Showing papers in "Composites Part A-applied Science and Manufacturing in 2005"

The effect of filler–filler and filler–elastomer interaction on rubber reinforcement

Abstract: The role of active fillers like carbon black and silica has been studied in the rubber matrix for a better understanding of the rubber performance and the mechanism of reinforcement. In particular the influence of basic properties of carbon blacks, such as specific surface area, structure and surface activity on the Payne-effect, was investigated with the Rubber-Process-Analyzer (RPA) which allows a testing of the strength of the filler network and the filler-polymer interaction in the green compound as well as in the vulcanizate in a wide range of shear amplitudes. A comparison between carbon black and the silica-silane system leads to further scientific findings for the understanding of the dynamic behavior of filled rubber compounds.

Influence of nano-modification on the mechanical and electrical properties of conventional fibre-reinforced composites

Abstract: Carbon nanotubes (CNTs) exhibit a high-potential for the reinforcement of polymers. The mechanical properties of potential matrices of fibre-reinforced polymers (FRP), such as epoxy resins, were significantly increased by low contents of carbon nanotubes (CNT) (tensile strength, Young's modulus and fracture toughness). Nano-particle-reinforced FRPs, containing carbon black (CB) and CNTs could successfully be manufactured via resin transfer moulding (RTM). A filtering effect of the nano-particles by the glass-fibre bundles was not observed. The glass-fibre-reinforced polymers (GFRP) with nanotube/epoxy matrix exhibit significantly improved matrix-dominated properties (e.g. interlaminar shear strength), while the tensile properties were not affected by the nano-fillers, due to the dominating effect of the fibre-reinforcement. The GFRP containing 0.3 wt% amino-functionalised double-wall carbon nanotubes (DWCNT-NH 2 ) exhibit an anisotropic electrical conductivity, whereas the conductivity in plane is one order of magnitude higher than out of plane.

A study of the effect of acetylation and propionylation surface treatments on natural fibres

Abstract: Recently, the incorporation of lignocellulosic materials as reinforcing agents or as fillers in polymer composites has received an increased attention. Although natural fibres have a number of advantages over glass fibres, the strong polar character of their surface is a limiting factor, as compatibility with strongly apolar thermoplastic matrices is very low. Such problems of incompatibility may be overcome with fibre pre-treatments, which can enhance compatibility, albeit having a negative impact on the economics. In the present study, two fibre pre-treatment methods, acetylation and propionylation, were applied on flax, hemp and wood fibres. The effect of esterification between the acetyl/propionyl groups and the hydroxyl groups of the fibre was examined by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS), while its extent was assessed by titration. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the crystallinity and the surface morphology of the untreated and esterified fibres. The highest extent of the esterification reaction was achieved for the wood fibres due to their high lignin/hemicelluloses content. The two spectroscopic methods revealed that the fibre surface chemistry was altered after the treatments, as the results indicated that ester bonds are present on the fibre surface. The SEM results revealed that both treatments resulted in a removal of non-crystalline constituents of the fibres, possibly waxy substances, and alter the characteristics of the surface topography. It was also shown that the fibre crystallinity decreased slightly as a result of esterification.

Thermo-mechanical properties of randomly oriented carbon/epoxy nanocomposites

Abstract: The thermo-mechanical properties of epoxy-based nanocomposites based on low weight fractions (from 0.01 to 0.5 wt%) of randomly oriented single- and multi-walled carbon nanotubes were examined. Preparation methods for the nanocomposites, using two types of epoxy resins, were developed and good dispersion was generally achieved. The mechanical properties examined were the tensile Young's modulus by Dynamic Mechanical Thermal Analysis and the toughness under tensile impact using notched specimens. Moderate Young's modulus improvements of nanocomposites were observed with respect to the pure matrix material. A particularly significant enhancement of the tensile impact toughness was obtained for specific nanocomposites, using only minute nanotube weight fractions. No significant change in the glass transition temperature of SWCNT/epoxy nanocomposites was observed, compared to that of the epoxy matrix. The elastic modulus of the SWNT-based nanocomposites was found to be slightly higher than the value predicted by the Krenchel model for short-fiber composites with random orientation.

