Abstract: A diagnostic methodology of artificial defects in a single stage gearbox operating under various load levels and different defect states is proposed in the present work based on vibration recordings as well as advanced signal analysis techniques. Two different wavelet-based signal processing methodologies, using the discrete as well as the continuous wavelet transform, were utilised for the analysis of the recorded vibration signals and useful diagnostic information were extracted out of them. Both wavelet analysis techniques provided the ability of distinguishing between the healthy and the artificially defected gears. In this way, the health monitoring potential of vibration monitoring in the case of rotating machinery and gearboxes obtains a new dynamic under the prism of sophisticated time-frequency signal processing schemes, rather than conventional FFT-based approaches.
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. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: This study focuses on the nondestructive monitoring of damage developed in carbon fiber reinforced polymers during fatigue loading via in situ acousto-ultrasonics (AU) measurements. Carbon nanotubes (CNTs) were used as modifiers of the epoxy matrix of quasi-isotropic carbon fiber reinforced laminates and consequently two composite material systems (one with matrix doped with CNTs and one with nondoped matrix) were manufactured and tested. The AU technique is utilized toward the monitoring of damage development, evolution, and accumulation during the fatigue tests. The AU waveforms acquired are processed using conventional (both in time and in frequency domain) as well as innovative wavelet-based signal processing techniques to extract parameters-indicators capable of monitoring and quantifying the damage accumulation into the test coupons. Based on the analysis of AU monitored signals, critical descriptors (parameters) were identified that show significant variation and monotonic behavior throughout the tests and thus can be proposed as potential candidate parameters for monitoring the damage development in composite materials. POLYM. COMPOS., 31:1409-1417,2010. (C) 2009 Society of Plastics Engineers
Abstract: The condition monitoring of a lab-scale. single stage, gearbox using different non-destructive inspection methodologies and the processing of the acquired waveforms with advanced signal processing techniques is the aim of the present work. Acoustic emission (AE) and vibration measurements were utilized for this purpose. The experimental setup and the instrumentation of each monitoring methodology are presented in detail. Emphasis is given on the signal processing of the acquired vibration and acoustic emission signals in order to extract conventional as well as novel parameters-features of potential diagnostic value from the monitored waveforms. Innovative wavelet-based parameters-features are proposed utilizing the discrete wavelet transform. The evolution of selected parameters/features versus test time is provided, evaluated and the parameters with the most interesting diagnostic behaviour are highlighted. The differences in the parameters evolution of each NDT technique are discussed and the superiority of AE over vibration recordings for the early diagnosis of natural wear in gear systems is concluded. (C) 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.
Abstract: A plane wave is scattered by an acoustically soft or hard sphere, covered by a penetrable non-concentric spherical lossless shell that disturbs the propagation of the incident wave field. The dimensions of the coated sphere are much smaller than the wavelength of the incident field. Low-frequency theory reduces this scattering problem to a sequence of potential problems, which can be solved iteratively. Exactly one bispherical coordinate system exists that fits the given geometry of the obstacle. For the case of a soft and hard core, the exact low-frequency coefficients of the zeroth and the first-order for the near field as well as the first- and second-order coefficients for the normalized scattering amplitude are obtained and the cross sections are calculated. Discussion of the results and their physical meaning is included. Copyright (C) 2009 John Wiley & Sons, Ltd.
Abstract: Purpose: The purpose of this Study was to evaluate the biomechanical results of meniscal repair in vitro by a nitinol suture and compare them with Ethibond (Ethicon, Somerville, NJ) and nylon. Methods: The 6 testing groups consisted of nitinol, Ethibond, and nylon sutures (No. 2-0 and No. 3-0). Sixty bovine menisci with a vertical longitudinal tear were repaired with I horizontal mattress Suture and were fully immersed into a water bath, adjusted to a tensile testing machine. All specimens were subjected to tensile testing, and force/displacement curves were obtained. Load to 5-mm gap, load to failure, tensile strength, stiffness, and mode of failure for each suture group were recorded. Statistical analysis included analysis of variance with Bonferroni correction for the post hoc multiple comparisons. Results: Nylon sutures achieved the lowest scores in all measurements. Nitinol achieved better scores, but not significantly better scores, than Ethibond in load to 5-mm gap and stiffness. The No. 2-0 and No. 3-0 nitinol suture repair showed the highest mean tensile strength and load to failure, with significant differences, being 36% and 45% stronger, respectively, than Ethibond. Modes of failure included pulling through the inner segment of meniscus and rupture of the suture at the knot. Conclusions: This study shows the superior load-to-failure and tensile strength characteristics of nitinol. However, in terms of stiffness and gap resistance force, the results were equivalent to those of Ethibond. Nitinol is an interesting and promising suture.
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.
Abstract: The present work, first of two parts, deals with three types ofwoven carbon/carbon (C/C) composites having differentiations during the manufacturing procedure, which influences their fibre/matrix interface. All material types were tested under tensile loading in a load-unload-reload configuration, with online acoustic emission monitoring. Unsupervised pattern recognition algorithms were utilized to classify the acoustic emission (AE) data recorded during the tests. The resulted Clusters, Concluded by the analysis of AE hits, are associated with the damage mechanisms of the material, activated at the different load levels, and significant remarks were extracted regarding the damage evolution and its differentiation according to the different fibre/matrix interfaces. Emphasis is given on the impact of the different interface types upon the total mechanical behavior and damage accumulation at the test coupons. A qualitative evaluation of the interfaces using non-destructive testing data is also attempted. This first apart intends to Propose methodologies and procedures to analyze data front online acoustic emission monitoring in order to extract useful information regarding the damage evolution within C/C materials. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: In the second part of this work, the acousto-ultrasonics waveforms acquired during the tests described in part I are processed using wavelet-based techniques in order to extract parameters capable of monitoring the damage accumulation in the C/C test coupons. A 10 level decomposition using the discrete wavelet transform is applied upon each one of the acquired acousto-ultrasonics waveforms. The continous wavelet transform was also used and the wavelet grey moment was calculated for each waveform. Some of the Parameters extracted from the wavelet-based analysis show all interesting monotonic variation and thus Call be Used as potential candidates for monitoring the damage development in the tested materials. (C) 2008 Elsevier Ltd. All rights reserved.
