Single Fiber Composite Research Articles

A method of stress analysis for interfacial property evaluation in thermoplastic composites

An analysis of stress transfer between the fiber and matrix in a single fiber reinforced composite which is useful to evaluate the effect of the properties of the interphase layer between the fibers and matrix is presented. The model is based on a concentric cylinder and considers an interphase surrounding the fiber to simulate a surface-treated layer as the product of processing conditions of short fiber reinforced thermoplastic composites. Stress and displacement fields in a single fiber composite under uniaxial tension in fiber direction are analyzed. The analysis presented here is used to predict the deformation of a short fiber in a thermoplastic composite. The analysis is compared with the experimental results obtained by using the micro-grid method.

Experimental studies on the interfacial shear-transfer mechanism in discontinuous glass-fibre composites

Interfacial stress/strain distributions in discontinuous glass-fibre/epoxy composites have been determined experimentally for the first time by the simultaneous use of tensile deformation measurements and Raman spectroscopy. Model single-fibre composites were used and the glass fibres were coated with a diacetylene-urethane copolymer which was thermally cross-polymerised. During composite deformation, the stress-induced band shifts of C≡C in the polydiacetylene phases of the coating were used to map the strain distribution along a fibre as a function of overall matrix strain up to fibre fracture. Measured strain distributions in fibres were analysed by using classical shear-lag theory to determine interfacial shear-stress distributions. Fibre fragmentation was monitored in detail up to saturation, and the limiting value of interfacial shear stress is shown to be determined by the shear yield stress of the matrix.

Measuring Interface Strength in Single Fiber Composites: The Effect of Stress Concentrations

ABSTRACTFiber-matrix interface strength is known to be a critical factor in controlling the long-term performance of structural composites. This parameter is often obtained by using the average fragment length data generated from the single-fiber fragmentation test (SFFT). The interfacial shear strength is then determined by using this data in a micro-mechanics model that describes the shear-stress transfer process between the matrix and the fiber. Recently, a non-linear viscoelastic micro-mechanics model was developed to more accurately account for the matrix material properties. This new model indicates that the interface strength is dependent on the testing rate. Experimentally, it has been shown that the final fragment length distribution in some systems is dependent on the testing rate. However, data analysis using the new model indicates that the distribution change with testing rate is promoted by the presence of high stress concentrations at the end of the fiber fragments. From the model, these stress concentrations were found to exist at very low strain values. Experimentally, the fragment distributions obtained from specimens tested by different testing rates were found to be significantly different at strain values well below the strain values required to complete the test. These results are consistent with the research of Jahankhani and Galiotis and finite element calculations performed by Carrara and McGarry. These authors concluded that stress concentrations can promote failure of the fiber-matrix interface on the molecular level. Our results support this conclusion. In addition, our research results suggest that altering the SFFT testing rate can lower the magnitude of these stress concentrations and minimize failure of the fiber-matrix interface.

Analysis of acoustic emission signals resulting from fiber breakage in single fiber composites

AbstractAcoustic emission techniques are able to sense micro failures in loaded structures for a wide range of materials. Moreover, a spread application of such techniques to structural health monitoring requires further efforts in the signal analysis in order to assess the relationships between the micromechanics of the failure and the signal features as recorded by a transducer. In the case of polymer matrix composites, the main sources of acoustic emission signals are fiber breakage, matrix cracking, and fiber/matrix debonding. In this work, single fiber composites have been tested to produce fiber breakage acoustic emission events. Three different fiber/matrix systems have been investigated: PCA polyster matrix/carbon fiber (6 μm diameter), ECA epoxy matrix/carbon fiber (6 μm diameter), and ECB epoxy matrix/carbon fiber (12 μm diameter). Acoustic emission signals recorded through a wide band transducer have been analyzed in the frequency domain in order to study the amplitude spectra characteristics. The irregular shape of the amplitude spectra suggested the fractal dimension as a feature. The Richardson fractal dimension resulted in a key parameter in the signal identification among the different fiber/matrix systems: DPCA = 1.28 ± 0.02, DECA = 1.46 ± 0.03, DECB = 1.42 ± 0.02. On the basis of this finding, the shape of the amplitude spectra has been analyzed. An algorithm able to extract shape similarities from a class of spectra has been developed. It permits the definition of the representative spectra together with the identification of the relevant frequencies for the fixed fiber/matrix system.

