Interfacial studies on carbon/thermoplastic model composites using laser Raman spectroscopy

Interfacial studies were carried out on a model composite system consisting of a short carbon fibre embedded in a polycarbonate matrix. While the composite was being strained, the local strain along the fibre was monitored using a Raman spectroscopic technique. The residual compressive strain in the fibre due to fabrication was found to be –0.45%. Subsequent loading of the composite up to 0.55% in tension resulted in a complex stress field consisting of tension at the fibre ends and compression in the middle of the fibre. The fibre strain at different levels of applied load was converted to interfacial shear stress (ISS) distribution along the fibre by employing a simple equilibrium analysis. The shape of the ISS profiles indicated a predominantly frictional type of load transfer from the matrix to the fibre. Finally, the maximum ISS value of 15 MPa was found to be unaffected by the amount of strain experienced by the composite.     

A study of mechanisms of stress transfer in continuous- and discontinuous-fibre model composites by laser Raman spectroscopy

The micromechanics of stress transfer in single-fibre/epoxy-resin model composites has been investigated. Two specimen geometries are examined incorporating both continuous and discontinuous fibres in epoxy resin blocks. In both cases, the point-by-point strain in the fibre is measured from the fibre Raman spectrum and its strain dependence. The continuous-fibre model composites (CFMC) are subjected to incremental tensile loading and the fibre fragmentation process is continuously monitored. The short-fibre model composites (SFMC) are loaded incrementally to levels of stress of sufficient magnitude to cause interfacial failure and the fibre strain profiles are obtained at each level of applied stress.

In addition to fibre strain measurements, the interfacial shear stress distribution is derived at each increment of applied stress by means of a balance-of-forces argument. The effects of fibre surface treatment and fibre modulus on the strain transfer profile and the distribution of the interfacial shear stress along the fibre are examined. The importance of parameters such as fibre/matrix debonding and interphase yielding in the vicinity of fibre breaks or fibre ends is discussed.     

Compressional behaviour of carbon fibres

Spectroscopic-mechanical studies have been conducted on a range of carbon fibres by bonding single filaments on the top surface of a cantilever beam. Such a loading configuration allows the acquisition of the Raman spectrum of carbon fibres and the derivation of the Raman frequency strain dependence in tension and compression. Strain hardening phenomena in tension and strain softening phenomena in compression were closely observed. The differences in the slopes of the Raman frequency versus applied strain curves in tension and compression respectively, have been used to obtain good estimates of the compression moduli. A method of converting the fibre Raman frequency versus strain data into stress-strain curves in both tension and compression, is demonstrated. Values of fibre stress and fibre modulus at failure in compression compare exceptionally well with corresponding estimates deduced from full composite data. The mode of failure in compression has been found to depend upon the carbon fibre structure. It is demonstrated that certain modifications in the manufacturing technology of PAN-based fibres can lead to fibres which show resistance to catastrophic compressive failure without significant losses in the fibre compressive modulus.     

Monitoring the micromechanics of reinforcement in carbon fibre/epoxy resin systems

The technique of laser Raman spectroscopy (LRS) was employed to obtain the interfacial shear stress (ISS) distribution along a short high-modulus carbon fibre embedded in epoxy resin at different levels of applied stress. Up to 0.6% applied strain, the ISS reached a maximum at the bonded fibre ends and decayed to zero at the middle of the fibre. At higher applied strains, the maximum value of the ISS distribution shifted away from the fibre ends towards the middle of the fibre. At the point of fibre fracture, fibre/matrix debonding was found to initiate at the fibre breaks. Further increase of applied strain resulted also in debonding initiation at the fibre ends. Current analytical stress-transfer models are reviewed in the light of the experimental data.     

Effects of interfacial bonding on sliding phenomena during compressive loading of an embedded fibre

The effects of interfacial bonding on the sliding phenomena at the fibre/matrix interface are considered for fibre-reinforced ceramic composites when an axial compressive stress is applied at the exposed end of an embedded fibre. Sliding occurs at the interface when the interfacial shear strength is exceeded. The interfacial shear stress, the stress transfer from the fibre to the matrix, the length of the sliding zone, and the fibre displacement are analysed in the present study. The results show that at a fixed load, the sliding length and the fibre displacement decrease with increase in the interfacial shear strength. Effects of interfacial bonding on the applied stress-fibre displacement relationship during compressive loading and subsequent unloading are also revealed.     

