PSI - Issue 10

Volume 10 • 201 8

ISSN 2452-3216

ELSEVIER

1st International Conference of the Greek Society of Experimental Mechanics of Materials,Athens, May 10-12, 2018

Guest Editors: Stavros K . Kourkoulis D imos T riantis

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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine P. Brandão a , V. Infante b , A.M. Deus c * a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 1 st International Conference of the Greek Society of Experimental Mechanics of Materials Editorial Stavros K. Kourkoulis a, *, Dimos Triantis b a Laboratory for Testing and Materials, Sch ol of Applied Mathematical and Physical Sciences, Department of Mechanics, National Technical University of Athens, 157 73, Athens, Greece b Laboratory of Electronic Devices and Ma erials, Univ rsity of West Attica, Greece © 2018 The Authors. Publis ed by Elsevi r Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of the 1 st International Conference of the Greek Society of Experimental Mechanics of Materials Keywords: Editorial; Experimental Mechanics of Materials; 1 st International Conference of the Greek Society of Experimental Mechanics of Materials; Short history of the Greek Society of Experimental Mechanics of Materials Among the most rapidly developing areas of Mechanics is the field of Experimental Mechanics of Materials. This is mainly due to the extended use of novel and innovative techniques for the detection, collection and recording of experimental data, and, also, due to the explosive development of computer science which enables rapid analysis and exploitation of huge amounts of data. On the other hand, the effective implementation of experimental protocols be es incr asingly difficult and expensive, rendering the cooperation of research gr ups of interdisciplinary background absolutely ece ary. In Greec , there are quite a few research groups, which are actively and quite uc cessfully involved in the area of Experimental Mechanics of Materials. Unfortunately, the interaction among these research groups is rather limited, resulting to waste of valuable resources, in both human and financial terms. As a matter of fact, similar experimental protocols are often implemented independently in various research centers, insti tutes or universities. In this context, the establishment of a closer communication and cooperation between the Greek research teams is imp rative. The foundation of the “ Greek So iety of Experimental Mechanics of Materials ” (GSEMM) is a first - even though small - step in this direction. One of the founding objectives of the GSEMM is to strengthen the inter action between the vario s research teams and to disseminate the outcomes of the experimental protocols. © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

* Corresponding author: Tel.: +30 210 7721263; fax: +30 210 7721264. E-mail address : stakkour@central.ntua.gr

2452-3216 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 10.1016/j.prostr.2018.09.001 2452- 3216 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of the 1 st International Conference of the Greek Society of Experimental Mechanics of Materials * Corresponding author. Tel.: +351 218419991. E-mail address: amd@tecnico.ulisboa.pt

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The GSEMM is already representing Greece in the European Structural Integrity Society (ESIS). The members of GSEMM encompass a unique group of experimentalists, development engineers, design engineers, test engineers and technicians, and research and development scientists from industry and educational institutions. According to its Statutes the GSEMM aims at production, promotion, gathering and dissemination of scientific knowledge on matters related to the field of Experimental Mechanics of Materials. Moreover, it aims at training, education and know-how transfer at all levels concerning Experimental Mechanics of Materials. In order to achieve its goals the GSEMM intends to: • Organize national and international scientific conferences and workshops. • Organize schools and seminars in the direction of training young scientists on subjects related to experimental research techniques. • Contribute in the organization of post-graduate courses in fields related to Experimental Mechanics of Materials. • Disseminate and promote the scientific work and achievements of its members. Provide to its members information concerning research directions of high priority for the development of the country. • Participate in the implementation of national and international research projects. • Promote collaboration between Greek and international scientific societies of similar scope. • Become member of European and international scientific associations of similar statutory purposes. In this direction, the GSEMM organized its “ 1 st International Conference of the Greek Society of Experimental Mechanics of Materials ” (Athens, May 10 -12, 2018). In this first Conference, organized under the aegis of ESIS, more than 100 articles were submitted for consideration, out of which 78 were finally accepted by the scientific committee for presentation, on the basis of their scientific soundness and the relevance of their content to Experimental Mechanics of Materials. The extended two-page abstracts of the papers accepted were published both electronically and, also, in a hard-copy volume (ISSN 2623-3541). The present special issue of “Procedia Structural Integrity” is an attempt to further disseminate the scientific papers presented during the “ 1 st International Conference of the Greek Society of Experimental Mechanics of Materials” . It is believed that this issue provides a comprehensive overview of the research work implemented in Greece in the field of Experimental Mechanics of Materials. In addition, it contains indicative contributions coming from abroad (such as, for example, UK, USA, Russia). All papers submitted were reviewed by two independent reviewers and finally forty four (44) papers were accepted to be included in this issue, again on the basis of their scientific soundness. The papers cover a wide scientific area of Experimental Mechanics of Materials, ranging from concrete and rock materials to metals and polymers. Papers dealing with novel sensing techniques, corrosion and degradation, experimental biomechanics, mechanical properties and simulation of materials used in restoration/conservation of monuments of Cultural Heritage are, also, included in this issue. Moreover, papers are included dealing with design and implementation of novel or improved experimental protocols, in the direction of characterizing materials, structures and systems. The editors would like to sincerely thank the reviewers of the papers. Their work guarantees the high scientific level of this issue. A special “thank you” goes to the Secretary of the GSEMM Board of Directors, Dr. Ermioni Pasiou, for taking excellent care of the final editing of the manuscripts. Based on the experience gathered during the “ 1 st International Conference of the Greek Society of Experimental Mechanics of Materials ” , and the full support of the European Structural Integrity Society (ESIS), it is anticipated that the next conference of GSEMM ( “ 2 nd International Conference of the Greek Society of Experimental Mechanics of Materials ” ) will appear as an important event in the forum of international scientific conferences, with significant contributions from many researchers from different countries, focusing on critical areas of special interest to re searchers and design engineers working in all areas of Structural Dynamics, Solid Mechanics and Materials Research.

