities and hygrothermal properties of epoxy based prepreg composites were improved notably, which was attributed to DICY crystal reduction in cured resin films resulted from enhanced solubility and dispersibility of L-DICY.

The bias-extension test for the analysis of in-plane shear properties of textile composite reinforcements and prepregs: a review

The bias-extension test is a rather simple experiment aiming to determine in-plane shear properties of textile composite reinforcements. However the mechanics during the test involves fibrous material at large shear strains and large rotations of the fibres. Several aspects are still being studied and are not yet modeled in a consensual manner. The standard analysis of the test is based on two assumptions: inextensibility of the fibers and rotations at the yarn crossovers without slippage. They lead to the development of zones with constant fibre orientations proper to the bias-extension test. Beyond the analysis of the test within these basic assumptions, the paper presents studies that have been carried out on the lack of verification of these hypothesis (slippage, tension in the yarns, effects of fibre bending). The effects of temperature, mesoscopic modeling and tension locking are also considered in the case of the bias-extension test.

Relief of Cure Stress in Prepreg Composites with Engineered Voids: A Solution to Warping in Composite Phenolic Resin/Fiberglass Laminates

The warp of a phenolic resin/fiberglass honeycomb composite panel is investigated using void and volatile analysis, and a solution to geometric warp is proposed and tested. The surface void content in semicured phenolic polymer composite facesheets was found to be an indicator for the average warp value of a fully cured composite laminate panel. Several potential sources in the resin formulation for producing voids in the phenolic resin/fiberglass composite sheets were investigated. Ultimately, high surface tension and polymer incompatible cosolvents are correlated to the population of voids. It is proposed that these voids act as sites for the relief of internal stresses that develop in the phenolic matrix during the final cure of the part. Void-inducing additives are added at the formulation stage which phase separate from the polymer matrix in partially cured fiberglass/phenolic composite sheets, and upon evaporation, lead to voids that prevent warping in honeycomb composite panels.

An anisotropic hyperelastic constitutive model for thermoplastic woven composite prepregs

A hyperelastic constitutive model is developed to characterize the anisotropic and large deformation behavior of woven thermoplastic composite prepregs during thermo-forming process. In the model, a strain energy function representing the material behavior of melting prepreg is additively decomposed into three parts nominally representing the energy contributions from the matrix, fiber and their interaction, respectively. The proposed constitutive model is demonstrated on a balanced 2 × 2 twill glass/PP prepreg. The specific forms of the strain energy functions are determined by fitting uni-axial tensile and bias extension tests of the prepregs at melting temperature. The developed model is then applied to simulation of a benchmark double dome thermo-forming, demonstrating that it is highly suitable for predicting the large deformation behavior of woven prepregs during thermo-forming process. The effects of matrix on forming simulation results are also investigated.

Scaling effects in the mechanical response of sandwich structures based on corrugated composite cores

Abstract This paper investigates the compression response of all-composite sandwich structures based on glass fibre/epoxy and carbon fibre/epoxy cores. The structures were manufactured by wrapping layers of composite prepreg around a series of adjacent steel cylinders. Prepreg surface layers were then attached to the upper and lower surfaces of these wrapped cylinders and the entire structure cured in a hot press. Co-curing the skins and the corrugated core in this fashion ensured a strong bond in the critical skin-core interfacial region. The mechanical response of the sandwich structures was modelled using the finite element method. Initial attention focuses on investigating the effect of varying key geometrical parameters, such as the corrugation thickness and the number of unit cells, on the mechanical properties of the sandwich structures. The failure mechanisms during compression loading are discussed and compared with the numerical predictions from the finite element models. The second part of this study investigates scaling effects in the compression response of both the carbon and glass fibre-based sandwich structures. In this part of the study, the geometry of the sandwich structures, as well as the relevant testing conditions, were varied in order to ensure a consistent scaling approach. Here, variations in compression strength as well as changes in failure mode were investigated with increasing scale size.

Analysis of Mechanical and Impact Properties of Prepreg Composites under Elevated Temperature

This paper presents an experimental investigation of mechanical and impact properties of carbon and Kevlar-glass composites prepared from pre-impregnated materials. Namely, flexural performance in three-point bending at different temperatures is evaluated. Moreover, Charpy impact test and low-velocity impact test are also conducted for classification of impact properties and character of rupture. These all properties are important for material design of sport bike rims and many sport and other applications.