Electromagnetic interference shielding effectiveness of carbon nanofiber/LCP composites

Abstract: Vapor grown carbon nanofiber reinforced liquid crystal polymer (LCP) composites were studied. The surface resistivity of LCP was lowered more than 11 orders of magnitude by incorporating 5 wt% of nanofibers. The electromagnetic interference shielding effectiveness (SE) at different frequencies was also studied. The composites were shown to exhibit up to 41 dB of SE. The shielding mechanism of the system was mainly reflection and multiple reflections. The thermal conductivity of the composites showed no significant enhancement with the inclusion of nanofibers.

A continuum mechanics-based non-orthogonal constitutive model for woven composite fabrics

Abstract: A non-orthogonal constitutive model is developed to characterize the anisotropic material behavior of woven composite fabrics under large deformation. A convected coordinate system, whose in-plane axes are coincident with the weft and warp yarns of woven fabrics, are embedded in the shell elements. Contravariant stress components and covariant strain components in a constitutive relation are introduced into the convected coordinate system. The transformations between the contravariant/covariant components and the Cartesian components of the stress and strain tensors provide an approach for deriving the global non-orthogonal constitutive relations for woven composite fabrics. By taking advantage of the tensile–shear decoupling in the constitutive equation under the convected coordinate system, the material characterization of woven fabrics is simplified. As an essential part for these transformations, a fiber orientation model is developed, by using some fundamental continuum mechanics concepts, to trace the yarn reorientation of woven fabrics during deformation. The proposed material characterization approach is demonstrated on a balanced plain weave composite fabric. The equivalent material properties are obtained by matching with experimental data of tensile and bias extension tests on the woven composite fabric. Model validation is provided by comparing numerical results with experimental data of bias extension test and shear test. The development of this non-orthogonal model is critical to the ultimate goal, i.e. using numerical simulations to optimize the forming of woven composite fabric sheets.

Mechanical properties of flax fibre/polypropylene composites. Influence of fibre/matrix modification and glass fibre hybridization

Abstract: The effect of fibre treatments and matrix modification on mechanical properties of flax fibre bundle/polypropylene composites was investigated. Treatments using chemicals such as maleic anhydride, vinyltrimethoxy silane, maleic anhydride-polypropylene copolymer and also fibre alkalization were carried out in order to modify the interfacial bonding between fibre bundles and polymeric matrix. Composites were produced by employing two compounding ways: internal mixing and extrusion. Mechanical behaviour of both flax fibre bundle and hybrid glass/flax fibre bundle composites was studied. Fracture surfaces were investigated by scanning electron microscopy. Results suggest that matrix modification led to better mechanical performance than fibre surface modification. A relevant fact is that silanes or MA grafted onto PP matrix lead to mechanical properties of composites even better than those for MAPP modification, and close to those for glass fibre/PP.

‘Bleeding composites’—damage detection and self-repair using a biomimetic approach

Abstract: The aim of this study is to develop a novel composite system which employs a biomimetic approach to perform a self-repairing function. Such a system can perform two functions; the visual enhancement of impact damage by the bleeding action of a highly conspicuous medium such as fluorescent dye, and the restoration of mechanical properties by a healing agent, stored within hollow fibres, infiltrating the damaged area and acting to ameliorate the effect of the damage. Impact indentation followed by four-point bend flexural testing was conducted to evaluate the strength restoration after self-repair. The results of mechanical testing have shown that a significant fraction (∼97%) of strength is restored by the self-repairing effect.

Dynamic mechanical behavior of short coir fiber reinforced natural rubber composites

Abstract: Dynamic mechanical behavior of natural rubber and its composites reinforced with short coir fibers has been studied. Maxima in tan δ, E″ and the middle point of E′ vs. temperature curves of the gum natural rubber compound at different frequencies almost coincide with one another. But the maxima in tan δ and E″ do not coincide in the case of composites. It is observed that as frequency increases the values of tan δ and E″ decrease whereas the values of E′ increase in the case of both gum and the composites. The values of E″ and tan δ increase with fiber incorporation, which indicates lower heat dissipation in the gum. Two prominent peaks are observed in the tan δ vs. temperature curve of these composites due to the dynamic mechanical behavior of matrix and fiber. The additional small peak represents the dynamic mechanical behavior at the interface. The effect of chemical treatment of coir fiber on damping of composites was studied and it was found that composite with poor interfacial bonding tend to dissipate more energy than that with good interfacial bonding.