An improved analysis for axisymmetric stress distributions in the single fibre pull-out test

Elastic stress transfer in the fibre pull-out problem has been investigated quite extensively using various shear lag analyses. These analyses grossly underestimate the severity of the stress concentration at the fibre-matrix interface. In this study, by using the total complementary energy approach, it is found that a stress concentration zone exists at the interface near the fibre entry. Compared to shear lag analysis, the interfacial radial stress at the fibre entry is found to be much higher and the interfacial shear stress reaches a maximum not at the entry end of the fibre but in a few fibre diameters from it. It is also shown that the magnitudes of these stress peaks reduce with increasing b/a and L/a ratios. At large b/a and L/a ratios, the maximum radial and shear stresses at the interface reach a plateau and are independent of the loading methods considered. Finally, the implications of these stress concentrations on the failure of the matrix and interface and the specimen geometry in determining the interfacial shear strength are discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Interfacial stress transfer and property mismatch in discontinuous nanofiber/nanotube composite materials

Novel nanotubes/nanofibers with high strength and stiffness did not lead to high failure strengths/strains of nanocomposite materials. Therefore, the interfacial stress transfer and possible stress singularities, arising at the interfacial ends of discontinuous nanofibers embedded in a matrix, subjected to tensile and shear loading, were investigated by finite element analysis. The effects of Young's moduli and volume fractions on interfacial stress distributions were studied. Round-ended nanofibers were proposed to remove the interfacial singular stresses, which were caused by high stiffness mismatch of the nanoscale reinforcement and the matrix. However, the normal stress induced in the nanofiber through interfacial stress transfer was still less than 2 times that in the matrix. This stress value is far below the high strength of the nanofiber. Therefore, the load transfer efficiency of discontinuous nanofibers or nanotube composites is very low. Hence, nanofibers or nanotubes in continuous forms, which also preclude the formation of singular interfacial stress zones, are recommended over discontinuous nanofibers to achieve high strengths in nanocomposite materials.     

The progressional approach to interfacial failure in carbon reinforced composites: elasto-plastic finite element modelling of interface cracks

A rigorous stage-by-stage ‘Progressional Approach’ to interfacial failure is described in which elasto-plastic finite element (FE) analysis has been used to model the initiation and subsequent propagation of an interface crack in a tensile loaded single carbon fibre composite specimen. The non-linear effects of frictional stress transfer across the debonded interface and the residual stresses thermally induced due to curing have been included. Such effects were found to play a key role in determining the reinforcement efficiency of the fibre as failure at the interface progressed, during incremental loading. Previously published laser Raman spectroscopy measurements for fibre stress distributions have been compared with the FE predictions, for a number of stress levels in the composite. Good correlation was obtained with the experimental data, for a number of interface crack lengths and for a coefficient of friction 0.8–0.9 at the cracked interface. The fibre stress distributions obtained were found to be characteristic of this mode of interfacial failure.     

Interfacial failure in poly(p-phenylene benzobisoxazole) (PBO)/epoxy single fibre pull-out specimens

The pull-out behaviour of poly(p-phenylene benzobisoxazole) fibres from an epoxy resin has been shown to follow that predicted by the elastic stress transfer shear-lag model at low applied strains, but at higher matrix strains a partial debonding model was more suitable. Debonding of the fibre/matrix interface led to interfacial failure where only friction resisted fibre extraction. Raman spectroscopy was able to quantify this level of friction and together with in situ optical microscopy proved an excellent method for the close monitoring of the frictional pull-out process. The effect of fibre surface treatment was also studied. The interfacial shear stress values from the heat-treated and corona-treated fibres showed only small differences. The failure processes were examined further using scanning electron microscopy and clean fibre pull-out was observed with the heat-treated fibre whereas fracture of the free fibre occurred with the corona-treated fibre.     

The compression strength of unidirectional carbon fibre reinforced plastic

A simple compression test, suitable for quality control measurements on unidirectional carbon fibre composite, is described. The specimen, a plane bar, with aluminium end tabs attached, is compressed by applying shear forces over the ends. With either type 1 or type 2 treated fibre the failure mode is one of shear over a plane at approximately 45° to the fibre axis. With untreated type 1 material failure is due to delamination. The variation of the compression strength of treated material with fibre volume loading is linear, the values being considerably below those predicted by buckling theory. Increasing void content causes a steady decrease in compression strength, and off-axis strength values are above those given by the maximum work criterion. The present work supports the recently proposed view that the compression strength of unidirectional carbon fibre composites at room temperature is not governed by fibre buckling but is related to the ultimate strength of the fibre.     

