Hybrid composites based on polyethylene and carbon fibres Part 3: Impact resistant structural composites through damage management

Damage tolerance and impact resistance have become key parameters for composite materials in structural applications. In this paper a toughening concept for structural composites based on the hybridization of carbon fibres with high performance polyethylene (HP-PE) fibres is presented. Impact behaviour of hybrid HP-PE/carbon laminates was studied using a falling weight impact test. The effect of the addition of HP-PE fibres as well as the effect of the adhesion level of these fibres on the impact resistance of hybrid HP-PE/carbon structures was investigated. Hybridization results in structural composites exhibiting a significantly better resistance to impact damage than all-carbon laminates due to a change in energy absorption mode. After hybridization more energy is stored in the HP-PE component and consequently less energy is available for damage in the structural carbon component, resulting in a reduction in impact damage and improved post-impact properties.     

Penetration impact resistance of hybrid composites based on commingled yarn fabrics

The penetration impact resistance of hybrid composites based on commingled yarn fabrics was investigated. The commingled yarn fabrics were composed of E-glass fibres (GF) and thermoplastic fibres blended together within the fibre bundles. Various thermoplastic fibres such as polypropylene (PP), polyamide (PA) and modified polyethylene terephthalate (mPET) were studied. Various resin matrices with different cure cycles were studied such as Quickcure polyester, Cycom X823 RTM epoxy, and Shell Epikote 828 epoxy resin. Depending on the crystalline melting temperature (T_m) of the thermoplastic fibres, the hybrid composites can be categorised as fibre-hybrid composites or matrix-hybrid composites. Fibre-hybrid composites refer to those in which the thermoplastic fibres remain in the fibre form after curing, for example the GF–PP and GF–PA hybrid composites. For matrix-hybrid composites, the thermoplastic fibres melt and dissolve into the thermosetting matrix during curing such as the GF-mPET hybrid composites. The results from the penetration impact showed that the total absorbed energy of the fibre-hybrid composites were significantly higher than for the plain glass composites. Plastic deformation in the thermoplastic fibres is the key factor that improves the absorbed energy of the hybrid composites. When the thermoplastic fibres dissolved into the thermosetting matrix as in matrix-hybrid composites, the total absorbed energy was similar to that of the plain glass fibre composites. This suggests that the total absorbed energy is dependent on the properties of the fibres rather than the matrix. However, the fibre-hybrid composites showed slight differences in the total absorbed energy for different matrices. The differences are thought to be related to the differences in the bonding between the thermoplastic fibres and the thermosetting matrix which have yet to be investigated.     

Early prediction of the failure of textile-reinforced thermoplastic composites using hybrid yarns

In this study, the manufacturing of core-sheath hybrid yarns consisting of steel fibres and glass filament yarn (GFY) using a friction spinning technique and their usability for failure prediction in composites is reported. With the DREF-2000 friction spinning technique, it is possible to manufacture hybrid yarns having a core-sheath structure. Steel fibres are used as the sheath and the GFY is used as the core of the yarns. These hybrid yarns are embedded between two layers of glass/polypropylene (GF/PP)-based knitted fabric composites. By varying the steel fibre content, it is possible to adjust the initial resistance as well as the sensitivity of the hybrid yarns to measure the interphase strain in the thermoplastic-based knit composite during tensile loading. The hybrid yarns with lower steel fibre content are found to be more sensitive in the prediction of the early damage in the composite. By performing a quasi-static and gradual increase of loading during the tensile tests, it is possible to identify the critical load for the composite. The before mentioned hybrid yarns show their suitability for the structural health monitoring and the potential to be integrated into thermoplastic-based composites by textile processing.     