Athens, August 2018

Stavros K. Kourkoulis Dimos Triantis

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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine P. Brandão a , V. Infante b , A.M. Deus c * a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 1 st International Conference of the Greek Society of Experimental Mechanics of Materials A comparative thermomechanical study of ferrite/polymer nanocomposites A. Sanida, S.G. Stavropoulos, G.C. Psarras* Smart Materials & Nanodielectrics Laboratory, Department of Materials Science, School of Natural Sciences, University of Patras, Patras 26504, Greece Abstract The thermomechanical response of epoxy nanocomposites filled with five different ferrite ’ nanoparticles, were studied in the present paper. The morphology of the specimens was investigated via Scanning Electron Microscopy (SEM). The thermomechanical characterization was conducted via static tensile tests and Dynamic Mechanical Analysis (DMA). © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific c mmittee of the 1 st I t rnational Conference of the Greek So i ty of Experi e tal Mechanics of Mater als Keywo ds: Thermomechanical properti s; tensile strength; polymer nanoco posites; ferrites 1. Introduction Organic-inorganic nanocomposites combine the advantages of the inorganic materials (mechanical strength, elec trical and magnetic properties and thermal stability) and the organic polymers (flexibility, dielectric, ductility and processability), which are difficult to be obtained from individual components (Elsayed et al. (2011); Kanapitsas et al (2013); Sc adleret al. (2007); Hanemann and Szabó (2010)). Furthermore, the magnetic properties of the nanocom posite can be simultaneously improved by using a suitable particulate material, such as ferrites (Ramajo et al. (2009)). Ferrites can be divided into three important classes based on their specific crystal structure, namely: a. Soft ferrites with the garnet structure such as the microwave ferrites (e.g: YIG). b. Soft ferrites with the cubic spinel structure such r The therm evier Ltd. responsibi t r e m with the garnet structure such as the mi © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

* Corresponding author. Tel.: +30 2610 969347; fax: +30 2610 969372. E-mail address: G.C.Psarras@upatras.gr Received: April 25, 2018; Received in revised form: July 10, 2018; Accepted: July 17, 2018

2452-3216 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 10.1016/j.prostr.2018.09.036 2452- 3216 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of the 1 st International Conference of the Greek Society of Experimental Mechanics of Materials t * Corresponding author. Tel.: +351 218419991. E-mail address: amd@tecnico.ulisboa.pt