On the effect of shear stresses on the fibre failure behaviour in CFRP

Parts made of carbon fibre reinforced plastics (CFRP) are being increasingly used in high performance parts subjected to very high loads. Nevertheless, the failure behaviour of these materials is still not understood completely. One important open question is whether superimposed shear stresses influence the fibre failure due to high fibre parallel tension or compression. Under these loading conditions the predictions of common failure criteria, e.g. Tsai/Wu, Hashin and Puck, are inconsistent with one another. At the Institute of Plastics Processing (IKV) in Aachen, Germany, extensive mechanical tests are conducted to clarify this situation by experimental results. The outcome so far shows that there is no significant effect of a shear stress on the fibre parallel tensile strength. The effect on the fibre parallel compression strength is not clear without ambiguity. A low shear stress does not cause a reduction of the strength, whereas shear of moderate magnitude seems to promote the development of a compression failure of the fibres. Nevertheless, this statement needs to be verified, because the loading case of fibre parallel compression combined with high superimposed shear stress could not be examined so far.     

Interfacial shear stress distribution in model composites: the effect of fibre modulus

The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis.A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.     

Thermal loading of short fibre composites and the induction of residual shear stresses

A study has been undertaken to identify the effect of thermal residual stresses on the stress transfer between a short fibre and resin using the photoelastic method. As expected, it was observed that fibre fracture occurred at a higher applied load for the fibre embedded in the thermally (80 °C) cured epoxy matrix than in the room-temperature-cured epoxy. Under plane polarised light bright birefringent patterns were observed in the hot-cured epoxy matrix around the fibre-ends prior to loading. These were not present in the room-temperature-cured epoxy, indicating that thermal residual stresses had been induced during thermal-curing. On loading, the birefringent patterns in the hot-cured matrix at the fibre-ends were almost extinguished but at a particular stress reappeared as a bright region, and increased in intensity on further loading.Using a phase-stepping polariscope, four images of the fibre-ends were captured simultaneously so that detailed contour maps of fringe order could be created. To examine the micromechanical response in the matrices at the interfaces the profile of interfacial shear stress at the fibre-ends was calculated. Under a given external load the shear stress at the interface in the hot-cured matrix was significantly lower than that in the cold-cured epoxy matrix. The thermal load which is applied to a resin on cooling from manufacture requires a shear stress at the interface to put the fibre into compression. At the fibre-ends a residual shear stress of opposite sign (to that induced mechanically) leads to extension of birefringent patterns on loading.     

Predicting the tensile strength of natural fibre reinforced thermoplastics

The tensile strength of short natural fibre reinforced thermoplastics (NFRT) was modeled using a modified rule of mixtures (ROM) strength equation. A clustering parameter, requiring the maximum composite fibre volume fraction, forms the basis of the modification. The clustering parameter highlights that as fibre loading increases, the available fibre stress transfer area is decreased. Consequently, at high volume fractions this decrease in stress transfer area increases the brittleness of the short fibre composite and decreases the tensile strength of the material. A key parameter, the interfacial shear strength, was determined by fitting the micromechanical strength model to tensile strength data at low fibre loading (10 wt%) where there is minimal fibre clustering.To test the modified ROM strength model, compression molded specimens of high-density polyethylene (HDPE) reinforced with hemp fibres, hardwood fibres, rice hulls, and E-glass fibres were created with fibre mass fractions of 10–60 wt%. The modified ROM strength model was found to adequately predict the tensile strength of the various composite specimens.     

Development of the tensioned push-out test for study of fibre/matrix interfaces

Finite element modelling has been used to explore the basic single-fibre push-out test and a variant in which in-plane tension is simultaneously applied. Comparison with experimental photoelastic data has shown that shear-lag based analytical models of the test are unreliable, even for the simplest cases. It has been shown that the loading geometry and, particularly, the presence of thermal residual stresses can affect the interfacial stress state, which often tends to vary significantly along the length of the fibre. For a metal-matrix composite, the peak shear stress is predicted to occur near the bottom of the fibre, which is where debonding is expected to initiate. The proportion of normal and shear stresses at the interface can be altered by the application of in-plane tension. Some previously published experimental tensioned push-out data for the SiC/Ti system have been interpreted in the light of finite element modelling, allowing an estimate to be made of the interfacial coefficient of friction.     

