Fracture behaviour of prepreg laminates studied by in-situ SEM mechanical tests

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Structural Integrity Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2018) 000–000

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Procedia Structural Integrity 13 00 (2018) 1442–1446 Structural Integrity Procedia (2016) 000–000

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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity XV Portuguesebehaviour Conference onof Fracture, PCFlaminates 2016, 10-12 February de Arcos, Portugal Fracture prepreg studied2016, by Paço in-situ SEM

Fracture behaviour of prepreg laminates studied by in-situ SEM mechanical testspressure turbine blade of an Thermo-mechanical modeling of a high mechanical tests a a a gas turbine engine Simon Barda, Martinairplane Demleitner , Markus Häublein , Volker Altstädtaa* a a Simon Bard , Martin Demleitner , Markus Häublein , Volker Altstädt * University of Bayreuth, Polymer Engineering, Universitätsstr. 30, 95444 Bayreuth, Germany a b c P. Brandão , V. Infante , A.M. Deus * Germany University of Bayreuth, Polymer Engineering, Universitätsstr. 30, 95444 Bayreuth, a a

a

Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa,

Abstract Portugal Abstract b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, By using a miniature testing device placed in a Scanning Electron Microscope (SEM) it was possible to investigate the fracture Portugal By cusing ainminiature device placed in a Scanning Electron Microscope was possible to investigate the fracture CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, deitLisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, toughness mode I oftesting graphite modified prepreg laminates. Different toolsUniversidade have(SEM) been developed to expand the possibilities of the Portugal toughness in mode I of graphite modified prepreg laminates. Different have been developed expand the possibilities the commercially available tool. In-situ deformation studies can therebytools be used to evaluate the to crack behavior of carbonoffiber commercially available tool. In-situ deformation studies can thereby be used to evaluate the crack behavior of carbon fiber reinforced composites (CFRP). The matrix of the CFRP has been optimized for higher thermal and electrical conductivity to be reinforced composites (CFRP). The The matrix of the CFRP has been the optimized and electrical to be used in electrically driven aircrafts. crack propagates through matrix for andhigher shows thermal crack deviation at the conductivity graphite particles, Abstract used incould electrically driven aircrafts. Themechanical crack propagates through the matrix and shows crack deviation at the graphite laminates particles, which be shown in in-situ SEM tests. Also the toughening mechanism of thermoplastic interleaved which could be shown in in the in-situ mechanical Alsotransition the toughening of thermoplastic interleaved laminates have been investigated SEMSEM mechanical tests.tests. A crack betweenmechanism different layers was clearly visible in SEM. During operation, modern aircraft engine are subjected to increasingly demanding operating conditions, have been their investigated in the SEM mechanical tests. components A crack transition between different layers was clearly visible in SEM. especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent © 2018 The Authors. Published by Elsevier B.V. one of which is model using the finite element method (FEM) was developed, in order to be able to predict © degradation, 2018 The Authors. Published bycreep. ElsevierAB.V. © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. the creepunder behaviour of HPT blades. data records (FDR) for a specific aircraft, provided by a commercial aviation Peer-review responsibility of the ECF22Flight organizers. Peer-review responsibility the ECF22 company, under were used to obtainofthermal and organizers. mechanical data for three different flight cycles. In order to create the 3D model Keywords: Carbon CFRP;afracture toughness, Mode toughness needed SEM; for the FEMFiber; analysis, HPT blade scrap wasI fracture scanned, and its chemical composition and material properties were Keywords: SEM; Carbon Fiber; CFRP; fracture toughness, Mode I fracture toughness 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

Nomenclature overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a Nomenclature model can be useful in the goal of predicting turbine blade life, given a set of FDR data. DIC Digital Image Correlation DIC ImagePublished Correlation © 2016 Digital The Authors. by Elsevier GIC Strain energy release rate mode IB.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. GIC Strain energy release rate mode I ILSS Interlaminar Shear Strength ILSS Interlaminar Shear Microscope Strength SEM Scanning Electron Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. SEM Scanning Electron Microscope