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Nomenclature DMA

Dynamic Mechanical Analysis

E ՛ Ε″

storage modulus loss modulus

phr

parts per hundred resin per weight Scanning Electron Microscopy

SEM

T

temperature

as ZnFe 2 O 4 , and Fe 3 O 4 ferrites. c. Hard ferrites with the magnetoplumbite (hexagonal) structure such as Ba and Sr hexaferrites. Due to their good magnetic and electrical properties, ferrites are used predominately in three areas of electronics: low level applications, power applications, and Electro-Magnetic Interference (EMI) suppression (Pullar (2012)). The breadth of application of ferrites in electronic circuitry continues to grow. The wide range of possible geometries, the continuing improvements in material characteristics and their relative cost-effectiveness make ferrite components the choice for both conventional and innovative applications (Scarlatache et al. (2012) ; Özgür et al. (2009)). There is a growing demand for multifunctional composites to meet special requirements of electronic components (Ramajo et al. (2009)). A probable drawback of these devices could be not their electromagnetic behavior but their thermomechanical stability, which is the exact purpose of this study. In the present study, series of nanocomposite systems consisting an epoxy resin as matrix and five different magnetic oxides nanoparticles (YIG, ZnFe 2 O 4 , Fe 3 O 4 , BaFe 12 O 19 and SrFe 12 O 19 ) as reinforcing phase, have been prepared and studied, varying the filler content. Specimens ’ m orphology was assessed via Scanning Electron Microscopy (SEM). The thermomechanical characterization was conducted via Dynamic Mechanical Analysis (DMA) and static tensile tests. Five-different ferrite/epoxy systems were prepared with the same method, by employing commercially available materials. Epoxy resin and curing agent with trade names Epoxol 2004A and Epoxol 2004B, (both provided by Neotex S.A., Athens, Greece), and the magnetic iron oxide nanoparticles (YIG, ZnFe 2 O 4 , Fe 3 O 4 , BaFe 12 O 19 and SrFe 12 O 19 ) (Sigma Aldrich), were used for the preparation of the composite systems. The particle diameter of nano powders, as denoted by the supplier, was less than 100 nm. The preparation procedure involved mixing of the resin with the curing agent in a 2:1 (w/w) ratio and then adding various amounts of the nanoparticles. Filler’s con tent was 1, 3, 5, 10, 15 20 phr (parts per hundred resin per mass). The mixture was stirred at a slow rate in a sonicator for 10 minutes. Subsequently, the mixture was poured into silicon molds and cured at ambient for 7 days. The post curing took place for 4 hours at 100 o C. The morphology of the produced specimens was checked for the presence of clusters, while the quality of the filler dispersion within the polymer matrix was examined via SEM (EVO MA 10, ZEISS). The thermomechanical investigation was conducted by Dynamic Mechanical Analysis (DMA) in the temperature range from room temperature to 100 o C with 5 o C/min heating rate, using TA Q800 device, provided by TA Instruments. Additionally, the mechanical properties of nanocomposites were examined with static tensile tests using an Instron 5582 apparatus provided by Instron, at room temperature with 5 mm/min tension rate. 2. Experimental protocol

3. Results and discussion

3.1. Morphological characterization

The quality of the specimens was examined via SEM. Representative images are shown in Fig.1 for three of the examined systems. The nanoparticles dispersion can be considered as successful, since fine nanodispersions can be detected and the formation of large aggregates has been avoided, moreover in all specimens limited or even no agglomeration is present.

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Fig. 1. SEM images for the nanocomposites with 5 phr in (a) Fe 3 O 4 ; (b) (a) BaFe 12 O 19 and (b) SrFe 12 O 19 content.

3.2. Mechanical characterization

The mechanical properties of the fabricated nanocomposites were tested using an Instron 5582 tensile tester. The tensile modulus was calculated as the slope of the stress/strain curve in the elastic region. Figs.2-6 show the value of the elastic modulus and tensile strength for the epoxy nanocomposites with different concentrations of the nanoinclu sions. Errors bars represent the standard deviation of the representative result. The measured modulus for the neat epoxy was 1.86 GPa. The general trend for the elastic modulus is to increase almost monotonously with the addition of the nanoparticles for all systems examined. The increment of tensile modulus is attributed to the reinforcing ability and rigidness of the ceramic nanoparticles and to the strong interfacial bonding between the matrix and the nanoinclusions. The nanocomposites filled with SrFe 12 O 19 demonstrated the highest values of Y oung’s modulus in nearly all con centrations fabricated, reaching 3.71 GPa for the 20phr sample a whopping 100% increase compared to the neat epoxy. The maximum values of Y oung’s modulus and their increas e compared to the pure matrix for each system were:  For the ZnFe 2 O 4 systems E  (20 phr) = 2. 34± 0.12 GPa (25.5% increase)  The Fe 3 O 4 nanocomposites E  (20 phr) = 2. 90± 0.14 GPa (56% increase)  The BaFe 12 O 19 systems E  (20 phr) = 2.68 ± 0.13 GPa (44% increase)  The Y 3 Fe 5 O 12 nanocomposites E  (15 phr) = 2.79 ± 0.14 GPa (49.6% increase) Since no surface treatment was conducted in any of the employed filler types, moreover no substantial differences in the dispersion quality were revealed during the morphological characterization via SEM, the variation of the Young’s modulus values with filler type could be attributed to the intrinsic reinforcing capability of each ferrite and the adhesion between different nanoparticles and the polymer matrix. Typically, hexaferrites exhibit higher modulus values than spinels, which was indirectly confirmed in this study by the results for the SrFe 12 O 19 filled nanocomposites. The lower values for the other hexaferrite/polymer system can be attributed to the presence of hematite in the BaFe 12 O 19 nano powder, as confirmed by the XRD spectra in a previous study (Kanapitsas et al. (2015)).

