Single-fibre Model Composites Research Articles

Characterization of a maleic anhydride-modified polypropylene as an adhesion promoter for glass fiber composites

Isothermal and nonisothermal crystallizations of isotactic polypropylene (iPP), maleic anhydride (MAH)-grafted PP, and MAH-modified iPP were studied by differential scanning calorimetry (DSC), to evaluate the influence of a small amount of MAH-grafted PP in iPP on its crystallization behavior. Isothermal crystallization was followed in the temperature range from 391 K to 403 K, and the rate constant and Avrami exponents were determined. Nonisothermal crystallization was carried out at different cooling rates (1-20 K/min). It was found that the crystallization kinetics of iPP was significantly altered by modification with the MAH-grafted polymer. A decreased equilibrium melting temperature, as well as decreased surface energy of folding and critical dimensions of a growing nucleus, was determined for the MAH-modified iPP, indicating faster growth of lamellae and a higher rate of crystallization. The improved nucleation ability of the modified polymer was shown to cause a shift in the crystallization peak temperature towards higher values (from 393.7 K to 399.6 K, at a cooling rate of 1 K/min), resulting in crystal structures less disposed to recrystallization. Model composites of iPP and MAH-modified iPP with glass fibers were also analysed. The apparent shear strength of single-fiber model composites with MAH-modified iPP was drastically increased compared with homo-iPP.

Definition and measurement of the shear-lag parameter, β, as an index of the stress transfer efficiency in polymer composites

The shear-lag parameter, β, employed in various problems of shear-lag analysis of composites is an unknown parameter which, in certain cases, is impossible to define. In this paper, a new methodology is proposed for the definition and subsequent experimental measurement of β for various single-fibre model composites. It is argued that, if β is defined as a fitting parameter for the solution of the shear-lag differential equation, then it can effectively serve as a stress-transfer efficiency index. The dependence of β upon the conditions prevailing at the fibre–matrix interface will be demonstrated by measuring β as a function of the fibre sizing in a carbon–epoxy composite system. © 1998 Chapman & Hall

Determination of the interfacial normal strength using single fibre model composites

A technique is presented which can be used to measure the interfacial normal strength (INS) in transversely loaded composites. The technique combines the measurement of local deformations during tests in a scanning electron microscope and numerical simulations, in which the measurements are used as boundary conditions. Deformations are measured by placing a grid of markers on the surface of a single fibre specimen. The displacements of these markers are measured during loading of the specimen. Numerical simulations show that the radial and tangential interface stresses increase towards the specimen surface, causing the initiation of debonding at the surface. This effect is supported by tests under an optical microscope. The INS is defined to be the maximum radial interface stress just before the initiation of debonding. The INS values are determined for carbon fibres with surface treatment levels varying from 0% to 100% commercial treatment level. The results show an increasing INS for increasing treatment level. Based on the presented results, it can be concluded that the presented technique can be used for the measurement of the interfacial normal stress.

Photo-elastic analysis of fibre-reinforced model composite materials

The phenomena of matrix cracking and fibre/matrix load transfer in single-fibre model composites have been studied. A special device was developed which allows in situ observation of fibre fracture during loading. Load- and stroke-controlled stress/strain curves of the test samples were measured. The local load and strain distributions were determined by photo-elastic analyses. This allowed a study of the micromechanical interdependence between the local fibre/matrix load transfer and crack propagation in the matrix starting from the location of fibre failure.

Stress distribution in single-fibre model composites with perfect bonding

A new method is presented for calculation of the stress distribution in a fragmented single fibre embedded in an epoxy matrix. The proposed method is based on the shear-lag analysis in order to describe both the dependences of the normal stress distribution in the fibre and the shear stress distribution in the fibre/matrix interface. The influence of the non-linear elastic behaviour of the epoxy matrix, externally applied stress and the fibre fragment length will be discussed.

