Calcium phosphate bone cements have gained significant scientific and commercial attention due to their outstanding biological properties. In this work the cement's paste prepared by mixing a-TCP with sodium phosphate solution and the... more
Calcium phosphate bone cements have gained significant scientific and commercial attention due to their outstanding biological properties. In this work the cement's paste prepared by mixing a-TCP with sodium phosphate solution and the final hardening occurred after immersion in Ringer's solution. Their internal structure evolution from the hydrolysis of a-TCP to the formation of calcium deficient hydroxyapatite was observed my scanning electron microscopy. Time temperature superposition principle was applied in order to investigate their time- and temperature dependent dynamic response. It was found that time temperature superposition can be applied with success and that the material dynamic stiffness is time-and temperature dependent. Also, a nanostructure of hydroxyapatitic platelets and needles evolves within ten days, after specimen immersion for maturing in a Ringer’s solution. The resulting nanostructure was verified by means of scanning electron microscopy and x-ray diffraction techniques.
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In the Mediterranean Sea, the species Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis often coexist, occupying different subareas of the same habitat. The mechanical and chemical properties of their calcitic skeletons... more
In the Mediterranean Sea, the species Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis often coexist, occupying different subareas of the same habitat. The mechanical and chemical properties of their calcitic skeletons are affected both by their microstructural morphology and chemical composition. The present study describes the main morphologic features and the possible temporal differences in elemental composition of the test and spines of the three species, while also determining the molar ratio of each element of their crystalline phase. Scanning electron microscopy showed major differences in the ultrastructure of the spines, while minor differences in the test were also noticed. More specifically, the spines of all three sea urchins possess wedges, however A. lixula exhibits bridges connecting each wedge, while barbs are observed in the wedges of S. granularis. The spines of P. lividus are devoid of both microstructures. Secondary tubercles are absent in the ...
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A methodology for predicting the nonlinear viscoelastic behaviour of unidirectional fibre composites is proposed. The prediction, which is based on a modified Schapery formulation, is easy to apply by using a combination of analytical... more
A methodology for predicting the nonlinear viscoelastic behaviour of unidirectional fibre composites is proposed. The prediction, which is based on a modified Schapery formulation, is easy to apply by using a combination of analytical formulations and numerical procedures. In addition a generic function is developed for the description of the stress dependence of the creep-recovery response over the whole stress
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The non-linear viscoelastic response of a unidirectional carbon-fibre-reinforced polymer composite has been studied. For the needs of the present study, creep and recovery tests in tension for different stress levels were executed while... more
The non-linear viscoelastic response of a unidirectional carbon-fibre-reinforced polymer composite has been studied. For the needs of the present study, creep and recovery tests in tension for different stress levels were executed while measurements were made of the creep and recovery strain response of the composite system. For the description of the viscoelastic behaviour of the material, Schapery's non-linear viscoelastic
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Abstract In the current study, the non-linear viscoelastic response of a polymer matrix composite under multiple creep/creep recovery loading has been investigated. Starting from Schapery’s constitutive equation for the description of the... more
Abstract In the current study, the non-linear viscoelastic response of a polymer matrix composite under multiple creep/creep recovery loading has been investigated. Starting from Schapery’s constitutive equation for the description of the non-linear viscoelastic response at single step loading, the influence of creep/creep recovery multiple step loading in the overall viscoelastic and viscoplastic strain responses is examined, and an attempt to model this behaviour is performed. Based on the modelling of the parameters of non-linearity, the effect of the number of creep/creep recovery cycles in the strain response is examined. It is evident that multiple creep/creep recovery loading has an impact on both viscoelastic and viscoplastic strain responses of polymers and their composites. Good to excellent agreement between the experimental data and the model values of the non-linear parameters is also very encouraging for the application of the model to the non-linear time dependent response of polymer composites.
