PSI - Issue 13

Simon Bard et al. / Procedia Structural Integrity 13 (2018) 1442–1446 Simon Bard / Structural Integrity Procedia 00 (2018) 000–000

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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 (G IC ) and mode II (G IIC ). 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 (G IC ) and mode II (G IIC ), toughening effects from filler can be observed. In the herein presented G IC 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, Epikote TM RESIN 496) from Hexion Inc. (Columbus, USA) and amine hardener (XB3473 TM , 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 G IC 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]