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.
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.
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.
Abstract: Carbon nanotubes (CNTs) because of their properties are alleged to be the key candidate additives for improving the mechanical properties of polymers and carbon fibre reinforced polymers (CFRPs). Nevertheless in order for the reinforcing effect of the nanotubes to be of practical use, the CNTs have to be mixed with the matrix material In Me current work an effort was made to chemically graft Multi-Wall Carbon Nanotubes (MWCNTs) on the actual reinforcing Carbon Fibres (CFs) in order to skip the sometimes complex mixing stage. Two different solutions were used in order to treat/prepare the CNTs; a) an acid solution of H2SO4/HNO3, and. b) a toluene solution. The treated CFs were added to each solution, sonic bathed with deionised water and Own dried in an oven. The resulting CFs were examined under SEM and both the solutions used proved to) be reasonable successful with further investigations/optimisation to be necessary.
Abstract: The present work deals with the application of an innovative in-house developed wavelet-based methodology for the analysis of the acceleration responses of a human head complex model as a simulated diffused oedema progresses. The human head complex has been modeled as a structure consisting of three confocal prolate spheroids, whereas the three defined regions by the system of spheroids, from the outside to the inside, represent the scull, the region of cerebrospinal fluid, and the brain tissue. A Dirac-like pulse has been used to excite the human head complex model and the acceleration response of the system has been calculated and analyzed via the wavelet-based methodology. For the purpose of the present analysis, a wave propagation commercial finite element code, LS-DYNA 3D, has been used. The progressive diffused oedema was modeled via consecutive increases in brain volume accompanied by a decrease in brain density. It was shown that even a small increase in brain volume (at the level of 0.5%) can be identified by the effect it has on the vibration characteristics of the human head complex. More precisely, it was found that for some of the wavelet decomposition levels, the energy content changes monotonically as the brain volume increases, thus providing a useful. index of monitoring an oncoming brain oedema before any brain damage appears due to uncontrolled intracranial hypertension. For the purpose of the present work and for the levels of brain volume increase considered in the present analysis, no pressure increase was assumed into the cranial vault and, associatively, no brain compliance variation.
Abstract: The acoustic emission (AE) technique is a powerful nondestructive tool for health monitoring of structures and mechanical components, especially due to its sensitivity to capture high frequency signals, which are associated with the early stages of damage development and evolution. The aim of the present work is twofold. The first is the evaluation of a new concept of transducer mounting on rotating structures without the use of the expensive solution of the slip ring. The new concept is realized in a single stage in-house built gearbox setup. The second is the evaluation of the potential of the acquired with the new concept AE signals in distinguishing between different types of artificially induced damage on the gears. Run-in tests were carried out to study the effect of gear damage on the AE recordings. The acoustic emission signature of the healthy gears is first acquired. Then artificial defects are seeded and the acquisition is repeated. The AE signals are analyzed, and their root-mean-square values are calculated. The capability of the new approach of AE acquisition in discriminating between different loading and damage states is shown and discussed. Interesting findings on the effect of the oil temperature on AE recordings only speculated theoretically so far are also presented.
Abstract: The fracture toughness of carbon fiber reinforced polymer (CFRP) laminates doped with carbon nanofibers (CNF) and/or piezoelectric (PZT) particles were investigated. An increase of 100% in fracture energy was observed after the addition of 1% CNF in the matrix of the laminates. The investigation of the fracture surfaces showed extensive fiber bridging because of the presence of CNFs, which verifies the enhanced fracture properties. On the other hand the introduction of PZT particles led to reduction in fracture toughness, mainly due to the brittle character of the particle inclusions. (c) 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. (c) 2006 Elsevier Ltd. All rights reserved.
