Alkiviadis Paipetis

Abstract: 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. © 2010 Acoustical Society of America.

Abstract: The goal of the present study was to investigate the influence of multi-wall carbon nanotubes (MWCNTs) on the impact and after impact behaviour of carbon fiber reinforced polymer (CFRP) laminates. About 0.5% per weight MWCNTs were dispersed via a high shear device in the epoxy matrix (Bisphenol A) of carbon reinforced quasi-isotropic laminates. Subsequently, the modified CFRPs were subjected to low-energy impact and directly compared with unmodified laminates. In previous studies, the beneficial effect of the MWCNT inclusion to the fracture properties of CFRPs has been demonstrated. In terms of the CFRP impact performance, enhanced performance for the CNT doped specimens was observed for higher energy levels. However, the after-impact properties and more specifically compression after impact were improved for both the effective compression modulus and the compression strength. In addition, compression-compression fatigue after impact performance of the CNT modified laminates was also improved, by extending the fatigue life. © 2009 Elsevier Ltd. All rights reserved.

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. © Institute of Materials, Minerals and Mining 2009.

Abstract: The environmental durability of carbon nanotube (CNT)-modified carbon-fibre-reinforced polymers (CFRPs) is investigated. The key problem of these new-generation composites is the modification of their polymer matrix with nanoscaled fillers. It was recently demonstrated that the damage tolerance of these materials, as manifested by their fracture toughness, impact properties, and fatigue life, can be improved by adding CNTs at weight fractions as low as 0.5%. This improvement is mainly attributed to the incorporation of an additional interfacial area between the CNTs and the matrix, which is active at the nanoscale. However, this additional interface could have a negative effect on the environmental durability of the aforementioned systems, since it is well known that the moisture absorption ability of a matrix is enhanced by the presence of multiple interfaces, which serve as an ingress route to water. To examine this problem, CNT-modified CFRPs were exposed to hydrothermal loadings. At specified intervals, the composites were weighted, and the water uptake vs. time was recorded for both the modified and a reference systems. The electrical conductivity of the composites was registered at the same time intervals. After the environmental exposure, the interlaminar shear properties of the conditioned composite systems were measured and compared with those of unmodified composites, as well as with the shear properties of unexposed laminates. © 2009 Springer Science+Business Media, Inc.

Abstract: 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. © SAGE Publications 2009.

Abstract: 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. © 2009 SAGE Publications.

Abstract: The current study focuses on the fracture toughness behaviour of A359 aluminium matrix reinforced with 31 wt. % SiC particulates subjected to different heat treatment conditions. Unreinforced aluminium alloy fracture properties have been also determined for reference purposes. Three different heat treatment conditions have been applied to the Al/SiC_p composites and the fracture toughness values have been determined for all specimens. Infrared thermography was used to monitor the plane crack propagation behaviour of the materials and validate the fracture toughness testing. As expected, the obtained K_IC values were found to be lower than those of the unreinforced matrix alloy. However, heat treatment considerably improved the fracture toughness of the composites. In particular, the specimens heat treated under the T6 condition exhibited enhanced fracture toughness compared to the other two conditions. This behaviour can be attributed to a mechanism related to alterations in the microstructure at the vicinity of the interface induced by the heat treatment. This mechanism was associated with precipitates accumulated at the interfacial region resulting in material hardening.

Abstract: The present paper describes the acoustic emission (AE) behavior of concrete under four-point bending. Steel fibres of varying content were used as reinforcement in concrete slabs and their influence on the fracture process and the acoustic activity was investigated. The total acoustic emission (AE) activity was found to be directly proportional to the fibre content. Analysis revealed that particular AE parameters change monotonically with the progress of damage and can be used for the characterization of the failure process. © 2009 Elsevier Ltd. All rights reserved.

Abstract: 'Damage tolerance' is used to describe the attribute of a structure associated with the retention of the required residual strength throughout its service life, while irreversible damage mechanisms are active within the structure itself. 'Design for damage tolerance' is based on the identification and quantification of the various damage mechanisms that result in the alteration (mainly deterioration) of the material properties. These may alter the material response to thermomechanical loads. In the present paper, transparent glass fibre reinforced epoxy laminates were used to study the damage evolution sequence under tensile loading. Acoustic emission was employed as a non-destructive technique for the in situ monitoring of the active damage mechanisms until the final failure of the material. Pattern recognition algorithms were utilised to classify the acquired acoustic emission signals and associate them to active damage mechanisms. Experimental findings were compared to theoretical model predictions. © Institute of Materials, Minerals and Mining 2009.

