Frattura ed Integrità Strutturale 2019-09-21T11:46:25+00:00 Francesco Iacoviello Open Journal Systems <p>Frattura ed Integrità Strutturale (Fracture and Structural Integrity) is the International Journal of the Italian Group of Fracture (ISSN 1971-8993). It is an open-access journal published online every three months (January, April, July, October).&nbsp;<br>Frattura ed Integrità Strutturale encompasses the broad topic of structural integrity, which is based on the mechanics of fatigue and fracture and is concerned with the reliability and effectiveness of structural components. The aim of the Journal is to promote works and researches on fracture phenomena, as well as the development of new materials and new standards for structural integrity assessment. The Journal is interdisciplinary and accepts contributions from engineers, metallurgists, materials scientists, physicists, chemists, and mathematicians.</p> <p><strong>More details:</strong></p> <p>- The Journal is financially supported by the <a href="">Italian Group of Fracture (IGF)</a> and by crowdfunding. It is completely free of charge both for readers and for authors. Neither processing charges nor submission charges are required.</p> <p>- Papers can be published only after a preliminary plagiarism/autoplagiarism check and a blind peer-review process (two reviewers, at least). More than one reviewing rounds are possible.</p> <p>- The Journal is well indexed (e.g., Scopus, since 2012, and WoS, since 2015).</p> <p>- All the papers are published with their Visual Abstracts (2 minutes max videos with the "cores" of the papers). All the Visual Abstracts are also available in a dedicated <a href="">YouTube channel</a>.&nbsp; All the issues are also published in a browsable version. All the issue in browsable version are also published in a dedicated website (<a href="">LINK</a>).</p> Numerical simulation of subsurface defect identification by pulsed thermography and improvement of this technique for noisy data 2019-09-21T11:37:06+00:00 Oleg Plekhov Anastasiya Kostina Sergey Aizikovich <p>Pulsed thermography is an active non-destructive technique which uses optical excitation source to stimulate heating of the object under investigation. This work is devoted to the simulation of the pulsed thermography method in a steel plate with the ceramic coating containing artificial defects of various depths and sizes. The simulation has been carried out on the base of the model which takes into account complex heat exchange of the sample with the surrounding by convection, conduction and radiation. Comparison of the temperature contrast with the experimental data has shown that the results are in a good qualitative and quantitative agreement in all stages of the cooling process. Due to the fact that the temperature contrast is often susceptible to the surface noise of various nature the Kalman-based signal processing technique was developed. The comparative analysis has shown that the proposed filtration technique provides better value of signal-to-noise ratio in comparison to the considered well-known techniques of signal reconstruction when proper calibration of the filtration parameters is carried out</p> 2019-09-20T02:17:40+00:00 Copyright (c) 2019 Oleg Plekhov, Anastasiya Kostina, Sergey Aizikovich Experimental characterisation of fatigue crack growth based on the CTOD measured from crack tip displacement fields using DIC 2019-09-21T11:37:29+00:00 José Manuel Vasco Olmo F.A. Díaz F.V. Antunes M.N. James <p>The current work presents an experimental study on the use of the crack tip opening displacement (CTOD) to evaluate its ability to characterise fatigue crack growth. A methodology is developed to measure and to analyse the CTOD from experimental data. The vertical displacements obtained by implementing digital image correlation (DIC) on growing fatigue cracks are used to measure the CTOD. Two fatigue tests at stress ratios of 0.1 and 0.6 were conducted on compact tension (CT) specimens manufactured from a 1 mm thick sheet of commercially pure titanium. A sensitivity analysis to explore the effect of the position selected behind the crack tip for the CTOD measurement was performed. The analysis of a full loading cycle allowed identifying the elastic and plastic components of the CTOD. The plastic CTOD was found to be directly related to the nonlinear zone (i.e., plastic deformation) generated at the crack tip during fatigue propagation. Moreover, a linear relationship between <em>da</em>/<em>dN</em> and ΔCTOD<em>p</em> independent of the stress ratio was found. Results show that the CTOD can be used as a viable alternative to the stress intensity factor range (<em>ΔK</em>) in characterising fatigue crack propagation since the parameter considers the fatigue threshold and crack shielding in an intrinsic way.