Comparing single-walled carbon nanotubes and samarium oxide as strain sensors for model glass-fibre/epoxy composites

The study of the interfacial stress transfer for glass fibres in polymer composites through the fragmentation test requires certain assumptions, such as a constant interfacial shear stress. In order to map the local interfacial properties of a composite, both Raman spectroscopy and luminescence spectroscopy have been independently used. Unlike other polymer fibre composites, the local strain state of a glass fibre cannot be obtained using Raman spectroscopy, since only very broad and weak peaks are obtainable. This study shows that when single-walled carbon nanotubes (SWNTs) are added to the silane sizing as a strain sensor, it becomes possible to map the local fibre strain in glass fibres using Raman spectroscopy. Moreover, if this model glass fibre contains a small amount of Sm₂O₃, as one of the components, luminescence spectroscopy can be simultaneously used to confirm this local fibre strain. A combined micromechanical properties study of stress transfer at the fibre–matrix interface using luminescence spectroscopy, together with Raman spectroscopy, is therefore reported. The local strain behaviour of both Sm³⁺ doped glass and SWNTs in the silane coating are shown to be consistent with a shear-lag model. This indicates that Sm³⁺ dopants and SWNTs are excellent sensors for the local deformation of glass fibre composites.     

Investigation of the long term effects of moisture on carbon fibre and epoxy matrix composites

The aim of this work is to investigate the long term effects of moisture on the interface between a carbon fibre and an epoxy matrix. High modulus carbon fibres were used to prepare single fibre model composites based on an epoxy resin. The samples were immersed in the seawater and demineralised water and their moisture uptake behaviour was monitored. The equilibrium moisture content and diffusion coefficients for the samples were determined. DSC has been used to analyse the moisture effects on glass transition temperature and thermal stability of the pure epoxy specimens. These results showed a reduction in the glass transition temperature (T_g) after moisture absorption. Tensile tests were also carried out for the epoxy specimens and a general decrease in the mechanical properties of the epoxy matrix was observed. Raman spectroscopy was used to observe the effects of moisture on the axial strain of the carbon fibre within the composite and stress transfer at the interface as a function of exposure time. The results show that the decrease in the mechanical and interfacial properties of the model composites under the seawater immersion is more significant than under demineralised water immersion.     

Fatigue life evaluation for carbon/epoxy laminate composites under constant and variable block loading

This paper presents the results of current research on the fatigue life prediction of carbon/epoxy laminate composites involving twelve balanced woven bidirectional layers of carbon fibres and epoxy resin manufactured by a vacuum moulding method. The plates were produced with 3 mm thickness and 0.66 fibre weight fraction. The dog bone shape specimens were cut from these plates with the load line aligned with one of the fibre directions. The fatigue tests were performed using load control with a frequency of 10 Hz and at room temperature. The fatigue behaviour was studied for different stress ratios and for variable amplitude block loadings. The damage process was monitored in terms of the stiffness loss. The fatigue life of specimens submitted to block loading tests was modelled using Palmgren–Miner’s law and taking in to account the stress ratio effect. The estimated and experimental fatigue lives were compared and good agreement was observed.     

Deformation micromechanics of a model cellulose/glass fibre hybrid composite

Interfacial stress transfer in a model hybrid composite has been investigated. An Sm³⁺ doped glass fibre and a high-modulus regenerated cellulose fibre were embedded in close proximity to each other in an epoxy resin matrix dumbbell-shaped model composite. This model composite was then deformed until the glass fibre fragmented. Shifts of the absolute positions of a Raman band from the cellulose fibre, located at 1095 cm⁻¹, and a luminescence band from a doped glass fibre, located at 648 nm, were recorded simultaneously. A calibration of these shifts, for both fibres deformed in air, was used to determine the point-to-point distribution of strain in the fibres around the breaks in the glass fibre. Each break that occurred in the glass fibre during fragmentation was shown to generate a local stress concentration in the cellulose fibre, which was quantified using Raman spectroscopy. Using theoretical model fits to the data it is shown that the interfacial shear stress between both fibres and the resin can be determined. A stress concentration factor (SCF) was also determined for the regenerated cellulose fibre, showing how the presence of debonding reduces this factor. This study offers a new approach for following the micromechanics of the interfaces within hybrid composite materials, in particular where plant fibres are used to replace glass fibres.     