Energy absorbing mechanisms in short-glass-fibre-reinforced polypropylene

The tensile load elongation curves to failure of samples cut along different directions from a extruded sheet of short-glass-fibre-reinforced polypropylene having good fibre alignment along the extrusion direction, were obtained at a strain rate of 0·01 min⁻¹. The work of rupture was calculated from the area under the stress extension curves. The contribution made by some of the energy absorbing mechanisms like plastic deformation of the matrix, debonding, and fibre pull-out to the total work have been considered in some detail. In the extrusion direction, and at angles up to 30° to this direction, the interfacial shear stress and load transfer are high. Consequently, the plastic deformation of the matrix througout the specimen volume, and particularly around fibre ends, makes a dominant contribution to the work of rupture, with a small but significant contribution from debonding and also from fibre pullout at the fracture surfaces. At larger angles, the interfacial shear stress and load transfer are relatively lower, and the work of rupture is considerably less. Supporting evidence for these observations from acoustic emission studies and electron micrographs of raptured surfaces is also presented.     

Fracture of plasma-sprayed thick thermal barrier coatings under multiaxial stress states

Failure behaviour of free‐standing plasma‐sprayed coatings was investigated under combined axial and shear loading. Thin‐walled tubular specimens were loaded with various combinations of tension/compression and torsion. This allows the failure surface to be established for loading situations where the two principal stresses are of opposite signs. Specimens failed in one of the two modes, a tensile failure perpendicular to the maximum principal stress or a compression shear failure through the thickness. Failure data were adequately described by the maximum principal stress theory. Stress–strain curves fall within a single scatter band depending on the failure mode. In situ deformation tests showed that the mechanism was microcrack closing and sliding in compression and microcrack opening, coalescence and the development of new microcracks in tension.     

Stress transfer in the fibre fragmentation test

The micromechanics of stress transfer has been analysed in a multiple-fibre composite which was subjected to uniaxial tension. The model composite was treated as a three-cylinder assemblage, which consisted of a central fibre, a matrix annulus and a composite medium. Analytical solutions have been derived for all major stress components for the composite with fully bonded fibre-matrix interface. A parametric study was performed on a carbon fibre-epoxy matrix composite. The result suggests that the principal effect of a stiff composite medium surrounding a discontinuous isolated fibre due to the high fibre volume fraction and stiff matrix, is to reduce the efficiency of stress transfer over the central potion of the fibre, while promoting the fibre-matrix interface shear stress concentration at the fibre end region. Practical implications of this observation with respect to fibre fragmentation and interfacial debonding are discussed.     

Stress transfer in the fibre fragmentation test

An improved micromechanics model has been developed of the stress transfer for a single fibre embedded in a matrix subjected to uniaxial loading. Debond crack growth is analysed based on the shear strength criterion such that when the interfacial shear stress reaches the shear bond strength, debonding occurs; and the average strength concept based on Weibull statistics is considered for fibre fragmentation. The influences of the interfacial shear bond strength and the fibre strength on the stress distributions in the composite constituents are evaluated. Depending on the relative magnitudes of these two strength parameters and given the elastic constants and geometric factors, three distinct conditions of the fibre-matrix interface are properly identified which include full bonding, partial debonding and full frictional bonding. Also quantified are the necessary criteria which must be satisfied in order for each interface condition to be valid. Finally, the mean fibre fragment length is predicted as a function of applied strain using a model composite of carbon fibre-epoxy matrix. The parametric study suggests that the critical transfer length predicted when the applied strain (or stress) required for further fibre fragmentation approaches infinity, can be regarded as a material constant, which is the sum of the bonded and the debonded lengths for the model composite.     

Interfacial shear stress in FRP-plated RC beams under symmetric loads

A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre-reinforced polymer (FRP) plates to their soffits. An important failure mode of such plated beams is debonding of the FRP plates from the concrete due to high level interfacial stresses near the plate ends. A closed-form rigorous solution for the interfacial stresses in simply supported beams bonded with thin plates and subjected to arbitrary loads has been found, in which a non-uniform stress distribution in the adhesive layer was taken into account. This paper uses the rigorous solution to investigate the impact of symmetric loading configurations on the interfacial shear stress distributions, and concludes that the bending moments on the cross sections at the plate ends play a significant role in terms of stress concentration, while the shear forces on the same cross-section contribute little to the concentration. On the basis of this observation, this paper proposes a simplified approximate solution to the shear stress along the interface between concrete and adhesive layer. Compared with the rigorous and other approximate solutions, the simplified solution exhibits sufficient accuracy in terms of stress distribution and stress concentration localized near the plate ends. Due to its compact feature, the simplified solution is more suitable for engineering applications using a portable calculator and to be adopted in the codes of practices.