The hypotheses that reactive uncured, thermoset bicomponent fibres can be prepared and mixed with reinforcing fibres and ultimately used in preparation of a composite was tested and is described. It is thought that such fibres have the two potential advantages: (1) to enable manufacturing with particle doped resins e.g. nanocomposites which add functionality to composites and (2) increased efficiency of structural composite manufacturing by increasing the level of automation. The structure of the thermoset fibres comprises of a sheath of thermoplastic and a core of uncured thermoset resin. Once manufactured, the fibres were wound with a reinforced fibre onto a plate, consolidated and cured. The resulting composite was examined and compared to other composites made with the same manufacturing method from commercially available materials. The results show that a laminate can be produced using these reactive bicomponent fibres. The resin system successfully impregnates the reinforcing carbon fibres and that the thermoplastic separates from the epoxy resin system during consolidation. In comparison to reference material, the bicomponent laminate shows promising characteristics. However, the processes developed are currently on a lab-scale and considerable improvement of various bicomponent fibre properties, such as the strength, are required before the technology can be used on a larger scale.

The paper deals with an indirect industry-friendly method for identification of the interfacial shear strength (IFSS) in a fully bio-based composite. The IFSS of flax fiber/starch acetate is evaluated by a modified Bowyer and Bader method based on an analysis of the stress-strain curve of a short-fiber-reinforced composite in tension. A shear lag model is developed for the tensile stress-strain response of short-fiber-reinforced composites allowing for an elastic-perfectly plastic stress transfer. Composites with different fiber volume fractions and a variable content of plasticizer have been analyzed. The apparent IFSS of flax/starch acetate is within the range of 5.5-20.5 MPa, depending on composition of the material. The IFSS is found to be greater for composites with a higher fiber loading and to decrease with increasing content of plasticizer. The IFSS is equal or greater than the yield strength of the neat polymer, suggesting good adhesion, as expected for the chemically compatible constituents.

During the manufacturing process of composite structural parts, layer of fabrics or unidirectional prepreg may have to be cut in order to fulfil production requirements. From a general mechanical point of view, cutting fibres in a composite part has a large negative impact on the mechanical properties. However, such interventions are necessary in particular cases, for example due to draping of complex geometries. A rather extensive test program was launched to investigate the effects of defects that typically could arise during manufacturing. The overall purpose of the test program was to determine knock-down factors on strength for typical manufacturing defects that occasionally arise and sometimes are hard to avoid in production: cuts/gaps and fibre angle deviations. Four types of specimens were tested, reference, intersection of cuts in adjacent layers combined with a bolt hole, cut in a zero degree ply combined with a bolt hole and specimens with misaligned fibres. The specimens with misaligned fibres were tested with three different fibre angles. In addition to the experimental procedure, FE-analyses utilising cohesive elements were conducted, and after mechanical tests, Non Destructive Investigation (NDI) and fractographic investigations were performed. An excellent correlation between analyses and experiments were obtained.

Two approaches to modelling the effects of embedded defects and impact damage in composite aerostructures are presented. These differ in the manner in which the damage is represented; one as an equivalent delamination and the other as a soft inclusion with non-linear homogenized material properties. These techniques are applied to study the effects of defects and impact damage on the performance of composite panels.

This paper introduces the concept of structural power composite materials and their possible devices and the rationale for developing them. The paper presents a comprehensive review of the state-of-the-art, highlighting achievements related to structural battery and supercapacitor devices. The research areas addressed in detail for the two types of material devices include: carbon fibre electrodes, structural separators, multifunctional matrix materials, device architectures and material functionalization. Material characterisation, fabrication and validation are also discussed. The paper culminates in a detailed description of scientific challenges, both generic as well as device specific, that call for further research. Particular reference is given to work performed in national and European research projects under the leadership of the authors, who are able to provide a unique insight into this newly emerging and exciting field.