Electrical and thermal behavior of polypropylene filled with copper particles

Abstract: Thermal conductivity, diffusivity, effusivity and specific heat of polypropylene matrix filled with copper particles of two different sizes were investigated. A parallel study of the evolution of the electrical conductivity was also carried out. The highest heat transport ability was observed for the composites filled with the smaller particles. Electrical conductivity investigations showed that the size of fillers also influences the percolation threshold. The Agari's model provides a good estimation of the thermal conductivity of composites for all filler concentrations. It was used in order to give a comparative analysis of both electrical and thermal properties of such two-phase systems. Nevertheless, the physical meaning of the two fitting parameters C1 and C2 of Agari's model has to be completed.

A comparison of compounding processes and wood type for wood fibre—PP composites

Abstract: The structure–properties relationship of wood fibre (WF)–polypropylene (PP) composites was studied in relation to WF types (hardwood and softwood) and compounding techniques, namely two-roll mill, high-speed mixer (agglomerator) and twin-screw extruder. It was shown that twin-screw extruder compounded composites had higher mechanical properties than those compounded in a two-roll mill or a high-speed mixer. The use of compatibilisers (maleated PP of different maleic anhydride content) in the compounding step improved the mechanical properties of the composites in dry conditions regardless of the compounding process, however, in wet conditions a decrease in tensile and flexural strength was observed for all composites. It was shown that the use of a compatibilisers significantly improved the hydrophobicity of WF surface. However, the bulk of WF remains unmodified and accessible to water. The dependence of the moisture uptake of WF was evaluated with respect to WF and compatibiliser type. The effect of the size and shape of the composite samples used in the experiments on the equilibrium water content was also tested.

Analysis of the vacuum infusion moulding process: I. Analytical formulation

Abstract: The present work is primarily concerned with the analytical formulation of governing equations for flow of incompressible fluids through compacting porous media and their application to vacuum infusion (VI) of composite materials. The literature on VI and the effects of compacting media on permeability and flow is reviewed. A complete development of the proposed governing equation is shown along with a suggested numerical solution. The proposed model is subsequently used to quantify the effect of process parameters such as inlet and outlet pressures, fibre architecture and lay-up. Implications for industrial production are discussed.

Polyimide/BaTiO3 composites with controllable dielectric properties

Abstract: The polyimide/barium titanate (BaTiO3) composites were successfully synthesized through a colloidal process. In this process, the preparing suspension of fine BaTiO3 particles in poly(amic acid) solution, the film casting of the suspension, and the imidization with heat treatment are involved in sequence. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), thermal-gravimetric analyses (TGA) and electrometer were used to characterize the structure and properties of the obtained composites. It was found that the BaTiO3 particles in the size of 100 nm were dispersed homogeneously in the polyimide matrix without aggregation. Thermal stability of the composites could be improved with the incorporation of BaTiO3. The dielectric constant (e) and the dielectric loss (tan δ) of these composites increased with the increase of the volume fraction of BaTiO3 particles. e, tan δ was 35 and 0.0082, respectively at 10 kHz as the composite contains 50 vol% BaTiO3. Also, the dielectric properties of the composites displayed good stability within a wide range of temperature or frequency.

Resistance welding of thermoplastic composites - an overview

Abstract: This paper presents an extensive overview of resistance welding of thermoplastic composites. The objective is to provide a deeper insight into the nature of the resistance welding process and a summary of the vast experimental investigative effort put into it over the years. The main focus is set on the parameters that govern the welding process and the principal phenomena that affect the quality of the joint. The standard experimental procedure, the experimental set-up and the main evaluation methods are also looked at in detail. Finally, several alternative resistance welding methods that involve non-thermoplastic materials and offer possibilities for future applications are briefly reviewed.