Stress Transfer Model for Single Fibre and Platelet Composites

A model for elastic stress transfer across the interface in composites has been developed in particular to overcome the known problem of the matrix effective radius. Stresses were determined using a two dimensional method by introducing a novel hypothesis of structural response of the matrix. This is in contrast to previous approaches, which mainly consider that the axial derivative of axial stress does not depend upon the radius. This leads to a new form of the stress transfer equation, free of an adjustable parameter. This analysis was applied to linear elastic single fibre and single platelet composites. The comparison of the two reinforcements allowed the identification of a structural parameter for which the two geometries are shown to be equivalent. The model was validated using literature results obtained by Raman spectroscopy means or photo-elastic methods. The agreement between measured data and the model showed the relevance of the matrix structural response approach to stress transfer.

Fracture resistance of short-glass-fiber-reinforced and short-carbon-fiber-reinforced polypropylene under Charpy impact load and its dependence on processing

Abstract In this paper, short-glass-fiber-reinforced and short-carbon-fiber-reinforced polypropylene composites were investigated with respect to the work of fracture (WOF), here viz the notched Charpy impact energy. Short glass fibers and short carbon fibers were incorporated into polypropylene with a twin-screw extruder and all of the specimens were injection molded into dumbbell-shaped tensile bars in a twin-screw injection-molding machine. Rectangular impact bars were obtained from tensile bars by removing the two clamping parts. Charpy impact tests were performed with a Charpy impact tester on specimens with V-notches. The values of the notched Charpy impact energy were obtained from each group of eight specimens. The effects of glass and carbon fiber volume fractions on their lengths in single and hybrid short fiber composites were investigated. The composite WOF was studied qualitatively by taking into account the effects of fiber volume fraction and fiber length distributions using the total WOF theory. Fiber length distributions have been previously shown to depend on the processing conditions. On the other hand, the composite impact resistance was shown to depend on fiber length and hence on processing. The dependence of the composite impact energy on processing is discussed.

繊維破断を有する単一繊維複合材料の応力伝達解析

Stress transfer between fiber and matrix in a single fiber reinforced composite with fiber breaks is analyzed. The model is based on a concentric cylinder model and considers an interphase layer between the fiber and matrix. Account is taken of thermal residual stresses arising from a mismatch between the fiber, matrix and interphase. In addition, Poisson's ratio mismatches are also taken into account. Stress and displacement fields in a single fiber composite under uniaxial tension in fiber direction are analyzed. The analytical results are shown for a single fiber composite with interphase of various elastic properties. Young's modulus reduction of composite due to fiber break is also predicted. The analysis would be useful for interpreting exprimental data of fragmentation tests.

複合材料 単繊維埋蔵横方向引張試験における初期損傷に及ぼす界面相特性の影響

The embedded single fiber transverse tensile test has been proposed as a new experimental evaluation method on composite interfaces. The effect of surface treatment on microscopic damage, such as the interfacial debonding and matrix cracking around a fiber was investigated by in-situ SEM observation, when the transverse tensile load to the fiber longitudinal direction was applied to an embedded single fiber composite. The experimental result revealed that the surface treatment condition of fibers has an influence on the initiation and propagation of interfacial debonding and matrix cracking around a single fiber. In addition, the effect of interfacial properties on microscopic damage was investigated by a finite element analysis. The analysis was carried out by using a three layers model, which was modeled separately, fiber, matrix and interphase into the test specimen. The property for the interphase was assumed to be orthotropic, i.e. both tensile and shear moduli of the interphase are dealt independently to express the role of interphase for stress bearing and stress transfer separately. The effect of interphase properties was investigated by changing the tensile and shear moduli of the interphase. The reasonable elastic properties of the interphase were obtained by comparing the computational results with experimental ones. The Hoffman's failure criterion was used for judging of the damage in the interphase and the matrix. The validity of this analytical method is verified by comparing the analytical results with the experimental ones.

