Stress Transfer Function for Interface Assessment in Composites with Plasma Copolymer Functionalized Carbon Fibres

Radio-frequency-induced plasma copolymerization of acrylic acid/1,7-octadiene was used to produce a range of functionalized plasma copolymer coatings with controlled degree of adhesion. The single-fibre fragmentation test was used to characterize the adhesion of plasma copolymer coated fibres to epoxy resin. The cumulative stress transfer function (CSTF) and Kelly-Tyson approaches were used to evaluate the degree of adhesion. By continuous monitoring of the fragmentation process, it was found that the mechanical performance of a composite material could be evaluated using the CSTF methodology at strain well below saturation. The degree of debonding was a good measure of relative interface/interphase adhesive strength. The trend in the CSTF is consistent with the propagation of interfacial debonds during the test. For a completely debonded fibre a normalized CSTF value, referred as stress transfer efficiency (STE), was found to provide a more consistent analysis that was able to differentiate between fibres with similar degrees of debonding. The calculated values of interfacial shear strength (IFSS) were only valid for a fully debonded fibre (1,7-octadiene plasma homopolymer coating) where the assumption of a constant shear stress, as in the Kelly-Tyson model, applied. However, IFSS did not provide the same ranking. Where debonding does not occur, the stress transfer efficiency also provides a sensitive measure of the interface/interphase performance. Improved adhesion over the untreated-unsized carbon fibre was observed for both of the plasma copolymer-coated and commercially treated carbon fibres. Since there is a concentration dependence of carboxyl groups on adhesion, the mechanism appears to relate to covalent bond formation with the epoxy group. Plasma copolymer coatings on carbon fibres also causes an increased tensile strength and Weibull modulus.     

Stress Transfer Function for Interface Assessment in Composites with Plasma Copolymer Functionalized Carbon Fibres

Radio-frequency-induced plasma copolymerization of acrylic acid/1,7-octadiene was used to produce a range of functionalized plasma copolymer coatings with controlled degree of adhesion. The single-fibre fragmentation test was used to characterize the adhesion of plasma copolymer coated fibres to epoxy resin. The cumulative stress transfer function (CSTF) and Kelly-Tyson approaches were used to evaluate the degree of adhesion. By continuous monitoring of the fragmentation process, it was found that the mechanical performance of a composite material could be evaluated using the CSTF methodology at strain well below saturation. The degree of debonding was a good measure of relative interface/interphase adhesive strength. The trend in the CSTF is consistent with the propagation of interfacial debonds during the test. For a completely debonded fibre a normalized CSTF value, referred as stress transfer efficiency (STE), was found to provide a more consistent analysis that was able to differentiate between fibres with similar degrees of debonding. The calculated values of interfacial shear strength (IFSS) were only valid for a fully debonded fibre (1,7-octadiene plasma homopolymer coating) where the assumption of a constant shear stress, as in the Kelly-Tyson model, applied. However, IFSS did not provide the same ranking. Where debonding does not occur, the stress transfer efficiency also provides a sensitive measure of the interface/interphase performance. Improved adhesion over the untreated-unsized carbon fibre was observed for both of the plasma copolymer-coated and commercially treated carbon fibres. Since there is a concentration dependence of carboxyl groups on adhesion, the mechanism appears to relate to covalent bond formation with the epoxy group. Plasma copolymer coatings on carbon fibres also causes an increased tensile strength and Weibull modulus.     

Experimental and numerical investigation into fatigue damage mechanisms in multifibre microcomposites

Abstract

Polarised light microscopy has been used to investigate the influence of stress level, interfibre spacing, and fibre–matrix adhesion on the fatigue micromechanisms in carbon–epoxy model composites consisting of a planar array of five intermediate modulus carbon fibres. It was found that an increase in fatigue stress results in an increase in the number of fibre breaks, a more coordinated fibre fracture pattern as a result of fibre–fibre interaction, and extensive interfacial damage. In addition, it was shown that a smaller interfibre spacing results in a higher level of fibre–fibre interaction. Finally, in the case of surface treated carbon fibres (good fibre–matrix adhesion), a more coordinated fibre failure pattern was observed owing to stronger fibre–fibre interaction, whereas in the case of untreated carbon fibres (poor fibre–matrix adhesion), extensive debonding was observed which resulted in a more random fibre failure pattern. Finally, the experimental results were validated by means of a three-dimensional finite element analysis.     