Fig. 2. (a) Young’s modulus ; (b) tensile strength as a function of concentration, for the nanocomposites with Y 3 Fe 5 O 12 nanoparticles.

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Fig. 3. (a) Young’s modulus ; (b) tensile strength as a function of concentration, for the nanocomposites with ZnFe 2 O 4 nanoparticles.

Fig. 4. (a) Young’s modulus ; (b) tensile strength as a function of concentration, for the nanocomposites with Fe 3 O 4 nanoparticles.

Fig. 5. (a) Young’s modu lus; (b) tensile strength as a function of concentration, for the nanocomposites with BaFe 12 O 19 nanoparticles.

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Fig. 6. (a) Young’s modulus ; (b) tensile strength as a function of concentration, for the nanocomposites with SrFe 12 O 19 nanoparticles.

In heterogeneous polymer systems the mechanism of micromechanical deformations and consequently, the macro scopic properties of the polymers are determined by local stress distribution around the inclusions (Abraham et al. (2009)). Because the adherence of filler to polymer in the nanocomposites, the nanoparticles form a network that is able to carry the load. The tensile strength of the nanocomposites at low contents was similar, or even slightly improved than that of the neat epoxy thermoset. Stress distribution is created around the particles, increasing the nanocomposite strength further. The nanocomposites showed a small increment in its ultimate stress value, while higher filler content induced a decrease of tensile strength in most cases. A problem in particulate filled composites is the poor stress transfer at the filler-polymer interface because of the poor adherence of the filler to the polymer. The nanocomposites with high concentration in ceramic nanoparticles, exhibited lower strength than the neat epoxy thermoset which can be interpreted in terms of an increasing susceptibility of the aggregation of nanoparticles and weak interfacial adhesion between them and the epoxy matrix. From Figs.2-6 it is observed that higher nanoparticle content favours their aggregation, and the aggregates may cause a high stress concentration and premature failure. Overall, SrFe 12 O 19 nanocomposites exhibit the optimum mechanical behaviour in almost all examined cases. The mechanical improvement is also confirmed by DMA tests, as depicted in Figs.7 and 8. In the spectra of all systems, a step like decrease of storage modulus (Figs.7a, 8a) is observed at 45 to 70 o C indicating the presence of  - relaxation process, which is attributed to the glass to rubber transition of the polymer matrix. The incorporation of nanoinclusions resulted in an increase of the storage modulus over the whole temperature range for all examined systems. Indeed, all the nanocomposites showed a higher storage modulus than the neat epoxy thermoset, both in the glassy ( T < T g ) and rubbery states ( T > T g ). This kind of transitions are expressed with the formation of peaks in the loss modulus diagrams. The characteristic glass transition temperature was determined by peak maxima of the loss modulus diagram. The presence of nanoparticles at low concentrations seems to shift the transition range to higher temperatures. Apparently, glass transition temperature (T g ) is expected to increase with filler content. Shifting T g to higher values with the addition of nanometric particles is considered as a strong indication for good adhesion between matrix and inclusions and suggests the occurrence of attractive interactions between macromolecules and nanoparticles hindering the macromolecular motion. At high concentrations the results are mixed with ZnFe 2 O 4 and BaFe 12 O 19 systems exhibiting higher T g values, while SrFe 12 O 19, Fe 3 O 4 and Y 3 Fe 5 O 12 filled nanocomposites demonstrating lower values even than the ones of the neat matrix because the nanoparticles are adjacent to each other making particle-particle interactions stronger thus lifting part of the mobility hindrance.

4. Conclusions

Five different series of nanocomposites consisting of epoxy resin and ferrite nanoparticles (YIG, ZnFe 2 O 4 , Fe 3 O 4 , BaFe 12 O 19 and SrFe 12 O 19 ) were successfully fabricated and characterized morphologically via SEM. Their thermomechanical properties were studied via DMA and static tensile tests. From the experimental data, it seems that the incorporation of the ceramic nanoparticles enhances significantly both the storage and the tensile modulus of

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Fig. 7 (a) Storage modulus; (b) Loss modulus as a function of temperature, at 3 phr concentration of all nanocomposite systems.

Fig. 8. (a) Storage modulus; (b) Loss modulus as a function of temperature, at 15 phr concentration of all nanocomposite systems.

all systems. As far as tensile strength is concerned, a small increase is observed at low filler content for mostly all examined system followed by a decrease in higher concentrations. Overall, SrFe 12 O 19 nanocomposites exhibit the best mechanical behaviour in almost all examined cases.