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In the present work, mixtures of mill scale (MS), an industrial by-product derived from the flaky surface of hot rolled steel, and lignite power station fly ash (FA), both originating from Greek industries, are examined as 100 % the... more
In the present work, mixtures of mill scale (MS), an industrial by-product derived from the flaky surface of hot rolled steel, and lignite power station fly ash (FA), both originating from Greek industries, are examined as 100 % the starting materials for the sustainable development of ceramics employing powder metallurgy (P/M) fabrication procedures. It should be emphasized that the safe management of such low price and largely available industrial secondary resources by their utilization as 100 % the feedstock for another industrial sector (ceramic industry) is strongly prioritized by current EU policies. FA and MS were mixed in various proportions (30–80 % wt. in MS), cold compacted at 20 tn using an automated hydraulic press to form a series of 5 cm diam. disc-shaped specimens, and then sintered at three different peak temperatures (1000°C, 1100°C and 1140°C) for 3 h. The experimental results are encouraging, showing that the mechanical performance (diametral tensile strength) o...
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The use of aluminium sandwich panels has been increased in a certain number of engineering applications from infrastructure systems and transportation to aircraft and naval engineering. Due to their structural efficiency these materials... more
The use of aluminium sandwich panels has been increased in a certain number of engineering applications from infrastructure systems and transportation to aircraft and naval engineering. Due to their structural efficiency these materials are ideal for applications where ratio of strength to weight is of crucial importance. In the current study the investigation of the strength characteristics of aluminium sandwich panels with aluminium honeycomb core and different types of skins is performed using both analytical models and experimental procedures. A series of strength tests such as tension, shear, three point bending and double cantilever beam were conducted on aluminium honeycomb-cored sandwich panel specimens with five different skins in order to examine the mode of failure and the mechanical behaviour of the structural elements. The experimental findings are compared to theoretical values while an attempt for the explanation of the mechanisms leading to failure such as buckling, ...
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Simple Summary: Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis play a key role in many sublittoral biocommunities of the Mediterranean Sea. However, their skeletons seem to differ, both morphologically and in chemical... more
Simple Summary: Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis play a key role in many sublittoral biocommunities of the Mediterranean Sea. However, their skeletons seem to differ, both morphologically and in chemical composition. Thus, the skeletal elements display different properties, which are affected not only by the environment, but also by the vital effect of each species. We studied the microstructural morphology and crystalline phase of the test and spines, while also examining the effect of time on their elemental composition. Results showed morphologic differences among the three species both in spines and tests. They also seem to respond differently to possible time-related changes. The mineralogical composition of P. lividus appears to be quite different compare to the other two species. The results of the present study may contribute to a better understanding of the skeletal properties of these species, this being especially useful in predicting the...
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Abstract Kalymnos island is the last bulwark of the sponge fishery. The economy of the island was based almost exclusively on the harvesting and treatment of commercial species of sponges. Nowadays, 30 years after the desertification of... more
Abstract Kalymnos island is the last bulwark of the sponge fishery. The economy of the island was based almost exclusively on the harvesting and treatment of commercial species of sponges. Nowadays, 30 years after the desertification of all the sponge fishing grounds, the populations recover even in shallow waters (5–15 m) where the species Spongia (Spongia) officinalis Linnaeus, 1759 and Hippospongia communis (Lamarck, 1814) are mainly found. Signs of the disease were detected only in 0.6% of the harvested sponges. In this paper the traditional method for whitening bath sponges is described. In order to identify three sponge species Spongia (Spongia) officinalis, Spongia (Spongia) zimocca Schmidt, 1862 and Hippospongia communis, classical and molecular methods were used. Concerning the classical methods, scanning electron microscopy (SEM) was used and the diameter of the sponges’ secondary fibers was measured. The softness, the elasticity and the resistance in tension are important parameters in order to define the commercial value of the sponges. Physical parameters, as tensile strength, module of elasticity, and the chemical composition can be strongly correlated with the commercial value of these sponges. We measured the tensile strength, the module of elasticity, and the elemental composition of the three sponge species before and after the chemical treatment used for the “whitening procedure”. We give a short description of reliable morphological characters used for the taxonomy of the species involved in our experiments. In addition we include preliminary results from the application of a molecular method based on mitochondrial DNA analysis to distinguish between S. (S.) officinalis and H. communis.