This paper introduces the concept of structural battery composite materials and their possible devices and the rationale for developing them. The paper presents an overview of the research performed in Sweden on a novel structural battery composite material. The research areas addressed include: carbon fibre electrodes, structural separators, multifunctional matrix materials, device architectures and material functionalization. Material characterization, fabrication and validation are also discussed. The paper focuses on a patented battery composite material technology. Here, carbon fibres are employed as combined negative battery electrodes and reinforcement, coated with a solid polymer electrolyte working simultaneously as electrolyte and separator with ability to transfer mechanical loads. The coated fibres are distributed in a conductive positive cathode material on an aluminium electron collector film. Efficient Li-ion transport between the electrodes is achieved by the solid polymer electrolyte coating being only a few hundred nanometres thick. Finally some outstanding scientific and engineering challenges are discussed. Such challenges, calling for further research are related to manufacture, development of new solid polymer electrolytes for improved multifunctionality and the lack of material models. 

In this study different concepts to attain a material that can reduce its stiffness upon external stimulation were evaluated regarding their suitability for traffic safety applications. All concepts rely on resistive heating of a carbon fibre reinforcement upon application of electric current through the fibres. The stiffness reduction is achieved by a phase transformation due to heating of the material. The phase transformation takes place either in a thermoplastic interphase, in a thermoplastic matrix or in a thermoset matrix, depending on the concept. The different concepts were studied regarding their thermomechanical and processing properties and their ability to reduce their stiffness upon application of an electric current was tested. Moreover, the materials were evaluated regarding their potential for fast activation, which is crucial for applications in traffic safety. Stiffness-reduction was achieved upon application of an electric current, where the activation temperature was between 60 and 120°C and the extent of stiffness-reduction varied between 50 and 90%, depending on the material. The response time was found to depend to a large extent on the amount of material, which leads to the conclusion that smart design solutions are required for larger parts. It is concluded that the concepts vary in their thermal, mechanical and processing properties as well as in their extent of stiffness-reduction upon activation. The results presented in this work prove the feasibility of the studied materials for traffic safety applications and the concepts allow further optimization of the materials for specific applications

Electrocoating at constant current is less sensitive to moisture and oxygen than electrocoating at controlled potential, which makes it more interesting for industrial implementation. The galvanostatic electrocoating of carbon fibres with Poly(methylmethacrylate) (PMMA) was therefore studied and compared to the well researched potentiostatic electrocoating procedure. The influence of different experimental parameters on the coating efficiency was investigated in order to identify the mechanisms that are involved in the cathodic electrocoating at constant current. It could be confirmed that the involved mechanisms differ from potentiostatic electrocoating and it was found that galvanostatic electrocoating is more efficient at ambient conditions compared to potentiostatic electrocoating. Polymer layers that cover the entire carbon fibre surface could be achieved in a continuous process by galvanostatic electrocoating under ambient conditions. 

The European Green Vehicle project ENLIGHT aims to advance highly innovative lightweight material technologies for application in structural vehicle parts of future volume produced Electric Vehicles (EVs) along four axes: performance, manufacturability, cost effectiveness and lifecycle footprint. The main target is to develop viable and sustainable solutions for medium production volume up to 50.000 EVs destined to reach the market in the next 8-12 years. The specific objectives of the ENLIGHT project are on holistic and integrated conceptual design and manufacturing concerning how the technologies and materials addressed can be combined into a representative medium-volume EV. The solutions will be demonstrated in five modules: a front module and central floor module, a front door, a sub-frame and suspension system as well as a cross-car beam. In this paper, a summary of the major results obtained up to the 3rd project year will be presented. © 2016 The Authors.

Intralaminar cracking in layers of a quasi-isotropic carbon fiber NCF laminate in tension-tension cyclic loading is studied experimentally. Methodology based on modified Weibull analysis is suggested to combine quasi-static and fatigue testing to identify parameters in the crack density growth model. The validity of the assumptions for the given material is experimentally confirmed. The suggested methodology can lead to significant time and material savings in composites fatigue behaviour characterization. 

