A. Paipetis

Enhanced Fracture Properties of Carbon Reinforced Composites by the Addition of Multi-Wall Carbon Nanotubes

In the present study, the fracture energy of hybrid carbon fiber reinforced polymers was investigated. The composites were modified by the addition of multi-walled carbon nanotubes into the matrix material. The interlaminar fracture properties under Mode I and Mode II remote loading were studied as a function of the carbon nanotube content in the matrix. With the addition of carbon nanotubes in the epoxy matrix, a significant increase in the load bearing ability as well as in the fracture energy was observed, for both Mode I and Mode II tests. It is speculated that carbon nanotubes due to their large aspect ratio have a significant toughening effect since extra energy is needed in order to pull them out from the matrix and start the crack propagation following a kinking out pattern at nanoscale.

Effect of fibre sizing on the stress transfer efficiency in carbon/epoxy model composites

The micromechanics of reinforcement of a model composite consisting of continuous high-modulus fibre embedded in epoxy resin has been investigated as a function of fibre sizing. The composite was subjected to incremental tensile loading up to full fragmentation, while the stress in the fibre was monitored at each level of applied strain with the new technique of remote laser Raman microscopy. The two systems exhibited differences in the residual stress field with the unsized fibre being in compression. The average stress in the fibre increased linearly with applied matrix strain up to first fracture. After fracture, the stress in the fibre was found to build from the tips of the fibre breaks, reaching a maximum value at the middle of each fragment. The shape of the stress transfer profiles indicated minor differences between the two systems at moderate strains. At high strains, the stress transfer profiles of the two systems were distinctly different possibly owing to the presence of two different interfacial failure modes in the two types of model composites. The maximum interfacial shear stress for both systems was of the order of 40 MPa with the sized system exhibiting slightly better adhesion. SEM examination of the fracture surfaces revealed clear interfacial failure for the unsized system whereas the sized system indicated areas of good adhesion.

Damage Monitoring of Carbon Fiber Reinforced Laminates Using Resistance Measurements. Improving Sensitivity Using Carbon Nanotube Doped Epoxy Matrix System

In this study, carbon nanotubes (CNTs) were used as additives in the epoxy matrix of unidirectional (UD) carbon fiber reinforced laminates. CNTs were employed not only for improving the mechanical performance of composite, but also for providing an innovative way for monitoring the damage accumulation during the loading of the laminate via the monitoring of the changes in the conductivity of the material. The CNT inclusion improved the transverse conductivity rendering the UD composite more electrically isotropic. Both plain and modified laminates were subjected to monotonic and cyclic tensile loading with simultaneous monitoring of the longitudinal resistance. The resistance change due to mechanical loading was more pronounced for the laminates with the modified matrices. As it was expected, the presence of the electrically conductive CNT network acted as a direct sensor of matrix related damage phenomena, which was complementary to changes related to failure of the reinforcing phase. This was not the case for the reference composite laminates where the monitored changes of the electrical conductivity mirrored the damage exclusively related to the reinforcing phase i.e., the carbon fibers.

Use of NIR for structural characterization of urea–formaldehyde resins

Abstract In this paper, the effect of pH and temperature on the structure of urea–formaldehyde resins was studied. GPC, NMR and Raman measurements were performed to elucidate the structural characteristics of the resin systems. Fourier Transform Near Infrared (FT-NIR) spectroscopy via optical fibers was used to monitor the reaction progress in situ. It was found that the reactions of urea and formaldehyde at different temperatures and pH values result in resins with different structures and properties: Resins produced at high temperatures and acidic pH values exhibit higher degrees of condensation, presumably because of the development of more cross-linked structures.

REMOTE LASER RAMAN MICROSCOPY (RERAM). 1 : DESIGN AND TESTING OF A CONFOCAL MICROPROBE

A confocal remote fibre-optic probe has been designed and tested. The theoretical design rationale for the development of the probe is presented in detail. Tailor-made optics have been introduced at both input and output positions of each fibre optic to ensure laser collimation, maximum efficiency and enhancement of Raman scattering. The probe design takes advantage of the pinhole nature of the optical fibre to apply the principles of confocal microscopy. In addition, interchangeable optics provide variable depth discrimination. The incorporation of a miniaturized video camera on the body of the microprobe allows simultaneous optical imaging during Raman spectra acquisition. The efficiency and versatility of the microprobe for a whole range of materials are demonstrated.