Compressive behaviour of rigid rod polymer fibres and their adhesion to composite matrixes

Abstract

The deformation behaviour of the new high performance polymer fibres, poly(p-phenylene benzobisoxazole) (PBO) and polypyridobisimidazole (PIPD) and their adhesion to an epoxy composite matrix have been investigated. Both fibres give well defined Raman spectra, and the deformation micromechanics of PBO and PIPD single fibres and composites were studied from stress induced Raman band shifts. Single fibre stress-strain curves were determined in both tension and compression, thus providing an estimate of the compressive strength of these fibres. It was found that the PIPD fibre has a higher compressive strength (~1 GPa) than PBO (~0·3 GPa) and other high performance polymer fibres, because hydrogen bond formation is possible between PIPD molecules. It has been shown that when PBO and PIPD fibres are incorporated into an epoxy resin matrix, the resulting composites show very different interfacial failure mechanisms. The fibre strain distribution in the PBO-epoxy composites follows that predicted by the full bonding, shear lag model at low matrix strains, but deviations occur at higher matrix strains due to debonding at the fibre/matrix interface. For PIPD-epoxy composites, however, no debonding was observed before fibre fragmentation, indicating better adhesion than for PBO as a result of reactive groups on the PIPD fibre surface.     

A comparative investigation of interfacial adhesion behaviour of polyamide based self-reinforced polymer composites by single fibre and multiple fibre pull-out tests

Abstract

Interfacial adhesion of composite materials plays an important role in their mechanical properties and performance. In the present investigation, analysis of the interfacial properties of self-reinforced polyamide composites by using microbond multiple fibre pull-out test is emphasised. Microbond specimens prepared through thermal processing are tested for their interfacial properties by multiple fibre bundle pull-out tests and compared with that of traditional single fibre pull-out test specimens. Multiple fibre pull-out addresses the volume fraction as well as eliminates the possibility of fibre breakage before matrix shear. Higher scatter in the data in the samples is addressed in the present studies. FTIR and Fractographic studies are carried out for deep understanding of the post pull-out interfacial adhesion.     

Finite element analysis of load transfer at a fibre–matrix interface during pull-out loading

Load transfer ability of the fibre–matrix interface is well known to mainly control the mechanical behaviour of fibre-reinforced materials. This load transfer phenomenon is of great importance in dentistry when a post is used for fixing a ceramic crown on the tooth. The pull-out test has been well accepted as the most important micromechanical test for evaluating the interaction properties between the fibre and matrix. In this study, a finite element model is developed to analyse the pull-out process of a steel fibre from an epoxy matrix. Based on the pull-out force–displacement curves, developed in our previous experimental work, specific load transfer laws at the fibre–matrix interface have been proposed for each stage of the pull-out process, i.e., before and after fibre–matrix debonding. Predicted initial extraction forces for different implantation lengths were fitted to experimental values and an initial interference fit of 4 μm was determined. An interfacial shear strength of 21 MPa was then determined by fitting the predicted debonding forces for different implantation lengths to the experimental values. According to the load transfer laws considered, analysis of the interfacial shear stress indicates that fibre–matrix debonding initiates simultaneously at both the lower and upper extremities of the interface.     

