PSI - Issue 38

Sara Eliasson et al. / Procedia Structural Integrity 38 (2022) 631–639 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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2. Material The material used in the study was a stitch-bonded UD NCF from Zoltek with a fast-curing epoxy resin from DOW. The material was received in 300x650 mm plates, with a thickness of approximately 2 mm. The plates were manufactured with Resin Transfer Molding (RTM) using 8 layers of the fiber fabric, all stacked with the fibers in the same direction. The UTS of the material in the fiber direction is 1295 MPa and was determined with static tension testing according to ASTM D3039/D3039M-17 (ASTM International, 2017). 3. Fatigue Testing Tension-tension fatigue testing was conducted according to ASTM D3479/D3479M-12 (ASTM International, 2012). The fatigue test rig used was a Schenk Hyrdopuls with a 100 kN load cell. The grips on the test rig were hydraulic with the ability to adjust the clamping pressure. The clamping pressure was set to, 30-45 kN (Eliasson, et al., 2019). The testing frequency was set to 5 Hz and a load ratio, , of 0.1 was used for all tests. Two million cycles were set as the runout limit for the specimens; however, some tests were run further. The fatigue testing was performed at a load level of 70 – 96 % of the UTS. The specimens were manufactured according to ASTM D3039/D3039M-17 (ASTM International, 2017) and the specimen geometry was chosen according to Eliasson et al. (2019) (Fig. 2).

Fig. 2. The specimen geometry used in the fatigue testing (L spec = 250 mm; w spec = 15 mm; L tab = 50 mm; w tab = 20 mm).

3.1. Fatigue Test Setup The fatigue testing methodology, measuring in-situ full-field displacements of the fatigue specimens was automated such that no operator input was required after initialization. The measurement setup was disconnected from the fatigue rig and could be connected to any testing system. Surface displacements, at peak load for a given cycle of the ongoing fatigue test, were captured in-situ with a High-Speed Camera (HSC) and the images were analyzed with Digital Image Correlation (DIC). Following sections outline the test setup, and for a more thorough description see the thesis of Wenner Berg (2020). 3.2. Automated In-Situ Full-Field Displacement Measurements DIC is an image processing technique that tracks surface displacement. It generates full-field measurements without contact to the specimen. 2D DIC was utilized in this study to track the development of fatigue induced displacement and strain. A speckle pattern in black and white was applied to the specimen to get high contrast images (LePage, et al., 2017). The DIC measurements were compared with a second measurement from an extensometer, to verify the correctness of the data. The HSC was used to capture the peak loading of a cycle without pausing or slowing down the fatigue test. Each captured peak was compared to follow the stiffness degradation, i.e. the fatigue damage growth. The HSC was configured with the software MotionProX Studio and was triggered by closing a circuit with a transistor switch. The sampling rate was 1000 frames per second and for every trigger event, 150 images were captured. Analyzing the data, the peak loads were compared to an image with zero load taken before the test was started. The camera settings are found in Table 1.