Characterization of the surface and interphase of plasma-treated HM carbon fibres

The surface chemistry and morphology of HM carbon fibres, treated with pulse-voltage excited air, oxygen, nitrogen and acrylonitrile plasmas, are characterized by high-resolution X-ray photoelectron spectroscopy and transmission electron microscopy. Oxygen- and nitrogen-containing species are generated on the surface under plasma treatment without significant changes in the fibre morphology. Comparable interfacial shear strengths are found for single-fibre model composites prepared with amine-cured epoxy resin, with a highest value of 61.4 MPa for air-plasma-treated fibres. The surface chemistry of the fibres if found to influence the degree of epoxy consumption at the earlier stages of the cure process, as determined by Fourier transform infra-red microscopy and differential scanning calorimetry.

Critical length and distribution of fibrous fillers and composites

The adhesion of UKN-01 carbon fibers to a PA-12 polymeric matrix is investigated by the method of testing single-fiber model composites. The distribution of critical fiber length is constructed from measurements of fragment lengths formed in the final stage of testing. Variation in the distribution of critical length is established as a result of surface treatment of the reinforcing fiber. A bimodal distribution corresponds to the initial carbon fiber, and a monomodal distribution to the treated fiber. This is explained by replacement of a physicomechanical type of interaction of the phase interface by a physicochemical interaction owing to electrochemical treatment of the fiber surface. Analysis of the results indicated that each type of interphase interaction has its own characteristic critical length. The selection of critical length has been confirmed for calculation of the interphase shear strength by the Kelly-Tyson formula.

Study of carbon-fibre strain in model composites by raman spectroscopy

A linear shift of carbon-fibre Raman frequencies is created by strain applied to their molecular structure. The magnitude of this shift is the greater, the better organised is the fibre structure—from −7cm −1/% for the T800HB intermediate modulus fibre to −11 cm −1/% for the M40B high modulus fibre. Raman spectroscopy is a useful tool for identifying the different interfacial load transfer types from strain profile observation in single-fibre model composites under tensile loading i.e. fibre debonding with (HMS fibre) or without (M40B fibre) friction, and elastic stress transfer well described by the Cox model. The interfacial shear stress distribution, the possible friction coefficient, and the longitudinal thermal residual strain—found to be smaller than the theoretical value—are derived from the strain profiles.

Interfacial micromechanics in thermoplastic and thermosetting matrix carbon fibre composites

The interfacial micromechanics of carbon fibres in thermoplastic matrices [poly(methyl methacrylate) (PMMA) and polycarbonate (PC)] has been investigated by determining the distribution of interfacial shear stress along fibres in single-fibre model composites using Raman spectroscopy. The variations of fibre strain with position along the fibre in these composites are almost linear, indicating that stress transfer from matrix to fibre in the system is predominantly by frictional shear. It was found that the maximum values of interfacial shear strength for the PMMA and PC model composites are 3.1 MPa and 8.4 MPa, respectively, and much lower than the value of 30 MPa obtained for the same fibres in a thermosetting epoxy resin matrix. These low values of interfacial shear stress in thermoplastic systems can be explained by the lack of chemical bonding between the fibre and matrices, and possibly the effect of residual solvent. The interfacial adhesion in the systems stems primarily from mechanical interlocking, which can be enhanced by preparing the composites at higher temperatures. It is shown for PMMA that the maximum interfacial shear stress correlates very well with the radial pressure on the fibre as a result of thermal mismatch between the fibre and matrix.

Interfacial failure in ceramic fibre/glass composites

Abstract The interfacial micromechanics have been investigated for single-fibre model composites consisting of single PRD-166 alumina/zirconia fibres, both carbon-coated and uncoated, in a glass matrix using fluorescence spectroscopy to follow axial fibre deformation. It has been shown that the presence of the carbon coating leads to a weaker interface. The fibre—matrix interface for the coated fibres is found to break down at lower levels of matrix strain than for the uncoated fibres when the composite is subjected to axial tensile deformation. The maximum interfacial shear stress is also found to be lower for the carbon-coated fibres.

