Dionisios Katerelos

Abstract:

Abstract: Stretching effects on the morphology of polypropylene/carbon nanotube composites is the focus of this work. The material under investigation was composed of isotactic polypropy lene (iPP) and multiwall car bon nanotubes (MWCNTs) in amount of 0.5 wt.%. The iPP and CNTs were mixed under sonication in a solvent, and the homogenized mixture was melted and pressed. The rectangular plates produced from the material were stretched by a constant load at a fixed temperature in order to obtain extended specimens. The scanning electron microscopy, a thermal gravimetric analysis, and the differential scanning calorimetry were employed to study variations in the structural morphology of the material. A dynamic mechanical analysis showed that the strain-induced crystallization of polypropylene and the possible Stone-Wales transformation of the carbon nanotubes due to stretching improved the mechanical performance of the nanocomposites considered. © 2009 Springer Science+Business Media, Inc.

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: Composites ability to retain functionality in the presence of damage is a crucial safety and economic issue. Generally the first damage mode in composite laminates is matrix cracking, which affects the mechanical properties of the structure long before its load-bearing capacity is exhausted. In this paper, a detailed analysis of the effect of matrix cracking on the behaviour of cross-ply [0/90]_s and unbalanced symmetric [0/45]_s glass/epoxy laminates loaded statically in tension is performed. Theoretical predictions of stiffness reduction due to damage are based on the Equivalent Constraint Model (ECM), which takes into account concurrent matrix cracking in all plies of the laminate, although matrix cracking under consideration is developing only within the off-axis ply of the laminates. The longitudinal Young's modulus predictions are compared to experimentally derived data obtained using laser Raman spectroscopy (LRS). The good agreement between predicted and measured values of the reduced longitudinal Young's modulus validates the ECM model and proves that its basic assumptions are accurate. Thus, the predictions for all the mechanical properties by the ECM model are within a realistic range, while experimental evidence is required for further validation. © 2007 Elsevier Ltd. All rights reserved.

Abstract: The energy dissipated in cross-ply laminates during loading-unloading loops is obtained from stress-strain curves for cross-ply laminates and used in an energy based approach to predict the development of matrix cracking. The dissipated energy is correlated to the crack density growth data recorded for a reference laminate. The critical strain energy release rate, G_c obtained in this way is increasing with the applied strain. This phenomenon reflects the statistical nature of G_c distribution in the 90-layer: the first cracks (lower strain) develop in positions with lower fracture toughness. The obtained G_c data are in a good agreement with fracture toughness data obtained using LEFM based "compliance calibration" model in which the stiffness change with increasing strain is used. Finally, the matrix cracking development is successfully simulated using in the LEFM model, the data for critical strain energy release rate and an earlier derived stiffness-crack density relationship. It has been demonstrated that knowing the laminates geometry and measuring the laminate stiffness reduction with strain or (alternatively measuring the dissipated energy) the damage evolution may be simulated, thus reducing the necessity for optical observations to validation only. © 2007 Elsevier Ltd. All rights reserved.

Abstract: A Raman spectroscopic technique was used to investigate the elastic modulus degradation and the residual strain development in cross-ply and [0/45₂/0]_T glass fibre/epoxy laminates. Glass has a poor Raman signal and, therefore, embedded aramid fibres were used as strain sensors for mapping of the internal strain development within the 0° ply due to matrix cracking in the off-axis ply. The elastic modulus and the residual strain were derived from the changes of the distance between well identified strain "peaks" on the aramid fibres. The elastic modulus reduction and the residual strain development are described by a closed form model. The modulus reduction predictions are accurate, whereas the experimental residual strain is higher than that predicted. Since both phenomena are governed by the same elastic parameters, the discrepancy cannot be explained by purely elastic analysis. Development of inelastic strains in the off-axis layer is suggested as a possible cause. © 2006 Elsevier Ltd. All rights reserved.

Abstract: Matrix cracking models developed for cross-ply composite laminates cannot easily be applied to more complicated geometries. In this paper a detailed analysis of the effect of matrix cracking on the longitudinal Young's Modulus of a [0/45]_s plate under uniaxial tension is attempted. The theoretical approach, based on a semi-analytical generalized plane strain model, is compared to experimental data obtained by microscopic strain measurements on a fiber sensor using the technique of laser Raman Spectroscopy. The experimental results are in a good agreement with theoretical stiffness degradation predictions obtained using the semi-analytical model. © Springer 2006.

Abstract: Among the principal damage modes in composite laminates, is delamination. Design details, such as free, straight or curved, edges, induce large local out-of-plane loads, generating interlaminar stresses. In the present work, the effect of ply thickness and the angle between two adjacent layers on the inter laminar stresses developed at the vicinity of straight and curved free edges in composite laminates under thermomechanical loading is examined. The results are obtained by the application of a 3-dimensional Finite Element Analysis.

Abstract: The matrix cracking models developed for cross-ply composite laminates have been poorly extended in the past to more complex geometries used in practice, and they are still under development. In this paper, a new detailed analysis of the effect of matrix cracking on the behaviour of cross-ply and [0/45] _s laminates under uniaxial tension is attempted. The model used in this work is applicable both to cross-ply laminates and unbalanced systems. It gives exact closed-form expressions for all thermomechanical properties of a general symmetric laminate with cracks in arbitrary layers. The theoretical approach is backed by experimental data obtained by microscopic strain-state variation measurements within a specimen, with using the technique of laser Raman spectroscopy. Glass-fibre-reinforced epoxy systems were investigated. Embedded aramid fibres-sensors within the 0° ply and near the 0°/θ ° interface were necessary due to the poor Raman signal of glass. Using experimental Raman data, the residual strain and the stiffness reduction are determined as functions of increase in crack density. The stiffness reduction is predicted with a high accuracy, whereas the measured residual strains are larger than predicted. The good results for the reduction in the elastic modulus show that the basic assumption of the model is accurate. The difference is explained by the viscoelastic-viscoplastic behaviour of the off-axis layer in shear, which in creases the "apparent" residual strain. © Springer Science+Business Media, Inc. 2006.

