A microdebonding test for in situ assessment of fibre/matrix bond strength in composite materials

A test technique which gives a quantitative measure of the in situ fibre/matrix bond strength in fibre-reinforced composite materials is described. The test involves the compressive loading of a fibre or region of fibres on a polished specimen surface to produce debonding. Results are presented for the debonding load for glass/epoxy, aramid/epoxy and graphite/epoxy composites. The change in debonding load is also followed as the interface degrades during moisture conditioning at different points through the thickness of the material. It is concluded that refinements to the technique are needed to simplify interpretation of the debonding force in terms of interface shear strength, and to make the test more reproducible.     

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.     

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.     

The role of the hydrogen bonding network for the shear modulus of PIPD

Ab initio total energy calculations at the DFT-GGA level for PIPD are reported. Both the monoclinic crystal with a bi-directional hydrogen-bond network and the triclinic crystal with a sheet-like network are studied. It is concluded that the latter is the more plausible microstructure for the fibre based on the following: (i) After optimisation of the lattice parameters and atomic positions it has a lower energy. (ii) The calculated internal shear modulus agrees better with experiment. (iii) The minimal shear stiffness constant, which is interpreted as the upper limit on the compressive strength, compares favourably with the experimental compressive strength.The hydrogen bonding network plays a crucial—but indirect—role in explaining the high compressive strength. It replaces the weak components of the lateral bonding, such as present in many high performance polymer materials with low compressive strength, e.g. PBO and PBZT, with much stronger hydrogen bonds. This makes that in PIPD the relatively strong π-π interaction has the weakest resistance against shear.     

Investigation of hygrothermal aging phenomena in model composites by Raman spectroscopy

Abstract

Raman spectroscopy combined with gravimetry has been used to investigate hygrothermal aging in model composites based on an epoxy resin containing a single filament of a reinforcing fibre. Three fibre types, namely PBO, M5 and Twaron, were investigated using two specimen configurations - the diffusion slab (DS) and the double fibre pullout (DFPO) geometry. Variables such as the fibre type, surface treatment in the case of Twaron fibres, and the penetrant type (water vapour and liquid phase), were screened by establishing development of the Raman strain profiles ?(x, t) and the water uptake M(t) with time. Experimental data were successfully modelled using relationships that describe Fickian diffusion into a solid parallelepiped. Relative water uptakes determined gravimetrically were reduced to time dependencies controlled by the specimen geometry, diffusion coefficient of the matrix D and the equilibrium uptake of water, M(∞). Using simple micromechanical models, the Raman strain profiles were rationalised in terms of the midfibre strain ?(x=0, t) and the dimensionless parameter n, which controls the stress transfer along the interface. The fibre strain energy G(t) was also obtained from the area under the Raman profile. At longer exposure times and for some fibres, debonding was observed and the time dependence of the propagation of the debond length L _d was quantified. Generic conclusions on composite behaviour were drawn by eliminating the exposure time. The swelling coefficient β was derived from the linear dependence of midfibre strain on the water uptake and a change in the fibre strain energy was related to the debond propagation.     

Deformation studies of single rigid-rod polymer-based fibres. Part 1. Determination of crystal modulus

This series of papers covers several aspects related to the influence of external stresses on the crystalline microstructure of rigid-rod polymer-based fibres. For the study, the main three fibres of this type have been selected, namely poly(p-phenylenebenzobisoxazole) (PBO fibres), poly(p-phenylenebenzobisthiazole) (PBT or PBZT fibres) and the novel poly{2,6-diimidazo[4,5-b:4′-5′-e]pyridinylene-1,4(2,5-dihydroxy)phenylene} (PIPD or M5 fibres). Synchrotron radiation was employed to record high-quality wide-angle X-ray scattering patterns from single fibres. The present paper deals mainly with the evaluation of lattice strain along the fibre axis (c-)direction. Crystal moduli of the different fibres were calculated from the variation with stress of the lattice strain determined from the shift of the major meridional (00l) reflections. This procedure rendered values of approximately 440 GPa for the crystal modulus of PIPD and PBO fibres, and 350 GPa for the PBT one. The difference between these two values was explained in terms of specific molecular conformation of the monomers in the unit cell. Discrepancies between the crystal and macroscopic (calculated from tensile tests) moduli are due to imperfections generated during the manufacture of the fibres.     