Transverse impact damage and energy absorption of 3-D multi-structured knitted composite

Knitted composites have higher failure deformation and energy absorption capacity under impact than other textile structural composites because of the yarn loop structures in knitted performs. Here we report the transverse impact behavior of a new kind of 3-D multi-structured knitted composite both in experimental and finite element simulation. The knitted composite is composed of two knitted fabrics: biaxial warp knitted fabric and interlock knitted fabric. The transverse impact behaviors of the 3-D knitted composite were tested with a modified split Hopkinson pressure bar (SHPB) apparatus. The load–displacement curves and damage morphologies were obtained to analyze the energy absorptions and impact damage mechanisms of the composite under different impact velocities. A unit-cell model based on the microstructure of the 3-D knitted composite was established to determine the composite deformation and damage when the composite impacted by a hemisphere-ended steel rod. Incorporated with the unit-cell model, a elasto-plastic constitute equation of the 3-D knitted composite and the critical damage area (CDA) failure theory of composites have been implemented as a vectorized user defined material law (VUMAT) for ABAQUS/Explicit. The load–displacement curves, impact deformations and damages obtained from FEM are compared with those in experimental. The good agreements of the comparisons prove the validity of the unit-cell model and user-defined subroutine VUMAT. This manifests the applicability of the VUMAT to characterization and design of the 3-D multi-structured knitted composite structures under other impulsive loading conditions.     

Impact and post-impact damage characterisation of hybrid composite laminates based on basalt fibres in combination with flax,hemp and glass fibres manufactured by vacuum infusion

The impact and flexural post-impact behaviour of ternary hybrid composites based on epoxy resin reinforced with different types of fibres, basalt (B), flax (F), hemp (H) and glass (G) in textile form, namely FHB, GHB and GFB, has been investigated. The reinforcement volume employed was in the order of 21–23% throughout. Laminates based exclusively on basalt, hemp and flax fibres were also fabricated for comparison. Hybrid laminates showed an intermediate performance between basalt fibre reinforced laminates on the high side, and flax and hemp fibre reinforced laminates on the low side. As for impact performance, GHB appears to be the worst performing hybrid laminate and FHB slightly overperforms GFB. In general, an increased rigidity can be attributed to all hybrids with respect to flax and hemp fibre composites. The morphological study of fracture by SEM indicated the variability of mode of fracture of flax and hemp fibre laminates and of the hybrid configuration (FHB) containing both of them. Acoustic emission monitoring during post-impact flexural tests confirmed the proneness to delamination of FHB hybrids, whilst they were able to better withstand impact damage than the other hybrids.     

Experimentally based strategy for damage analysis of textile-reinforced composites under static loading

For a reliable design of components made of textile composites, a deep knowledge of their failure behaviour and of realistic damage models is necessary. Such models require the onset of damage and the evolution of different damage phenomena to be determined experimentally. In this context, an experimental damage analysis strategy is proposed here that combines crack density measurements, acoustic emission analysis and optical microscopy with the recording of stiffness degradation by ultrasonic wave speed measurements. The correlation between the results of quasi-static tests is discussed for two selected examples of textile composites: multi-layered flat bed weft-knitted glass fibre–epoxy composites and woven glass fibre–polypropylene composites made of hybrid fabrics.     

The effect of fibrous reinforcement on optical and impact performance of fibre-reinforced transparent glass composites

Fire-resistant laminated glass composite containing intumescent silicate as an interlayer between two glass sheets is a widely used transparent building material. To improve the impact and other mechanical properties of this composite structure, a transparent silicate matrix has been reinforced with alkali- and UV-resistant synthetic (polypropylene, polyamide 66, glass) and metallic (steel) fibres as of nonwoven webs or woven meshes. The refractive indices (RIs) of the fibres and the matrices were measured and the transparency of the laminated composites depended upon fibre RI as well as reinforcement structure. All fibres were successful in significantly enhancing impact properties of laminated glass composites with alkali-resistant glass fibres showing the best performance.     

Moderate speed impact damage to 2D-braided glass–carbon composites

Hybrid glass–carbon 2D braided composites with varying carbon contents are impacted using a gas gun by impactors of masses 12.5 and 44.5 g, at impact energies up to 50 J. The damage area detected by ultrasound C-scan is found to increase roughly linearly with impact energy, and is larger for the lighter impactor at the same impact energy. The area of whitening of the glass tows on the distal side corresponds with the measured C-scan damage area. X-ray imaging shows more intense damage, at the same impact energy, for a higher-mass impactor. Braids with more glass content have a modest increase in density, decrease in modulus, and reduction in the C-scan area and dent depth at the impact site, particularly at the higher impact energies. Impact damage is found to reduce significantly the compressive strength, giving up to a 26% reduction at the maximum impact energy.     