Acknowledgements

This research has been financially supported by the General Secretariat for Research and Technology (GSRT) and the Hellenic Foundation for Research and Innovation (HFRI) (Scholarship Code: 2383).

References

Abraham, R., Thomas, S.P., Kuryan, S., Isac, J., Varughese, K. T., Thomas, S., 2009. Mechanical properties of ceramic-polymer nanocomposites. Express Polymer Letters 23, 177-189. Elsayed, A.H.,. Mohy Eldin, M.S,. Elsyed, A.M,. Abo Elazm, A.H, Younes, E.M., Motaweh, H.A., 2011. Synthesis and properties of polyaniline/ferrites nanocomposites. International Journal of Electrochemical Science 6 (1), 206-221. Hanemann, T. , Szabó , D.V., 2010. Polymer-nanoparticle composites: From synthesis to modern applications. Materials 3(6), 3468-3517. Kanapitsas, A., Tsonos, C., Psarras, G.C., Kripotou, S., 2015. Barium ferrite/epoxy resin nanocomposite system: Fabrication, dielectric, magnetic and hydration studies. Express Polymer Letters 10(3), 227-236. Kanapitsas, A., Tsonos, C., Zois, H., Delides, C.G., Psarras, G.C., 2013. Thermal and mechanical characterization of epoxy resin nano composites. Journal of Advanced Physics 2, 25-28.

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Özgür, Ü ., Alivov, Y. , Morkoç , H., 2009. Microwave ferrites, Part 1: Fundamental properties. Journal of Materials Science: Materials in Electronics 20(9), 789-834. Pullar, R.C., 2012. Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics. Progress in Materials Science 57 (7), 1191-1334. Ramajo, L.A., Cristóbal , A.A., Botta, P.M., Porto López , J.M., Reboredo, M.M., Castro, M.S., 2009. Dielectric and magnetic response of Fe 3 O 4 /epoxy composites. Composites Part A: Applied Science and Manufacturing 40(4), 388-393. Scarlatache, V.A, Olariu, M., Ursache, S, Ciobanu, R. C., Pasquale, M., 2012. Magnetic and dielectric losses of a nanocomposites polymer matrix reinforced with ferromagnetic powders. International Conference and Exposition on Electrical and Power Engineering, IEEE, 125-128. Schadler, L.S., Brinson, L. C, Sawyer, W. G., 2007. Polymer nanocomposites: A small part of the story. Jom 59(3), 53-60.

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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine P. Brandão a , V. Infante b , A.M. Deus c * a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 1 st International Conference of the Greek Society of Experimental Mechanics of Materials Acoustic emission monitoring of marble specimens under uniaxial compression. Precursor phenomena in the near-failure phase D. Triantis* Laboratory of Electronic Devices & Materials, University of West Attica, Greece Abstract In the present study Acoustic Emission (AE) indices are used to characterize the damage process in marble rock specimen that is subjected to axial compressive stress. The I b -value which depends on the amplitude distribution of the AE hits, shows a consistent trend of decreasing from the level of 70% of the ultimate compressive strength. A steep decrease is observed when the stress exceeds the 95% of the ultimate strength reaching a value near 1, a fact that can be considered as a warning of the coming event of fracture. Additionally, the average frequency of the AE hits shows a severe shift to lower values by about 200 kHz when the dominant failure mechanism is related to shear type of failure. Similarly, RA which is defined as the ratio of the waveform Rise Time over the Amplitude in μs/V , shows strong peaks at the moments near the fracture, indicating shear action. © 2018 The Authors. Published by Els vier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativec mmons.org/licenses/by-nc- d/3.0/). Peer-revi w under responsibility of the scientific committe of the 1 st International Conference of the Greek Society of Experimental Mechanics of Materials Keywords: Acoustic Emission; marble; I b -value; RA and AF values; crack classification 1. Introduction In the literature there are several recordings and studies of the Acoustic Emissions (AE) that are attributed to mechanical damages caused by microcracking in rocks (L ckner (1993); Shiotani et al. (2001); Yoshikawa and Mogi (1989)). When a material is subjected to external mechanical loading, the elastic mechanical energy is released in the form of Acoustic Emissions. Τ he change of the stress field insi the m terial which is related to the deformation 1 st t ti l t i t i t l i t i l . r t r f l tr i i s t ri ls, i rsit f st tti , r t t I t r t t ti i i ( ) i i r t r t ri t r i r l r i that i j t t i l r i tr . I b - l i t lit i tri ti f t it , i t t tr f r i fr t l l f f t lti t r i tr t . t r i r t tr t f t lti t tr t r i l r , f t t t i r r i f t i t f fr t r . iti ll , t r fr f t it r ift t l r l t t i t f il r i i r l t t r t f f il r . i il rl , i i fi t r ti f t f r i i r t lit i / , tr t t t r t fr t r , i i ti r ti . t rs. lis ls i r Ltd. is is ss rti l r t -NC-ND license ( tt :// r ti o s. r /li s s/ - - / . /). r-r ie r responsi ility of th s i ntific committee f th 1 st I t r ti l f r f t r i t f ri t l nics of Materi ls r s: sti issi ; r l ; I b - l ; l s; r l ssifi ti 1. Introduction I t lit r t r t r r r l r r i t i f t ti i i ( ) t t r ttri t t i l i r r i i r ( r ( ); i t i t l. ( ); i i ( )). t ri l i j t t t r l i l l i , t l ti i l r i r l i f t tr fi l i i t at ri l i i r l t t t f r ti © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. t f r f ti i i .