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. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: In the present work, a methodology is presented for the assessment of bridging laws for continuous fibre-rein forced ceramic matrix composites based on material properties as well as micromechanics of fibre deformation and failure. A load-displacement model is initially formulated that utilizes weakest-link statistical concepts to analyse and relate the individual contributions of matrix, intact/bridging and failed/pull-out fibres during the composite fracture process. The total and individual contributions to the bridging law and crack growth resistance of the material are determined by identifying the non-elastic part of displacement as crack opening. The model is validated against the experimentally recorded load-displacement behaviour of a notched SiC-fibre-reinforced glass-ceramic matrix composite tested under monotonic tension. The output parameters of the converged regression procedure remain within a small scattering range from the corresponding mean values that compare favourably with known material properties. A parametric analysis of the effect of fibre volume fraction, Weibull modulus of fibres and interfacial shear stress in overall composite performance is presented in view of the ability of the model to serve as an a priori fracture prediction tool. (c) 2006 Acta Materialia Inc. Published by 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 degrees 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. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: The aim of the present work is to develop a non-destructive, non-invasive technique for the early diagnosis of an oncoming brain edema based on the variation of vibration characteristics of the head system (i.e. eigenfrequency spectrum and modal damping). Besides the theoretical model that supports the basic principle, the proposed technique has been verified experimentally in animal tests. The advantage of such an approach is that the relative information is available well in advance an increase of intracranial pressure is detected. The uncontrolled intracranial hypertension is associated with increased mortality or vegetative state in head trauma. Traumatic lesions located on temporal lobe render particularly impeding the transtendorial herniation. From the medical point of view, intracranial pressure (ICP) monitoring represents an effective way for early consideration of neurological decompensation in various neurosurgical conditions particularly in the head-injured setting. However, the use of ICP monitoring is not an effective way of brain edema detection, since ICP increase very often causes irreversible problems to the patient's brain. Therefore, the determination of an earlier, less invasive and more sensitive indicator of the oncoming intracranial hypertension and of the impeding neurological deterioration is of profound importance. The present work aims at experimental verification of both eigenfrequency shifting and modal damping increase of the spectral response of the head system of rabbits, wherever a mass increase in the content of cranial shell appears. The conducted analysis concludes that the eigenfrequency spectrum and its modal damping characteristics are sufficiently sensitive parameters in order to characterize mass increase in the cranial shell. Therefore the combination of both the above parameters could be used with confidence for the early diagnosis of brain edema. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: A cadaveric biomechanical study was performed to investigate the fracture energy absorbed by strips of bone from the proximal femur in relation to age and gender, under impact loading conditions. Four groups (young mate, young female, old mate, old female) of four cadaveric proximal femurs were used in each case. Four bone strips were taken from the neck and four from the subtrochanteric area and these were tested under dynamic-impact conditions using the Charpy impact test. The fracture energy was calculated as the energy needed to achieve fracture per unit area, and expressed in J/m(2). Bone specimens from young mates are significantly tougher under impact conditions to those of females (p = 0.001), whereas between the old mate and female groups, fracture energy does not significantly differ (p = 0.165). There was also significant difference (p < 0.0005) between the young and the old groups in both genders. The fracture energy absorption of the subtrochanteric area compared to that of the femoral neck for the same group of age and gender is in general slightly higher for all groups. In conclusion, gender in the young age group played a significant role in bone resistance in breaking whereas in the older age group it played a less important role. (c) 2004 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.
Abstract: The monitoring of the elastic properties of Al2O3/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 R-curve, bridging stress profile and the corresponding bridging law of C/C woven composites were calculated for two different specimen configurations: compact tension (CT) and double-edge-notch (DEN). Monotonic tensile loading as well as cyclic loading was performed on CT specimens and the R-curve was evaluated from the change of specimen compliance in the presence and absence of the bridging zone by quantifying the total damage in the material through an effective crack length approach. The bridging stresses and laws for the DEN specimens tested under monotonic tension were calculated directly through the elastic correction of the measured displacement. The comparison of the R-curves and bridging laws between the two configurations was performed after identification of a common damage extent measure, the local crack opening displacement. The rising part of the R-curve was found similar for the two configurations, with the mean initiation and plateau values being R-0 similar to 1.3 and R-infinity similar to 9.5 kJ/m(2), respectively. In contrast, the bridging laws evaluated for the two configurations were in disagreement. It was concluded that the investigated material, although characterised as Class-III in the literature, exhibits crack growth and bridging, however of a Small rather of a large scale and that the calculated R-curve can serve as an intrinsic parameter that characterises the fracture behaviour of the material independently of configuration, and - thus - independently of dimensions and geometry. (c) 2004 Elsevier Ltd. All rights reserved.
Abstract: The mechanical performance of gas turbine material systems consisting of a Ni-base superalloy substrate with three different MCrAlY overlay coatings with various Al contents was assessed, close to service temperature spectrum, by means of stepwise tensile tests. Furthermore, acoustic emission (AE) activity was monitored during the tensile testing. Tensile tests were performed for all three types of coated systems at four different temperatures (300, 500, 700 and 900 degreesC). The AE data were analysed by means of a standardized k-means algorithm with respect to nine parameters ( such as amplitude, number of counts, duration, rise time, energy, etc.). In conjunction with in situ video imaging of the specimen surface and postmortem microstructural analysis of the samples, the classification of AE data has provided a means for damage identification and performance assessment of the substrate-coating material systems.
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.
Abstract: The aim of the present work is the classification of the characteristics of elastic waves, which are generated by fibre failures during quasistatic tensile fibre bundle tests and captured by the use of acoustic emission (AE) method. In addition, elastic waves generated out of the fibre bundle gauge length and propagated through the bundle at different stages of the loading process using an acousto-ultrasonic (AU) technique are also classified. To this target, a large number of tests were conducted on organic, ceramic and carbon fibre bundles according to DIN 53942. An in-house developed analysis and quantification methodology of the captured AE and AU waveforms is proposed in order to identify the frequency content of the fibre failure event, to characterize the medium of propagation and to investigate the effect of the acquisition system on the monitored signals. In fact, the application of the proposed analysis on the results of the conducted mechanical tests leads to the development of a useful database, concerning the ranges of AE features and the representative sets of frequency values that correspond to fibre fracture. Furthermore, the proposed database offers valuable knowledge for the role of material parameters, such as fibre structure and properties, on the characteristics of the recorded waveforms, constituting in this way a valuable tool that enables a better understanding of elastic waves initiation and propagation through continuous fibres. (C) 2003 Published by Elsevier Ltd.