Mechanical performance optimization through interface strength gradation in PP/glass fibre reinforced composites

A new design for thermoplastic composites based on the gradation of the interlaminar interface strength (IGIS) has been developed with the aim of coupling high impact resistance with high static properties. IGIS laminates have been prepared by properly alternating layers of woven fabric with layers of compatibilized or not compatibilized polymeric films. To prove the new concept, polypropylene (PP) and glass fibres woven fabrics have been used to prepare composites by using the film stacking technique. Maleated PP, able to compatibilize polypropylene with glass fibres, has been used to manage the interface strength layer by layer.The flexural and low-velocity impact characterizations have shown that the presence of the coupling agent in conventional composite structures (prepared with fully compatibilized polymeric layers) improves the static flexural properties through the strengthening of the matrix/fibre interface but considerably lowers the low velocity impact resistance of the composite, in terms of maximum load before fibre breakage and recovered energy after impact. The use of the IGIS design, that grade the interface strength through the laminate thickness, allows to prepare composites with both high flexural properties and high impact resistance, without affecting the balance and type of the reinforcement configuration.     

A study of the mechanical behaviour on fibre reinforced hollow microspheres hybrid composites

This paper presents the results of an investigation into the effects of hollow glass microsphere fillers and of the addition of short fibre reinforcements on the mechanical behaviour of epoxy binding matrix composites. Properties like flexural stiffness, compressive strength, fracture toughness and absorbed impact energy, were studied. The specimens were cut from plates produced by vacuum resin transfer moulding having a microsphere contents of up to 50% and with fibre reinforcement up to 1.2% by volume. The tests performed with unreinforced composites show that flexural and compressive stiffness, maximum compressive stresses, fracture toughness and impact absorbed energy decrease significantly with increasing filler content. However, in terms of specific values, both flexural and compressive stiffness and impact absorbed energy increase with microsphere content. The addition of glass fibre produces only a slight improvement in the flexure stiffness and fracture toughness, while increasing significantly the absorbed impact energy. In contrast, the addition of a small percentage of carbon fibres produces an important improvement in both fracture toughness and flexure stiffness, when hybrid composites with 0.9% carbon fibre are compared to unreinforced foam, but did not improved absorbed impact energy.     

Strain-sensitive Raman spectroscopy and electrical resistance of carbon nanotube-coated glass fibre sensors

This paper presents the development of glass fibres coated with nanocomposites consisting of carbon nanotubes (CNTs) and epoxy. Single glass fibres with different CNT content coating are embedded in a polymer matrix as a strain sensor for composite structures. Raman spectroscopy and electrical response of glass fibres under mechanical load are coupled for in situ sensing of deformation in composites. The results show that the fibres with nanocomposite coating exhibit efficient stress transfer across the fibre/matrix interface, and these with a higher CNT content are more prone to fibre fragmentation at the same matrix strain. A relationship between the fibre stress and the change in electrical resistance against the fibre strain is established. The major finding of this study has a practical implication in that the fibres with nanocomposite coating can serve as a sensor to monitor the deformation and damage process in composites.     

Modelling of the Mechanical Properties of Composite Materials at High Temperatures. Part 3. Textile Composites

The present paper is devoted to extending the model suggested previously (Dimitrienko, 1997) to textile composite materials at high temperatures. The model describes a degradation of elastic moduli of polymer-matrix composites in heating to high temperatures (2000°C). With the help of the model, analytical relations between elastic moduli of textile composites and elastic characteristics of their matrices and fibres, as well as geometrical structural parameters of the composites at high temperatures have been derived. Calculated results have been compared with experimental data for carbon/ and glass/phenolic composites as examples. Heat expansion and shrinkage of the composites in heating up to high temperatures have been also considered.     