* Tel.: +30 210 5385357 E-mail address : triantis@teiath.gr Received: March 25, 2018; Received in revised form: June 26, 2018; Accepted: July 04, 2018 l.: - il r ss : tri tis t i t . r i i r is f r : J , ; t : J l , i : r , ;

2452-3216 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 1st International Conference of the Greek Society of Experimental Mechanics of Materials. 10.1016/j.prostr.2018.09.003 2452- 3216 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of the 1 st International Conference of the Greek Society of Experimental Mechanics of Materials - t r . li l i r t . is is ss rti l r t - - li s ( tt :// r ti s. r /li s s/ - - / . /). r-r i u r r s si ilit f t s i tifi itt e f t st I t r ti l f r f t r i t f ri t l i s f t ri ls * Corresponding author. Tel.: +351 218419991. E-mail address: amd@tecnico.ulisboa.pt

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processes, crack growth, dislocations movement, inclusions cracks, etc, constitute the AE sources. Thus the AE technique is developed and further improved in order to be used as a valuable tool for the monitoring and under standing of the mechanisms of dynamic processes, and warning of the upcoming failure (Rao and Lakschmi (2005); Colombo et al. (2003); Cox and Meredith (1993)). Fracturing process involves nucleation, growth and micro-cracks coalescence to the final breaking. The state of stress on rock materials generates a field in the specimen under study. The AE are mostly produced by the growth of microcracks. Generally, the AE are generated, at different spatial and temporal scales. It must be noted that the release of seismic waves related to an earthquake and the AE released from solids during fracture process have similarities. The initial link between AE and seismology was attempted by Ono and Ohtsu (Ohtsu et al. (1991); Ono (1993); Ohtsu (1994)), by transferring earthquake data processing techniques to AE data processing, resulting to the adjustment of several seismological techniques to the field of civil engineering. During the AE, different fracture modes generate different types of AE signals, of various frequency ranges and amplitudes. In general, micro-cracks generate a large number of small amplitude AE, while macro-cracks generate fewer events, but of higher amplitudes. A popular method used for damage quantification of a material under mech anical loading is the b-value analysis. The b-value was originally defined in seismology and then it was expanded and used in the AE signals in engineering materials (Colombo et al. (2003); Kurz et al. (2005)). The method of analysis of b-value has been recently modified using statistical values such as mean and standard deviation of each amplitude, and the most modern method is known as “improved b - value” (I b -value) (Rao and Lakschmi (2005)). In parallel with the study of the I b -value, two additional AE parameters have been used to analyze the failure mode: the average frequency (AF) of the AE signals that is defined as the number of counts divided by the duration, and the parameter RA (Rise Time/Amplitude) (Ohtsu and Tomoda (2008); Aggelis et al. (2011)). It is generally accepted that cracks generating ΑΕ signals of relatively high AF and lower RA are related to tensile cracking mode (Mode I), while shear mode cracking (Mode II) corresponds to lower AF and high RA. In this work, the acoustic emission (AE) study focuses on the change of key ΑΕ parameters, such as the b-value, the RA (Rise Time/Amplitude) and the AF (Average Frequency), when quasi-brittle materials as marble, are subjected to uniaxial compression. Emphasis is placed on the ΑΕ recorded near failure, when different types of AE signals with varying frequency ranges and amplitudes are observed. 2. The material and experimental process A prismatic sample of Dionysos marble, of dimensions 40 mm x 40 mm x 100 mm was used. Detailed description of the mechanical and physical properties of the specific material can be found in previous papers by Kourkoulis et al. (1999, 2010) and Pasiou and Triantis (2017). Initially, the strength of similar samples against uniaxial compression was checked, and failure was observed within a range from 62 MPa to 65 MPa. Preliminary tests conducted on similar specimens, extracted from the same rock volume, show that the transition from the linear to the non-linear region of the mechanical behavior of the material occurs at about 70% to 75% of the fracture stress. Mechanical loading was applied during four distinct stages until failure, as follows: During the first stage (A), the stress increased at a constant rate of 0.44 MPa/s up to 60.5 MPa. A second loading stage (B) of duration exceeding 100 sec followed, during which the stress remained constant at 60.5 MPa. Given that at the end of stage B the AE activity was very limited, a slight increase of the stress by 3 MPa followed (stage C), at the same rate (0.44 MPa/s). During the last stage (D), the stress was kept constant at 63.5 MPa. The sample failed after 60 sec. The continuous temporal recording of the applied stress during all four stages is depicted in Fig.1. An acoustic sensor recording the AE hits was placed in the middle of the side surface of the sample and coupled on the specimen using vacuum grease (see Fig.2a). One preamplifier with 40 dB gain with analogue band-pass filters in the range of 20-400 kHz was also used. The equipment and the software used were by Mistras Group, Inc.