Abstract: Vapor growth carbon nanofibers (CNF), lead zirconate titanate piezoelectric (PZT) particles, as well as a combination of these two were added in an epoxy resin (EP), and their influence on the mechanical quasi-static properties was investigated. Moreover, the prepared samples were characterised by dynamic thermal mechanical analysis, and optical and scanning electron microscopy. Enhancement of the mechanical properties was observed by the addition of the CNF. The uncured mixtures were also used as matrix material for manufacturing unidirectional carbon fiber reinforced laminates. © 2006 Elsevier Ltd. All rights reserved.

Abstract: The goal of the present study was to investigate the influence of carbon nanofibers (CNF) and/or piezoelectric (PZT) particles on the fracture behaviour of carbon fiber reinforced polymer laminates. For this purpose the fillers were added as dopants in the epoxy matrix of the laminates. An increase of 100% in fracture energy was observed after the addition of 1% CNF in the matrix of the laminates, while the introduction of PZT particles led to reduction in fracture energy, mainly due to the brittle character of the particle inclusions. In addition, the acoustic emission technique was used for monitoring the fracture process of the laminates. © 2006 Elsevier Ltd. All rights reserved.

Abstract: The present work deals with the development of anisotropic damage in alumina/alumina continuous fiber ceramic composites (CFCCs). The composites were isothermally exposed to a corrosive/high temperature environment at 1100°C, which simulates the working conditions of a gas turbine. Stiffness matrix components and strength were experimentally defined as a function of exposure duration by means of ultrasonic stiffness measurements and quasi-static tensile tests. In order to determine the stiffness matrix components, a new ultrasonic stiffness characterisation technique was employed. According to this method, the through transmission phase velocities are measured using a custom built immersion set-up. The experimental data are subsequently used in order to solve the inverse scattering problem and reconstruct the stiffness matrix of the composite at successive thermal exposure levels. The stiffness matrix of the composite was assumed to be orthotropic. Damage functions were formulated to describe the high temperature/corrosive exposure effect on the stiffness matrix of the composite. Finally, quasi-static tensile tests were used to assess the stiffness reduction of the composite and compare the values to those acquired non-destructively. The effect of exposure time on the strength of the composite was determined in the same way. © 2005 Elsevier Ltd. All rights reserved.

Abstract: The monitoring of the elastic properties of Al₂O ₃/Al₂O₃ composites during the exposure at high temperature environment that simulates the working conditions of a gas turbine has been performed non-destructively using ultrasonics. The applied methodology is based on velocity measurements of the elastic waves that propagate in an orthotropic medium. These were estimated experimentally using a custom pulser-receiver setup which allows control of the angle of the incident pulse on the sample, while the latter is immersed in a water bath. The derivation of the elastic constants in order to reproduce the stiffness matrix of the composite is an inverse wave propagation problem described by the Christoffel equation. The damage initiation and propagation as depicted by the deterioration of the moduli of the material was described using deterministic and stochastic approaches. Finally, the damage accumulation process was simulated as a Markov process.

Abstract: The fatigue behaviour of composite panels that have been subjected to low-velocity impact was studied. Impacted specimens were tested under compression-compression fatigue. A delamination propagation model based on the derivation of the strain energy release rate was used. The stress distribution around the initially induced delamination was derived analytically. The shape of the delamination was experimentally monitored by c-scan imaging and is assumed to be an ellipse. The orientation and aspect ratio of the ellipse were used to calculate the corresponding strain energy-release rates, which were subsequently used to predict the direction of delamination.propagation. © 2004 Blackwell Publishing Ltd.

Abstract: 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. © 2003 Elsevier Ltd. All rights reserved.

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. © 2003 Elsevier Ltd. All rights reserved.

Abstract: This study involved the investigation of the micromechanics of reinforcement of model carbon fibre-epoxy composites using the technique of remote laser Raman microscopy. The technique allows in situ axial stress monitoring in highly crystalline fibres, such as carbon or Kevlar®. Model composites were subjected to incremental tensile loading, while the stress in the fibre was monitored at each level of applied strain. The stress-transfer regime was studied in the elastic domain using a model single-fibre composite, where a fibre of finite length (i.e. of a length smaller than the coupon gauge length) was embedded along a resin coupon. A shear lag approach was employed to model the stress-transfer efficiency of the interface through the use of the shear-lag parameter β. The stress build-up in the fibre in the presence of energy dissipation mechanisms such as fibre fractures was modelled, and the stress-transfer efficiency was quantified at different levels of applied composite strain. Parallels between the interfacial efficiency of single-fibre systems and practical composites are drawn.