Creep behavior of interfaces in fiber reinforced metal–matrix composites

The elevated temperature deformation behavior of interfaces in model single fiber composites was isolated and studied using a fiber push-down approach, whereby the interface is loaded in shear. Two fiber–matrix systems, one with no mutual solubility (quartz–lead) and the other with limited mutual solubility (nickel–lead), were investigated. In both systems, the matrix and fiber underwent sliding relative to each other, with the interface acting as a high diffusivity path. The mechanism of sliding was inferred to be interface-diffusion-controlled diffusional creep with a threshold stress (Bingham flow). The behavior was modeled analytically using a continuum approach, and an expression for the constitutive creep behavior of the interface was derived. The model provided a physical basis for the observed threshold behavior, which was found to be directly related to the normal (radial) residual stress acting on the fiber–matrix interface. The results are deemed to be significant because: (1) in some instances, interfacial sliding may be instrumental in determining the overall creep/thermal cycling response of a composite; and (2) they offer an alternative rationalization of threshold behavior during diffusional flow (besides interface reaction control) and may be useful in understanding creep in multi-phase systems with internal stresses.

Evaluation of interface fracture energy for single-fibre composites

Raman and luminescence spectroscopy have been used for the first time to determine the interface fracture energy for single-fibre composites. By using the measured fibre stress distributions in single-fibre fragmentation composite specimens and a simple energy-balance scheme, the energy for the initiation of interfacial debonding has been estimated for carbon (T50) and α-alumina (PRD-166 and Nextel 610) fibres embedded in epoxy resins. It has been found that the interface fracture energy shows good sensitivity to changes in the level of fibre/matrix adhesion due to surface treatment and sizing of the fibres. It is also found that the values of interface fracture energy correlate well with measured values of interfacial shear strength determined for the same fibre/matrix systems.

Effect of friction between fiber and matrix on the fracture toughness of the composite interface

A method of evaluating the interfacial fracture toughness using a single-fibre composite test is proposed. In contrast with the existing techniques, the method takes into account the phenomenon of friction between the fibre and matrix in the debonding zone. A general mathematical solution of the problem and modelling of the friction phenomenon are presented. Finite-element analysis using a “contact” statement is utilized for numerical evaluation of the stress–strain state. The influence of the coefficient of friction and interfacial debonding length is analysed in detail. It is shown that the friction reduces the calculated value of the elastic strain energy release rate for a given debonding length, relative to that obtained when friction is neglected. The magnitude of the difference depends on the coefficient of friction, the elastic properties of the fibre and matrix, and the characteristics of the debonding mechanism. Experimental data on debonding in a series of glass-epoxy single-fibre composites are analysed using the proposed numerical technique to obtain the effects of fibre surface treatments and fibre strain-to-break on the interfacial fracture toughness. © Kluwer Academic Publishers

Light transmittance of continuous fibre-reinforced composites: Analysis, model experiment and parametric study

Abstract The light transmittance of unidirectionally aligned continuous fibre-reinforced composites at visible wavelengths were studied. Analysis was made of the light transmittance of a single-fibre composite, and the results were extended to the analysis of a multiple-fibre composite. A model experiment was carried out using a continuous SiO2-fibre-reinforced epoxy matrix composite. The experimentally obtained light transmittance of the composite perpendicular to the fibre axis was compared with the developed theoretical analysis and good agreement was obtained. Numerical simulations were carried out to obtain the higher light transmittance of the composite. The effects of firstly the refractive index mismatch between the fibre and the matrix, secondly the fibre radius and thirdly, the volume fraction on the transmittance are discussed using the developed analytical model. Key guidelines for obtaining high light transmittance of fibre-reinforced composites are discussed using the analytical results.

Single fibre fragmentation test for assessing adhesion in fibre reinforced composites

The single fibre fragmentation test for measuring the properties of the fibre–matrix interface in fibre-reinforced composites is reviewed. Special emphasis has been paid to the recent stress transfer models in single fibre composites and its application to the development of a suitable data reduction technique for the fragmentation test. The complexities of the correlation of the micromechanical results to the properties of the macrocomposites have been highlighted.