</p> 2019-09-19T03:17:32+00:00 Copyright (c) 2019 José Manuel Vasco Olmo, F.A. Díaz, F.V. Antunes, M.N. James Structural failure process of schistosity rock under microwave radiation at high temperatures 2019-09-21T11:37:52+00:00 Leping He Yucheng Gu Qijun Hu Yuan Chen Junsen Zeng <p>The effects of high temperature induced by microwave radiation on the schistosity structural rock were investigated. A 1.45 kW commercial microwave system was employed to irradiate specimens to a designed temperature (300–800 °C) for 15 minutes. Cracking and local melting initially appeared in the biotite enrichment area at 500 °C. Macro-cracks in the dark area were parallel to the schistosity trend, owing to the weak connection in a direction perpendicular to the schistosity plane. The composition of the rock did not significantly change before and after microwave radiation. The diffraction peak intensity of the biotite decreased with temperature increase, owing to melting. The average peak stress decreased significantly with increasing temperature. It is concluded that the high temperature induced by microwave radiation promotes hard rock breakage and the schistosity structure of rock significantly affects the cracking pattern.</p> 2019-09-14T13:41:01+00:00 Copyright (c) 2019 Leping He, Yucheng Gu, Qijun Hu, Yuan Chen, Junsen Zeng Comparative study on the deterioration of granite under microwave irradiation and resistance-heating treatment 2019-09-21T11:38:15+00:00 He Zi Shou Qijun Hu Junsen Zeng Leping He Hexi Tang Bingsheng Li Shunzhang Chen Xirui Lu <p>To investigate the deterioration of granite under microwave irradiation and heat transfer, granite specimens were heated up to 400-1000 °C and then kept for 15 min. Uniaxial compressive strength testing results demonstrate a similar variation in two groups in 400-900 °C, which is initial strengthening (less than 500 °C), subsequent weakening (500-600 °C) and final stabilizing (600-900 °C). Furthermore, the specimen irradiated by microwaves presented a second decline at 1000 °C. Compared to heat transfer, microwave irradiation can reduce the strengthening due to localized transition plasticity and further promotes the deterioration of rock structure in weakening stage. TG/DSC results indicate that the strengthening is related to the iron mineral transition. The formation of porous glass substance which is mainly composed of feldspar and biotite. Furthermore, temperature-controlled microwave irradiation induced the variation of feldspar crystallinities, which is consistent with the corresponding UCS data, especially the plagioclase. In practical application, microwaves can be used to irradiate the vulnerable positions (surface edge and cleavage) and kept the whole rock mass around 600 °C.</p> 2019-09-13T04:29:57+00:00 Copyright (c) 2019 He Zi Shou, Qijun Hu, Junsen Zeng, Leping He , Hexi Tang, Bingsheng Li, Shunzhang Chen, Xirui Lu J integral computation and Limit load analysis of bonded composite repair in cracked pipes under pressure 2019-09-21T11:38:39+00:00 Mohamed Belhamiani Djamel Eddine Belhadri Wahid Oudad Omar Mansouri Wahiba Nessrine Bouzitouna <p>In this paper, an additional criterion was introduced to evaluate the composite repair systems using the limit load analysis. The plastic collapse pressure of API 5L X65 PSL2 steel pressure vessel structure with crack defect is numerically investigated after the structure was been repaired by composite overwrap. The objective of this study was the analysis of the efficiency of composite repair systems using this additional criterion to gain more confidence on it taking into account, the cracks and the overwrap geometries. The material of the pipe is elastic perfectly plastic for the plastic collapse pressure criterion and elastic-plastic using the Romberg Osgood model for fracture mechanic criterion. The additional criterion allows us to compare the uncracked and cracked pipe to estimate the repair efficiency. Moreover, the composite overwrap could restore 90% of the plastic collapse pressure for cracked pipes.