Abstract: The effect of specimen geometry upon the parameters that govern the stress transfer (transfer length, interfacial shear strength, positively affected length and stress concentration factor) in a carbon fibre/epoxy composite was examined in detail. Five frequently employed composite geometries were considered: single fibre composite coupons incorporating discontinuous and continuous carbon fibres, multi-fibre composite tapes with a controlled inter-fibre separation, four-ply unidirectional coupons, and, finally, impregnated fibre tows. The chemistry and the curing characteristics of the matrix were kept unaltered regardless of specimen geometry. All fibre stress measurements were conducted by means of Raman microscopy. The experimental results showed that the transfer length, positively affected length (PAL) and the interfacial shear stress obtained at the location of a fibre fracture are not considerably affected by specimen geometry. In contrast, the residual fibre stress of the unloaded specimens and the stress concentration factors obtained in fibres adjacent to a fibre fracture site were found to be significantly dependent upon fibre volume fraction and specimen geometry. Copyright ©1999 John Wiley & Sons, Ltd.

Abstract:

Abstract: 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. © 1998 Chapman & Hall.

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 inter facial 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. © 1997 Elsevier Science Limited.

Abstract: 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.

Abstract: The fiber/matrix interaction of high modulus (M40) and intermediate modulus (T800) carbon fibers with a bismaleimide resin has been studied by means of three micromechanical techniques involving a single fiber, namely, fragmentation, Raman spectroscopy, and pullout. A number of chemical treatments aimed at improving the fiber/matrix stress transfer at elevated temperature were tested. The stress transfer proved to be reduced by the temperature in the same way for all interfacial conditions. The limitations to the micromechanical characterization of model composites in temperature are emphasized.

Abstract: The micromechanics of stress transfer in single-fiber as well as multi-fiber composites were investigated. The material system under investigation consisted of high modulus carbon fibers embedded in an epoxy resin. The point-by-point stress in the fiber was measured using the newly developed technique of remote laser Raman microscopy (ReRaM). The composite specimens were loaded incrementally in tension and the stress transfer profiles emanating from fiber discontinuities, such as fiber breaks, were closely monitored. At each applied stress level, the interfacial shear stress (ISS) distribution was derived by means of a balance of shear-to-axial forces argument. In the single-carbon fiber/epoxy system, a maximum interfacial shear stress of 30 MPa was reached at the point of first fiber fracture. In the multi-fiber carbon fiber/ epoxy system, the maximum interfacial shear stress developed at the point of first fiber fracture was of approximately the same magnitude. Finally, the local stress concentration in the intact fibers, as a result of an adjacent fiber fracture, was determined as a function of distance from the fiber fracture for three distinct levels of applied stress. A maximum stress concentration of approximately 1.2 was measured at the maximum applied composite strain level of 0.5%. This value compared well with existing analytical models.

Abstract: A new stress/strain sensor for localised measurements in polymer based composites, has been developed and tested. The stress/strain dependent property is the frequency of the atomic vibrations of reinforcing fibres which can be probed with laser Raman spectroscopy. Measurement can be conducted in reinforcing fibres on the surface of laminates. For measurements in the bulk of composites, the exciting laser light has to be transported to the reinforcing fibres via an embedded fibre optic cable. The backscattered light is transmitted through the same fibre optic and is sent to the Raman spectrometer for analysis. The effect of the direction of the fibre optic cable with respect to the axis of the reinforcing fibres is examined. Finally, the relationships between the local fibre stress or strain obtained form the Raman sensor and the far field stress or strain measured conventionally, are established.

Abstract: 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.