With an increased use of composite materials within the aerospace industry follows a need for rational and cost-effective methods for composite manufacturing. Manual operations are still common for low to medium manufacturing volumes and complex products. Manual operations can for example be found in material handling, when picking prepreg plies from a cutter table and stacking them to form a plane laminate in preparation for a subsequent forming operation. Stacking operations of this kind often involves a great number of different ply geometries and removal of backing paper and other protecting materials like plastic. In this paper two different demonstrator cells for automated picking of prepreg plies and stacking of plane laminates are presented. One demonstrator is utilizing a standard industrial robot and an advanced end-effector to handle the ply variants. The other demonstrator is using a dual arm robot which allow for simpler end-effector design. In combination with a previously developed system for automated removal of backing papers both systems have shown to be capable of automatically picking prepreg plies from a plane surface and stack them to generate a flat multistack laminate. The dual arm approach has shown advantageous since it result in simpler end-effector design and a successive lay down sequence that result in good adhesion between the plies in the laminate. null.

With an increased use of composite materials follows a need for rational, cost-efficient manufacturing processes. This paper explores how off-the-shelf solutions, developed for other purposes than composite manufacturing, can be used to build systems for automated composite manufacturing. Three demonstrators, each of them dealing with a specific type of material and all of them representing different manufacturing technologies for automated composite manufacturing, are presented and analyzed to find aspects that affect the ability to use off-the-shelf solutions. The three demonstrators target low to medium manufacturing volumes of complex products and they have been developed in collaboration with industrial partners within the aerospace industry. The conclusions drawn from the development of the demonstrators are that it is technically feasible to use off-the-shelf solutions in the three cases while adhering to the high quality standards of the industry. Furthermore three groups of aspects, quality aspects, product aspects and system aspects, which affect the ability to use off-the-shelf solutions for automated composite manufacturing, are identified.

Within the aerospace industry the manufacturing of composite components with complex shapes, such as spars, ribs and beams are often manufactured using manual layup and forming of prepreg material. Automated processes for prepreg layup and efficient forming techniques like vacuum forming are sometimes difficult to employ to these type of products due to technical limitations. This paper describes the development of tools and the forming sequence needed to automate sequential forming of a complex shape using an industrial robot. Plane prepreg stacks are formed to the final shape using a dual-arm industrial robot equipped with rolling tools. Tests show that the developed tools and the employed sequence can be used to form stacks to the desired shape with acceptable quality.

The present contribution details the development and implementation of dedicated material models for the finite element computation of the thermal and mechanical response of polymer compositestructures subjected to fire. The material models are developed so that mechanical and thermal properties at the ply level can be calculated from the constituent's properties, therefore allowing for a greater flexibility in architecture and reduced testing programs. The degradation of the resin during fire is predicted during the thermal analysis. Its effect, together with the effect of temperature, on the mechanical response is accounted for in the material model. The models are validated against a mini furnace experiment.

Manufacturing and delamination testing, of textile reinforced Duroplast beams, have been performed to determine critical fracture energies in mode I, mode II and mixed mode loading. This includes values for growth initiation as well as propagation. For mode II and mixed mode loading, a large scatter in values were observed and no values corresponding to stable crack growth was obtained. Furthermore, an XFEM enriched shell element, which internally can represent multiple interlaminar cracks, have been used to simulate the DCB test. In the numerical simulation, data from the experimental tests have been used as input for a bilinear cohesive zone model. The load-displacement curve from the FE analysis shows good agreement with the corresponding measured curves, although with a somewhat higher stiffness. 

A model for the longitudinal response of laminated fibre-reinforced composites during compressive damage growth is implemented in a Finite Element (FE) package and validated for mesh objectivity. The current work details the FE implementation of the fibre kinking model and in particular challenges associated with mesh objectivity. The numerical way to solve the stress equilibrium and stress compatibility equations simultaneously in an FE framework is also presented. The results show that the current model can be used to predict the kinking response and thus account for the correct energy absorption.