</p> 2019-09-12T02:32:59+00:00 Copyright (c) 2019 Mohamed Belhamiani, Djamel Eddine Belhadri, Wahid Oudad, Omar Mansouri, Wahiba Nessrine Bouzitouna Stress intensity factor for small embedded cracks in weldments 2019-09-21T11:39:03+00:00 Paolo Livieri Fausto Segala <p>In the present work, the stress intensity factor (SIF) of embedded small cracks placed at the weld toe is calculated by means of two procedures based on the Oore-Burns integral. In the first approach, the defect is considered as a circular disk and the SIF is evaluated by means of the Oore-Burns weight function. By taking advantage of a suitable change of variable, the singularity of the weight function on the crack border can be removed. In this way, the numerical evaluation of the SIF is possible without the use of specific integration algorithms, although the nominal stress field becomes singular when the crack approaches a V-sharpe notch. As an example, the obtained equations are applied to a defect located in the neighbourhood of a weld toe with an opening angle of 135 degrees under mode I loading. Subsequently, for a crack similar to a star domain with a border expressed by means of the Fourier series, the SIF is given by means of an explicit equation based on the Oore-Burns weight function.</p> 2019-09-11T17:53:04+00:00 Copyright (c) 2019 Paolo Livieri, Fausto Segala Experimental fracture resistance study for cracked bovine femur bone samples 2019-09-21T11:39:26+00:00 M.R.M. Aliha Hussein Ghazi Farkhondeh Ataei <p>Crack growth and brittle fracture is one of the major failure modes in bone materials and therefore understanding the fracture behavior and affecting parameters on the crack growth resistance of bone is necessary for biomechanics researchers. In this paper, mode I fracture toughness value for the left and right femur bones of same bovine were measured experimentally using several single edge notch bend beam specimens (SENB) subjected to three-point bend loading in dry condition. The SENB specimens were cut along the longitudinal axis of bone but from different hoop directions. Fracture toughness results of sample prepared from the frontal part of bone were higher than the back or side sections. Depending on the location of sample, the fracture toughness of femur bone was varied from 5 to 10 MPa . Furthermore, the results obtained for similar location of both left and right femur bones were nearly identical. The fracture energy (<em>G</em><sub>f</sub>) of the tested specimens was also measured and it was found that a linear relation can be fitted to the (<em>K</em><sub>Ic</sub>)<sup>2</sup> versus <em>G</em><sub>f</sub> results of the tested bovine bone.</p> 2019-09-11T04:34:33+00:00 Copyright (c) 2019 M.R.M. Aliha, Hussein Ghazi, Farkhondeh Ataei Effect of Environmental Conditions on the Resistance of Damaged Composite Materials 2019-09-21T11:39:49+00:00 Sidi Mohammed Amine Khiat Ramdane Zenasni Mawloud Hamdi <p>The present paper proposes a strength model for unidirectional composites with Lin/Epoxy. The model assumes that, a central core of broken fibers flanked by unbroken fibers which are subject to stress concentrations from the broken fibers. The approach of the model consists of using a modified shear lag model to calculate the ineffective lengths and stress concentrations around fiber breaks. In this paper, we attempt to incorporate in the proposed model the unidirectional composite property variation with temperature and moisture in order to predict even composite strength degradation. Strength degradation is often seen as a result of changes in ineffective lengths at fiber breaks. Subsequently, damage to the material can be estimated at the micromechanical scale under the effect of temperature and humidity.</p> 2019-09-09T00:00:00+00:00 Copyright (c) 2019 Sidi Mohammed Amine Khiat Behavior of precast reinforced concrete columns subjected to monotonic short-term loading 2019-09-21T11:40:13+00:00 Sebastião Salvador Real Pereira Hermes Carvalho João Victor Fragoso Dias Victor Roberto Verga Mendes Pedro Aires Montenegro <p>This paper presents a case study on the behavior of precast reinforced columns with non-conventional cross-section. An experimental program was developed to verify the serviceability and ultimate limit states of these columns, loaded with eccentric compression forces. In addition, results obtained through a theoretical model based on the Brazilian standard ABNT NBR 6118:2014 and a Finite Element numerical model using ANSYS software were presented. Analysis of the results showed that the proposed models satisfactorily predicted column behavior. However, the ultimate strength of the column could not be determined due to limitations in the formulation of the theoretical model. The numerical model presented good agreement with the experimental results, both in serviceability and in ultimate limit state.