Abstract: Bonded repair offers significant advantages over mechanically fastened repair schemes as it eliminates local stress concentrations and seals the interface between the mother structure and the patch. However, it is particularly difficult to assess the efficiency of the bonded repair as well as its performance during service loads. Thermography is a particularly attractive technique for the particular application as it is a non-contact, wide field non destructive method. Phase thermography is also offering the advantage of depth discrimination in layered structures such as in typical patch repairs particularly in the case where composites are used. Lock-in thermography offers the additional advantage of on line monitoring of the loaded structure and subsequently the real time evolution of any progressive debonding which may lead to critical failure of the patched repair. In this study composite systems (CFRP plates) with artificially introduced defects (PTFE) were manufactured. The aforementioned methods were employed in order to assess the efficiency of the thermographic technique. The obtained results were compared with typical C-scans. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Abstract: Acoustic emission is a powerful technique for identifying and monitoring the evolution of service induced degradation in structural components and localising damage. The present study is dedicated to the investigation of model composite systems in order to identify, locate and quantify service induced damage. These systems are cross ply translucent glass fibre reinforced composite materials. In cross ply composites, service induced primary damage is manifested in the form of matrix cracking of the off-axis layers. For the purposes of this study, the cross ply composite were subjected to step loading with the concurrent recording of the acoustic activity. At specific intervals of the loading process the propagation characteristics of ultrasonic waves were also recorded using the acoustic emission sensors in a pulser-receiver setup. The acoustic emission activity has been successfully correlated to damage accumulation of the cross ply laminates, while specific acoustic emission indices proved sensitive to the various modes that evolve during the loading. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Abstract: The acoustic emission (AE) behaviour of steel fibre reinforced concrete is studied in this paper. The experiments were conducted in four-point bending with concurrent monitoring of AE signals. The sensors used, were of broadband response in order to capture a wide range of fracturing phenomena. The results indicate that AE parameters undergo significant changes much earlier than the final fracture of the specimens, even if the AE hit rate seems approximately constant. Specifically, the Ib-value which takes into account the amplitude distribution of the recent AE hits decreases when the load reaches about 60-70 % of its maximum value. Additionally, the average frequency of the signals decreases abruptly when a fracture incident occurs, indicating that matrix cracking events produce higher frequencies than fibre pull-out events. It is concluded that proper study of AE parameters enables the characterization of structural health of large structures in cases where remote monitoring is applied. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Abstract: This study deals with new generation composite systems which apart from the primary reinforcement at the typical fiber scale (~10 μm) are also reinforced at the nanoscale. This is performed via incorporation of nano-scale additives in typical aerospace matrix systems, such as epoxies. Carbon Nanotubes (CNTs) are ideal candidates as their extremely high aspect ratio and mechanical properties render them advantageous to other nanoscale materials. The result is the significant increase in the damage tolerance of the novel composite systems even at very low CNT loadings. By monitoring the resistance change of the CNT network, information both on the real time deformation state of the composite is obtained as a reversible change in the bulk resistance of the material, and the damage state of the material as an irreversible change in the bulk resistance of the material. The irreversible monotonic increase of the electrical resistance can be related to internal damage in the hybrid composite system and may be used as an index of the remaining lifetime of a structural component. © 2009 SPIE.

Abstract: This work deals with the AE behavior of concrete under four-point bending. Different contents of steel fibers were included to investigate their influence on the load-bearing capacity and on the fracture mechanisms. The AE waveform characteristics revealed that, although tension was the dominant mechanism of fracture for the plain material, the increase in the fiber content resulted in extension of the shear failure due to improvement of the weak tensile properties of concrete. Appropriate AE indices employed for early warning prior to macroscopic failure can lead to more suitable design of the reinforcement, in order to withstand the specific stresses. © 2009 SPIE.

Abstract: This work deals with the development of a novel non-destructive technique based on nonlinear acoustics, enabling real-time monitoring of material degradation in aerospace structures. When a sinusoidal ultrasonic wave of a given frequency and of sufficient amplitude is introduced into a nonlinear or an-harmonic solid, the fundamental wave distorts as it propagates, so that the second and higher harmonics of the fundamental frequency are generated. The measurement of the amplitude of these harmonics provides information on the coefficient of the second and higher order terms of the stress-strain relation for a nonlinear solid. It is demonstrated here that the material bulk nonlinear parameter for titanium alloy samples at different fatigue levels exhibits large changes compared to linear ultrasonic parameters such as velocity and attenuation. However, the use of bulk ultrasonic waves has serious disadvantages for the health monitoring of aerospace structures since it requires the placement of ultrasonic transducers on two, perfectly parallel, opposite sides of the samples. Such a setup is hardly feasible in real field conditions. For this reason, surface acoustic waves (SAW) were used in this study enabling the in-situ characterization of fatigue damage. The experimental setup for measuring the material nonlinear parameter using SAW was realised and the feasibility of the technique for health monitoring of aerospace structures was evaluated. © 2008 American Institute of Physics.