The compression-after-impact strength of woven and non-crimp fabric reinforced composites subjected to long-term water immersion ageing

The current work examines the durability of composites reinforced with glass fibre woven fabric as well as non-crimp fabrics (NCF) immersed in water at 43, 65 and 93 °C for up to 2.5 years. Low velocity normal impact has been induced at various time intervals before and after water immersion at energy levels of 2.5, 5 and 10 J. Following impact the plates were tested statically in compression to determine the residual strength for assessment of damage tolerance. The compression strength suffered significant reductions from the water absorption and the low velocity impact with values being dependent to the time of immersion and the water temperature. A parallel behaviour was monitored, in terms of strength reduction over time, of plates impacted prior to water immersion with the plates that contain no damage. For specimens where impact damage introduced after water immersion lower compression-after-impact (CAI) strength was observed at the same energy levels. An increase in damage diameter was evident, regardless the reinforcement type, though the gradually produced greater density of through thickness damage was responsible for the significant lower compression strength values. The presence of 0° fibres for the NCF composites as the main load bearing element dictated the sensitivity to impact as well as the corresponding residual strength. For composites with woven reinforcement, damage was contained and localized by the fabric weave and effective stress redistribution seemed to be the mechanism for the relatively higher residual strengths obtained. A semi-empirical model has been used with high accuracy in fitting the given experimental data and draw conclusions from the comparisons.     

Shear mechanism modelling of heavy tow braided composites using a meso-mechanical damage model

Heavy tow braid reinforced composites are a potential substitute for metals in automotive and other transport applications. These composites, if properly designed, can provide lightweight efficient load bearing structural members that can also absorb high specific energy under impact and crash loading. Many of these components are ‘beam like’ members that must resist large transverse deformations at high force levels, thereby absorbing high levels of energy. This class of composite component is particularly considered in this paper.An effective means to achieve high energy absorption is careful design of the fabric architecture so that shearing mechanisms of the fibre/matrix interface, without premature fibre failure, are possible. Characterisation and modelling of progressive shear damage and failure occurring in biaxial carbon and glass braided composites are investigated. Fibre re-orientation and fibre/matrix interface damage is measured using an optical strain measuring method based on digital image correlation (DIC). This is then used to provide input to a meso-mechanical damage model in an explicit finite element code. A modelling approach using coupled layers of equivalent unidirectional plies is used to represent the biaxial braid composite and validation of the approach has been performed against test coupons and beam structures loaded transversally to failure.     

Impact loading on fibre metal laminates

Static identation and low and high velocity impact tests are conducted on specimens with a circular clamped test area. Monolithic A1 2024-T3 and 7075-T6, various grades of Fibre Metal Laminates (FML), and composites are tested. The energy to create the first crack for FML with aramid and carbon fibres is comparable to fibre reinforced composite materials and is relatively low compared to A1 2034-T3 and FML with R-glass fibres (GLARE). GLARE laminates can show a fibre critical or aluminium critical failure mode. The dent depth after impact of FML is approximately equal to that of the monolothic aluminium alloy. The damage size of FML after impact is considerably smaller than of glass and carbon fibre composite materials. The maximum central deflection during low velocity impact loading is approximately equal to the static deflection at the same energy (i.e., area under load-deflection curve). This deflection can be modelled using the simplified Von Kármán equations neglecting the contribution of the in-plane displacements. For these calculations the shape of the specimen under load was measured. This shape was approximately independent of the central deflection and the type of material.     

The role of reinforcement architecture on impact damage mechanisms and post-impact compression behaviour

This review considers the link between the damage tolerance of composite laminates and the nature and organization of the fibre reinforcement. This embraces composites made from unidirectional prepregs through composites based on a variety of textile forms such as woven fabrics, multiaxial fabrics, braids and knits. The objective has been firstly to detail how the differing varieties of composite exhibit different properties under impact conditions and under subsequent loading after impact. This includes both fracture mechanisms and data such as energy absorption, and peak failure loads. The second objective is to describe the links that have been found between these properties and the specific fibre architectures and damage development processes in the various composite forms. The post impact compression properties are highlighted as this is the area of greatest interest by end-users. The review describes the different forms of textiles that are used for composite reinforcement, considers different impact conditions (e.g. low velocity and ballistics), general materials variables such as fibre and resin type, and ultimately looks at specific textile systems. Some consideration is also given to the value and role of numerical modelling in the field of damage formation and damage tolerance. Clear differences have been found in the literature between composites based on different textile forms in terms of damage states after impact and the consequences of this damage on subsequent properties. While the literature is clearly incomplete at this time there is sufficient information available to indicate that control of fibre organization by the use of textiles may be an effective method of optimizing composite properties for specific end use properties.     