The durability of entirely bio-based composites with respect to the exposure to elevated humidity was evaluated. Different combinations of bio-based resins (Tribest, EpoBioX, Envirez) and cellulosic fibers (flax and regenerated cellulose fiber rovings and fabrics) were used to manufacture unidirectional and cross-ply composite laminates. Water absorption experiments were performed at various humidity levels (41%, 70%, and 98%) to measure apparent diffusion coefficient and moisture content at saturation. Effect of chemical treatment (alkali and silane) of fibers as protection against moisture was also studied. However, fiber treatment did not show any significant improvement and in some cases the performance of the composites with treated fibers was lower than those with untreated reinforcement. The comparison of results for neat resins and composites showed that moisture uptake in the studied composites is primarily due to cellulosic reinforcement. Tensile properties of composites as received (RH = 24%) and conditioned (RH = 41%, 70%, and 98%) were measured in order to estimate the influence of humidity on behavior of these materials. Results were compared with data for glass fiber reinforced composite, as a reference material. Previous results from study of unreinforced polymers showed that resins were resistant to moisture uptake. Knowing that moisture sorption is primarily dominated by natural fibers, the results showed that some of the composites with bio-based resins performed very well and have comparable properties with composites of synthetic epoxy, even at elevated humidity. 

In this work the thermal conductivity, λ, of composite materials is investigated. The experimental results are from the experimental transient hot wire method and the experimental transient plate source method. The measurements are carried out on pure RTM6 epoxy resin and glass fibre and carbon fibre composites. The material is investigated both in its fully cured, pristine shape as well as during curing, consolidation and degradation. The λ-values for pure epoxy, glass fibre composite and material during degradation has been determined.

In this paper, the major outcomes from a recently completed research program with ambition to develop polyimide carbon fibre composites with temperature ability above 360°C are reported. Data from characterisation of the processing properties such as viscosity and cure behaviour are presented alongside with data on the mechanical properties at room temperature of quasi-isotropic composites based on the developed resin and 8-harness satin weave carbon fibre fabrics. The paper also contains a demonstration of the use the material system in a demonstrator component.

The current communication present results from work on polymeric composites with extreme temperature performance. This study focuses on carbon fibre composites based on a new phenyl ethynyl terminated polyimide formulation NEXIMID® MHT-R (Nexam Chemicals AB, Sweden) based on hexafluoroisopropylidene bisphthalic dianhydride (6-FDA), 4-(Phenylethynyl)Phthalic Anhydride (4-PEPA) and ethynyl bis-phthalic anhydride (EBPA). In particular influence of post-cure conditions such as time, temperature and atmosphere on Tg of the composites is investigated. In addition to this monitoring and analyses of the consequences of post-cure on mass loss and occurrence of micro-cracks is carried out. Three different post-curing temperatures are considered in this study: 400°C, 420°C and 440°C. Two different atmospheres, air and inert by nitrogen, were also investigated. In summary the results reveal that remarkably high Tg, up to around 460°C, is achieved with only very limited mass loss. It was also observed that some, but limited amounts of, micro-cracks are developed within the laminates due to the inevitable high thermal stresses generated upon cooling from cure temperature.

The non-linear and time-dependent stress-strain response of NCF [±45]s laminates in tension is studied. Testing methodology is suggested to separate and quantify the effect of damage development, non-linear viscoelastic effects, and viscoplasticity on the inelastic response. This is achieved by decomposition of viscoelastic and viscoplastic response, both of them being affected by microdamage accumulated during the service life. Material model based on Schapery's work on viscoelasticity and Zapas viscoplastic function with added damage terms is presented and used. Simulation is performed and validated with constant stress rate tensile tests, identifying the non-linear viscoelasticity and viscoplasticity as the major sources of the non-linear response.

A new concept, Re-Fib, was developed within an EU project, REFORM, to recycle carbon and glass fibres from polymeric composite structures, aiming to reduce energy consumption and degradation of fibre properties during recycling. The optimized thermolysis treatment, 24 h at 380 °C, was verified able to recover clean fibres from most tested composite structures containing different thermoset resins (epoxy, vinyl ester, and polyester) and various core materials such as polyvinyl chloride (PVC), polyurethane (PU), and wood. Single-fibre test was performed in dynamic mechanical analysis (DMA). The reduction of strength was found around 26% for carbon fibres and 34–40% for glass fibres. Thermally recycled glass fibres were melt-compounded with recycled polypropylene (rPP); the resultant composites showed promising mechanical properties.