Length-scale dependence of variability in epoxy modulus extracted from composite prepreg

Property variability in conjunction with morphological variability are important sources of uncertainty in composite modeling. While image processing of experimental microstructures has enabled accurate quantification of morphological variability, the characterization of material variability is not as well established. In this study, the local material properties of epoxy extracted from a prepreg sheet was determined using nanoindentation with a spherical indenter tip with a radius of 50 μm. Indentations were carried out at four different indentation depths to evaluate the change in the variability of epoxy modulus with the sampling volume. For each length scale studied, 40 indentations were carried out to determine the variability in epoxy modulus. A significant decrease was observed in the coefficient of variation as the indentation depth increased. The corresponding modulus distributions were quantified. The results suggest that, similar to morphological variability, material variability is length-scale dependent and the appropriate variability associated with the selected length scale must be considered for stochastic modeling of composite structures.

An approach to identify complex CNT reinforcement effect on the interlaminar shear strength of prepreg composites by Taguchi method

Several factors affect the interlaminar shear strength of carbon nanotube reinforced carbon fiber composites in multi-scale. A combined route of CNT toughening through manufacturing a custom made prepreg system with CNT dispersion in epoxy and integrating vertically aligned CNTs as nanofibers to bridge in between prepreg plies is applied to achieve enhanced interlaminar shear strength (ILSS). Depending on the CNT loading, effectiveness of dispersion and CNT bridging between prepreg plies, the mechanical enhancement is achieved up to 15%. A new approach of identifying the critical parameters of nano-engineered and nano-reinforced composites through Taguchi method is a novel route for investigating the combined effects of several factors in a complex mechanical enhancement problem with reduced number of experiments as studied in here. Taguchi L8 orthogonal array is used to obtain maximum ILSS by short beam shear (SBS) test. The parametric study has shown that the most significant parameters affecting the ILSS results are type of epoxy, fabrication procedure and CNT loading.

Development and thermal properties of carbon nanotube-polymer composites

The favorable conductive properties of carbon nanotubes (CNTs) offer opportunities for constructing CNT-based nanocomposites with improved thermal conduction for a range of potential applications. Such lightweight composite materials are expected to have thermal properties that depend on their CNT volume fraction and operating temperature. The lack of available CNT processing methods that are compatible with multi-laminated composite structures is one of the largest challenges facing the construction of CNT-based nanocomposites.The aim of the work is to develop enhanced thermal properties in carbon nanotube-polymer composites that can replace traditional aerospace metallic materials to reduce the weight in space structures. Dispersing the carbon nanotube onto prepreg composite structure, increases the thermal storage, increases the thermal transport and supports scientific instrumentation. The investigated composites were processed and characterized using Raman spectroscopy, thermogravimetric analysis, thermal diffusivity and differential scanning calorimetry.Through varying the concentration of single-walled carbon nanotubes (SWCNTs) up to 30 wt% to the IM7 prepreg composite, its heat capacity increased as much as 30% greater than the tested temperature range and its through-thickness thermal diffusivity increased by 30% compared to the virgin composite material. The addition of randomly oriented SWCNTs yielded an increase in the in-plane thermal conductivity ranging from 120 to 150 percent greater than the temperature range of 120–470 K and 30% in the through thickness. This may be due to interfacial resistance between the SWCNTs, the 8552 epoxy and the IM7 composite. The developed methods provide the opportunity for enhancing the thermal properties of a composite through the use of CNTs as additives. These improvements would be particularly beneficial applications such as solar arrays, fairings and thermal radiators.

Translaminar fracture toughness of NCF composites with multiaxial blankets

In this paper, the translaminar initiation fracture toughness of a carbon–epoxy non-crimp fabric (NCF) composite laminate was measured using a compact tension (CT) test. The translaminar fracture toughness of the individual unidirectional (UD) fibre tows was related to that of the NCF laminate and the concept of a homogenised blanket-level translaminar fracture toughness was introduced. Using an approach developed for UD-ply prepreg composites, it is demonstrated that the translaminar fracture toughness of off-axis fibre tows/NCF blankets can be analytically related to that of axially-loaded fibre tows/NCF blankets with a difference between experimentally-measured and predicted values lower than 5%.