Modification of epoxy resin by polysulfone to improve the interfacial and mechanical properties in glass fibre composites. II. Adhesion of the epoxy-polysulfone matrices to glass fibres

Adhesion of epoxy-polysulfone (PSF) matrices to glass fibres of 12–30 μm in diameter was studied under both quasi-static and cyclic loadings. A pull-out technique was used for adhesion measurement. It was shown that incorporation of PSF into epoxy resin changed its adhesion to fibres. A maximum was observed in the adhesion strength vs. PSF content dependence at 10 wt% thermoplastic concentration. The results obtained were compared with the data on the epoxy-PSF matrices adhesion to thick steel wire (d = 150 μm) and Nylon-6 fibres (d = 250 μm). Similar values of the adhesion strength increase (22–25%) confirmed that all the changes at the interface were connected primarily with the matrix. A new preferably non-destructive cyclic loading technique was used to test the systems under cyclic loading at varying force amplitudes, frequencies and displacement amplitudes. In this technique the interphase behaviour is characterised by two variables: by the phase angle between the deformation applied to the matrix and the force transferred by the matrix to the fibre, and also by the amplitude of this force. Minimal force amplitudes were observed for the joints with 10 wt% polysulfone. Moreover, phase-angle values for epoxy-10% polysulfone joints were minimal among all the systems investigated. Increase in the number of loading cycles caused much more damage to unmodified epoxy matrix than that to epoxy-polysulfone matrices. Thus, modification of epoxy resin by polysulfone enhanced its adhesion to fibres under both quasistatic and cyclic loadings, especially for epoxy-10% polysulfone matrix. The possible mechanism of the phenomenon observed is discussed.     

Effect of annealing and silylation on the strength of melt-spun Ni–Mn–Ga fibres and their adhesion to epoxy

Single crystals of ferromagnetic Ni–Mn–Ga shape memory alloys show magnetic-field and stress induced twinning, leading to shape memory. Adaptive composites can thus be produced by embedding single crystalline particles or bamboo-structured Ni–Mn–Ga fibres into a polymer matrix. To guarantee a durable performance of these composites, adhesion between reinforcement phase and matrix should be quantified and optimised. The influence of annealing and surface treatment with an aminosilane of melt-spun Ni–Mn–Ga fibres on their strength and adhesion to an epoxy matrix was investigated using single fibre tension and fragmentation tests. Annealing of the melt-spun Ni–Mn–Ga fibres changed the surface from a “pimpled” to a smooth microstructure. This resulted in a reduced adhesion of the annealed fibres in comparison to the as-spun fibres embedded in an epoxy matrix. As-spun fibres exhibited an interfacial shear strength (IFSS) comparable to the shear strength of the epoxy matrix so that the silylation did not change the adhesion significantly. For the annealed fibres, the silane treatment improved the IFSS by 67%. Furthermore, the silylation increased the fracture strength of the Ni–Mn–Ga fibres due to surface flaw healing or forming of a protective surface coating.     

Raman spectroscopy study of high-modulus carbon fibres: effect of plasma-treatment on the interfacial properties of single-fibre-epoxy composites: Part II: Characterisation of the fibre-matrix interface

High-modulus carbon fibres from different precursors were submitted to an oxygen plasma-treatment under similar conditions. Single-fibre epoxy composites were prepared from them, and fragmentation tests were performed in order to characterise fibre-matrix interfacial adhesion. Raman spectroscopy has been used in the present work to map the strain along the fibre during tensile loading of the matrix. The strain distributions obtained agreed well with the prediction of analytical models used conventionally to describe load transfer at interfaces. Interfacial shear stress distributions were then obtained from these distributions according to the conventional force-balance concept. The interfacial shear strength (IFSS) and frictional shear stress (τ_f) values were calculated to quantify the degree of fibre-matrix adhesion. It was found that both parameters increased dramatically after the surface treatment, confirming the ability of cold plasma oxidation to improve the adhesion of carbon fibre to epoxy matrices. A dependence of the IFSS on the degree of surface order, as given by the structural order parameter I_D/(I_D+I_G), calculated from the relative intensities of the D and G bands of Raman spectra, was found. This supports the role played by the graphitic structure in fibre-matrix adhesion.     

Influence of Matrix Properties on Fragmentation Test

Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface in order to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T_g = 166°C) are altered by annealing below T_g . Neat resin samples with identical thermal histories are tested. All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.     