Abstract: The Young's modulus determination of different types of ceramic fibres using both conventional quasi-static tensile tests and non-destructive elastic wave propagation techniques is the scope of the present work. A sufficient number of tensile tests were conducted on a number of different types of fibres such as carbon. Al2O3 and SiC fibre bundles and the elastic modulus was obtained. In parallel a method to determine the elastic modulus via a non-destructive approach was of primary concern due to the uncertainties of the conventional approach. To this direction the acousto-ultrasonics method proved quite capable to provide time-of-flight measurements in a rather simple experimental setup. The results seem remarkably close to the manufacturer's data and verify the method's applicability. Both techniques are analytically described with the emphasis lying on the non-destructive one. Deviations obtained from the conventional approach are thoroughly discussed and the advantages of the non-destructive technique are highlighted.
Abstract: The present work deals with the determination of the viscoelastic properties of composite laminates based on the assumption of viscoelastic behaviour of the single lamina. A viscoelastic lamination theory is developed assuming the lamina stiffness matrix fully complex and frequency dependent. For the measurement of the complete set of frequency dependent viscoelastic properties of the single lamina (i.e. complex moduli and Poisson's ratio) a new methodology has been introduced, based on the free and forced vibration of unidirectional and/or [45/ - 45, 0](ns) composite laminates. All the measurements have been performed in air. Since the vibration amplitude was very small, in order to fulfil linearity requirements, the results do not have any significant dependence on air damping. The effects of experimental errors on the evaluation of the viscoelastic properties of the single lamina have also been discussed analytically. (C) 2003 Elsevier Science Ltd. All rights reserved.
Abstract: In the present work, a new class of oxide/oxide composites made of Nextel((TM)) 720 fibre reinforcement and a mullite-based matrix, fabricated by using liquid polymer infiltration process, were studied. A fibre coating was applied via sol-gel in order to achieve improved damage tolerant behaviour. Mechanical properties were investigated at ambient temperature under quasi-static loading in the presence of continuous Acoustic Emission (AE) monitoring. Statistical pattern recognition analysis is the proposed tool for the classification of the monitored AE events. Lacking an a priori knowledge of different signal classes, unsupervised pattern recognition algorithms were used. A complete methodology including descriptor selection methods, procedures for numerical verification and cluster validity criteria is followed. Cluster analysis of AE data was achieved and the resulted clusters were correlated to the damage mechanisms of the material under investigation. This process was assisted by systematic microscopic examination. Furthermore, the initiation and evolution of each mechanism is described by plotting the cumulative hits of each class as a function of the applied load. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: The micromechanics of tension-tension fatigue loading in model single-fibre composite geometries is investigated in this paper. In an attempt to emulate the conditions encountered in full carbon fibre composites, the fibres were prestrained prior to the curing process to ensure that they were free of high residual compressive stresses as a result of resin shrinkage. The resulting specimens were grouped into two categories depending on the level of the initial fibre prestrain (case A low, case B high). The cyclic load is designed to be well below the endurance fatigue limit of the polymer matrix (similar to0.6%), and to have a frequency low enough to avoid unwanted thermal post curing. Throughout the preparation procedure, as well as during fatigue loading, the fibre stress (strain) was constantly monitored by means of laser Raman spectroscopy. The fibre axial stress distributions at each fatigue step were converted to interfacial shear stress (ISS) distributions, from which important parameters such as the maximum ISS the system can accommodate, the transfer length for efficient stress built-up and the length required for the attainment of maximum ISS were obtained. The results showed that, up to 2 x 106 loading cycles, the main parameters which affected the stress transfer efficiency at the interface were the fibre fracture process itself and the viscoelastic behaviour of the matrix material.
Abstract: The energy absorption during impact provided by a motorcycle safety helmet is always of critical importance in order to protect the rider against head injury during an accident. In the present study, a parametric analysis has been performed in order to investigate the effect of the composite shell stiffness and the damage development during impact, on the dynamic response of a composite motorcycle safety helmet. This kind of parametric analysis may be used as a tool during helmet design for minimising testing needs. The LS-DYNA3D explicit hydrodynamic finite element code was used to analyse a detailed model of the helmet-headform system (composite shell/foam liner/metallic headform) and to simulate its dynamic response during impact. A significant part of the work was focused on the modelling of the mechanical behaviour of the composite materials, including damage and delamination development. The dynamic response of the different helmet-headform systems was judged in terms of the maximum acceleration monitored at the centre of gravity of the headform and the maximum value of head injury criterion. It was shown that composite shell systems exhibiting lower shear performance provide additional energy absorbing mechanisms and result to better crashworthiness helmet behaviour.
Abstract: As is well known, repair, rehabilitation and strengthening of old concrete or seismically vulnerable structures is a technique that is used very often in order to extend the operational life of buildings and concrete structures. Such techniques are usually based on traditional, often inefficient techniques and on conventional materials, which may soon exhibit new durability problems. For structural upgrading, reinforced concrete jacketing and surface-bonded steel plates are mostly used. However, during the last decades high-performance advanced composites of the type routinely used in aerospace, such as carbon fibre reinforced plastics, are entering the stage, despite their much higher material costs. In the present work different methods of repairing, using CFRP laminates, of a concrete beam having a center crack are discussed analytically. The stresses and the deformations calculated are analytically discussed and the different repairing methods are evaluated systematically. Problems concerning the dynamic characteristics of the repaired structures are also presented, and a new approach for the repair strategy is introduced.