</p> 2019-09-04T00:00:00+00:00 Copyright (c) 2019 Sebastião Salvador Real Pereira, Hermes Carvalho, João Victor Fragoso Dias, Victor Roberto Verga Mendes, Pedro Aires Montenegro Multi-response optimization of CuZn39Pb3 brass alloy turning by implementing Grey Wolf algorithm 2019-09-21T11:40:36+00:00 Nikolaos Fountas Angelos Koutsomichalis John Kechagias Nikolaos Vaxevanidis <p>Machinability of engineering materials is crucial for industrial manufacturing processes since it affects all the essential aspects involved, e.g. work­load, resources, surface integrity and part quality. Two basic ma­chin­ability para­meters are the surface roughness, closely associated with the functional and tribological performance of components, and the cutting forces acting on the tool. Knowledge of the cutting forces is needed for estimation of power re­quirements and for the design of machine tool elements, tool-holders and fix­tures, adequately rigid and free from vibration. This work in­ve­stigates the in­flu­ence of cutting conditions on machinability indicators such as the main cutting force <em>Fc</em> and surface roughness parameters <em>Ra</em> and <em>Rt</em> when longitudinally turning CuZn39Pb3 brass alloy. Full quadratic regression models were de­veloped to correlate the machining conditions with the imparted machinability characteristics. Further on, an advanced artificial grey wolf optimization algorithm was implemented to optimize the aforementioned responses with great success in finding the final optimal values of the turning parameters.</p> 2019-09-03T03:25:39+00:00 Copyright (c) 2019 Nikolaos Fountas, Angelos Koutsomichalis, John Kechagias, Nikolaos Vaxevanidis Damage evolution in marble under uniaxial compression monitored by Pressure Stimulated Currents and Acoustic Emissions 2019-09-21T11:40:59+00:00 Ilias Stavrakas Stavros Kourkoulis Dimos Triantis <p>The spatiotemporal evolution of damage in marble speci­mens under uniaxial compression is monitored using Pressure Stimulated Currents (PSCs) and Acoustic Emissions (AEs). The novelty of the study is the use of an integrated grid of sensors (instead of a single pair of electrodes) to detect the weak electrical signals, emitted during loading. The use of such a grid of sensors does indeed enhance the capabilities of the PSC technique providing valuable information about the initiation and propagation of micro-fracturing at the interior of the specimens. The experimental results indicate that both the im­proved b-value of the AE hits and the energy of the PSCs offer information about the proximity of the applied stress to that causing fracture. It is thus con­­cluded that both quantities could be considered as pre-failure indicators.</p> 2019-09-03T03:17:32+00:00 Copyright (c) 2019 Ilias Stavrakas, Stavros Kourkoulis, Dimos Triantis Restoring stone monuments: Enlightening critical details by the combined use of innovative sensing techniques 2019-09-21T11:41:22+00:00 Ermioni Pasiou <p>The experience gathered during the implementation of long series of laboratory experimental protocols, aiming to study the mechanical response of restored structural elements, is summarized. Conclusions are drawn con­cerning the proper exploitation of the laboratory results in field applications in the frame of on-going restoration projects of ancient stone monuments. The need of continuous bidirectional interaction between the scientific per­son­­nel working in the site and the scientists working in the laboratory is empha­sized. The role of the Digital Image Correlation technique in quantifying para­sitic effects influencing the laboratory data is proven decisive. The need to use modern sensing techniques, providing data from the interior of loaded res­tored complexes (simulating restored structural el­ements), like the Pressure Stimu­lated Currents and the Acoustic Emissions ones, according to a com­bined manner is highlighted. The capability of these two techniques to monitor the damage evolution within the mass of the elements tested and to provide clear pre-failure indicators renders them flexible tools in the hands of engineers designing the restoration projects.