Delamination resistant laminates by Z-Fiber® pinning: Part I manufacture and fracture performance

Abstract: The article provides an introduction into the technology of through-the-thickness reinforcement of thermosetting composites by the insertion of Z-Fiber® pins. The manufacture of the raw materials is described, as is the method of pin insertion. Delamination tests samples were prepared from unidirectional continuous carbon fibre/epoxy prepreg (IMS/924), made into 3 mm thick unidirectional laminates with and without a block of Z-pins in the crack path. Fracture testing was carried out under Mode I (standard DCB test) and Mode II (3-point-ENF) loading conditions. The data presented here were obtained by using the commonly used data reduction schemes; the applicability of such data treatment is considered.

Micromechanical FE analysis of UD fibre-reinforced composites with fibres distributed at random over the transverse cross-section

Abstract: A methodology is established in this paper for the micromechanical finite element analysis of unidirectionally fibre-reinforced composites having fibres distributed at random over the transverse cross-section, a characteristic that most UD composites in engineering applications bear. Regular packing is often assumed in micromechanical analyses. While this simplifies the analysis greatly, it will never provide understanding of aspects of material's behaviour directly related to the distributions of fibres, such as clustering of fibres over the transverse cross-section. Within the limited results available in the literature, it appears to be difficult to assess the errors due to unrealistic boundary conditions prescribed to the representative volume element (RVE) under consideration, as a uniform displacement or traction along the border of a regularly shaped RVE would not be realistically achievable in general. According to the St Venant's principle, one would expect the effects of such incorrectly prescribed boundary conditions affect only a limited zone next to the boundary. A quantitative analysis on the effects of such incorrectly prescribed boundary conditions is provided in this paper using a numerical means. It has been found that the error due to incorrect boundary conditions diminishes over a couple of times of the distance of fibre spacing (centre-to-centre). It is generally true that uniformly prescribed displacement gives a short decay length than that associated with uniformly prescribed traction. Applying this to the analysis of UD composites with fibres distributed at random over the transverse cross-section, an RVE can be introduced and analysed, within which a sub-domain is defined appropriately distant to the boundary. The deformation and stresses obtained in this sub-domain can be considered to be free from the effects of incorrectly prescribed boundary conditions for the RVE analysed and the results extracted only from this sub-domain can, therefore, be considered correct representation of the behaviour of materials of this type. Examples of application of such a procedure are shown in the paper, through which some interesting perspectives of UD composites with fibres distributed at random over the transverse cross-section have been revealed. A significant improvement in predicted transverse Young's modulus has been obtained as compared with those by assuming regular packing in relation to experimental data. Rather different hardening characteristics between the UD composites with regular and random fibre packing have been found in the plastic regime.

Low-velocity impact damage progression in woven E-glass composite systems

Abstract: The objective of this experimental study is to obtain a detailed understanding of the effects of reinforcement geometry on damage progression in woven composite panels under repeated drop-weight impact loading conditions. The composite systems included a 2D plain-woven laminate, a 3D orthogonally woven monolith, and a biaxially reinforced warp-knit. The radial spread of damage was smallest for the 2D laminates and largest for the 3D woven composites. The 3D composites had the greatest resistance to penetration and dissipated more total energy than the other systems. This is due to unique energy absorption mechanisms, which involve the crimped portion of z-tows in the 3D composites, and implies that failure can be controlled by manipulation of the properties of the z-tows. The 3D systems provide both an inherent capability to dissipate energy over a larger area and a greater perforation strength than other systems with comparable areal densities and fiber-volume-fractions.

Low velocity impact behavior of composite sandwich panels

Abstract: Composite sandwich structures are susceptible to low velocity impact damage and thorough characterization of the loading and damage process during impact is important. The objective of this work is to study experimentally the low velocity impact behavior of sandwich panels consisting of woven carbon/epoxy facesheets and a PVC foam core. Instrumented panels were impacted with a drop mass setup and the load, strain, and deflection histories were recorded. Damage was characterized and quantified after the test. Results were compared with those of an equivalent static loading and showed that low velocity impact was generally quasi-static in nature except for localized damage. A straightforward peak impact load estimation method gave good agreement with experimental results. A contact force–indentation relationship was also investigated for the static loading case. Experimental results were compared with analytical and finite element model analysis to determine their effectiveness in predicting the indentation behavior of the sandwich panel.