3. Experimental results and discussion

During the whole experimental procedure up to the fracture of the specimen, 3335 ΑΕ hits were recorded with amplitudes equal or higher than 40 dB. After collecting the AE data, filtering work was conducted on the recorded AE hits, aiming at excluding hits probably attributed to noise. In this context, AE hits of less than 10 ms duration and count less than 2 were excluded (Calabrese et al. (2012)).

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3

75

4

(C)

(D)

(B)

3

50

2

25

(A)

Ib - value

stress (MPa)

1

stress Ib-value

0

0

0 25 50 75 100 125 150 175 200 225 250 275 300 325 time (s)

Fig. 1. Temporal recording of the applied stress and the I b -values during all four stages.

(a)

(b) Fig. 2. Indicative photos of the specimen prior to loading (a) and following to failure (b).

The number of the AE hits, as well as the mean rate of occurrence of the ΑΕ hits (mean value of hits per second) during the four stages previously described are presented in Table 1. The third column of Table 1 includes the average hit rate. The values in the parentheses in stages B and D, correspond to the values observed in the first 10 seconds following to the stabilisation of stress at 60.5 MPa and 63.5 MPa, respectively.

Table 1. AE hits and average hits rate during four distinct stages. stage Number AE hits Average hit rate A 893 7.2 B 730 5.8 (8.8) C 174 33.7 D 1538 25.1 (17.5)

3.1. The variability of the I b -value Provided that the ratio of weak to strong ΑΕ hits can be clearly described through the b-value analysis, it is quite common for the engineers working in the area of rock mechanics to use AE based b-value analysis in order to study the damage and fracture process in rocks (Main (1989); Rao and Lakschmi (2005)). In 2001, Shiotani et al. proposed the “improved b - value” (I b -value). The I b -value uses statistical values as mean and standard deviation of AE ampli tude that varies during the test. The I b -value defines as:

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log ( N

) log ( N

)    

  

 

(1)

I



1

2

b

(

)     

1

2

where σ is the standard deviation, μ is the mean value of the amplitude distribution, α 1 is the coefficient related to the smaller amplitude, α 2 is the coefficient related to the fracture level. The values of α 1 , α 2 vary between 0.5-5.0. Con sidering that changing the values of α 1 and α 2 did not significantly affect the I b -value, their value was here assumed constant and equal to 1 (α 1 =α 2 =1) (Rao and Lakschmi (2005)). In order to demonstrate the variability of the I b -value during the experiment, the total number of AE hits were divided into groups of sequential hits. Each group included n=100 hits from which the first 99 belonged to the previous group and the last one to the next. The I b -values that were calculated using equation (1), were attributed to a time value that corresponds to the mean time of the moments of occurrence of the sequential hits of each group. In Fig.1 the I b -value time history is depicted for all stages. During stage A the I b -values increase until the moment t=70 s, where the corresponding axial stress ( σ ) is not higher than 50% of the failure stress ( σ f ). Then, and until the time instant t=100 s ( σ≈0.70σ f ), the I b -values show a trend to remain constant at values higher than 2.5. This kind of I b -values variations during the first stages of loading may be attributed to AE generated due to the closure and friction phenomena of pre-existing microcracks in the material. At the following time instants (t>100 s) while the elastic de formation stage is practically exceeded, a gradual decrease of the I b -values starts, maintaining high values exceeding 2.0. This behaviour is continued until the time instant of t=130 s ( σ≈0.90σ f ). In this time period, the AE are mainly related to the generation of a number of new microcracks that are spread in the whole bulk of the tested specimen. While approaching the end of stage A, and as the stress reaches the value of 60.5 MPa ( σ≈0. 96 σ f ), a sudden decrease of the I b -values, is observed (see Fig.3) and a minimum value (Ib≈1.0) is recorded. This means that during this time period, a transitional state from formation, to growth stage of newly formed cracks takes place, at the end of which a preliminary crack-coalescence stage is possible.