Effect of alkali treatment on properties of unidirectional isora fibre reinforced epoxy composites

Abstract

Unidirectional isora fibre reinforced epoxy composites were prepared by compression moulding. Isora is a natural bast fibre separated from Helicteres isora plant by retting process. The effect of alkali treatment on the properties of the fibre was studied by scanning electron microscopy (SEM), IR, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Mechanical properties such as tensile strength, Young's modulus, flexural strength, flexural modulus and impact strength of the composites containing untreated and alkali treated fibres have been studied as a function of fibre loading. The optimum fibre loading for tensile properties of the untreated fibre composite was found to be 49% by volume and for flexural properties the loading was optimised at ～45%. Impact strength of the composite increased with increase in fibre loading and remained constant at a fibre loading of 54·5%. Alkali treated fibre composite showed improved thermal and mechanical properties compared to untreated fibre composite. From dynamic mechanical analysis (DMA) studies it was observed that the alkali treated fibre composites have higher E' and low tan δ maximum values compared to untreated fibre composites. From swelling studies in methyl ethyl ketone it was observed that the mole percentage of uptake of the solvent by the treated fibre composites is less than that by the untreated fibre composites. From these results it can be concluded that in composites containing alkalised fibres there is enhanced interfacial adhesion between the fibre and the matrix leading to better properties, compared to untreated fibre composites.     

Effect of silane treatment on the Curaua fibre/polyester interface

ABSTRACT

Good adhesion at fibre/matrix interface of lignocellulosic fibres is crucial when substituting synthetic fibres in polymer composites. The great variability presented by those fibres requires diverse characterisation studies for better insights on fibre surface treatments and resin systems interactions. In this work, Curaua fibres were treated using silane coupling agents to improve their interfacial properties with polyester. The fibres were pre-treated using 4?wt% solution of NaOH and then treated with 5?wt% solution of (3-aminopropyl) trimethoxysilane (AMPTS) or triethoxymethylsilane (TEMS). Characterisation of the treated fibres was carried out using infrared spectroscopy, X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Fibre wettability and adhesion towards polyester was investigated using contact angle measurements and pull-out tests, respectively. The AMPTS treatment yielded a significant result of 20.2?MPa in interfacial shear strength (≈2.5 times that of the untreated fibre), attributed to the increase in availability of binding sites with polyester.     

The effect of fibre sizing and compatibilizer of polypropylene/poly(butylene terephthalate) blends on the mechanical and interphase properties of basalt fibre reinforced composites

The effect of basalt fibre sizing on the mechanical and interphase properties of fibre‐reinforced composites was studied. Two different chemical preparations of the fibre surface (PBT‐compliant and PP‐compliant) were used. The polymer matrix was prepared from polypropylene/poly(butylene terephthalate) (PP/PBT) immiscible polymer blend and the effect of different compatibilizers on the composite properties was evaluated. SEM hints at improved fibre adhesion to the polymer matrix when a PP‐compliant sizing is applied. SEM also reveals improved compatibilization effects when block copolymer instead of multiblock copolymer is used for the PP/PBT blend preparation. The pull‐out test was applied to quantitatively evaluate the interface adhesion between the fibres and matrices. It showed a high value of the interfacial shear strength between basalt fibres modified with PP‐compliant sizing and polymer blend compatibilized by block copolymer, thus confirming good adhesion. One possible explanation of such good mechanical properties can be related to the chemical interactions between functional groups, mainly maleic anhydride on basalt fibres and the polyolefin component (PP) of the polymer matrix. © 2017 Society of Chemical Industry     

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.     

深潜壳体用高模量树脂基体的研究——基体模量对复合材料抗压抗剪性能的影响

制备了高模量树脂基单向复合材料，测试了单向复合材料的纵向压缩性能和平面剪切性能。研究了基体模量对单向复合材料抗压强度及复合材料平面剪切性能的影响，结果表明：单向复合材料的抗压强度与基体模量成线性比例关系，随基体模量的提高而提高，复合材料的平面剪切性能与基体模量基本上呈线性关系，平面剪切强度亦随基体模量的提高而提高。以模量达５．３６ＧＰａ的环氧树脂作为复合材料的树脂基体制备的，单向玻璃纤维增强复合材料其抗压强度高达１．２９５ＧＰａ，碳纤维增强的复合材料抗压强度高达１．３７２ＧＰａ，与普通环氧树脂的单向复合材料相比，分别提高了５５％和４５．８％；复合材料的平面剪切强度达６４．５ＭＰａ，比普通环氧树脂复合材料的平面剪切强度提高了４４．３％，满足了深潜壳体对复合材料抗压强度的要求。     