Abstract: We investigated the strain pattern developed in the anterior and posterior part of the fixed patella during knee motion. Eight fresh cadaver knees were used but two were excluded because of non reliable measurements due to misplacement of gauges. Two strain gauges were bonded in the midline of the anterior and two in the posterior surface of the patella. Threaded steel rods were cemented into the intramedullary femoral and tibial canals. The knee was placed on a special device. The quadriceps tendon was gripped and a 4.5 kg weight was attached to the tibial rod 16.5 cm distal to the joint line. Ten flexion/extension cycles were performed before testing. Initially the intact patella was tested. A transverse osteotomy was performed before being stabilized by the AO recommended tension band technique. The knee was retested again as above. Finally an additional circular wire was passed around the patella and the knee was tested again under the same loading configuration. The intact patella showed weak tensile strain on the anterior and compressive strain on the posterior surface through the range of knee motion. Tension band fixation produced weak tensile strains in the first few, degrees of flexion and then weak compressive strains in the posterior surface. The presence of the additional circular wire significantly increased the compressive strain. The classical tension band is highly effective for the fixation of the fracture patella but is improved by an additional circular wire. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: Continuous Fibre Ceramic (Matrix) Composites (CFCCs) have found during the last decade numerousof industrial applications in a variety of technological areas, where structural components are subjected to high temperature combined with significant mechanical loading. The present work deals with the application of an innovative design methodology for the development of an industrial gas turbine combustor chamber made of oxide/oxide composite materials. Oxide/oxide composites offer high-temperature structural stability without the need of any kind of oxidation protection and thus permit the increase of working temperature of the gas turbine, increasing the efficiency of the system and decreasing NOx emissions. Since, oxide/oxide composites degrade their structural properties as a function of the operating temperature (for temperature higher than 10000 degreesC) and the exposure time, an incremental approach has been introduced as a structural design methodology for the combustion chamber, where each increment represents a thermal exposure stage. The data set required for the application of the present design approach, was obtained through an extensive material characterization program based on the measurement of the anisotropic properties of oxide/oxide composites using ultrasonic techniques.
Abstract: A model single carbon fibre/epoxy composite geometry was subjected to cyclic loading at a maximum strain below the critical fatigue limit of the matrix material. The carbon fibres were pre-strained prior to incorporation in the resin to ensure that they were free of thermally induced compression stresses in the axial direction. A strain controlled cyclic experiment from 0 to 0.5% applied strain was performed up to a maximum life of 10(6) cycles. At discrete fatigue levels of 10(0). 10(3), 10(4) 10(5), 5 x 10(5) and 10(6) cycles the fibre normal stress distributions of a specified window of observation were obtained by means of remote laser Raman microscopy. The fibre normal stress distributions at each fatigue level were converted to interfacial shear stress (ISS) distributions from which important parameters, such as the maximum ISS the system could accommodate, the transfer length for efficient stress built up and the length required for the attainment of maximum ISS were obtained. The results showed that up to the level of full fibre fragmentation the main fatigue damage parameter that affected the stress transfer efficiency at the interface was the fibre fracture process itself. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: The fracture behavior of carbon/carbon composites is mainly governed by the activation of multiple matrix cracking, shear band formation, fiber debonding and fiber pull out, promoting the stress redistribution and offering a kind of "stress shielding effect" against crack propagation. In order to understand these effects and quantify the evolution of different damage mechanisms in time, experiments have been conducted on compact tension (CT) test coupons, accompanied by continuous acoustic emission (AE) monitoring. Then, fracture mechanics analysis was applied on the experimental results and a signal pattern recognition classification process was developed for the AE data, supported by extensive microscopical examination and systematic ultrasonic inspection. Correlation between clusters, resulting from the classification algorithm of AE data, and damage mechanisms activated by load increase during the CT experiment was accomplished, using classification algorithm parameters. A relation between the felicity ratio and the effective crack length was introduced and the multiple matrix cracking resulting by the shear failure of carbon matrix was found to act as the dominant stress redistribution mechanism. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: A systematic presentation of the concept of a well-defined stochastic damage accumulation model for fatigue calculation is attempted; the model is applicable for both organic and ceramic matrix composites. Stationary and nonstationary, with one or more blocks, discrete time and finite state Markov chain models are employed. They are found appropriate for yielding fatigue life probability distributions and damage evolution information at different stress levels. The structure, applicability, flexibility, and limitations of the model are examined in detail. The theoretical concepts are elucidated by the incorporation of data from a comprehensive testing program.
Abstract: The validity of indentation tests for the characterization of the mechanical properties of coatings relies greatly on the indentation depth. Deep indentation concludes to unreliable results due to the substrate effect on the measured properties. At shallow depths the size effect can also be an important error factor. The purpose of the present study is the determination of the critical ratio of coating thickness to indentation depth, up to which the substrate properties have a negligible effect on the force versus indentation depth curve and thus on determined mechanical properties of the coating. The analysis required, was conducted using finite element method. A spherical (ball) indenter was used and a three dimensional model of the indenter/coating-substrate system was applied. The effect of the coating to substrate Yield strength ratio, on the critical coating thickness to indentation depth ratio, was investigated for three different coating to substrate Young's modulus ratios. The results of this work provide considerable insight for the determination of the confidence indentation depth during micro-indentation for layered systems with different properties.
Abstract: The free vibration response of [+/- Theta](ns) orthogonal, simply supported (all around) composite laminated plates was studied. assuming linear viscoelastic behaviour of die constitutive lamina material. The problem was solved numerically and the results were verified ill the special case of [0](2n) laminate against the analytical solution. which is also provided. Stability and convergence problems were discussed analytically. Eigenfrequency values and modal damping characteristics were calculated for the first nine mode shapes. Furthermore. keeping constant the plate thickness, the effect of the lamina thickness on the vibration characteristics of the plate was examined.