</p> 2019-09-03T03:09:48+00:00 Copyright (c) 2019 Ermioni Pasiou Comparison of the mechanical response of B400c and B450c dual phase steel bar categories, in long terms 2019-09-21T11:41:45+00:00 Charis Apostolopoulos Argyro Drakakaki Alkiviadis Apostolopoulos Konstantinos Koulouris <p>In this work, the effects of chloride-induced corrosion on temp­core B400c and B450c steel grades are evaluated, in terms of corrosion resis­tance and mechanical characteristics, after the performance of Tensile and Low Cycle Fatigue Tests. Both steel categories, characterized by high strength and high ductility, have been used in existing structures, indicating that they demonstrate different performance against the ascribed corrosion environ­ments. B450c steel grade seems to preserve higher energy reserves, ensuring higher expectancy to the corresponding reinforced concrete struc­tures. Ad­ditionally, due to buckling and buckling reversal, both steel grades dem­onstrate limited ductility at 4%. Finally, when cyclically stresses occur, crack nucleation is taking place, at the areas where sulphides, FeS and MnS can be found, leading to sub-surface crack propagation, interacting with external pits.</p> 2019-09-03T03:00:31+00:00 Copyright (c) 2019 Charis Apostolopoulos, Argyro Drakakaki, Alkiviadis Apostolopoulos, Konstantinos Koulouris Fracture precursor phenomena in marble specimens under uniaxial compression by means of Acoustic Emission data 2019-09-21T11:42:09+00:00 Dimos Triantis Stavros Kourkoulis <p>Marble specimens are subjected to a specially designed stepwise load­­ing protocol, in an attempt to detect fracture precursor phenomena taking advantage of Acoustic Emission (AE) data. The analysis is carried out in terms of the number of acoustic hits recorded and the time evolution of the improved b-value (I<em>b</em>-value), the cumulative energy of the acoustic signals and the F-fun­c­tion. During the stage of in­creasing load, intense acoustic activity is detected as the cor­responding stress reaches the transitional phase from the linear to the nonlinear mechanical re­sponse of the material. When the stress is stabilized at levels exceeding 95% of the material’s compressive strength, the acous­tic activ­ity is drastically reduced. During the first seconds of the stress stabil­iz­ation stage the reduction follows an exponential law. Special attention is paid to the phases, where the occurrence of AE hits shows a strong increase. During these phases acoustic signals of low frequency and high RA are re­corded, in­di­cating that the micro-cracking process is of shear rather than of opening mode.</p> 2019-09-03T00:00:00+00:00 Copyright (c) 2019 Dimos Triantis, Stavros Kourkoulis Use of by-products for partial replacement of 3D printed concrete constituents; rheology, strength and shrinkage performance 2019-09-21T11:42:32+00:00 Michail Papachristoforou Vasilios Mitsopoulos Maria Stefanidou <p>In this paper, fly ash, ladle furnace slag and limestone filler were utilized in concrete used as material for additive manufacturing (3D printing). Fly ash and ladle furnace slag were used as a replacement of cement (30% wt.) and limestone filler as a replacement of siliceous aggregates (50% wt.). Work­ability of fresh concretes that contained these by-products was measured 0, 15 and 30 minutes after mixing. Three different workability tests were conducted and compared: flow table, ICAR rheometer and an experimental method that measures the electric power consumption of the motor that rotates the screw extruder. Workability parameters that were measured were evaluated regarding printability of mixtures. Density, ultrasonic pulse velocity, compressive and flexural strength were measured on hardened concrete. Additionally, relative like­­li­hood of cracking of different concrete mixtures was estimated by per­forming restrained shrinkage test (ASTM C1581). Results showed that use of fly ash or ladle furnace slag as binder, and limestone filler as aggregate decreases slightly the mechanical properties of concrete but improve its durability re­garding cracking potential. Monitoring of electric power consumption of screw extruder motor was found to be an effective method for measuring easily real-time workability and define if a mixture is printable or not.