Research on Fuzzy Control Based Flexible Composite Winding System

In the process of composite resin prepreg tape winding, the presence of pores or voids among the layers of composite can result in reduced strength of winding. To alleviate this problem, it is required that the composite tape winding machines be designed such that the layers of composite are evenly wound on the previous one. The paper presents a novel design of flexible winding system for composite tape winding. Based on the analysis of errors in winding process, the novel winding system eliminates winding point error and winding angle error based on the speed controlled flexible roller. This paper also presents the kinetic analysis of the novel system and its controller design. Experiments are conducted on the novel winding system. The experimental results illustrate that the novel flexible winding system has a good performance in winding accuracy.

Study on Compression Molding of an Intercrossed Prepreg Glass Fiber-Reinforced Plastic

In this paper, a specimen of 80 × 80 × 3.8 mm dimensions is cut out from the prepreg intercrossed glass fiber-reinforced plastic (GFRP) and subjected to hot compression molding. The experimental flow front of the specimen is compared to the one predicted by the finite element method (FEM) based on the Galerkin method. Since the prepreg intercrossed GFRP composite is made of stacking plain weaved prepregs, the fibers’ entanglement prevents the specimen slip across the mold surface. It is shown that the flow results are strongly affected by the mold-specimen friction: the slip parameter α =100 used in the FEM flow analysis provides a close correlation of the calculated and experimental results.

Improvement of the impact and crack resistance of epoxy thermosets and thermoset-based composites with the use of thermoplastics as modifiers

Main methods for the modification of epoxy composites that aim to increase their impact and crack resistances are considered with the main attention focused on modification with the use of thermally stable thermoplastics as additives. The influence of various factors (the modifier–matrix chemical bond and the morphology and molecular weight of the modifier) on the physicomechanical characteristics of the composites is discussed. New approaches to the toughening of polymeric composite materials based on epoxy thermosets that involve the use of modified binders or the introduction of a modifier between layers of the composite prepreg are advanced.

Combining Additive Manufacturing with Advanced Composites for Highly Integrated Robotic Structures

The combination of additive manufacturing with advanced composites offers potentials in the development of highly integrated lightweight structures. This paper investigates a novel manufacturing process route where binder jetting is used to produce a water soluble sand core for hand layup of autoclave prepreg composites. The novel approach is applied to a hollow high performance robotic part demonstrating the following two design potentials: First, binder jetting of water soluble sand material is a suitable technology for the production of very complex composite parts at low tooling costs. Second, tailored load introduction elements made of selective laser melting enable lightweight designs. Weight savings of 54.3% compared to a state-of-the-art aluminum robotic part indicate that the approach is competitive for complex low volume parts.

Cure monitoring of CFRP composites by dynamic mechanical analyser

The cure characteristics of carbon/MTM44-1 epoxy composite prepreg and neat MTM44-1 epoxy resin are monitored using a dynamic mechanical analyser (DMA). This study also assesses whether the simple containment device recommended by the DMA manufacturer for powder and gel is suitable for prepregs. The device is a disposable 0.1-mm thick stainless steel pocket. The cure behaviour of the packaged materials is compared with that of the unpackaged prepreg. All the samples are cured following the manufacturer's 180°C-isothermal schedule with heating rates of 2 °C/min and 5 °C/min. The tests highlight that: (i) the cure of composite prepregs and resin can be monitored using a DMA; (ii) the stainless steel pocket does not influence the cure of the prepreg and resin; and (iii) the softening and melting of the uncured resin, the resin low viscosity state and the subsequent gelation-vitrification-solidification of the resin are detected with this method.

Thermomechanical analysis, modelling and simulation of the forming of pre-impregnated thermoplastics composites

In this paper, the viscoelastic behaviours of pre-impregnated (prepreg) thermoplastic composites are analysed using the bias-extension test. A new constitutive model is proposed in order to simulate the forming of thermoplastic composite prepregs at the macroscopic scale. The model is based on a continuous approach. Hyperelastic behaviours are associated with dry reinforcements. Four principal deformation modes, all considered independent, define the hyperelastic potential: the elongation in warp direction, the elongation in weft direction, the in-plane shear strain and the bending contribution. Experience shows that viscoelastic behaviour is mainly associated with the in-plane shear deformation. A non-linear visco-hyperelastic model based on the generalisation of Maxwell rheological model is considered for this type of deformation mode. The finite element simulation of a stamping case using this model is introduced. The influence of temperature on the forming stage and the performance of the model are evaluated.