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The biological and mechanical nature of calcium phosphate cements (CPC's) matches well with that of bone tissues, thus they can be considered as an appropriate environment for bone repair as bone defect fillers. The current study... more
The biological and mechanical nature of calcium phosphate cements (CPC's) matches well with that of bone tissues, thus they can be considered as an appropriate environment for bone repair as bone defect fillers. The current study focuses on the experimental characterization of the mechanical properties of CPCs that are favorably used in clinical applications. Aiming on evaluation of their mechanical performance, tests in compression loading were conducted in order to determine the mechanical properties of the material under study. In this context, experimental results occurring from the above mechanical tests on porous specimens that were fabricated from three different porous additives, namely albumin, gelatin and sodium alginate, are provided, while assessment of their mechanical properties in respect to the used porous media is performed. Additionally, samples reinforced with hydroxyapatite crystals were also tested in compression and the results are compared with those of th...
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This study initially deals with the investigation of the induced strains during hardening stage of a self-setting calcium phosphate bone cement using fiber-Bragg grating (FBG) optical sensors. A complementary Scanning Electron Microscopy... more
This study initially deals with the investigation of the induced strains during hardening stage of a self-setting calcium phosphate bone cement using fiber-Bragg grating (FBG) optical sensors. A complementary Scanning Electron Microscopy (SEM) investigation was also conducted at different time intervals of the hardening period and its findings were related to the FBG recordings. From the obtained results, it is demonstrated that the FBG response is affected by the microstructural changes taking place when the bone cement is immersed into the hardening liquid media. Subsequently, the FBG sensor was used to monitor the absorption process and hygroscopic response of the hardened and dried biocement when exposed to a liquid/humid environment. From the FBG-based calculated hygric strains as a function of moisture concentration, the coefficient of moisture expansion (CME) of the examined bone cement was obtained, exhibiting two distinct linear regions.
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In the present study, pure and mixed calcium phosphate/gypsum cements were prepared and characterized. Chara-cterizations were performed using X-ray diffraction FTIR spectroscopy, thermogravimetric analysis, and Scanning Electron... more
In the present study, pure and mixed calcium phosphate/gypsum cements were prepared and characterized. Chara-cterizations were performed using X-ray diffraction FTIR spectroscopy, thermogravimetric analysis, and Scanning Electron Microscopy. Furthermore, the mechanical proper-ties of the materials were studied and it was found that they depend with the gypsum content. Bioactivity tests after incubation in 2xSBF solution revealed dense apatite structure on the samples containing α-tricalcium phosphate while the pure gypsum specimens showed sparse distributed tiny apatite crystals. Finally, 1% and 2% wt of the anti-inflammatory drug ibuprofen incorporated into the cement paste and the release behaviour of the drug was stu-died for 18 days in PBS solution. The results showed marked differences in the release profiles of ibuprofen for the different formulations. Complete release of the drug was noticed in the case of pure calcium phosphate cements within 18 days whereas ca. 35–40% of the drug was released from its congeners containing gypsum at the same timescale.
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In the present research, the combined utilization of fly ash (FA), derived from a lignite-fed power station, along with scrap-soil (SS), a steel industry by-product, is investigated, for the development of eco-friendly ceramics, thus... more
In the present research, the combined utilization of fly ash (FA), derived from a lignite-fed power station, along with scrap-soil (SS), a steel industry by-product, is investigated, for the development of eco-friendly ceramics, thus enhancing innovation and sustainability. The valorization of these low price and largely available industrial secondary resources as 100% the raw materials mixture in ceramic industry arises interesting technological, environmental and economical benefits. FA and SS were mixed in various proportions (0-70%wt. in SS), cold compacted at 20 tn load using an automated hydraulic press to form a series of 5 cm diameter disc-shaped specimens, and finally sintered at three different peak temperatures (1000oC, 1100oC and 1140oC) for 3h. Then, the specimen microstructure and physico-mechanical properties were characterized. According to the experimental results, a sintering temperature increase from 1000°C up to 1140oC significantly improves specimen densification, thus sharply enhancing the diametral tensile strength (DTS), from 0.5 MPa up to 12.8 MPa respectively for a 50-50%wt. FA-SS mixture. Mechanical strength also varies with the SS percentage in the raw materials. Physico-mechanical properties seem to be constant for specimens containing SS up to 60% at 1140oC.