</p> 2019-09-02T18:03:35+00:00 Copyright (c) 2019 Michail Papachristoforou, Vasilios Mitsopoulos, Maria Stefanidou Multi-parameter analysis of curing cycle for GNPs/glass fabric/ epoxy laminated nanocomposites 2019-09-21T11:42:55+00:00 Georgios Seretis Aikaterini Polyzou Dimitrios Manolakos Christopher Provatidis <p>In this study, a multi-parameter analysis, using Taguchi method for design of experiments, has been conducted to investigate the optimum curing conditions for GNPs/E-glass fabric/epoxy laminated nanocomposites. The independent variables in the L<sub>25</sub> Taguchi orthogonal array were heating rate, curing temperature and curing time, addressing five levels each. Tensile and 3-point bending tests were performed for each experiment number (run number) of the Taguchi L<sub>25</sub>. The analysis shown that the most significant para­meter for tensile strength is the time and for flexural strength is the tem­pe­ra­ture. Also, it shown that the optimum performance was obtained for tem­pera­ture values greater than the glass transition temperature T<sub>g</sub>.</p> 2019-09-02T17:53:46+00:00 Copyright (c) 2019 Georgios Seretis, Aikaterini Polyzou, Dimitrios Manolakos, Christopher Provatidis The use of acoustic emissions technique in the monitoring of fracturing in concrete using soundless chemical demolition agent 2019-09-21T11:43:18+00:00 Vassilios Saltas Despoina Peraki Filippos Vallianatos <p>Soundless chemical demolition agents (SCDAs) have been used during the last decades in the demolition of boulders and concrete structures as well as in open-surface and sub-surface rock excavation, as an alternative to the use of explosives posing safety risks. However, the knowledge of the governing fracture mechanisms in brittle materials is rather limited. In the present work, we thoroughly investigate the use of the acoustic emission technique to study the SCDA-induced fracture process in concrete blocks. Energy-related features and waveform parameters of the recorded AE activity are correlated to the fracture mode of the concrete where a quasi-static behavior is observed. Monitoring of the progressive fracture is also achieved by the 3D localization of the AE sources. The distribution of the inter-event times of the recorded hits is further analyzed in the context of non-extensive statistical physics.</p> 2019-09-02T17:44:55+00:00 Copyright (c) 2019 Vassilios Saltas, Despoina Peraki, Filippos Vallianatos Metal foaming by powder metallurgy process: investigation of different parameters on the foaming efficiency 2019-09-21T11:43:41+00:00 Ioannis Papantoniou Helena Kyriakopoulou Dimitrios Pantelis Dimitrios Manolakos <p>Aluminium foams, produced by powder metallurgy process, have significant potential applications for uses in weight-sensitive structural parts. Problems in the production of metal foams arise from the lack of knowledge in the control of process parameters. The results are frequently uneven and un­predictable variations in the structure and properties of the final foamed parts are observed. This paper aims to investigate the effect of different para­meters of the powder metallurgy with foaming agents process on the foaming efficiency. The parameters examined included the powder morpho­logy, the compaction pressure and the foaming temperature. During the foaming stage, for each set of parameters the porosity-time (P<sub>f-t</sub>) diagrams were created and the foaming efficiency was calculated (η=P<sub>fmax</sub>). Results indicated that the highest foaming efficiency was observed at the specimens with the fine alu­min­ium powder, with high (700 MPa) compaction pressure and high foaming temperatures (800 <sup>o</sup>C). Finally, compression tests were per­formed on the foamed specimens with the higher foaming efficiency in order to investigate their s-e response. Furthermore, average compressive strength and density were estimated and presented.</p> 2019-09-02T17:31:19+00:00 Copyright (c) 2019 Ioannis Papantoniou, Helena Kyriakopoulou, Dimitrios Pantelis, Dimitrios Manolakos Damage and failure of Orban’s gun during the bombardment of Constantinople walls in 1453 2019-09-21T11:44:04+00:00 Aristotle Kakaliagos Nikolaos Ninis <p>In this paper the bombardment of the Constantinople Theodosian Walls by the great cannon of Orban is numerically reproduced deploying Struc­tural Mechanics. Overall gun dimensions were assessed based on historical reports, whereby, the gun­powder charge p was estimated at 177 kg, and the gun was placed at 500 m from the Inner Walls. Gun ballistics and effect on target have been evaluated analytically. The analysis has verified Orban’s gun muzzle velocity, cannonball trajectory and its effect on Constantinople Walls by successfully calculating the length of the breach in the wall, referred in historical reports, as well as the cannon ball penetration into soil. The evaluated sound pressure level inside Constantinople, produced by the bombard, con­firmed the tremendous psychological effect of the cannon’s blast on the City’s population. A numerical effort was made to assess the combined effect of powder chamber internal pressure with associate temperature produced by powder ignition.