Matrix molecular orientation in fiber-reinforced polypropylene composites

A distinctive crystalline morphology which develops in certain fiber-reinforced thermoplastics, termed "transcrystallinity", occurs as the result of dense nucleation of polymer crystals at the surface of reinforcing fibers. As these fiber-sponsored nuclei grow, they impinge upon one another, such that crystal growth occurs essentially perpendicular to the fiber axis. Previous studies concerning transcrystallized composites have generally focused on single-fiber composites or model systems. Our interest is in elucidating the crystal orientation in conventional fiber-reinforced composites, and in quantifying the fraction of transcrystallized matrix, where present. In the present work, we develop a wide-angle X-ray scattering (WAXS) technique to investigate composites formed from an isotactic polypropylene (PP) matrix with practical loading levels of unidirectional pitch-based carbon, polyacrylonitrile (PAN)-based carbon, or aramid fibers. The transcrystalline fraction of the crystalline matrix approaches 0.95 in pitch-based carbon composites and 0.50 in the aramid composites near fiber loadings of 30 vol %. In addition, a previously-unreported mode of matrix orientation is observed in composites containing the non-transcrystallizing PAN-based carbon fibers, arising from restrictions on the isotropic growth of PP crystallites by the unidirectional fibers. This "constrained growth" orientation can coexist with the transcrystallized matrix at lower fiber loadings.

The Influence of Interface Structure and Composition on the Response of Single-Fiber SiC/Ti-6Al-4V Composites to Transverse Tension

The cruciform specimen geometry has recently been established to investigate the transverse tensile behavior of single-fiber or multiple-fiber titanium matrix composites; however, the results on only relatively few commercially-available fibers have been reported to date. The present study reports the transverse behavior of a range of SiC fibers prepared by different manufacturers and with different surface coatings. The mechanical response of the composite and the damage present at the interfacial region have been documented. In general, the stress–strain behavior was found to be sensitive to the chemical and structural nature of the fiber–matrix interfacial region. Fibers with carbon-rich coatings were found to have a range of interfacial strengths depending on the structure of the interface layers, while uncoated fiber interfaces have a high strength. This study demonstrates the value of the single-fiber transverse cruciform test for quantitatively comparing the behavior of various fibers and coatings, and shows that it can be useful for coating development studies.

Fiber-matrix interface studies on bioabsorbable composite materials for internal fixation of bone fractures. II. A new method using laser scanning confocal microscopy.

In this study, a new visual characterization method was developed using laser scanning confocal microscopy (LSCM) to study morphologic properties, particularly at the fiber-matrix interface, by optical sectioning of bioabsorbable single-fiber composites. The interface gap width (IGW) between the fiber and matrix, and the changes in IGW after in vitro hydrolysis, named the gap rate (Rg), were measured from images obtained using the LSCM. Higher values for IGW and Rg showed faster degradation of the fiber-matrix interface. These parameters were used to investigate the effects of strain, wicking, different reinforcing fibers, and gamma-irradiation on the fiber-matrix interface morphology. The component materials used were nonbioabsorbable AS4 carbon (C) fibers, bioabsorbable calcium phosphate (CaP), poly(glycolic acid) (PGA), and chitin fibers, and bioabsorbable poly(L-lactic acid) (PLLA) matrix. The application of strain on CaP/PLLA composites increased the IGW up to about 15%, after which there was no change up to 25%. The Rg for CaP/PLLA composites with the fiber ends exposed in vitro (permitting wicking) was greater than for CaP/PLLA with the fiber ends embedded completely within the matrix (preventing wicking). Open-end C/PLLA composites had the slowest rate of interface degradation in vitro, followed by chitin/PLLA, PGA/PLLA, and CaP/PLLA. The exposure of closed-end CaP/PLLA composites to 4 Mrad of gamma-irradiation, in air at room temperature or in vaccuum at 77K, accelerated the rate of interface degradation in vitro. In conclusion, an effective new visual characterization method was developed using LSCM, and it was used to show that (a) moderate strain could accelerate the degradation of the interface, (b) fiber-matrix interface wicking could accelerate the rate of degradation of the interface, (c) the rate of interface degradation depends on the type of fiber used, and (d) gamma-irradiation could accelerate the rate of interface degradation. Furthermore, the results of LSCM analysis of different reinforcing fibers with a PLLA matrix agree with measurements of interfacial shear strength (IFSS) and single-fiber tensile strength reported in Part I of this study.

Test Methods for Estimation of Interfacial Normal Strength in Unidirectional Fiber Reinforced Composites

In this work, two specimen geometries, namely, model single fiber composite and block specimen have been suggested for estimating the interfacial normal strength in unidirectional fiber reinforced composites. Analytical modeling of both the specimens has also been done to estimate the interfacial stresses in terms of constituent material properties. Finally, some failure criteria have been employed in conjunction with the experimental observations to estimate the normal bond strength of the interface. This methodology is applied to several fiber/matrix combinations with appropriate variations of the fiber-matrix interfacial bond strength. The results indicate good agreement between the two proposed specimen designs for estimating the normal strength of the strongly bonded interface.