In the research on durability of polymer matrix composite systems, methods in which short-term testing results are extrapolated in order to make predictions on the long-term behaviour become more and more important. Beside the development... more
In the research on durability of polymer matrix composite systems, methods in which short-term testing results are extrapolated in order to make predictions on the long-term behaviour become more and more important. Beside the development of the mathematical tools for modeling material behaviour, the research in this field is also directed towards the study of accelerating factors for these short-term tests. COSARGUB is currently investigating the interaction effects of combined loadings, e.g. cyclic-static, static-temperature,... on the behaviour of composite structures in an attempt to make more reliable predictions on the structural behaviour under complex loadings. Static creep and creep-recovery tests, each taking 168 hours, were carried out on [90] 12 Fibredux 920-C-TS-5-42 laminates. The material clearly exhibited non-linear behaviour, even at moderate stress-levels. This behaviour was modelled using Schapery's constitutive equation. To this equation, a term that represents the viscoplastic strain is added as suggested by [3] and [4]. The nonlinearizing parameters in the equation are obtained by curve-fitting of the creep and creep-recovery curves. The master curve, showing the evolution of the linear compliance of the material, was obtained by applying the classical TTSP-technique on data obtained by Dynamic Thermal Mechanical Analysis. INTRODUCTION Schapery's single-integral representation was chosen for the modelization of the non-linear behaviour. This is a single integral formulation with four nonlinearizing functions reducing to the classical constitutive equation for linear viscoelastic behaviour (Boltzmann integral equation) if those nonlinearizing functions are put equal to one. In the case of uniaxial loading, Schapery's equation is given by : f d[g ?a] s(t) = gQS 0a + g, . I AS (ψ-ψ ' ) . d~ .dt' ο (1) The so-called reduced times ψ and ψ' are given b y :
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Composite materials used in technically advanced structures are subjected to constant aging from exposure to changing environmental conditions. Studying the effects on such materials due to exposure to varying temperature, humidity,... more
Composite materials used in technically advanced structures are subjected to constant aging from exposure to changing environmental conditions. Studying the effects on such materials due to exposure to varying temperature, humidity, ultraviolet radiation, etc. reveals the impact on their mechanical behavior. This study assesses alterations in static, dynamic, and viscoelastic response of polymer matrix woven carbon fiber lamina composites upon exposure to varying environmental conditions recreated in a climatic chamber. Therein, specimens suffered temperature changes from –35 to +40℃ and humidity variations from <10% to 95% RH (noncondensing) over a period of up to 30 days. Alternating cycles simulating conditions of actual 3–4 h flights were specified. Additionally, specimens of the same material were subjected to thermal shock under similar (to the aging scenario) temperature extremes. All specimens were comparatively assessed via experimental procedures involving three-point bending tests performed in both static and dynamic mechanical analysis for a range of temperatures and frequencies, frequency and thermal scans, and finally impact tests. Results indicate that aged materials exhibit increased dynamic stiffness (expressed by the storage moduli) and decreased material damping ability (expressed by the tan δ parameter). Macroscopic assessment of impact test data was performed via stochastic model-based damage detection methodologies. Results indicate that differences in the impact behavior between pristine and aged specimens are statistically detectable and quantifiable, without input from mechanical testing analysis. More importantly, this assessment of aging-induced effects on the specimens corroborates the findings on storage moduli and tan δ from mechanical testing analysis, thus validating the latter.