The advantage of using thin plies is a well-known feature in laminated composite plates, since the homogenized properties usually improve, and in turn, the performance of the structure. In the literature, several works deal with the study of the ply thickness effect for different structures and loading conditions. However, fewer studies have been performed to understand the structure response under out-of-plane loading, such as the low-velocity impact event. The apparition of high quality manufactured ultra-thin plies, such as the composite material product TeXtreme of Oxeon AB, requires a detailed analysis of their damage resistance and tolerance performance under impact loading. The present work deals with a discussion of a large experimental test campaign of drop-weight impact tests and Compression After Impact (CAI) tests on ultra-thin ply based composite laminates. The composite material analysed is a plain-weave fabric with HTS45 fibers and 20 mm wide yarns, used with HexFlow RTM 6 mono-component epoxy system, and manufactured out-of-autoclave. Two ply thicknesses are considered: 0.08 mm and 0.16 mm. For each case, the same laminate thickness and stacking sequence is considered in order to define the same in-plane stiffness. The study considers different impact energy levels.

We study the growth of self-assembled InGaSb/InAs quantum dots (QDs) and investigate how gallium can be used to reduce the optical transition energy in the InSb QD system. InGaSb QDs were grown on InAs (0 0 1) substrates by metal-organic vapor-phase epitaxy (MOVPE) and the material was characterized by photoluminescence (PL) measurements. A PL peak wavelength is demonstrated beyond 8 μm at 77 K, which is significantly longer than what has been reported for InSb QDs. The results suggest that InGaSb QDs can be grown at a larger size than InSb QDs leading to reduced confinement in the QDs.

A material model to predict kink-band formation and growth under a 3D stress state is proposed. 3D kinking theory is used in combination with a physically based constitutive law of the material in the kink-band, accounting for friction on the microcracks of the damaged material. In contrast to existing models, the same constitutive formulation is used for fibre kinking and for the longitudinal shear and transverse responses, thereby simplifying the material identification process. The full collapse response as well as a crush stress can be predicted. The model is compared with an analytical model, a micromechanical finite element analysis and crushing tests. In all cases the present model predicts well the different stages of kink-band formation and crushing.

Compression moulding experiments of sheet moulding compound, visual observations of a vacuum test with prepregs and numerical models with two main approaches for computational fluid dynamics simulations of the mould filling phase are presen ed. One assumes that there are layers near the mould surfaces with much less viscosity and the other only use one viscosity model. The numerical experiments showed that the pressure could be accurately predicted with both approaches. The property ne essary to predict correct pressure with altered mould closing velocities was that the bulk material had to obey shear-thinni g effects. Preheating effects before compression were neglected, but altering the heating time until the prepreg was assumed to start flow had a significant effect. The experiments confirmed that the pressure is predominantly affected by the mould c osing velocity. Regardless of the considered process settings, a first pressure top always appeared approximately at the logarithmic strain 0.25. A second top was associated with a slowdown of the press. The location of this was affected by the velocity and the vacuum, the latter indicating that vacuum assistance prevents a build-up of back pressure. Furthermore, heated prepreg above a critical temperature is observed to swell immediately as vacuum assistance is applied.

As a route to increase the efficiency of electric vehicles, weight reductions through composite building materials are constantly being introduced. To further aid this effort focus has been put on structural batteries, where the composite is multifunctional serving both as energy storing as well as load bearing unit. In an attempt to reduce the high ionic resistances solid polymer electrolytes introduces, carbon fibres have been individually coated with polymeric layers ranging from <500 nm to >3 µm in thickness. This study investigates the feasibility of using such coatings in structural applications with respect to mechanical load cycling. The coated fibres were subjected to cyclic load up to approximately 1 % strain for up to 70,000 cycles. The polymer coatings were found not to be visibly affected by the prolonged mechanical fatigue. No cracks were observed in the coatings which makes the coating technique promising for future structural battery applications. 