Fragmentation of aramid fibres in single-fibre model composites

Raman spectroscopy has been used to monitor the state of axial stress along fragmented, high-modulus Kevlar 149 aramid fibres in an epoxy resin matrix by monitoring the peak position of the strain-sensitive 1610 cm−1 aramid Raman band along individual fragments. It is shown that the interfacial shear stress along each fragment, derived from the strain distribution profiles, is not constant as assumed by conventional fragmentation analysis. The fragmentation process of as-received Kevlar 149 fibres is compared to that of irradiated Kevlar 149 fibres exposed to ultraviolet light where the tensile strength and modulus of the fibres have been reduced. It is found that the derived interfacial shear stress and interfacial shear strength values are higher for those fibres exposed to ultraviolet light compared with the as-received fibres. It is also clearly demonstrated that the values of interfacial shear strength calculated at high matrix strains from conventional fragmentation analysis are considerably lower than the maximum value of interfacial shear stress prior to fibre fracture that was found to be close to the shear yield stress of the resin matrix. Hence the determination of the interfacial shear strength following the saturation of the fragmentation process may give rise to misleading results.

The effect of Cyclic Loading on the micromechanics of the Interface

The effect of fatigue on the interfacial properties of carbon fibre model composites was investigated, employing the fragmentation test. Continuous and discontinuous single carbon fibre model composites were manufactured and subjected to strain controlled fatigue loading. Whilst no effect of cyclic loading on the fibre/matrix interface of continuous fibres was detected, a significant reduction in the interfacial strength of the discontinuous fibre specimens was measured.

Effect of temperature and strain rate on interfacial shear stress transfer in carbon/epoxy model composites

The amount of stress transferred in a composite from the matrix to the fibre has been determined in an epoxy/carbon system by using a fragmentation test on single-fibre model composites (microcomposites), as a function of temperature and strain rate. Experiments, performed on microcomposites containing commercial carbon fibres, with and without sizing agent, and an epoxy resin of low glass transition temperature (T g), indicate that the interfacial shear strength, 〈τ〉, decreases sharply as the test temperature approaches the T g of the constituent surrounding the fibre. Moreover, while in desized fibre composites the interfacial shear strengths compare well with the matrix shear strength, in sized fibre composites the measured 〈τ〉 values appear to be strongly influenced by the presence of an interphase. 〈τ〉 values measured for sized fiber microcomposites are higher than the matrix shear strength. The contribution to adhesion of frictional stresses, exerted by the matrix on the fibre as a result of thermal and Poisson contractions, appears to be negligible with respect to the 〈τ〉 values obtained from the fragmentation test.

The difference between transcrystallization and shear-induced cylindritic crystallization in fibre-reinforced polypropylene composites