Use of FT-NIR spectroscopy for on-line monitoring of formaldehyde-based resin synthesis

A new method is being developed for the fast and reliable assessment of the pathway(s) followed during formaldehyde-based resin synthesis, both at laboratory and industrial scale. The method is based on Fourier transform Near Infrared (FT-NIR) chemometrics. No sample manipulation is necessary and the complete evaluation can be performed on- or off-line in less than 1 min. FT-NIR chemometrics were found to be valuable in providing a fast and consistent way of monitoring directly the effects of a change of resin formulation when evaluating new procedures at laboratory scale. Similarly, during industrial production, NIR will soon become a standard tool for ensuring reproducibility and improving overall quality. Measurements are performed on-line and deviations from the standard synthesis pathway can be detected early, allowing the necessary steps to be taken in order to return to the desired pathway. Furthermore, NIR methodologies have been developed to identify and check the conformity of raw materials and final products from urea and UFC solutions to laminated paper produced by impregnation with formaldehyde-based resins. This can prove particularly useful in applications (such as in laminated paper production) where the reproducibility of production and the effects of storage are both questionable and difficult to assess.

Acoustic emission monitoring of degradation of cross ply laminates

The scope of this study is to relate the acoustic activity of damage in composites to the failure mechanisms associated with these materials. Cross ply fiber reinforced composites were subjected to tensile loading with recording of their acoustic activity. Acoustic emission (AE) parameters were employed to monitor the transition of the damage mechanism from transverse cracking (mode I) to delamination (mode II). Wave propagation measurements in between loading steps revealed an increase in the relative amplitude of the propagated wave, which was attributed to the development of delamination that confined the wave to the top longitudinal plies of the composite.

Multistage fatigue life monitoring on carbon fibre reinforced polymers enhanced with multiwall carbon nanotubes

Abstract In this study, CNTs were used as modifiers of the epoxy matrix of quasi-isotropic carbon fibre reinforced laminates. The prepared laminates were subjected to tensile loading and tension–tension fatigue and, the changes in the longitudinal resistance were monitored via a digital multimeter. In addition, acoustic emission and acoustoultrasonic techniques were used for monitoring the fatigue process of the laminates. The nanoenhanced laminates, on the one hand, exhibited superior fatigue properties and on the other hand, they demonstrated the full potential of the material to be used as an integrated sensor to monitor the fatigue life.

Definition and measurement of the shear-lag parameter, β, as an index of the stress transfer efficiency in polymer composites

The shear-lag parameter, β, employed in various problems of shear-lag analysis of composites is an unknown parameter which, in certain cases, is impossible to define. In this paper, a new methodology is proposed for the definition and subsequent experimental measurement of β for various single-fibre model composites. It is argued that, if β is defined as a fitting parameter for the solution of the shear-lag differential equation, then it can effectively serve as a stress-transfer efficiency index. The dependence of β upon the conditions prevailing at the fibre–matrix interface will be demonstrated by measuring β as a function of the fibre sizing in a carbon–epoxy composite system.

A study of the stress-transfer characteristics in model composites as a function of material processing, fibre sizing and temperature of the environment

Abstract The micromechanics of reinforcement of a model composite consisting of a high-modulus fibre embedded in epoxy resin has been investigated as a function of processing conditions, namely thermal stresses, fibre sizing, and temperature. The residual stresses on single-fibre coupons were monitored for both long- and short-fibre geometries with the technique of remote laser Raman microscopy (ReRaM). The systems studied consisted of sized and unsized fibre/epoxy systems at room temperature as well as a sized system at 60 °C. Each composite was subjected to incremental tensile loading up to full fragmentation, while the stress in the fibre was monitored at each level of applied strain. The three systems exhibited differences in the residual stress field, with the unsized fibre being in compression. The average stress in the fibre increased linearly with applied matrix strain up to first fracture. After fracture, the stress in the fibre was found to build from the tips of the fibre breaks, reaching a maximum value at the middle of each fragment. Two different interfacial failure modes were identified, depending on the possible initiation of a mixed-mode matrix crack. At room temperature, the maximum interfacial shear stress for both systems was of the order of 40 MPa with the sized system exhibiting slightly better adhesion. At 60 °C, the sized system exhibited interfacial shear stress values of the order of 20 MPa.