The degradation of the elastic properties of composite laminates with intralaminar cracks is caused by reduced stress in the damaged layer which is mainly due to two parameters: the crack opening displacement (COD) and the crack sliding displacement (CSD). In this paper these parameters are measured experimentally providing laminate stiffness reduction models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edges of a [0/ +704/ -704]s glass fiber/epoxy laminate specimens with multiple intralaminar cracks is studied and the COD and CSD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). By studying the displacement discontinuities, the crack face displacements were measured. A comparison between the COD and the CSD (for the same crack) is performed

The previously developed closed form expressions for thermo-elastic properties of laminates with intralaminar cracks (Lundmark and Varna, 2005) contain crack surface opening and sliding as main local parameters. The dependence of these parameters on various material and geometrical characteristics was in (Lundmark and Varna, 2005) described by power functions valid only for noninteractive cracks in a given layer (low crack density). In this article the 90-layer crack interaction in terms of its effect on the crack opening displacement (COD) is discussed. The effect on COD is described by the introduced ‘interaction function’ which is determined fitting results of finite element (FE) analysis for cross-ply laminates. To simplify the application in stiffness predictions, the numerically found weak dependence of the interaction on geometrical and material parameters is neglected and the interaction function is presented as a function of crack density only. Using the interaction function to determine the COD, the previously developed calculation scheme (Lundmark and Varna, 2005) has been used to predict stiffness reduction in the entire crack density region. The results are validated comparing with tests and FE simulations. 

Today's society is a society in constant motion with a strong focus on rapid development. The desire for rapid development and continuous growth causes Earth's resources to be consumed in an increasingly fast pace. Constituents in conventional fibre-reinforced composite sandwich structures which are widely used in marine applications, sports, aerospace and construction, such as carbon fibre, polyester, vinyl ester, epoxy resins, PVC- and PET foam are usually oil-based. It is therefore of great importance for our society that we are trying to replace these petroleum-based products with bio-based equivalents derived from renewable CO2-neutral plants. Cellulose, the primary structural component of plants, is the most ubiquitous and abundant organic compound on the planet. When cellulose fibrils are processed under carefully controlled conditions, it is possible to release highly crystalline nano-particles known as “nano crystalline cellulose (NCC)”. NCC has some very interesting mechanical properties: a single NCC fibre typically has a Young's modulus of around 150 GPa and a strength of 10 GPa. MELODEA and the Hebrew University of Jerusalem recently developed a unique technique for self-assembling NCC into highly ordered layered cellular structures, i.e. foams, for use as lightweight core materials for biocomposite sandwich constructions. Characterisation of foams in respect of their mechanical performance compared to fossil based equivalent has been conducted. Results for the study show that the NCC-foams' mechanical properties are sufficient for composite sandwich structures. By comparing with fossil oil-based PET equivalent it is seen that when the mechanical properties are in the same region, the density of the NCC-foams is slightly higher. The intense research and development on the NCC-foam indicates that in the future it will match the oil-based PET foam on all properties. Overall conclusion of the work is that the environmental friendly NCC-foam has potential as core material for composite sandwich structures. 

Functional composite materials allowing stiffness reduction upon external stimulation are being developed within the European projects ENLIGHT and SafeEV. The aim is to develop material concepts to reduce the severity of injuries involving vulnerable road users. The current work addresses the development of textile composite material models aimed to be employed in a car front structure during static loading to low velocity impact situations. The modelled material is a non-crimp fabric reinforced thermoplastic (LPET) composite, in which the stiffness reduction relies on resistive heating of the carbon reinforcement upon application of electric current through the fibres. The stiffness reduction is achieved by a phase transformation of the thermoplastic matrix material. In this paper it is shown how a micro- and mesomodelling methodology in concert with only a few simple DMTA measurements can be utilized to model the macroscopic stiffness response of an impacted beam at various temperatures and loading rates. The material models used for simulation of the material show a good correlation with the experimental data despite the exclusion of a damage model and failing to account for the temperature variation within the specimens used in the experimental testing. The peak loads are well predicted. 