Effect of fibre length and chemical modifications on the tensile properties of intimately mixed short sisal/glass hybrid fibre reinforced low density polyethylene composites

Hybrid composites prepared by the incorporation of two or more different types of fibres into a single polymer matrix deserve much attention. This method of hybridisation of composites offers a profitable procedure for the fabrication of products while the resulting materials are noted for their high specific strength, modulus and thermal stability. The influence of the relative composition of short sisal/glass fibres, their length and distribution on the tensile properties of short sisal/glass intimately mixed polyethylene composites (SGRP) was examined. Different compositions of sisal and glass such as 70/30, 50/50 and 30/70 have been prepared with varying fibre lengths in the range of 1–10 mm. Emphasis has also been given to the variation of fibre–matrix adhesion with several fibre chemical modifications. Chemical surface modifications such as alkali, acetic anhydride, stearic acid, permanganate, maleic anhydride, silane and peroxides given to the fibres and matrix were found to be successful in improving the interfacial adhesion and compatibility between the fibre and matrix. The nature and extent of chemical modifications were analysed by infrared spectroscopy while improvement in fibre–matrix adhesion was checked by studying the fractography of composite samples using a scanning electron microscope. Assessment of water retention values has been found to be a successful tool to characterize the surface of the stearic acid modified fibres. It was found that the extent of improvement in tensile properties of SGRP varied with respect to the nature of chemical modifications between fibre and matrix. Improved mechanical anchoring and physical and chemical bonding between fibre and polyethylene matrix are supposed to be the reasons for superior tensile strength and Young's modulus in treated composites. Several secondary reasons such as high degree of fibre dispersion and reduced hydrophilicity in chemically modified fibres also are believed to play a role. Among the various chemical modifications, the best tensile strength and modulus was exhibited by the SGRP with benzoyl peroxide treated fibres. This is attributed to the peroxide‐initiated grafting of polyethylene on to the fibres. Copyright © 2004 Society of Chemical Industry     

Characterization of the adhesion of single-walled carbon nanotubes in poly(p-phenylene terephthalamide) composite fibres

Poly(p-phenylene terephthalamide)/single-walled carbon (PPTA/SWNT) composite fibres with different draw ratios have been spun using a dry-jet wet spinning process and their structure and deformation behaviour analysed using Raman spectroscopy. The dispersion of nanotube has been examined by Raman scattering intensity mapping along the fibre. The nanotubes improved the polymer orientation in composite fibre with a draw ratio of 2 but degraded the orientation at higher draw ratios. The mechanical reinforcing effect by nanotubes is related to the change of polymer orientation, suggesting a dominant role of polymer orientation in mechanical performance of the composite fibre. High efficiency of stress transfer within the strain range of 0-0.35% and breakdown of the interface at higher strains has been found in the composite fibres through an in situ Raman spectroscopic study during fibre deformation. Cyclic loading applied on the fibre has indicated reversible deformation behaviour at low strain and gradual damage of the interface at high strains.     

Preparation of thermoset composites from natural fibres and acrylate modified soybean oil resins

Structural composites with a high content of renewable material were produced from natural fibres and an acrylated epoxidized soybean oil resin. Composites were prepared by spray impregnation followed by compression moulding at elevated temperature. The resulting composites had good mechanical properties in terms of tensile strength and flexural strength. Tensile testing as well as dynamical mechanical thermal analysis showed that increasing the fibre content, increased the mechanical properties. The resin can be reinforced with up to 70 wt % fibre without sacrifice in processability. The tensile modulus ranged between 5.8 and 9.7 GPa depending on the type of fibre mat. The study of the adhesion by low vacuum scanning electron microscopy shows that the fibres are well impregnated in the matrix. The aging properties were finally evaluated. This study shows that composites with a very high content of renewable constituents can be produced from soy bean oil resins and natural fibres. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Tensile,flexural and impact properties of jute fibre-based thermosetting composites