Hybrid effects in thin ply carbon/glass unidirectional laminates: Accurate experimental determination and prediction

Experimental results are presented which allow the hybrid effect to be evaluated accurately for thin ply carbon/epoxy–glass/epoxy interlayer hybrid composites. It is shown that there is an enhancement in strain at failure of up to 20% for very thin plies, but no significant effect for thicker plies. Hybrid specimens with thick carbon plies can therefore be used to measure the reference carbon/epoxy failure strain. The latter is significantly higher than the strain from all-carbon specimens in which there is an effect due to stress concentrations at the load introduction. Models are presented which illustrate the mechanisms responsible for the hybrid effect due to the constraint on failure at both the fibre and ply level. These results give a good understanding of how variability in the carbon fibre strengths can translate into hybrid effects in composite laminates.     

Characterisation of carbon fibres recycled from carbon fibre/epoxy resin composites using supercritical n-propanol

Three different PAN based carbon fibres (Toray T600S, T700S and Tenax STS5631) were recycled from epoxy resin/carbon fibre composites using supercritical n-propanol. The recycled carbon fibres were characterised using single fibre tensile tests, SEM, XPS and micro-droplet test. The tensile strength and modulus of the recycled carbon fibre was very similar to the corresponding as-received carbon fibres. However, the surface oxygen concentration decreased significantly, which caused a reduction of the interfacial shear strength with epoxy resin.     

Investigation of the influence of surface-activated carbon fibres on debonding energy and frictional stress in polymer-matrix composites by the micro-indentation technique

The influence on fibre/matrix adhesion of the acidic and basic nature of surface-activated carbon fibres in epoxy and PPS matrices has been investigated by means of the micro-indentation method. The fibre ‘push-out’ and ‘push-in’ techniques were used for this study. The debonding energy and frictional stress are calculated, and the adhesion behaviour is compared with the calculated thermodynamical work of adhesion (as derived from the fibre surface tension) and surface oxygen content. Influence of surface activation on the interfacial frictional stress is discussed.     

Evaluation of interfacial shear stress between multi-walled carbon nanotubes and epoxy based on strain distribution measurement using Raman spectroscopy

This study investigated the stress recovery of aligned multi-walled carbon nanotubes (MWCNTs) embedded in epoxy using Raman spectroscopy, and evaluated interfacial shear stress between MWCNTs and epoxy using shear-lag analysis. To this end, ultralong aligned MWCNTs (3.8 mm long) were embedded in epoxy to obtain Raman spectra at multiple points along the MWCNTs. Downshift of the G′-band due to tensile strain was measured from the nanotube end to the center, and the strain distribution of embedded MWCNTs was evaluated successfully. Interfacial shear stress was then estimated by minimizing the error between the shear-lag analysis and measured strain distribution. The maximum interfacial shear stress between the embedded MWCNTs and epoxy was 10.3–24.1 MPa at the failure strain of aligned MWCNT-reinforced epoxy composites (0.46% strain). Furthermore, the interfacial shear stress between an individual MWCNT and epoxy was investigated.     

Fragmentation analysis of glass fibres in model composites through the use of Raman spectroscopy

Raman spectroscopy has been used to monitor deformation micromechanics in a model discontinuous fibre composite comprising a single glass fibre in an epoxy resin. The glass fibre was coated with a diacetylene-containing urethane copolymer that was subsequently cross-polymerised thermally. During composite deformation, the stress-induced Raman band shifts of the polydiacetylene sequences in the cross-polymerised coating were used to map the distributions of strain along glass fibres inside the epoxy resin matrix. The fragmentation of the fibre has been followed in detail and the behaviour analysed using classical shear-lag analysis. Values of the interfacial shear stress along fibre fragments were determined from the measured fibre strain distributions and were shown to be limited by the shear yield stress of the matrix. The effect of adhesion between the coating and the fibre upon the strain distributions has been investigated in detail. The fibre strain distributions can only be determined accurately when the adhesion is good. However, in the case of poor adhesion, although the strain distribution in the coating follows that of the matrix, the fragmentation process can still be monitored.     