Lately it has been demonstrated that natural fibres may be an environmentally superior alternative for, e.g., glass fibres. In order to estimate properties of composite materials made of natural fibres, models designed for synthetic fibres are often used. The models usually do not account for irregularities in the material, e.g., suboptimal fibre orientation due to the twisting angle of fibres in yarns. Use of models without taking those features into account might lead to unreliable results. Methods to quantify the microstructural properties of natural fibre composites with X-ray microtomography and three-dimensional image analysis are demonstrated in this work. The methods are applied to flax fibre composites made from three different kinds of pre-forms. Microstructural parameters estimated with the methods are used in micromechanical models for the stiffness of the composite. Comparison between rule-of-mixtures and classical laminate theory is made, highlighting the requirement for accurate parameter estimation and use of a model that accounts for significant structural features of the material. 

In this paper, intrabundle failure initiation in NCF reinforced composite materials is predicted based on a finite element model built with shell elements. The full 3D stress state is estimated based on the shell results and used in a state of the art 3D failure criterion. The procedure considers predictions of the transverse shear and normal stresses from stress equilibrium. By using this approach on shell elements, more efficient modelling strategies suited to identify hot spots in larger structures can be pursued.

In this paper, a set of failure criteria for transverse failure in non-crimp fabric-reinforced composites is presented. The proposed failure criteria are physically based and take into account the orthotropic character of non-crimp fabric composites addressing the observed lack of transverse isotropy. Experimental data for transverse loading out-of-plane in combination with in-plane loads are scarce. Therefore, to validate the developed criteria, experimental data are complemented with numerical data from a representative volume element model using a meso-micromechanical approach. The representative volume element model also provides a deeper understanding of how failure occurs in non-crimp fabric composites. Strength predictions from the developed set of failure criteria show good agreement with the experimental and numerical data. © The Author(s) 2015.

The recent carbon fibers, with fiber architecture of the spread tow type, has shown radical improvements in critical material properties during use in the ongoing EU project CHATT (Cryogenic Hypersonic Advanced Tank Technologies), especially for the strain at onset of micro-cracking, transverse to the fiber direction. Liquid composite manufacture (wet filament winding, RTM) has been used in CHATT for these materials. The manufacturing challenge has been to achieve high quality and short cycle times. The processing issues have been solved using a combination of process simulation and manufacturing equipment modifications. The manufactured subscale demonstrator tubes have successfully been tested in CHATT towards the demanding loading conditions specified in the project, indicating that the TeXtreme® material performs similar to a load carrying liner material. 

This review paper gives an overview of test methods for multiaxial and out-of-plane strength of composite laminates, with special consideration of non-crimp fabrics (NCF) and other textile systems. Tubular and cruciform specimens can provide arbitrary in-plane loading, while off-axis and angle-ply specimens provide specific biaxial loadings. Tensile and compressive out-of-plane strength may be determined by axial loading of specimens with a waisted gauge section, while bending of curved specimens allow determination of the out-of-plane tensile strength. Tests suited for out-of-plane shear strength include the short beam shear test, the inclined double notch test and the inclined waisted specimen. Testing of arbitrary tri-axial stress states using tubular or cruciform specimens with superimposed through-the-thickness loading is highly complex and significant problems have been reported in achieving the intended stress states and failure modes. Specific tri-axial stress states can be obtained by uniaxial loading of specimens with constrained expansion, as in the die channel test. 

This article presents analytical models for predicting large mass impact response and damage in thin-ply composite laminates. Existing models for large mass impact (quasi-static) response are presented and extended to account for damage phenomena observed in thin-ply composites. The most important addition is a set of criteria for initiation and growth of bending induced compressive fibre failure, which has been observed to be extensive in thin ply laminates, while it is rarely observed in conventional laminates. The model predictions are compared to results from previous tests on CFRP laminates with a plain weave made from thin spread tow bands. The experiments seem to confirm the model predictions, but also highlight the need to include the effects of widespread bending induced fibre failure into the structural model

This chapter gives an overview of causes and effects of damage due to low and medium velocity impact. Typical features of impact damage are described, with focus on tape prepreg laminates. The relation between impact response and impactor/plate mass ratio is explained and appropriate analytical response models outlined. Experimental observations of the behaviour of impact damage under load and its effect on strength and buckling are described. A brief discussion is also provided on the application of computational methods to impact on composites, and issues needing attention highlighted. The chapter is concluded with a discussion on future trends and advice for further information.