Preparation of glass reinforced NBR–phenolic composite: study of fibre length distribution and burning properties

Abstract

A glass–NBR–phenolic composite was made by combining short glass fibres and NBR–phenolic on a two-roll mill. Fibre breakage during the manufacturing process was investigated and a fibre length distribution was obtained for various mixing times. A reduction in fibre length was observed as mixing proceeds and an average fibre length was determined for different mixing times. It was concluded that for a particular matrix, the processing conditions must be optimised in order to achieve the maximum reinforcing effect of the reinforcement. Various flame retardants were employed and their effect on the burning properties of the composite was evaluated using burning rate measurements as well as sample warpage after burning. Among the flame retardants used, samples with ammonium molybdate showed the minimum warpage and the best char consistency whereas magnesium hydroxide offered the lowest burning rate.     

Mechanical properties of maleic anhydride treated jute fibre/polypropylene composites

Abstract

The effect of maleic anhydride (MA) modification of jute fibre on the mechanical properties of jute/polypropylene (PP) composites was studied. Jute fibre, an environmental friendly, low-density renewable material was chemically modified with MA before the incorporation with PP to improve interfacial adhesion between them. Fourier transform infrared (FTIR) study showed that the C=C groups of MA attached to jute cellulose reacted with the PP matrix. Jute fibre/PP composite treated with MA displayed higher Young's modulus and dynamic storage modulus owing to the enhanced interfacial adhesion between the fibre and PP matrix. A scanning electron microscopy (SEM) study showed evidence of the enhanced adhesion and bridging in the interfacial region of the composite as the result of MA modification of jute fibre.     

Adhesion: Comparison Between Physico-chemical Expected and Measured Adhesion of Oxygen-plasma-treated Carbon Fibers and Polycarbonate

The adhesive interaction between oxygen-plasma-treated, polyacrylonitrile-based, high-tensile-strength carbon fibers and a polycarbonate matrix has been studied. Several models have been used to predict the impact of the plasma treatment process on the strength of adhesion between both jointing partners. These approaches have been the thermodynamic work of adhesion which was calculated from the solid surface tensions, based on the results of contact angle measurements versus test liquids, the contact angle which was directly obtained via polycarbonate melt droplets on single carbon fibers and the zeta (?)-potential data provided by streaming potential measurements. The results have been compared with the interfacial shear strength determined from the single-fiber fragmentation test. Additionally, the single-fiber tensile strength of the oxygen-plasma-treated carbon fibers was determined.

We confirmed that any physico-chemical method on its own fails to describe exactly the measured adhesion. However, for the investigated system, the conscientious interpretation of the data obtained from wetting measurements, in conjunction with the thermodynamic approach, is sufficient to predict the success of a modification technique which has been applied to one component in order to improve adhesion.     

Influence of Fibre and Matrix Modifications on Mechanical and Physical Properties of Flax Fibre Reinforced Poly(propylene)

In this work several types of flax were used as reinforcement in poly(propylene) based unidirectional composites. These flax types included non‐treated as well as treated (boiled) samples. On the other hand, two types of poly(propylene) were used as matrix: non‐modified poly(propylene) (PP), and maleic anhydride modified poly(propylene) (MAA‐PP). The influence of both fibre and matrix modification was studied through mechanical (flexural) and physical (density, sorption, and drying) tests. Combination of boiled flax with MAA‐PP proved to yield the best mechanical properties, combined with good physical properties. A 100% stress transfer between fibre and matrix could be calculated in this optimised case. Interlaminar shear strength tests were done in order to confirm this improved fibre‐matrix adhesion.     

Strain response and damage modelling of glass/epoxy pipes under various stress ratios

Abstract

This paper presents the stress–strain response and general lifetime damage modelling of glass fibre reinforced epoxy (GRE) composite pipes subjected to multi-ratios stress loadings at room temperature (RT). This particular modelling work was developed to predict the non-linear stress–strain response caused by the fatigue static and cyclic loading in the multiaxial ultimate elastic wall stress (UEWS) tests by considering the effects of matrix cracking within the laminates. Although the UEWS procedure is not a standard protocol used for qualification of GRE pipes, it appears to offer an option to existing procedures delineated in ASTM D2992. The ply properties initially expressed as a function of crack density was computed as a function of increasing stress and strain using shear lag approximation. In general lifetime damage model, the effects of stress developed in each ply from ultimate elastic wall stress (UEWS) test were expressed in a single quadratic term of axial and hoop stress. The term then solved to produce limits with respect to axial and hoop stress, which represented in a graphical form of failure envelope. The predictions from both models are found to be in good agreement with the data from the multiaxial UEWS tests of ±55° filament wound GRE pipes. These models thus enable for the long term performance prediction of the pipes under combined loadings.     