</p> 2019-09-02T17:24:10+00:00 Copyright (c) 2019 Aristotle Kakaliagos, Nikolaos Ninis Imaging performance of a CaWO4/CMOS sensor 2019-09-21T11:44:28+00:00 Niki Martini Vaia Koukou George Fountos Ioannis Valais Ioannis Kandarakis Christos Michail Athanasios Bakas Eleftherios Lavdas Konstantinos Ninos Georgia Oikonomou Lida Gogou George Panayiotakis <p>The aim of this study was to investigate the modulation transfer function (MTF) and the effective gain transfer function (eGTF) of a non-destruc­­tive testing (NDT)/industrial inspection complementary metal oxide semi­conductor (CMOS) sensor in conjunction with a thin calcium tungstate (CaWO<sub>4</sub>) screen. Thin screen samples, with dimensions of 2.7x3.6 cm<sup>2</sup> and thick­ness of 118.9 μm, estimated from scanning electron microscopy-SEM im­ages, were extracted from an Agfa Curix universal screen and coupled to the active area of an active pixel (APS) CMOS sensor. MTF was assessed using the slanted-edge method, following the IEC 62220-1-1:2015 method. MTF values were found high across the examined spatial frequency range. eGTF was found maximum when CaWO<sub>4 </sub>was combined with charge-coupled devices (CCD) of broadband anti-reflection (AR) coating (17.52 at 0 cycles/mm). The com­bi­nation of the thin CaWO<sub>4</sub> screen with the CMOS sensor provided very pro­mis­ing image resolution and adequate efficiency properties, thus could be also con­sidered for use in CMOS based X-ray imaging devices, for various applications.</p> 2019-09-02T17:14:40+00:00 Copyright (c) 2019 Niki Martini, Vaia Koukou, George Fountos, Ioannis Valais, Ioannis Kandarakis, Christos Michail, Athanasios Bakas, Eleftherios Lavdas, Konstantinos Ninos, Georgia Oikonomou, Lida Gogou, George Panayiotakis Double initial and caustic curves in diametrically compressed transparent discs - Application to the contact length 2019-09-21T11:44:51+00:00 Christos Markides <p>General formulae for double initial and caustic curves (reflected and transmitted) are obtained in the case of smooth contact of two cylindrical elastic bodies of arbitrary radii. Namely, based on the method of reflected and transmitted caustics, the conditions for the development of double initial and contact caustic curves are established as functions of six independent para­meters, while easy-to-use closed-form expressions are given for obtaining the contact length. An experimental protocol is then implemented in the case a thin cylindrical transparent disc is compressed between the jaws of the Inter­na­tional Society for Rock Mechanics suggested device for the execution of the Brazilian-disc test. The experimental method of caustics can provide the con­tact length quite accurately, even in the case of double curves which seem that are not always a consequence of a wide contact region.</p> 2019-09-02T17:05:45+00:00 Copyright (c) 2019 Christos Markides Effect of fly ash on the corrosion performance and structural integrity of stainless steel concrete rebars in acid rain and saline environments 2019-09-21T11:45:14+00:00 Angeliki Lekatou Sofia Tsouli Christos Nikolaidis Spyridon Kleftakis Ilias Tragazikis Theodoros Matikas <p>The corrosion behavior of 304L stainless steel rebars in an alkaline solution simulating new concrete subjected to acid rain attack and a mildly to slightly acidic solution simulating corroded cover concrete that ex­posed the reinforcement to direct acid rain attack, was investigated by reverse polarization. Both solutions contained Ca(OH)<sub>2</sub> and fly ash (0-25 wt.% of the dry mixture). Concrete cubes containing 0-25 wt.% fly ash and reinforced with 304L rebars were subjected to salt spraying for 4 m. Although the polar­ization behaviors in the two electrolytes were different, the relative trends with respect to the fly ash contents were similar. The beneficial effect of fly ash (up to 20 wt.%) on the corrosion resistance of 304L rebars was de­mon­strated. How­ever, a deteriorating effect was realized at 25 wt.% addition. Partial re­placement of cement by fly ash did not significantly affect the tensile properties of the 304L rebar before or after 4 m of salt spraying. The elastic modulus and percent elongation presented a slight decrease after 4 m of salt spraying, ir­respectively of FA content. Corrosion-wise, 304L can replace 316L stainless steel provided that FA has been added to the concrete mixture, even at low contents (10 or 15 wt.