Mechanical properties improvements in two-phase and three-phase composites using carbon nano-fiber dispersed resin

Abstract: Static strength tests were carried out for cured carbon nano-fiber (CNF) dispersed resin as tow-phase composites and for CFRP laminates using CNF dispersed resin as three-phase composites. To obtain these CFRP laminates, the CNF dispersed resin was impregnated to CF reinforcement and cured by hot press. The CNF used was a cup-stacked type of nano-fiber, CARBERE®, made by GSI CREOS Corporation, Japan. Two CNF aspect ratios of 10 and 50 were employed. These fiber lengths of the CNF were controlled about 1000 nm (AR10) and 5000 nm (AR50), respectively. The CNF was dispersed to EPIKOTE 827® epoxy resin in two values of CNF weight ratios, 5 and 10% to the resin. TORAYCA® C6343 plain woven fabric was used for reinforcement of the CFRP laminates. The cure condition with the agent of aromatic amine EPIKURE W® was 100 °C for two hours followed by a post cure of 175 °C for 4 h. The static strength tests led to the conclusion that the dispersion of CNF into epoxy improves mechanical properties of the tow-phase composites, and that CFRP laminates with CNF dispersed resin also exhibit higher compressive strength than CFRP laminates without CNF as control. Possibilities of improvement in mechanical properties were confirmed in the two and three-phase composites. Moreover, a proportional tendency in strength improvements to CNF weight content was found in the two present composites so far in the present test results.

A torque and thrust prediction model for drilling of composite materials

Abstract: In this paper, the authors present a mechanistic model for use in predicting thrust and torque during composite materials drilling. They specify the number of coefficients to be experimentally determined and provide a detailed analysis of the problems associated with the action of the chisel edge. Their theoretical approach was suggested by the observation that during a drilling process the prerequisites for orthogonal fiber cutting are met for an infinitesimal instant. Hence their decision to simulate a conventional drilling process by developing a model with only two semi-empirical coefficients. The method proposed is comparatively user-friendly since the semi-empirical coefficients concerned can be determined without difficulty by means of simple linear relations. The results obtained were satisfactorily validated by comparing the test findings with the corresponding theoretical data and it is possible to conclude that the model affords a focused approach to the definition of the most appropriate drill geometry and cutting parameters in composite materials drilling.

Novel biodegradable composites based on treated lignocellulosic waste flour as filler. Part I. Surface chemical modification and characterization of waste flour

Abstract: The chemical modification of lignocellulosic materials based on waste flour, using catalyst and solvent-free reactions with acetic and propionic anhydride, was studied. The reaction of esterification between the acetyl/propionyl groups and the hydroxyl groups of the flour was confirmed by Fourier transform infrared (FTIR) analysis, while its extent was assessed by titration. Thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the thermal stability, crystallinity and surface morphology of the untreated and esterified flours. The ester content of all three flours was higher for the treatment with acetic as compared to propionic anhydride. The highest extent of the esterification was achieved for the olive husk flour due to its high lignin/hemicelluloses content. The esterified flours exhibited a decreased hydrophilicity as indicated by their moisture content reduction, while their thermal stability was slightly decreased. It was also shown that the flour crystallinity decreased slightly as a result of esterification.

Effect of clustering on particle pushing and solidification behaviour in TiB2 reinforced aluminium PMMCs

Abstract: The behaviour of titanium diboride particles in molten aluminium was investigated by conducting casting experiments at different cooling rates and particle addition levels, starting with a master alloy containing in situ formed TiB2 particles. The particle pushing/engulfment phenomena and particle clustering effects were studied for two matrix alloy systems, commercial purity (CP) aluminium and an Al-4% Mg (A514) alloy. The critical velocity, which is the solidification velocity below which the particles are pushed and above which they are engulfed, was calculated. The results were validated against the predictions of the models available in the literature. The assumptions, limitations and comparative success of the different models were discussed. The critical velocity observed in CP-Al ranged between 4 and 8 μm/s at low particulate concentration ( The hypothesis that particle clustering could result in erroneous critical velocity predictions was explored and it was concluded that particle clustering must be included for accurate prediction of particle pushing in these alloys.