Effect of oxygen plasma treatment of PPTA and PBO fibers on the interfacial properties of single fiber/epoxy composites studied by Raman spectroscopy

The interfacial micromechanics of single poly(p-phenylene terephthalamide (PPTA) and poly(p-phenylene benzobisoxazole (PBO) fibers embedded in an epoxy resin has been investigated by determining the interfacial shear stress distributions along the fiber length. The effects of an oxygen plasma treatment on the interfacial shear stress of the fiber-epoxy systems are analyzed. Raman spectroscopy was used to map the stress distributions along the fiber when the composite is subjected to a small axial tensile strain (3.5% for PPTA and 2.5% for PBO). The quality of the interface or adhesion was improved after the surface treatment, supporting the ability of plasma oxidation to enhance the adhesion of high-performance fibers to epoxy resins. The tensile behavior of fiber-reinforced systems was different in each case. PPTA reinforcements underwent fragmentation, likely by fiber microfailure, whereas debonding or bridging is the most probable fragmentation mechanism in the case of PBO.     

Carbon nanotube grafted silica fibres: Characterising the interface at the single fibre level

Model polymer composites containing carbon nanotube (CNT) grafted fibres provide a means to investigate the influence of nanostructures on interfacial properties. Well-aligned nanotubes, with controllable length, were grown on silica fibres by using the injection chemical vapour deposition method, leading to a significant increase of the fibre surface area. In single fibre tensile tests, this CNT growth reaction reduced the fibre strength, apparently due to catalyst etching; however, the fibre modulus increased significantly. Contact angle measurements, using the drop-on-fibre method, indicated an excellent wettability of the CNT-grafted fibres by poly(methyl methacrylate) (PMMA). PMMA model composites were fabricated and studied using the single fibre fragmentation tests. A dramatic improvement (up to 150%) of the apparent interfacial shear strength (IFSS) was obtained for the composites containing CNT-grafted fibres. The improvement of IFSS was also influenced by the length and morphology of the grafted CNTs.     

Internal strain measurement and cure monitoring of 3D angle interlock woven carbon fibre composites

This study presents a methodology to accurately embed EFPI (Extrinsic Fabry Perot Interferometer) fibre optic sensors within the resin channels of 3D woven composite for the purpose of monitoring strain during tensile tests on a 3D woven composite. Using the same sensors the cure induced strain measurement was developed into a valid in situ cure monitoring technique for an epoxy matrix composite through correlation with rheological based cure strain data. Specific strain events were attributed to the gelation and vitrification phases of the epoxy cure cycle.     

The study of model polydiacetylene/epoxy composites

A mode composite system consisting of one polydiacetylene single crystal fibre in an epoxy resin matrix has been subjected to tensile strain parallel to the fibre direction. The strain at all points along the length of the fibre was determined by resonance Raman spectroscopy while that of the matrix was measured by conventional techniques. Comparison of the fibre and matrix strain showed two distinct regions. Below about 0.5% matrix strain the composite followed Reuss-type behaviour with equal stress in the fibre and the matrix. At higher matrix strain the composite followed Voigt-type behaviour with any increase in matrix strain matched by an equal increase in fibre strain. In this region the strain distribution along the length of the fibre could be approximately described by the shear-lag model of Cox. The critical length of the fibre was found to increase linearly with fibre diameter as predicted by that model. Good qualitative agreement was found with the predictions of a calculation based on finite element analysis over the full range of applied stress.     

Low velocity impact response and damage of laminate composite glass fibre/epoxy based tri-block copolymer

Two types of laminate composites made of glass fibre/epoxy matrix (EPO_FV) and glass fibre/epoxy modified tri-block copolymer (Nanostrength) matrix (EPONS_FV) were manufactured by compression moulding. Some AFM investigations have been done to identify the Nanostrength dispersion in the epoxy matrix and some DMA analyses have been performed, at different frequencies, to understand the frequency or the strain rate sensitivity of both composites. Compared to EPO_FV, EPONS_FV exhibits a significant frequency/strain rate sensitivity. Impact resistance of the composite was investigated by means of low velocity impact tests. The low velocity impact results indicate that the addition of Nanostrength leads to the improved impact resistance and an increase in absorbed energy, especially at high impact energy level. SEM observations, performed on ion polished samples, reveal the presence of micro-cracks for both composites. Micro-cracks consist of a coalescence of fibre matrix de-bonding. It was also observed that EPONS_FV contains a lower density of micro-cracks compared to EPO_FV, confirming the fact that the composite with Nanostrength absorbs more energy by Nanostrength micelles cavitation.     