This paper reviews findings on the type, morphology and constitutive behaviour of impact damage zones during loading after impact and their effect on the laminate strength and stability. The paper is limited to tape prepreg based monolithic laminates, although some similarities exist with impact damage in textile based laminates. Damage zones have a complex geometry with several damage types, which results in an interaction and competition between different failure mechanisms, e.g. local and global buckling, compressive failure, and delamination growth. Hence, simplified damage models may provide incorrect predictions of the failure load and failure mechanisms after impact. The constitutive behaviour of damage zones has been studied experimentally in tension and compression using an inverse method, and the results have been compared with detailed FE models of a generic impact damage. The paper is concluded with a discussion on analytical and computational models to predict the resulting strength of impacted laminates.

This paper presents an experimental and numerical study of impact response, damage and tension after impact of thin ply HTS45/RTM6 carbon/epoxy laminates, manufactured via resin transfer moulding. A plain weave from carbon fibre spread-tow bands was used in a quasi-isotropic layup. Finite element simulations were performed using layered shell elements accounting for in-plane damage mechanics, with cohesive surfaces between a few layers of shell elements to account for delamination. The damage was found to include a combination of fibre damage and delaminations, in contrast to a previous study on similar cross-ply laminates, where fibre damage dominated. The rate of decrease in tensile strength after impact was similar to prepreg laminates with conventional ply thickness, but the impacted strength was slightly higher due to a higher undamaged strength for thin ply laminates.

Thin-ply composites offer reduced or suppressed matrix cracking and higher strains to first ply failure. Initial tests indicate a significantly different impact damage than for conventional composites, with less delamination and more fibre fracture. The current paper presents models focused on the observed fibre damage, including an analytical model for the response and damage initiation during impact on thin ply composites as well as a finite element FE model for prediction of damage growth. The limitations and challenges of the analytical model and FE model are discussed and illustrated by comparisons with response histories and fractography for drop weight impact on a thin-ply composite laminate.

Core shear cracking induced by impact on sandwich panels is a critical failure mode causing severe loss of structural performance. This paper reviews previous experimental and theoretical work in the area and derives improved closed form expressions for initiation of skin rupture and core shear cracking during impact on sandwich panels with foam cores. The criterion for skin rupture is also applicable to laminates without a core. It is shown that the skin rupture load limits the achievable core shear load, and that core shear cracking can be prevented by selecting a core thickness above a certain threshold value. The criteria are successfully validated by comparison with experimental results for a range of thicknesses of skins and cores in panels with carbon/epoxy skins and a Rohacell foam core. The criterion for skin rupture is also validated for plain laminates.

This paper examines analytical models for hail impact on composite laminates and compares the predictions with finite element simulations and experiments. The crushing of the ice results in a distributed load and a much higher delamination threshold load than for impact by hard objects. Furthermore, prediction of the impact load by merely considering the mass flow of ice particles results in too low loads and a response in disagreement with experiments. The pressure acting on the impacted plate is fairly uniform within the contact area, but initial through-thickness waves during the first moments of the impact cause much higher stresses than the quasi-static values assumed in the analytical models. Hence the finite element models predict a different load history, with a much steeper initial increase in the contact load.

This paper presents experiments to support development of mesomechanics models for prediction of the strength of Non-Crimp Fabric (NCF) materials under tri-axial loading. Here we describe initial tests to characterise the failure envelope of the material in fibre bundles of a carbon fibre/epoxy NCF. Uniaxial loading of unidirectional off-axis specimens is used to obtain in-plane stress states ranging from highly shear dominated to fully transverse or axial tension, while future tests will include compressive stresses. The bundle material is represented by filament winding of unidirectional laminates and subsequent vacuum infusion of the resin. The resulting fibre volume fraction of 69% is representative of typical bundles in NCF materials. A modified Puck failure criterion provides reasonable predictions of the failure envelope.