Open hole compressive strength and damage mechanisms: maximum stress versus Hashin criteria

Abstract

The open hole compressive strength and damage mechanisms of various T800/924C carbon fiber reinforced plastic laminates were estimated by a three-dimensional progressive damage model (PDM). The emphasis was put on the effect of Hashin and maximum stress criteria on the PDM estimations. The maximum stress criterion provided better overall correlation with the experimental data in terms of the matrix damage onset, ultimate strength and fibre damage pattern at the ultimate load. Despite both criteria being essentially the same for fibre compression, a scattered fibre damage pattern was predicted by Hashin criteria including fibre tensile damage as a result of the shear stress contribution. The matrix damage onset was significantly underestimated by Hashin criteria also due to the shear stress contribution. Nevertheless, neither Hashin nor maximum stress criteria could capture a satisfactory agreement with the experimental matrix damage pattern at the ultimate load.     

Polymeric fibre adhesion to the cementitious matrix related to the fibres type,water to cement ratio and curing time

In this study adhesion of some polymeric fibres to a cement matrix was evaluated by both theoretical and experimental approach. The adhesion results are interpreted in terms of fibre nature, water to cement ratio and curing. It was found that practical work of adhesion (G) is ten times larger than thermodynamic work of adhesion (W) between fibre and cement matrix. By employing adhesion theory to the composite system, fibre/cement interactions are better described. The results indicated that chemical adhesion between cement matrix and fibres decreased with decrease in water to cement ratio while mechanical bonding increased. The application of adhesion theory to cement based composite leads to calculation of a new parameter which is called energy dissipation (loss function). It was found that the loss function of the fibre–cement system increases more than 21% by increase in the curing time of the matrix from 14 days to 28 days.     

Adhesion of Polypropylene Fiber to Cement Matrix

Abstract

In this research, the adhesion of polypropylene (PP) fibers to cementitious matrix has been investigated and the chemical bonding and mechanical interlocking between PP fiber and hardened cement paste has been studied. Furthermore, thermodynamic work of adhesion and loss-function (dissipation energy) has been calculated in the PP-cement matrix model system. To investigate the work of adhesion, the pull-out test has been used. Also, the surface free energy and contact angle of the PP monofilaments and cement matrix have been measured using a tensiometer and the fiber–cement interfacial interactions and thermodynamic work of adhesion and loss-function were calculated. Scanning electron microscopy (SEM) analysis was used to study the fiber–cement matrix interfacial transition zone (ITZ). The results showed that the application of theories of polymer–polymer adhesion in fiber–cement matrix systems was feasible. To verify the accuracy of the method, the adhesion of two other fibers (nylon 6,6 and acrylic polymer) was studied.     

Influence of Matrix Properties on Fragmentation Test

Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface in order to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T_g = 166°C) are altered by annealing below T_g. Neat resin samples with identical thermal histories are tested. All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.     

Experimental results of a pull-out test performed with single- and multi-fiber samples

Widely-used methods for characterising the fibre/matrix interface in polymeric composites are the fragmentation test and the droplet test as a special kind of the single-fibre pull-out test. A severe disadvantage of these tests is that non-realistic model samples are investigated which contain only one fibre in the matrix. In order to obtain data about the effect of the different residual stress situations for fibres in such samples and in composites, pull-out tests of E-glass fibres in polystyrene and polycarbonate are performed using samples, where the investigated fibre is surrounded by 0 to 3 other near fibres. Neighbouring fibres can increase the pull-out forces by a factor of three and the interfacial toughness by a factor of four. This has to be taken into account, if the tests are performed not only for comparison reasons but for measuring interface properties.