150

2.5

2.0

100

hits per sec Ib -value

1.5

50 hits per sec

Ib - value

1.0

0

0.5

285

290

295

300

305

310

315

time (s)

Fig. 3. The acoustic activity (hits per second) and the I b -values during the last seconds before failure. .

During stage B, the I b -values continuously rise and after 35 s they are stabilized -with some fluctuations- between 3.0 and 3.3, given that the occurrence rate of AE decreases and the ΑΕ hits are characterized by lower amplitudes. A similar pattern is also observed in stages C and D, except that in stage D, for t>290 s, instead of a further increase of I b -value, a gradual reduction appears. During this final stage, the AE activity described by the number of hits per second, starts becoming more intense (see Fig.3). Two to three seconds before fracture of the specimen, and while the rate of the AE hits remains constant, the I b – values show a rapid decrease from the value of 1.5 down to values lower than 1 ( ≈ 0.85) at the instant of fracture. This behaviour clearly shows that abrupt events take place. These events indicate the existence of dynamic and unstable cracks that lead to the macroscopic failure.

3.2. The variability of the average frequency and RA value

Considering that the Acoustic Emission technique, can be used for the characterization of the typical crack modes, the Recommendation of RILEM TC 212-ACD was followed, that has been satisfactorily applied in concrete (Ohtsu

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(2010)). In RILEM TC 212-ACD, the classification of cracks in Mode I and Mode II types of damage, the quantities used are RA and AF. In order to study the variability of the average frequency (AF) of the AE hits and the RA values throughout the experimental procedure, the moving average was calculated for 100 sequential hits, to allow the variability monitoring. It is recalled that it is recommended to calculate the RA and AF from the moving average of more than 50 hits (Ohtsu (2010)).

80

40

(C) (D)

(B)

60

30

stress

40

20

(A)

20 RA (μs/mV)

10

(kHz)

stress (MPa)

0

0

0 25 50 75 100 125 150 175 200 225 250 275 300 325 time (s)

Fig. 4. Stress, RA and AF history.

The average frequency of the AE hits shows a similar behavior with that of the Ib – values (see Fig.4 and Fig.1). A sharp drop of the values is observed when the stress in stage A exceeds 57 MPa (t>120 s), as well as while entering stage C, whereas, in stage D, 30 s prior to the final failure the values start to drop. During the last 3 s prior to the fracture of the specimen, the AF values decline rapidly from 20 to 10 kHz. Taking into consideration that low AF values indicate shear type of cracking, it can be concluded that while microcracks cluster to create macrocracks, frictional phenomena appear. Also, observing the temporal variation of the RA values (Fig.4), strong peaks with high RA greater than 60 μ s/mV appear, in the ranges where the applied stress is higher than 96% of the failure stress. This constitutes another indication of the existence of shear phenomena, which could be either due to Mode II cracking or due to friction between the bases of the specimens and the loading platens or due to friction between the material fragments already formed (see Fig.2a). The latter is unavoidable, even in case of “perfectly” smooth contact of the specimens and the loading platens, as it was observed by Vardoulakis et al. (1997) during axial com pression of cylindrical marble specimens which split axially when the formation of the familiar Mohr‟s cones was totally suppressed. In order to study the relation between the RA value and the AF for various crack types, diagrams are plotted, in which the vertical axis corresponds to the AF in kHz, while the horizontal corresponds to the RA in μ s/mV. Α straight line starting from (0,0) and of slope m=0.5 kHz ∙ mV/ μ s is drawn and the AE with m<0.5 kHz ∙ mV/ μ s, are considered to be related to shear type of cracking (mode II) or friction. The determination of the appropriate value for the slope m is an open issue for research since there is not a rule generally accepted. However, it is accepted that it should be determined depending on the type of the material and the type of the experiment (Aggelis (2011)).

Table 2. Proportion of AE corresponding to Mode-II cracking for two different values of the parameter m. stage A1 A2 Β C D1

D2

(σ/σf)≈1 hits for last 3 s

stress

0.7≤(σ/σf)≤0.9

0 .9<(σ/σf)≤0.96

(σ/σf)≈0.96

0.96<(σ/σf)≤1 (σ/σf)≈1 total hits

% hits for m≤0.5 (mode II) % hits for m≤0.2 (mode II)

6%

53%

13%

33%

40%

53%

28%

33%

8%

19%

20%

33%