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The objective of this experimental work is the investigation of the viscoelastic behavior of a hybrid matrix, fiber-reinforced building material. Hybrid matrix consisted of epoxy resin mixed with fine marble sand, whereas short steel... more
The objective of this experimental work is the investigation of the viscoelastic behavior of a hybrid matrix, fiber-reinforced building material. Hybrid matrix consisted of epoxy resin mixed with fine marble sand, whereas short steel fibers were used as reinforcement. The experimental procedure involved, first, the manufacturing of specimens using different hybrid matrix types and different reinforcement by weight ratios. Subsequently, bending relaxation experiments at room temperature were executed, under three-point bending test experimental configuration, at different strain levels and the variation of stress of the hybrid matrix material with time was monitored. The data obtained were used to (i) investigate the effect of reinforcement mass fraction contained in the composite on the viscoelastic behavior and (ii) to apply existed and newly developed viscoelastic models for the description of the observed viscoelastic behavior. More precisely, the four-parameter (Burgers) viscoelastic model and the modified Residual Property Model were calibrated and used to simulate the relaxation behavior of the materials manufactured and tested. Experimental results exhibited a clear influence of both reinforcement ratio and initial displacement on the viscoelastic behavior of the materials manufactured and tested, whereas the models proposed and used can adequately reproduce the variation of relaxation stress with time. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41429.
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Carpal tunnel syndrome (CTS) is a clinical disorder resulting from the compression of the median nerve. The available evidence regarding the association between computer use and CTS is controversial. There is some evidence that computer... more
Carpal tunnel syndrome (CTS) is a clinical disorder resulting from the compression of the median nerve. The available evidence regarding the association between computer use and CTS is controversial. There is some evidence that computer mouse or keyboard work, or both are associated with the development of CTS. Despite the availability of pressure measurements in the carpal tunnel during computer work (exposure to keyboard or mouse) there are no available data to support a direct effect of the increased intracarpal canal pressure on the median nerve. This study presents an attempt to simulate the direct effects of computer work on the whole carpal area section using finite element analysis. A finite element mesh was produced from computerized tomography scans of the carpal area, involving all tissues present in the carpal tunnel. Two loading scenarios were applied on these models based on biomechanical data measured during computer work. It was found that mouse work can produce larg...
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... 2 = ˙ /n1 is the deformation of the dashpot, both from the model of Maxwell, and 3 is the deformation according to the model of Kelvin (Eq. ... J. 14, 239 (2003) 8. JS Alfonso, RMN Jorge, PS Martins, Marly Da S. Soldi, OL Alves, B.... more
... 2 = ˙ /n1 is the deformation of the dashpot, both from the model of Maxwell, and 3 is the deformation according to the model of Kelvin (Eq. ... J. 14, 239 (2003) 8. JS Alfonso, RMN Jorge, PS Martins, Marly Da S. Soldi, OL Alves, B. Patricio, T. Mascarenhas, MGF Sartori, and MJ BC ...
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ABSTRACT
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In the current study the nonlinear viscoelastic response of a polymer matrix composite under multiple creep/creep-recovery loading is presented. The influence of creep/creep-recovery multiple step loading in the overall strain response... more
In the current study the nonlinear viscoelastic response of a polymer matrix composite under multiple creep/creep-recovery loading is presented. The influence of creep/creep-recovery multiple step loading in the overall strain response both viscoelastic and viscoplastic is examined and an attempt for modeling this behaviour is attempted. An increase in the parameters of nonlinearity is observed as well as in the viscoplastic response. Based on modeling of the parameters of nonlinearity the effect of the number of creep/creep-recovery cycles is examined. It is evident that multiple creep/creep-recovery loading has an impact on both the viscoelastic and viscoplastic response of polymers and its composites. Key-Words: composites, viscoelasticity, creep, nonlinear response, Schapery’s model, stress threshold
The softness, the elasticity and the resistance are important parameters in order to define the commercial value of the sponges. Physical parameters as tensile strength, module of elasticity and the chemical composition can strongly be... more
The softness, the elasticity and the resistance are important parameters in order to define the commercial value of the sponges. Physical parameters as tensile strength, module of elasticity and the chemical composition can strongly be correlated with sponges' commercial value. The aim of this study is to determine the tensile strength, the module of elasticity and the chemical composition of 4 commercial sponges before and after chemical treatment of the " whitening procedure " .