Abstract: Individual aramid Kevlar^® 49 fibers were embedded into the 0° ply of transparent cross-ply glass fiber reinforced polymer (GFRP) laminates and used as Raman sensors for the monitoring of the local strain field due to matrix cracking under quasi-static loading. The sensors were placed near the 0/90 interface, and the resulting strain within the 0° ply and its dependence on the geometry of the specimen due to 90° cracking were recorded. The decay of the strain magnification with the distance from the crack front, the residual strain and the axial modulus of elasticity reduction were derived from the strain distributions at each separate level of loading. The results obtained were compared with shear-lag calculations as well as numerical and analytical models. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Abstract: The design and construction is described of a vehicle bridge made of glass-reinforced polyester pultruded box beams. The bridge has a simply supported span 11·6 m long and 4 m wide. It has been designed as a Class 30 (300 kN load capacity) according to DIN 1072 and represents a single traffic lane. The composite bridge consists of a 3-D truss structure made of thick-wall fibre-reinforced plastic longitudinal box elements of hollow square cross-section. The bridge design proposed allows for fast construction as it consists of pre-fabricated, ready to assemble elements. The total of the composite bridge does not exceed 135 kN. © 2005 Institute of Materials, Minerals and Mining.

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: The initial mode of damage in multidirectional composites is the accumulation of matrix cracks in the off-axis plies. Remote laser Raman spectroscopy, using aramid (Kevlar 49®) fibers as Raman strain sensors, has been employed to monitor the local strains in cracked crossply composites. The strain magnification in the 0° ply caused by 90° matrix cracking are measured at different levels of loading. A relationship between the values of strain magnification and the distance between the position of the fibers sensors vis-à-vis the crack front has been established. The effect of damage progression within the 0° plies on the remaining/surviving glass fibers is modeled satisfactorily and verified against predictions on the basis of a generalized plane strain model by assuming that the 90° ply "expands" in relative size against the 0° ply thickness. © 2004 American Institute of Physics.

Abstract: A multifunctional non-destructive technique, based on the Raman response of aramid fibers, is employed for (a) assessing the interface integrity and the overall stress distribution in a unidirectional Kevlar 29^®/epoxy composite (b) measuring the stress concentration and its development with applied load in Kevlar 49^®/epoxy composites incorporating a circular notch and finally, (c) determining the strain arising in 0° plies due to partial and full crack growth within the 0° plies in multidirectional 0°/0°/0° composites. It is shown that this technique can be used effectively to study the damage development in composite materials that arise from the presence of different length scale discontinuities. © 2002 Elsevier Science Ltd. All rights reserved.

Abstract: When designing composite materials, the presence of stress concentration at locations such as circular notches is unavoidable. Such locations in structural elements arise from joints required to form a structure. The stress concentration, observed around the notch, is quantified by the stress concentration factor K. This quantity is normally calculated analytically and/or numerically and is an important design parameter. In this work, the experimental technique of remote laser Raman microscopy is used for the in situ measurement of K in Kevlar 49 fiber/epoxy composite plates containing a circular hole. The results obtained by this technique are compared with those calculated analytically and by finite element analysis. Both analytical and numerical methods underestimate the experimental results for maximum K by approximately 10 percent, which is considered reasonable within experimental error. In addition, very good agreement between analytical and experimental data is obtained for the decay of the stress concentration factor as a function of distance from the edge of the hole. The numerical results, however, overestimate the decay K with distance from the notch boundary and only converge at relatively large distances. When designing composite materials, the presence of stress concentration at locations such as circular notches is unavoidable. Such locations in structural elements arise from joints required to form a structure. The stress concentration, observed around the notch, is quantified by the stress concentration factor K. This quantity is normally calculated analytically and/or numerically and is an important design parameter. In this work, the experimental technique of remote laser Raman microscopy is used for the in situ measurement of K in Kevlar 49 fiber/epoxy composite plates containing a circular hole. The results obtained by this technique are compared with those calculated analytically and by finite element analysis. Both analytical and numerical methods underestimate the experimental results for maximum K by approximately 10 percent, which is considered reasonable within experimental error. In addition, very good agreement between analytical and experimental data is obtained for the decay of the stress concentration factor as a function of distance from the edge of the hole. The numerical results, however, overestimate the decay K with distance from the notch boundary and only converge at relatively large distances.

Abstract: Composite laminates are used for the construction of a wide range of products; from civil infrastructure to aeronautical or space structures. Since all these structures cannot be built at once, but they are composed by several structural elements connected in various patterns, the problem of stress concentrations strikes importantly. Additionally, the structures are often designed in a "fail safe" manner, which bespeaks the damage tolerance of the material. This means that the structural element and the structure are called to operate in the presence of various forms of damage. Damage locations within a structural element or a structure cause stress rising. In the present paper experimental investigation of the stress concentration arising in composite laminates, (a) around a circular notch and (b) due to damage onset and growth during their lifetime, is presented. The experimental results are compared to analytical model predictions.