Institut f(Jr Verbundwerkstoffe GmbH, Universit#t Kaiserslautern, Pf.3049, D-67663 Kaiserslautern, Germany It was recognized early on that the fibre/matrix interphase (or interface) strongly influences the mechanical properties of composite materials. One of the current research directions in the field of composites with semicrystalline thermoplastic ma- trices is devoted to the question of whether or not particular supermolecular arrangements of the ma- trix in the vicinity of reinforcing fibres can improve the overall mechanical performance. Interest is focused particularly on the potential benefits of a transcrystalline interphase [1-4], since it is believed that this kind of "physical coupling" would enhance the stress transfer between the fibre and the matrix. On the effects of the transcrystalline interlayer, however, rather contradictory reports have appeared. The formation of the transcrystalline interphase layer, originated by fibre surface induced heterogeneous nucleation, is generally studied by single fibre model composites [2, 3]. This has also been the case for glass fibre (GF) reinforced polypropylene (PP) [5-8], where a transcrystaUine- like supermolecular structure could only be pro- duced when the crystallizing PP melt was sheared by pulling the embedded GF. In our recent work [9, 10], it was demonstrated that the columnar structure developed by melt shearing is not tran- scrystalline, but row-nucleated cylindritic. We have shown further [9, 10] that when both the crystalliza- tion (Tc) and pulling temperature (Tpull) are set in the range of the formation of the/3-PP, i.e. between T,~ ~ 100 °C and T~, ~ 140 °C [11, 12], a /3-rich columnar structure appears. Partial melting of this supermolecular formation revealed a layer attached to the pulled GF. This sheared layer of oi-PP contained ol-row nuclei and originated/3-crystalliza- tion (c~-fl bifurcation of growth, [11]). So under the conditions To~ < To, Tpun < T~o: melt shearing by pulling of GF resulted in a complex polymorphous structure containing both the o~- (row-nuclei along the GF) and fl-form of PP (grown up onto the oL-row nuclei). Due to this "/3-overgrowth" the oc-layer along the fibre surface can be resolved on the optical level only after separate melting of the/3-phase [10]. In the cited works on the shear induced crystalliza- tion of PP by pulling the fibre [5, 6, 9, 10] the GF proved to be "inert", i.e. it did not show any o-nucleation ability. Therefore it seemed that it would be very interesting to study how the above scenario changes when ol-nucleating fibres are used. 0261-8028 © 1994 Chapman & Hall In this case the conditions of both transcrystalliza- tion (due to heterogeneous 0l-nucleation) and row- induced cylindritic crystallization (due to melt shear- ing) are met. One can easily differentiate between them by taking into account the polymorphism of the PP and thus choosing the following crystalliza- tion conditions: T~ < To, Tpull < T~. Under these conditions the development of an o~-transcrystalline front is evidence of transcrystallization, while the formation of a fl-PP rich columnar structure indic- ates cylindritic crystallization [9, 10]. As oL-nucleating fibre poly(ethylene-terephtha- late) (PET) fibre taken from a commingled PET/PP yarn supplied by Toyobo Co. (Japan) was used. The oL-nucleating ability of PET has been reported in several works (cf. [7, 13, 14]). The isotactic PP used in this study was a general-purpose injection-mould- ing grade (Tipplen H-523, Tisza Chemical Works, Hungary). The crystallization and melting behaviour of the GF/PP model composites were studied with a Leitz polarizing optical microscope equipped with a Mettler hot stage. All further experimental details can be found in our previous work [9, 10]. Figs 1-3 show the effect of PET fibre on the isothermal crystallization of PP in its quiescent melt. Fig. 1 demonstrates the supermolecular structures formed in 2-step isothermal crystallization. At higher T0 (Td = 134 °C, t~ = 30 min) only spherul- ites were grown sporadically (Fig. la). Reducing T~ after 30 min crystallization time (t~) to To2 -- 124 °C gives rise to o:-transcrystallization of PP along the PET fibre (Fig. lb and c). A well developed transcrystalline layer can be obtained in 1-step isothermal crystallization, provided Tc is low enough (To = 124 °C, Fig. 2). This indicates that for the transcrystallization growth necessary high surface nucleation density on the PET fibre is guaranteed only at Tc ~< 130 °C in the given PP/PET composi- tion. The strong ol-nucleating ability of the PET fibre can be demonstrated by using a/3-nucleated PP matrix (Fig. 3). Fig. 3 shows that the PET fibre preserves its c~-nucleating ability even in fl-PP (achieved by using a proprietary selective fl-nucleat- ing agent (in 0.1 wt %). The oL-transcrystalline layer on the PET fibre is obvious, whereas/3-spherulites characterize the bulk (Fig. 3a). This structure becomes even more striking after selective melting (cf. later) of the /3-phase at Tf = 158 °C (Fig. 3b). Figs 1-3 show that the PET fibre acts as an

Evaluation of interface parameters in push-out and pull-out tests

Push-out and pull-out tests with single-fibre model composites are performed to evaluate the frictional parameters of the interface. Analysis of the experimental data according to models based on the assumption of an ideal cylindrically shaped fibre does not satisfy all observed phenomena. Thus, a model is proposed to include the effects of roughness interaction during fibre sliding. The predictions of the model fit the experimental results very well.