Abstract: The present work deals with the determination of the viscoelastic properties of composite laminates based on the viscoelastic behaviour of the single lamina. A viscoelastic lamination theory is developed assuming that the lamina stiffness matrix is fully complex and frequency dependent. For the measurement of the complete set of frequency dependent viscoelastic properties of the single lamina (i.e. complex moduli and Poisson's ratio) a new methodology has been introduced, based on the free and forced vibration of unidirectional and [45/-45](ns) composite laminates. All measurements have been performed both in air and in vacuum. Since the vibration amplitude was very small, in order to fulfil linearity requirements, the results do not show any significant difference due to air damping. The effects of experimental errors on the evaluation of the viscoelastic properties of the single lamina have also been discussed.
Abstract: In order to investigate the ideal position in which the wrist should be immobilized during scaphoid fracture, treatment the strains which are developed in the carpal scaphoid for various wrist positions has been recorded in cadaveric wrists, using strain gauges. The data obtained shows that during radial deviation with neutral or slight palmar flexion of the wrist the waist of the scaphoid tends to compress because of the development of strong compressive strains, while the strain development parallel to the fracture site that tends to shift the scaphoid waist is minimum. This position seems to be the best for stable scaphoid fracture immobilization. (C) 2000 Elsevier Science Ltd. All rights reserved.
Abstract: Objective: To verify brain eigenfrequency shifting after the occurrence of a lesion producing mass effect into the cranial vault. Design: Experimental animal study. Setting: Laboratory of experimental surgery affiliated with a university critical care department. Subjects: Six adult male New Zealand white rabbits. Interventions: A Camino ICP monitor was placed in the parenchyma, and a 5-Fr balloon-tipped catheter and accelerometer were placed into the epidural space. Measurements: Before and after the introduction of successive 0.1-mL increments of autologous blood into the balloon, intracranial pressure (ICP) was recorded along with the accelerometer signal obtained during free vibration of the skull triggered by a calibrated hammer. Fast Fourier transformation of the digitized signal provided the eigenfrequency spectrum. The eigenfrequency showing the sharpest decrease after the initial 0.1-mL volume addition was considered as the best frequency, and its variation in response to subsequent 0.1-mL increments represents the brain eigenfrequency shifting. Main Results: Brain eigenfrequency shifting to lower values occurs for small blood volume increments (up to 0.2 mL). When volume addition becomes greater than or equal to 0.3 mL, brain eigenfrequency shifting to higher values is exhibited. The decrease in best frequency after the initial introduction of 0.1 mL is statistically significant (p = .003), in a range of volume in which no significant intracranial pressure difference appears. The respective variation of ICP is explained using a quadratic curve. For volumes of 0 to 0.1 mL, the change in ICP is not statistically significant (p = .08). Conclusions: Changes of the brain's physical characteristics by mass addition in the cranial vault can be expressed by brain eigenfrequency shifting. The method seems advantageous because it reliably detects mass additions at low levels where no ICP change occurs. Additionally, it provides serial measurements, and it is less invasive than the currently used methods for intracranial compliance.
Abstract: Fracture fixation using rigid plates leads to direct bone union, but it also may lead to complications because of stress protection osteopenia. This study aims to compare the mechanical characteristics restored during the callus formation after an osteotomy is fixed with two types of internal plate fixation. Twenty-four adult female sheep were divided randomly into three groups of eight each, which were euthanized at 2, 4, and 6 months after operation. Half of them had their osteotomized radius fixed with a seven hole dynamic compression plate, whereas in the remainder a sliding plate was used. The sliding plate consists of two halves connected together in such a way as to permit axial sliding of the one within the other, thus allowing cyclic axial load transfer at the fracture site. Bone strips obtained from the healthy (control) and the surgically treated side were subjected to four-point bending tests. The effective modulus of elasticity, ultimate bending strength, and energy absorption to fracture (toughness) were calculated. All parameters were restored more quickly in the sliding plate group, but there was no statistically significant difference observed at 6 months when all the osteotomies were united completely. Thus, the sliding plate, by allowing axial loading at the fracture site, led to a faster callus maturation and hence bony union, which, hopefully, will permit earlier full weightbearing and functional recovery of the injured limb.
Abstract: The effect of fatigue loading on the mechanical performance of 3-D SiC/SiC composites was investigated. A non-destructive macromechanical approach was applied which permits for the evaluation of the material damage state by monitoring its dynamic response as function of fatigue cycles. The correlation of the results provided by this method to that of other non-destructive techniques such as Acoustic Emission (AE), leads to a detail micromechanical- macromechanical monitoring of the material fatigue behaviour. The damage modes identification and their successive appearance, together with the evaluation of the material performance at the different stages of fatigue loading, is among the inspection capabilities that provides the above mentioned combination of non-destructive techniques. The proposed methodology applied in the case of a 3-D SiC/SiC ceramic matrix composite material and the effect of fatigue loading on the material integrity was evaluated by measuring the degradation of the dynamic modulus of elasticity and the increase of the material damping. Conclusions, concerning design aspects using these materials, as well as fatigue life prediction were provided. Finally, the sensitivity of the proposed methodology for the definition, the characterisation of the development and the separation of the different damage modes during fatigue loading has been discussed. (C) 1998 Elsevier Science Limited. All rights reserved.
Abstract: The present work aims toward the application of an innovative methodology for the analysis of acoustic emission (AE) activity monitored during the quasistatic tensile loading of centre-hole carbon/carbon. An in-house developed algorithm is proposed, which utilises the results of unsupervised pattern recognition classification of AE data. Correlation between clusters and specific material failure modes was achieved, using algorithm results and cluster activation parameters. During the analysis a dependence of AE activity on the hole diameter was observed. (C) 1998 Elsevier Science Ltd. All rights reserved.