Impact characterization of RTM composites: Part I Metrics

The use of textile based architectures and the dry compaction of preform layers prior to resin infusion creates the potential for highly tailored resin transfer-moulded composites that have significantly different response to impact than traditional prepreg-based composites. The response of a number of resin transfer moulding (RTM) composites is characterized using both traditional and new metrics of performance. Impact performance maps are used to differentiate between materials response at a fundamental level and key differences are traced to differences in preform fabric architecture. Global and local differences in response based on architecture are elucidated through the determination of damage and energy absorbance, and are related to materials' specific characteristics in an attempt to allow comparison of impact response of composites on a more quantitative basis. This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile fatigue behaviour of FRP and hybrid FRP sheets

This paper presents the fatigue behaviour of various fibre reinforced polymer (FRP) composites, namely, carbon, glass, polyparaphenylenl benzobisoxazole (PBO), and basalt fibres, including the effect of hybrid applications such as carbon/glass and carbon/basalt composites. A coupon test was conducted to examine the mechanical characteristics of the FRP composites subjected to monotonic and cyclic loads. Test parameters included the applied load range and different types of hybridization. Study results show that (1) the mechanical properties of the emerging PBO and basalt fibres are comparable to those of the conventional carbon and glass fibres; (2) the tensile modulus of the fibres influences the failure mode of the composite coupons; (3) the progressive damage propagation causes fatigue failure of the composites; (4) the hybrid composites of carbon/basalt significantly improves the fatigue resistance in comparison to the homogeneous basalt composite, whereas the resistance of the carbon/glass hybrid composites does not provide such effects.     

The effects of matrix microcracking on the oxidation behaviour of carbon-fibre/glass-matrix composites

Carbon-fibre/glass-matrix composites were fabricated using Fortafil fibres and two different glass matrices: a sodium-borosilicate glass (CGW 7740), and a calcium-aluminosilicate glass (CGW 1723). Upon cooling from the hot-pressing temperature used to fabricate the composites (approximately 1250°C), the glass matrices cracked due to differences in the coefficients of thermal expansion between the fibres and the matrix. At elevated temperatures these cracks serve as short-circuit diffusion paths for oxygen transport, and the majority of the weight loss from the cracked samples was caused by oxygen diffusing along these microcracks and reacting with the fibres. Because of the relatively large diameter of these cracks compared to the mean free path for diffusing oxygen, traditional gas kinetics can be applied to the various transport processes occurring in the oxidation reactions, and there is no need to allow for capillary size or to apply Knüdsen diffusion. The composites made of 1723 glass exhibited linear relationships between specific-mass loss ( mass/initial exposed surface area of carbon fibres) and time at all oxidation temperatures (450, 500, 550 and 600 °C). With the 7740 composites, a parabolic relationship between specific-mass loss and time was obtained. As the oxidation temperature approached or exceeded the glass-transition temperature, T _g, for the 7740 composites (560 °C), this parabolic relationship became more pronounced. Microstructural evidence revealed that at temperatures near or exceeding the T _g for the 7740 glass the microcracks in the matrix heal, thereby decreasing the amount of fibre surface area available for chemical reaction. Because the rate of oxidation is directly proportional to the amount of available fibre-surface area, the weight-loss data appear parabolic with time. Additionally, the mechanism for the oxidation of the carbon fibres does not appear to change once the fibres are placed in a glass matrix. The apparent activation energy for oxidation remained constant at approximately 174 kJ mol^–1.     

Development and evaluation of a novel integrated anti-icing/de-icing technology for carbon fibre composite aerostructures using an electro-conductive textile

The preliminary evaluation is described of a new electro-thermal anti-icing/de-icing device for carbon fibre composite aerostructures. The heating element is an electro-conductive carbon-based textile (ECT) by Gorix. Electrical shorting between the structural carbon fibres and the ECT was mitigated by incorporating an insulating layer formed of glass fibre plies or a polymer film. A laboratory-based anti-icing and de-icing test program demonstrated that the film-insulated devices yielded better performance than the glssass fibre insulated ones. The heating capability after impact damage was maintained as long as the ECT fabric was not breached to the extent of causing electrical shorting. A modified structural scarf repair was shown to restore the heating capacity of a damaged specimen.