* Corresponding author. Tel.: +49 (0) 921 55 7476; fax: +49 (0) 921 55 7473. * Corresponding Tel.: +49 (0) 921 55 7476; fax: +49 (0) 921 55 7473. E-mail address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility theby ECF22 organizers. * Corresponding author. Tel.: +351of 218419991. Peer-review under responsibility of the ECF22 organizers. E-mail address: [email protected]

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 B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.299

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1. Introduction The identification of failure mechanisms of components in most cases follows after the mechanical testing. The live-observation by video devices, also in combination with Digital Image Correlation (DIC), delivers more information, but is limited in magnification. Micro-testing stages as provided by Kammrath & Weiß (Dortmund, Germany) offer the possibility to carry out such tests in-situ under SEM or optical microscopes. The tests are so far limited to tensile and compression tests. Interesting results with the device were already found by Kaya et al. [1] in metallic foams. The tests on a macroscopic level (5x and 30 x magnification) are very useful to identify basic deformation mechanisms as bending and buckling of struts. Besides foams, copper wires have been tested in tensile tests by in-situ SEM. [2] Also glass fiber-reinforced composites have been tested under compression load using 250x magnification. [3] Canal et al correlated their values via Digital Image Correlation (DIC) and showed that at low magnification, the fibers themselves acted as the speckle pattern and the short elastic interactions between fibers as well as the sharp strain gradients at the fiber/matrix interfaces were completely smoothed out from the strain maps. DIC was able to accurately capture the displacement fields throughout the specimen. In general, very small plastic and elastic deformations can be observed in SEM. The manufacturer Kammrath & Weiß only provides tools for tensile and compression tests. Interesting results are expected from the transfer of the method to other mechanical tests, as tests for Interlaminar Shear Strain (ILSS) or fracture mechanical tests to determine energy release rate in mode I (GIC) and mode II (GIIC). In the literature, fillers or thermoplastic interleaves are used to increase the impact and fracture properties. These interleaves are thermoplastic veils, which can be placed between the carbon fiber layers before the lamination process. The in-situ tests can improve the understanding of the fracture behavior especially for filled composites and toughened composites. In energy release rate tests in mode I (GIC) and mode II (GIIC), toughening effects from filler can be observed. In the herein presented GIC mechanical tests, the toughening mechanism as crack pinning and crack deflection are aim of the research. Therefore, first static tests have been conducted. Then samples with an adapted sample size were produced and their fracture behavior has been observed in SEM. 2. Production and Experimental Setup Samples have been prepared from a high temperature thermoset resin (TGMDA, EpikoteTM RESIN 496) from Hexion Inc. (Columbus, USA) and amine hardener (XB3473TM, DETDA, hydrogen equivalent weight 43 g/eq) from Huntsman (Salt Lake City, USA). PAN based fiber 12K A-49 (DowAksa, Atlanta, USA) with a tensile strength of around 4900 MPa and a Young’s modulus of 250 GPa has been used for the prepreg production. Graphite platelets with average size of ~18 µm have been used as modificator of the matrix (Imerys Graphite&Carbon, Zurich, Switzerland). Thermoplastic interleave from PE (TFP Global, Schenectady, USA) with areal weight of 17 g/m² has been placed between the carbon-fiber layers before the lamination of the prepregs. Mechanical testing module with maximum force of 5kN (Kammrath & Weiß, Dortmund, Germany) was installed into SEM (LEO 1530 SEM by Zeiss, Oberkochen, Germany). Different self-made modules were prepared from steel. Pictures of the modules can be found in Figure 1. The module for Interlaminar Shear Strength (ILSS) in Figure 1 a) allows mechanical tests in accordance to DIN EN ISO 14130. The module in b) can be used to determine the interlaminar fracture toughness energy - Mode I/II similar to DIN EN 6033, but the sample sized needed to be reduced to fit into an SEM apparatus. The third module was designed for three-point bending tests according to DIN EN ISO 12125. Figure 2 shows the mechanical testing unit, which can be placed in the SEM. Samples were glued to the tools and sputtered using Cressington Coater 108 AUTO (Watford, UK). 3. Results and Discussion Table 1 shows the results from quasi-static tests. The GIC of the unmodified laminate is at 252 ± 2 J/m², which is in accordance to findings of other researchers. [4–7] The low standard deviation reflects the high quality of the samples produced. A straight crack was observed in SEM, as shown in Figure 3. The crack grows perfectly parallel to the fiber direction. Observations in the literature also show a straight crack growth in the sample [8]