%).</p> 2019-09-02T04:17:13+00:00 Copyright (c) 2019 Angeliki Lekatou, Sofia Tsouli, Christos Nikolaidis, Spyridon Kleftakis, Ilias Tragazikis, Theodoros Matikas A SEM-X-Ray assisted experimental approach for the determination of mechanical and thermal load – induced damage in MMCs 2019-09-21T11:45:38+00:00 Victor Kytopoulos Emilios Sideridis John Venetis Chrysoula Riga Alexandros Altzoumailis <p>An experimental technique is presented to evaluate mechanical and thermal load-induced microstructural damage, based on the Electron Probe Micro-Analysis (EPMA) principle, by which certain ana­lyti­cal potentialities of Scanning Electron Micro­scopy (SEM) are used. The aim of the study is to apply this technique in the case of metal matrix composites (MMCs). Based on earlier findings it is shown that by this technique the damage-con­trolled micro­­structural integrity distribution ahead of an edge-notch under tension is deter­mined with sufficient reliability. To demonstrate this fact two MMCs were tested. Dog-bone specimens with an edge notch were subjected to slow tension up to their ultimate stress, the loading process was terminated and the EPMA technique was applied. The procedure was also applied after sudden cooling of the specimens by immersion in liquid hydrogen. It is shown that the MMC with larger differences between the elastic and plastic constants of the inclusion and the matrix exhibit increased prone­ness to mechanical and thermal load-induced damage. The findings obtained are discussed on the basis of the dominating com­bined influence of macro-microscopic mechanical and thermo-elastic stress concentration processes on the matrix-particle interfacial fracture strength.</p> 2019-09-02T04:06:30+00:00 Copyright (c) 2019 Victor Kytopoulos, Emilios Sideridis, John Venetis, Chrysoula Riga, Alexandros Altzoumailis Experimental and numerical study on the influence of critical 3D printing processing parameters 2019-09-21T11:46:02+00:00 Nikoletta Chatzidai Dimitrios Karalekas <p>In the present work the temperature profile variations generated in rectangular specimens built using the Fused Deposition Modeling (FDM) process, at different printing speeds and orientations, were investigated. The temperature recordings were achieved by the integration of temperature sensors throughout the 1<sup>st</sup> and/or 21<sup>st</sup> building layer of the specimens. The experimental results show that the temperature values inside the specimen re-main above the glass transition temperature (T<sub>g</sub>) even at the end of the fabrica­tion process. Higher values were obtained when increasing the printing speed and decreasing the printing path. The experimental results were compared to the corresponding ones derived by simulation of the thermal diffusion problem via Finite Element Analysis. The calculated maximum temperature values were in good agreement with the experimentally recorded ones.</p> 2019-09-02T03:35:15+00:00 Copyright (c) 2019 Nikoletta Chatzidai, Dimitrios Karalekas A simplified damage evolution relationship and deformation character¬istics of a pozzolanic lime mortar when subjected to unloading-reloading cycles in the pre-peak region 2019-09-21T11:46:25+00:00 Kostas Kaklis Zach Agioutantis Stelios Mavrigiannakis Pagona Maravelaki-Kalaitzaki <p>Two series of uniaxial and triaxial compression tests including unloading-reloading cycles were performed under different confining press­ures, in order to study the stress-strain and the deformation behavior of a pozzolanic lime mortar subjected to cyclic loading. Each test included a cyclic loading sequence using five loops in the pre-peak region. The experi­ment­al results showed that the specimens exhibit a strain-softening behavior for uniaxial and low pressure triaxial tests and a strain-hardening behavior for higher triaxial compression tests. The mortar specimens subjected to triaxial compressive cyclic loading at higher confining pressures failed along a single or conjugate shear planes accompanied by considerable lateral ex­pansion. The marked Young’s modulus degradation behavior in the pre-peak region is related to damage that occurs in each specimen.</p> 2019-09-02T03:23:21+00:00 Copyright (c) 2019 Kostas Kaklis, Zach Agioutantis, Stelios Mavrigiannakis, Pagona Maravelaki-Kalaitzaki