Numerical analysis of cure temperature and internal stresses in thin and thick RTM parts

Abstract: Resin transfer molding (RTM) is a widely used manufacturing technique of composite parts. Proper selection of processing parameters is critical in order to produce successful molding and to obtain a good part. Notably, when thermosetting resins are processed, the shrinkage that results from resin polymerization increases the complexity of the problem. Numerical prediction of internal stresses during composite manufacturing has three objectives: (1) to improve knowledge about the process; (2) to analyze the effects of processing parameters on the mechanical integrity of the part; and (3) to validate the principles of thermal optimization. This investigation aims to predict residual stresses and part deformation (i.e. warpage) in thin and thick composites. Accurate characterization of materials is essential for effective numerical analysis of phenomena which determine the generation of processing stresses. For this purpose, a reaction kinetics model of the resin is presented, together with a description of mechanical properties as a function of the degree of polymerization and glass transition temperature. A linear model is used to predict volume changes in glass–polyester composites. A finite difference analysis is used to simulate the effect of thermal and rheological changes during the processing of sample plates. Classical laminate theory is applied to calculate the internal stresses that result from processing conditions. These stresses are compared to determine different curing strategies for thick composite parts. Finally, a thermal optimization algorithm is applied to demonstrate the advantages of transient heating and cooling, to minimize processing stresses and avoid thermal degradation of the material or composite delamination.

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 6. the properties of injection moulded long fibre PP at high fibre content

Abstract: The results of an investigation of the mechanical performance of injection moulded long glass fibre reinforced polypropylene with a glass fibre content in the range 0–73 wt% are presented. The composite modulus exhibited a linear dependence on fibre content over the full range of the study. Composite strength and impact resistance exhibited a maximum in performance in the 40–50 wt% reinforcement content range. The residual fibre length and fibre orientation in the samples has also been characterised. These parameters were also found to be fibre concentration dependent. Modelling of the composite strength using the measured fibre length and orientation data did enable a maximum in strength to be predicted. However, the position and absolute level of the predicted maximum did not correlate well with the experimental data. Further analysis indicated that deeper investigation of the dependence of the interfacial shear strength and fibre stress at composite failure on the fibre content are required to fully elucidate these results.

The influence of void content on the structural flexural performance of unidirectional glass fibre reinforced polypropylene composites

Abstract: Voids are often present in composite materials and have, as may be expected, negative effects on the mechanical properties. As a consequence, it is regarded to be important to optimise manufacturing techniques of composite materials towards minimum void content. However, it is not obvious that voids also have negative effects on the properties of a structure since those are not just determined by the properties of the material, but also by the dimensions. For a given mass of material, void will increase the dimensions and as a result, for example, the moment of inertia. Thus in principle, it is possible that void may actually have positive effects on the properties of a structure. In this paper this hypothesis is evaluated by studying the influence of void content on the flexural properties of beams manufactured by compression moulding multiple unidirectional commingled glass/polypropylene fibre tows. By varying the time under moulding pressure, beams with void contents between 1 and 14% could be manufactured (with the mass, width and length fixed, and thus only the height free). As expected, voids were found to have a negative effect on the flexural modulus and strength, which both decreased by about 1.5% for each 1% of voids. However, what is more interesting is the fact that voids actually had a clear positive effect on the beam stiffness EI ; which increased by about 2% for each 1% of voids. For example, beams which contained 14% porosity exhibited about 28% higher EI than beams with less than 1% porosity. Moreover, the flexural failure load did not decrease up to a void content of 14%. On the contrary, a least square fit suggested a weak increase with void content. It can therefore be concluded that, in this case, voids actually have positive effects on the structural flexural performance.

Unsaturated polyester composites reinforced with flax fibers: effect of cold plasma and autoclave treatments on mechanical and permeation properties

Abstract: In composite materials, fibers used as reinforcements are usually synthetic fibers such as glass. Since several years, for economic and environmental reasons, there has been an increasing interest in using plant fibers in composite systems. In this work, polyester composites reinforced by flax fibers submitted to helium cold plasma and/or autoclave treatments were investigated by means of water permeation measurements and mechanical tests. The analysis of the permeation and mechanical results has shown that plasma treatment improves fiber/matrix adhesion while autoclave treatment reduces water solubility in the fibers. For reinforced composites, therefore, autoclave treatment is more efficient in terms of water permeability and plasma treatment gives better stiffness in terms of mechanical properties. However, autoclave treatment followed by plasma treatment on flax fibers is recommended to obtain a good compromise for increasing both moisture resistance and stiffness of reinforced composites.