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a

b

c

Figure 1: Modules for (a) Interlaminar Shear Strength, (b) energy release mode I, (c) three point bending test.

Figure 2: Mechanical testing unit installed in SEM by Kammrath & Weiß Table 1. Results from quasi-static tests performed at a universal testing device Sample

Fiber content vol%

GIC J/m²

Unmodified laminate

34 ± 1

252 ± 2

Filled laminate (15 vol% Graphite)

38 ± 3

321 ± 32

Interleaved laminate

35 ± 2

626 ± 62

Figure 3: Straight crack in neat carbon-fiber reinforced laminate

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The laminate with graphite particles in the epoxy matrix are characterized by an energy release rate of 321 ± 32 J/m², which is already 27% higher than the reference. As no matrix-modification of prepreg laminates can be found in literature so far, the results can only be explained by modified epoxy resins without fibers. Herein the stress intensity factor (KIC) increased by the incorporation of macro-sized particles, resulting from crack deviation [9]. Higher energy release rates of toughened epoxy resins generally lead to higher energy release rates in carbon-fiber reinforced laminates. [10] So it can be expected that the toughening mechanism can be transferred from the filled matrix to the laminate. Is can be well seen from Figure 4, left, that the crack growth undulating and not straight as in Figure 3. Figure 4, right, shows the crack deflection at graphitic particles at higher magnification.

Figure 4: left: Undulation crack growth in laminate, right: Crack deflection at graphite particle in laminate

Figure 5: left: Crack transition in thermoplastic interleaved laminate, right: evolution of a second crack in laminate

Thermoplastic interleaves are well known from the literature to improve the fracture toughness of fibre reinforced laminates. [5–7, 11–14] The GIC increased from 252 ± 2 J/m² in the unmodified laminate to 626 ± 62 J/m² in the interleaved laminate, which corresponds to a relative increase of 248%. Although in the literature different crack mechanism have been suggested, they are poorly proven. From the R-curves, Beckerman [7] concluded that the crack grows with the path of the least resistance. He came to the conclusion that the crack can transition from the interlayer into other regions of the laminate, which seems one toughening mechanism. Figure shows that several cracks were observed in SEM. The crack is transitioned from one layer of carbon fibre to another layer and a new crack starts to grow. The magnification of the crack tip shows that Crack 1 stops growing (crack pinning) at the lower part of the interleave region and then a second crack (marked as Crack 2) starts at the upper part of the interleave. As explained by Beckerman [7], the crack takes the path of the least resistance and the interface between interleave region and carbon fibres seems to show low adhesion.

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4. Conclusion and Outlook The underlying research showed a straight crack growth in the unmodified laminate. When graphite is used as a filler for the matrix, the crack is strongly undulating and the crack deflection can be well seen in the in-situ SEM mechanical tests. In the interleaved laminate, crack pinning and crack transition could be proven in SEM. 5. Outlook It is suggested to use Digital Image Correlation (DIC) software to evaluate the deformation of the sample. As showed by Canal in compression tests, the surface can be sputtered with a coating to increase the contrast. Then a DIC surface was used to evaluate the deformations. [3] Also the application of the tests to mechanical tests in Mode II might give interesting insights in the crack mechanism under shear-loading. Acknowledgements Authors kindly thank to the German Ministry of Economy and Energy (BMWi) for the funding of the Lufo Project TELOS (FKZ 20Y1516D). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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