Abstract: The present work investigates the behaviour of 2D carbon/carbon (CIC) composites containing centre holes and proposes methodologies for the prediction of the residual strength of such C/C laminates. Two different concepts were adopted in order to understand the fracture behaviour of CIC composites in the presence of holes and to evaluate their strength. The first approach is based on the hypothesis of the existence of a damage zone around the hole, which originates during tensile loading. The second uses monitoring of the acoustic emission (AE) activity of the centre-hole specimen, loaded up to a stress level well below its failure strength, as an indication of damage formation, and combines it with a defined calibration curve. Both models predict very accurately the strength of notched laminates for a number of specimens tested. (C) 1998 Elsevier Science S.A. All rights reserved.
Abstract: Measurement of interface fracture toughness requires knowledge cf the appropriate K -calibration functions for the pertinent test geometries rind material combinations. The 4-point-bending geometry has been used in this study to calibrate the stress intensity factors in order to measure fracture toughness of metal/nitride ceramic, Si3N4/Fe/Si3N4: and metal/oxide ceramic, Al2O3/Au/Al2O3 joints over the full range of mode mixity. This Calibration was carried out via finite element analysis. Specific techniques were adopted to decouple the two modes of fracture at the interface crack-tip. The K -calibration functions for the pertinent geometry are strongly dependent on the-metal interlayer thickness, particularly for loading conditions close to the model I.. Experimental results are presented for the Si3N4/Fe/Si3N4 system. (C) 1998 Elsevier Science Ltd. All rights reserved.
Abstract: An acoustically soft or hard sphere which is covered by a penetrable concentric spherical shell disturbs the propagation of an incident wave field emanating from a point source. The source is located in the exterior of the coated sphere. The medium, occupying the shell, is considered to be lossy while the dimensions of the coated sphere are much smaller than the wavelength of the incident field. For the case of a soft sphere covered with a penetrable lossy shell, the exact low frequency coefficients of the zeroth and the first order for the near field as well as the first and second order coefficients far the normalized scattering amplitude are obtained. In the case of the coated hard sphere, the zeroth and the first order coefficients of the near field, as well as the leading nonvanishing coefficient of the normalized scattering amplitude, which is of the second order, are obtained. For both cases of the soft and the hard sphere, the scattering and the absorption cross sections are calculated. The effect of the coating is expressed in terms of specific constants. A detailed discussion of the results and their physical meaning is included. For a sphere with a soft core and a point source located more than five radii away from the scatterer, or for a sphere with a hard core and a point source located more than two radii away from the scatterer, the results obtained are almost the same as if the scatterer was excited by a plane wave. (C) 1998 Acoustical Society of America. [S0001-4966(98)06010-X]
Abstract: Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed. (C) 1998 Elsevier Science S.A. All rights reserved.
Abstract: Fracture toughness of ceramic-matrix composites cannot be provided by an intrinsic fracture parameter. The presence of a fracture process zone induces stress redistribution mechanisms that partially shield the crack tip area from the applied stress field. The shape and size of the process zone depends upon extrinsic factors such as specimen shape and thickness, load distribution on the propagation zone, etc. The present work deals with the determination of the crack growth resistance curve (R-curve) of carbon/carbon (C/C) composites using the standard compact tension geometry. Two different material thicknesses were used and both monotonic and loading/unloading test patterns were applied. The determination of the R-curve in all cases was based on a compliance calibration technique. The results show that the monotonic loaning leads to higher plateau values for the R-curve of the C/C material compared to that calculated from cyclic loading and referred to the same material thickness. Furthermore, the thicker specimens appear to have slightly higher plateau values. A precise examination of damage initiation and propagation was attempted to explain the monitored differences. (C) 1997 Elsevier Science Limited.
Abstract: A parametric study based on finite element analysis is performed in order to investigate the sensitivity of the eigenfrequency spectrum of the human head system upon variation of its constitutive material properties. This study stems from the ever increasing medical interest connected to early diagnosis of brain edema and the lack of existing accurate and non-invasive diagnostic methods to achieve it. The present computational work aims to resolve the question of whether or not such a spectrum shifting is detectable with current experimental procedures. The human head is modeled as a prolate spheriod consisting of confocal shell representing the scalp-skull system, in contact with the subarachnoidal space which surrounds the brain. The skull is modeled as an isotropic elastic material whereas the brain is assumed to be a frequency dependent viscoelastic body.
Abstract: A tension-tension fatigue damage analysis was performed using 3-d silicon carbide fibre reinforced (orthogonal) silicon carbide matrix (SiC/SiC) composites. Two groups of SiC/SiC specimens were tested. The first group consisted of samples without any oxidation protective top layer coating, whilst the latter one contained samples covered with a well fitting, chemical vapour deposited (CVD) SiC system. This coating is necessary for the material to sustain high temperatures. Both the coated and uncoated material had a fibre volume fraction of about 36% equally distributed in three rectangular directions. Load control fatigue tests were conducted at room temperature. The fatigue life was found to decrease by increasing the cyclic stress level. A power-law equation is proposed, which correlates the applied maximum stress during the fatigue test with the number of cycles to failure. In general, the presence of the coating layer decreases the static strength of the material. However, the nominal maximum cyclic stress for which the endurance fatigue limit appeared, remained unaffected by the presence of the oxidation protective SiC coating. Microstructural examination has also been performed on the fractured specimens and it reveals some of the failure mechanisms of the composite that appeared under quasi-static and dynamic loading.
Abstract: A fracture toughness calibration of the Si3N4/Fe joint system has been performed for various mixed-mode loading conditions and for different thicknesses of the metal interlayer. The asymmetric four-point-bend loading geometry was used. The values for the calibration function (Y) as well as the mode mixity (psi) of the system increase on increasing the thickness of the metal. As the loading conditions change from mode II to mode I the dependence of both parameters on metal thickness is more intensive.