Effect of Interphase Structure on the Debonding of Polycarbonate from S-2 Glass Fibers

The effect of interphase structure on the debonding of polycarbonate from S-2 glass fibers has been studied. The shear strength, fracture toughness and hydrolyic stability of the interphases were measured in a single fiber composite of a continuous S-2 glass fiber embedded in a polycarbonate matrix. Polycarbonate oligomers were chemically grafted onto the glass fiber surfaces through use of a silicon tetrachloride intermediary and the properties of the resulting interphases were compared with those of two commercial sizings and ozone-cleaned surfaces. Evaluation was accomplished by measuring the stress transmission across the interphase, τ, by carrying the embedded single fiber fragmentation test to saturation and by using computer simulations and a finite element analysis to calculate the strain energy release rate, G, of the observed fiber-matrix debonding accompanying the first fiber fracture. The oligomer-grafted interphase exhibited improved stress transmissibility and toughness, after 24 hours in boiling water. The tenacity of the tightly bound oligomers was confirmed via DRIFT, TGA and GC/MS experiments on Soxhlet-extracted fibers. The grafting reaction was modeled on a high surface area silica and studied using solid state NMR to determine reasons for the greater stability of the oligomer-treated surfaces. Measurements of chemical shifts and spin-lattice relaxation times indicate that the oligomers are chemically attached to the surfaces, providing for a well bonded, water resistant interphase. Parallel experiments on a monomeric Bisphenol A-primed silica surface provided evidence that chemical bonding was primarily responsible for the greater hydrolytic stability.

Fracture behavior and acoustic emission in bending tests on single-fiber composites

The bending fracture mechanisms and interfacial behavior of single-fiber composites (s.f.c.) with different fiber surface treatments and embedded fiber positions were investigated in three-point bending with simultaneous acoustic emission monitoring. Microfractures occurring at fiber breakages were examined by AE parameters and observations by a polarized microscope. As a result, it was found that AE signals in a bulk resin specimen were almost not detected, while many AE events were monitored in the s.f.c. bending specimens. The number of AE events was in good agreement with the number of fiber breakages, except for specimens with an embedded fiber near the compressive surface. Using AE parameters, especially the peak frequency and its power energy obtained by a power spectrum analysis, failure modes can be identified. A transition of failure mode from fiber break accompanied by a matrix crack and debonding to buckling is observed when the stress in the embedded fiber changes from tension to compression. The debond length is very long near the loading point for the specimens with a fiber near the tensile surface, but it decreases with increasing distance from the loading point. The debond length is small for the specimen with an embedded fiber near the neutral plane since the strain in the fiber decreases. Furthermore, a model for debonding failure is proposed and the maximum interfacial shear stress is derived. It is confirmed that fiber fragment lengths for the specimens with a fiber near the tensile side can be also expressed by the Weibull distribution as done in s.f.c. tensile tests.

Strain and hysteresis by stochastic matrix cracking in ceramic matrix composites

A theory is presented to predict the stress/strain relations and unload/reload hysteresis behavior during the evolution of multiple matrix cracking in unidirectional fiber reinforced ceramic matrix composites (CMCs). The theory is based on the similarity between multiple matrix cracking and fiber fragmentation in a single fiber composite, and determines the crack and strain evolution as a function of the statistical distribution of initial flaws in the material, the interfacial sliding resistance τ, and the thermal residual stresses in the composite. The model properly includes matrix fragments of all lengths, from lengths smaller than the current slip length δ(σ) to larger than 2δ(σ), at applied stress σ, and accounts for their respective and differing contributions to the overall strain and hysteresis behavior of the composite. The procedure by which experimental stress/strain and hysteresis data can be interpreted to derive values for the interfacial shear stress, thermal stresses, and intrinsic matrix flaw distribution is discussed. The actual physical crack spacing needs only to be determined at one load level, such as post-fracture, which greatly simplifies the data acquisition and analysis. Several detailed examples are presented, and the results compared with a widely-used approach in which the crack spacing is assumed constant and equal to the average spacing obtained directly from experiment. The discrepancy between the previous and present theories is manifest in an incorrect estimate for the interfacial sliding, but only by approximately 10%. The effect of changing temperature, and hence residual stresses, without changing either matrix flaws or interfacial sliding resistance, is studied.