Stefanos Zaoutsos

On the non-linear viscoelastic behaviour of polymer-matrix composites

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, Schaperys non-linear viscoelastic model was used. For the description of the non-linear viscoelastic response of the material and the determination of the non-linear parameters, an analytical method, based on a modified version of Schaperys constitutive relationship where a viscoplastic term was added, has been developed. The method has successfully been applied to the current tests and an estimation of the non-linear parameters was successful. Useful results and conclusions for the applicability of the new method were also extracted.

Prediction of the non-linear viscoelastic response of unidirectional fiber composites

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 range examined. All the parameters included in the proposed generic function used for the prediction of the nonlinear viscoelastic behaviour of unidirectional fibre composites, such as the stress threshold of nonlinearity and the ultimate tensile strength of the material, have a clear physical meaning. The accuracy of both the generic function and the numerical technique is checked by creep-recovery tests on 90° unidirectional carbon-fibre/epoxy-matrix composites in which predicted response is compared with measured response. Good-to-reasonable agreement between experimental and analytical results is observed. The method is presented in a generalised manner and could be applicable to a large class of UD fibre composite systems.

Further development of a data reduction method for the nonlinear viscoelastic characterization of FRPs

Abstract According to the well-known Schapery’s formulation, the nonlinear viscoelastic response of any material is controlled by four stress and temperature dependent parameters, g0 g1, g2 and aσ, which reflect the deviation from the linear viscoelastic response. Based on Schapery’s formulation, a new methodology for the separate estimation of the three out of four nonlinear viscoelastic parameters, g0, g1 and aσ, was recently developed by the authors. In the present article, a further development of the previously developed methodology is attempted leading to an analytical estimation of the fourth nonlinear parameter, g2, which additionally includes the viscoplastic response of the system. Thus, a full nonlinear characterization of the composite system under consideration is achieved. The validity of the integrated model was verified through creep-recovery experiments, applied at different stress levels to a unidirectional carbon fibre reinforced polymer.

Monitoring of hardening and hygroscopic induced strains in a calcium phosphate bone cement using FBG sensor.

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.

Finite element simulation of the mechanical impact of computer work on the carpal tunnel syndrome

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 large deformation fields on the median nerve region. Also, the high stressing effect of the carpal ligament was verified. Keyboard work produced considerable and heterogeneous elongations along the longitudinal axis of the median nerve. Our study provides evidence that increased intracarpal canal pressures caused by awkward wrist postures imposed during computer work were associated directly with deformation of the median nerve. Despite the limitations of the present study the findings could be considered as a contribution to the understanding of the development of CTS due to exposure to computer work.

Damage assessment of carbon fiber reinforced composites under accelerated aging and validation via stochastic model-based analysis

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.

Influence of artificially-induced porosity on the compressive strength of calcium phosphate bone cements.

The biological and mechanical nature of calcium phosphate cements (CPCs) 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 the above tested porous CPCs. The knowledge obtained allows the improvement of their biomechanical properties by controlling their structure in a micro level, and finds a way to compromise between mechanical and biological response.

Time Dependent Properties of Nanocomposite Hydroxyapatite Based Bone Cements

Calcium phosphate bone cements have gained significant scientific and commercial attention due to their outstanding biological properties. In this work the cements paste prepared by mixing a-TCP with sodium phosphate solution and the final hardening occurred after immersion in Ringers 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.

Sustainable Development Of P/M Ceramics From Steel Mill Scale And Lignite Fly Ash Mixtures

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) of the integral ceramic materials so-produced sharply increases, from 0.77 MPa up to as much as 13.42 MPa, with the temperature increase from 1000°C up to 1140°C, for a 50–50 %wt. FA–MS mixture. Scanning electron microscopy mapping enables a better understanding of the microstructural changes occurring at higher sintering temperatures. On the other hand, the coefficient of thermal conductivity increases with temperature and the MS content in the mixture.

Innovative Synergistic Valorization of Lignite Fly Ash and Steel Industry Scrap-Soil as Secondary Resources for Compacted Ceramics

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.