In situ fibre strength determination in metal matrix composites

Single fibre fragmentation tests were performed at room temperature on SiC/Ti-6242 specimens in order to estimate the in situ fibre strength. Tensile specimens were instrumented with two acoustic emission transducers and an extensometer in order to monitor the strain at which fibre breaks occurred. Data analysis utilized Monte Carlo simulations of fibre fragmentation. The fibre/matrix stress transfer profile near a fibre break was derived using a finite element analysis. Cohesive zone model is used to describe damage of the interfacial zone. Thermally induced residual stresses and matrix plastic deformations were accounted for. The results presented in this paper show that the in situ Weibull parameters of the fibre are smaller than the reference obtained on as received fibres. Analysis of data raised questions about the validity of the Monte Carlo simulation method.     

3D in situ observations of glass fibre/matrix interfacial debonding

X-ray microtomography was used for 3D in situ observations of the evolution of fibre/matrix interfacial debonding. A specimen with a single fibre oriented perpendicular to the tensile direction was tested at a synchrotron facility using a special loading rig which allowed for applying a load transverse to the fibre. Three distinguishable damage stages were observed: (i) interfacial debond initiation at the free surface, (ii) debond propagation from the surface into the specimen and (iii) unstable debonding along the full length of the scanned volume. The high resolution microtomography provides both qualitative and quantitative 3D data of the debonding initiation and propagation. Thus, microtomography is demonstrated as a promising technique which can assist micromechanical model development.     

Impact surface fractures of glass-fiber/epoxy lamina-coated glass plates by small steel-ball

Fiber orientation effects on the impact surface fracture of glass plates coated with a glass-fiber/epoxy lamina layer were investigated using a small-diameter steel-ball impact experiment. Four kinds of materials were used: soda-lime glass plates, unidirectional glass-fiber/epoxy layer (one ply, two plies) coated glass plates, crossed glass-fiber/epoxy layer (only two plies) coated glass plates. The maximum stress and absorbed fracture energy of these plates were measured by a single-grid strain gage bonded to the back surface of the glass plates during the impact of the steel ball. With increasing impact velocity, various surface cracks, such as ring, cone, radial and lateral cracks, occurred near the impact sites of the uncoated glass plates. Plates with glass-fiber coating had a plastic deformation zone between the fiber layer and the glass plate that formed around the impact site while the surface cracks in the plates drastically diminished. The principal direction of this plastic deformation and delamination followed the fiber orientation. The impact surface-fracture index expressed in terms of the maximum stress and the absorbed energy could be used as an effective evaluation parameter for surface resistance.     

Assessment of interfacial bonding between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests

Two experimental approaches were employed to assess the fibre/matrix adhesion between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests. The first approach was to measure interfacial bonding strength of the fibre/matrix interface in dog-bone-shaped tensile specimens by applying normal stress until failure, simulating the Mode I failure mode. The second approach was to determine the fibre/epoxy interfacial bonding strength in shear (simulating the Mode II failure mode) by means of a V-notched beam shear testing method, i.e. a modified Iosipescu test. In both methods, polymer threads were transversely incorporated in the middle section of the specimens. It was found that both methods were simple, reliable, and sensitive to changes in the fibre/matrix adhesion conditions, though interpretation of the test results was somewhat complex. The two experimental approaches were able to produce consistent results and can thus be adopted as alternative methods for determining the interfacial bonding properties between fibres and matrix in composite systems where conventional micro-mechanical or macro-mechanical testing methods cannot be used.     

Strain-sensitive Raman spectroscopy and electrical resistance of carbon nanotube-coated glass fibre sensors

This paper presents the development of glass fibres coated with nanocomposites consisting of carbon nanotubes (CNTs) and epoxy. Single glass fibres with different CNT content coating are embedded in a polymer matrix as a strain sensor for composite structures. Raman spectroscopy and electrical response of glass fibres under mechanical load are coupled for in situ sensing of deformation in composites. The results show that the fibres with nanocomposite coating exhibit efficient stress transfer across the fibre/matrix interface, and these with a higher CNT content are more prone to fibre fragmentation at the same matrix strain. A relationship between the fibre stress and the change in electrical resistance against the fibre strain is established. The major finding of this study has a practical implication in that the fibres with nanocomposite coating can serve as a sensor to monitor the deformation and damage process in composites.