Dielectric response and relaxation phenomena in composites of epoxy resin with BaTiO3 particles

Abstract: Dielectric properties and relaxation phenomena of composite materials (epoxy resin—barium titanate) were studied as a function of ceramic content. The composites were prepared mixing both components and pouring them into suitable moulds. Some composites were made using tetrahydrofuran (THF) because it helped to reduce matrix viscosity, facilitating the system mixture. Dielectric measurements showed that dielectric constant of composites depended on the filler amount, the temperature and the frequency. Dielectric losses also were influenced by the temperature and by the frequency through relaxation process, even though volume fractions did not have influence on losses.

Delamination monitoring of laminated composites subjected to low-velocity impact using small-diameter FBG sensors

Abstract: Small-diameter fiber Bragg grating (FBG) sensors were applied for the monitoring of delamination induced by low-velocity impact. The FBG sensors were embedded into carbon fiber reinforced plastic (CFRP) laminates [04/904/04]. Using a drop-weight impact tester, an impact loading was applied to the laminates at four impact energy levels. After the impact tests, the internal damages including delaminations were observed by ultrasonic C-scan, and the reflection spectra from the embedded FBG sensors were measured. The form of the spectrum changed sensitively depending on the delamination size. Furthermore, the spectra were calculated theoretically for confirmation of the measured spectra. Since the change in the measured spectrum was consistent with that in the calculated spectrum, the relationship between the delamination size and the form of the spectrum could be clarified. From the results, the present method using small-diameter FBG sensors was found to be effective for the monitoring of the delamination.

Carbon composites based on multi-axial multi-ply stitched preforms. Part 4. Mechanical properties of composites and damage observation

Abstract: Multi-axial multi-ply fabric (MMF) or non-crimp stitched carbon fabric (NCF) reinforced epoxy composites, produced by the resin transfer moulding process are described. The mechanical properties of the composites—tensile, in-plane shear and out-of-plane shear—are measured in a number of orientations relative to the stitching direction for different NCF based laminates: quadriaxial, biaxial ±45 and 0/90°, as well as unidirectional. As expected, the quadriaxial laminates show a quasi-isotropic behaviour while biaxial laminates show an an-isotropic behaviour. The mechanical properties of the biaxial ±45° laminates are similar to the 0/90° laminates in the corresponding directions. The in-plane shear properties of the materials are strongly dependent on the amount of reinforcement in the +45 and −45° direction relative to the loading direction. Absence of a significant difference in stiffness between experimental results and classical laminate theory predictions reveals that stitching has minor effects on the stiffness of this material. Tensile tests on 0/90° laminates were accompanied by acoustic emission registration. The results were correlated with X-ray and C-Scan damage observation. Early (at 0.3% applied strain for load in fibre direction) initiation of damage has been detected. Damage sites are correlated with the resin-rich zones created by the stitching.

Finite element forming simulation for non-crimp fabrics using a non-orthogonal constitutive equation

Abstract: The forming behaviour of non-crimp fabric (NCF) was simulated using finite element (FE) analysis incorporating a non-orthogonal constitutive model. NCFs feature asymmetric shear behaviour caused by the stitching used to hold the tows together. This asymmetric shear property causes an asymmetric draping pattern of NCF, even when formed over a symmetrical hemispherical forming tool. Current work focuses on the feasibility of a continuum mechanics model to simulate the asymmetric forming behaviour of NCF. The constitutive equation consists of two parts: the tensile contribution from fibre reinforcement and the shear stiffness. For the fibre directional properties, a non-orthogonal equation originally developed for woven fabric was adopted. The shear stiffness was modelled through a constitutive equation incorporating picture-frame shear data. Both a picture-frame shear test and forming of NCF over a hemisphere tool were simulated by commercial finite element software with the current constitutive model implemented within a user material subroutine. The virtual picture-frame test confirmed the validity of the constitutive equation in simulating planar deformation behaviour of NCF. Furthermore, the numerical analysis of hemispherical forming suggests that increasing blank-holder force decreases the asymmetry of the draped pattern.