Abstract: In this work an attempt is made to study the dynamic characteristics of the human dry skull. The analysis is based on the three-dimensional theory of elasticity and the representation of the displacement field in terms of the Navier eigenvectors. The frequency equation was solved numerically and the results obtained are fairly good, in comparison to the experimental ones. Copyright (C) 1996 Elsevier Science Ltd
Abstract: A superposition of a longitudinal and a transverse plane wave, propagating in an isotropic and homogeneous purely elastic medium, impinges upon a penetrable body exhibiting thermoelastic coupling of the Plot type. The incident field penetrates the body exciting both a displacement and a temperature field in the interior. Transition conditions imposing continuity of the displacement and the traction fields and boundary conditions of vanishing temperature are assumed on the interface. We provide integral representations for the interior and the exterior fields as well as for the scattering amplitudes, scattering cross-section and absorption cross-section. We then examine the low frequency response of this scattering problem by reducing it to a sequence of elastostatic and thermal stresses problems that can be solved iteratively. We show that the thermoelastic character of the interior of the scatterer does not affect the low frequency approximations of the exterior field of order less or equal to the second. Therefore, the thermoelastic coupling is a third order effect in the theory of low frequency scattering, which is reflected as a fourth order effect for the scattering amplitudes.
Abstract: Plate fixation depends mainly on the holding power of the screws. In the present study the pattern of screw loosening was investigated. Thirty-two adult female sheep divided into four groups were used. A mid-diaphyseal transverse osteotomy was made on the right radius of each animal, and then plated on the anterior (tension) surface. Half of each group were plated using a standard narrow 7-hole AO Dynamic Compression Plate (DCP), whereas in the remaining animals a 6-hole newly designed sliding plate (SP) was applied. The required torque of tightening intraoperatively, as well as for releasing the screws after killing the animals was recorded with a tension-calibrated screwdriver. The pattern of loosening was similar for all the screws and the three phases could be recognized. An initial loosening was observed 1 month after the operation. A slow recovery of the torque was measured from the second month onwards, becoming highest by the fourth month after the operation, whereas a slow decrease in torque was observed from the fourth to the sixth month. In the SP group, the overall loosening was much lower than the DCP group (P < 0.05), whereas there was no difference in the loosening between the proximally placed screws and the distally placed ones (P > 0.05) for both plates.
Abstract: Operative management of long bone fractures is considered to be the treatment of choice all over the world. This could be achieved either by intramedullary nailing or plating. The former allows delivery of stresses into the fracture site, but it requires a well-equipped operating theatre and an experienced surgeon who is, unfortunately, continuously exposed to radiation. On the other hand, plates could easily be applied in every orthopaedic clinic, but the existing ones are rigid and protect the fracture site from stresses to which it is ordinarily exposed. Recently, various experimental attempts have been made to provide plates which allow partial loading on the fracture site and enhance callus formation (secondary fracture healing). Bearing that in mind, a two-part sliding plate (SP) has been developed at the Orthopaedic Clinic of Patras University, which allows intermittent loading into the fracture site. Both the conventional AO rigid plate and SP were applied to osteotomized sheep radii. The dynamic properties of the callus were estimated with its two dynamic mechanical characteristics (dynamic modulus of elasticity and the relative loss factor), whereas its static properties (strength) were evaluated by measuring the ultimate bending strength. A superiority of callus produced by the sliding plate was observed concerning all the parameters under consideration.
Abstract: A two-dimensional carbon-carbon composite material consisting of stacked carbon cloths densified with a carbon matrix has been characterized with respect to its dynamic mechanical properties. Unprotected specimens and some protected against oxidizing conditions were investigated over the temperature range of subambient (- 100-degrees-C) to 500-degrees-C. The reproducibility of the calculated storage modulus as well as the loss factor was within 5% at any one temperature over the entire temperature range. Results concerning protected and unprotected specimens were compared analytically and the difference of the relative plots above 400-degrees-C reflects the bad corrosion resistance of the unprotected samples under oxidizing conditions, where the carbon matrix material is more sensitive to oxidation than the fibre. The transition regions which appeared on the loss factor plots are strongly connected with the relevant secondary transition regions of the antioxidation protected material used. This technique, which has been demonstrated to be non-destructive for the sample analysed, proved that no differences exist in the dynamic mechanical properties of the specimens with respect to fibre orientation (warp or weft direction). Details of the experimental technique and assumptions made are also presented.
Abstract: A method of analysis for an adhesively-bonded metal-to-composite scarf joint is presented, by which both the metal and the adhesive exhibit non-linear behaviour. The analysis leads to a numerical procedure capable of evaluating stresses developing by shear failure of the joint both for tensile and compressive loading, and eventually to a failure criterion proposed for the particular type of scarf joint.
Abstract: A general thermoelastic plane wave with harmonic time dependence is incident upon a rigid triaxial ellipsoid which is either maintained at zero temperature or it is thermally insulated. Low frequency expansions are introduced and the thermoelastic scattering problem is reduced to an iterative coupled scheme for elastostatic problems in the presence of thermal stresses and for problems of thermal equilibrium in the presence of elastic sources. Rayleigh expansions for the elastic and the thermal fields, as well as for the corresponding scattering amplitudes, for each one of the two described scattering problems are given. The boundary value problems for the displacement and the temperature field that determine the corresponding Rayleigh coefficients are solved and the six thermoelastic scattering amplitudes are explicity evaluated. The special cases of spheroids, spheres, needles and circular discs are also discussed.