Abstract

The present study reports static and impact mechanical properties of jute fibre-based thermosetting composites using woven and flat braided jute fabrics. Tensile, three-point flexural and low-to-medium energy drop-weight impact tests were conducted and mechanical properties were evaluated to study their dependence upon surface modifications of the fibre materials due to bleaching and coating treatments. Full-bleaching (longer and rigorous) treatments improved interfacial bonding and tensile strength properties of the woven jute composites compared to unbleached and half-bleached counterparts. Bleaching treatments did not seem to improve the flexural strength of composites. Unbleached (natural) jute composites have relatively better flexural strength due to reduced microstructural waviness or fibre crimping to facilitate flexural failure. With coated jute yarns, the tensile properties of the resultant flat braided composites slightly degraded, whereas the flexural properties showed clear improvements. The changes in the mechanical properties were broadly related to the accompanying modifications and to the state of microstructural imperfections, namely fibre/matrix interfacial adhesion, severity of resin matrix shrinkage during the curing process, fibre/matrix debonding and distribution of disbonds within the matrix region, and also to the relative fibre filament density along the loading axis, in the cured composite structure. There was a clear indication that natural woven jute composites could be more effective in applications requiring better impact damage resistance, energy absorption capability and improved progressive crushing behaviour.     

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.     

Surface modification of boron fibres for improved strength in composite materials

When boron fibres are combined with an organic matrix, such as an epoxy resin, a high-performance composite structure is created. This study investigates the surface chemistry of plasma- and organosilane-treated boron fibres with the key aim to improving the adhesion properties between the boron fibre and the epoxy matrix. Optimisation of this interfacial region plays a critical role in influencing the mechanical behaviour of composite materials and has considerable industrial applications in the aerospace and manufacturing industries. The surface chemistry of a model boron surface and boron fibres was monitored using a combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Initial investigation of the as-received fibres showed the presence of silicone contamination on the fibre surface, which would affect adhesion. Removal of this contaminant through solvent cleaning and plasma oxidation provided an ideal surface for attachment of the organosilane adhesion promoter. A model for the interaction of the organosilane with a boron surface is proposed. The pull-out strength of boron fibres, with different surface treatments, embedded in the epoxy resin was measured using a custom designed adhesiometer. Compared with as-received boron fibres, a 6-fold improvement in the apparent interfacial shear strength was achieved for the organosilane treated fibres. Optical microscopy was used to determine the failure mechanisms between the fibre and epoxy resin. Typically, as the surface treatment improved adhesion, the locus of failure changed from the boron–epoxy interface to failure within the epoxy and ultimately fibre breakage.     

Curing Characteristics and Mechanical Properties of Oil Palm Wood Flour Reinforced Epoxidized Natural Rubber Composites

Abstract

The paper reports on the curing characteristics and mechanical properties of oil palm wood flour (OPWF) reinforced epoxidized natural rubber (ENR) composites. Three sizes of OPWF at different filler loadings were compounded with a two roll mill. The cure (t ₉₀) and scorch times of all filler size decrease with increasing OPWF loading. Increasing OPWF loading in ENR compound resulted in reduction of tensile strength and elongation at break but increased tensile modulus, tear strength and hardness. The composites filled with smaller OPWF size showed higher tensile strength, tensile modulus and tear strength. Scanning electron microscope (SEM) micrographs showed that at lower filler loading the fracture of composites occurred mainly due to the breakage of fibre with minimum pull-out of fibres from the matrix. However as the filler loading is increased, the fibre pull-out became very prominent due to the lack of adhesion between fibre and rubber matrix.     

Blends of poly(ethylene terephthalate) and epoxy as matrix material for continuous fibre reinforced composites

Abstract

The morphology and mechanical properties of poly(ethylene terephthalate) (PET)–epoxy blends and the application of these blends in continuous glass fibre reinforced composites have been investigated. Epoxy resin was applied as a reactive solvent for PET to obtain homogeneous solutions with a substantially decreased melt viscosity. The epoxy resin in these solutions was cured using an amine hardener according to two different schedules. In the first, high temperature curing at 260°C preceded low temperature crystallisation of the PET at 180°C. In the second, the PET was allowed to crystallise prior to low temperature curing at 180°C. After cure, all blends revealed a phase separated morphology of dispersed epoxy in a continuous PET matrix. The flexural strength and failure strain of all cured blends showed an increase with increasing epoxy content, whereas the high temperature cured blends exhibited overall lower flexural properties than those cured at the lower temperature. Microstructural analysis and flexural properties of continuous glass fibre reinforced PET–epoxy laminates showed that the composites obtained had a low void content. These PET–epoxy laminates had increased inplane shear strength in comparison with unmodified PET based laminates, indicating considerably increased fibre–matrix adhesion.