PSI - Issue 38

S. Häusler et al. / Procedia Structural Integrity 38 (2022) 230–237

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S. Häusler et al. / Structural Integrity Procedia 00 (2021) 000 – 000

4. Conclusion With the aim of characterising the stiffness degradation of GFRP commonly utilised in wind turbine rotor blades, two layups have been fatigue tested at R = -1 and R = 0.1. In these experiments the loss in stiffness was obtained and the results were averaged to one representative curve for each test. While the shear degradation shows only little scattering and no visible amplitude dependency, the loss in stiffness of the tests with Θ = 90° specimens strongly depended on the damage development and needed to be differentiated. Hereby the application of the DIC for strain analysis worked reliable. In combination with the backlight images, a better understanding of the damage development and the stiffness degradation in fatigue was accomplished. The measurement with a major crack outside the DIC analysis area suggests that the conventional measurement with strain gauges can miss the specimen’s degradation , if not placed directly upon a crack, which than however would bear the risk of destroying the gauge. Nevertheless, also the speckle pattern is susceptible of damages close to or under it, so affected datasets need to be further investigated. By monitoring the surface temperature, it was observed during the R = -1 shear fatigue tests that damage causes a local overheating. Due to the limited area of this heating a thermocouple as suggested in the standards is unlikely to detect this effect. A full-scale measurement system like thermography is recommended. Acknowledgement The authors would like to thank the German Federal Ministry for Economic Affairs and Energy for the financial support of the Project DynaWind² (contract number: 0325228G). References American Society of Testing and Materials (2019). Standard Test Method for Tension-Tension Fatigue of Polymer Matrix Composite Materials. ASTM International, D3479/D3479M – 19. DOI: 10.1520/D3479_D3479M-19 Cadavid, M. Ospina, Al-Khudairi, O., Hadavinia, H., Goodwin, D., Liaghat, G. H. (2017): Experimental Studies of Stiffness Degradation and Dissipated Energy in Glass Fibre Reinforced Polymer Composite under Fatigue Loading. In: Polymers and Polymer Composites 25 (6), pp. 435 – 446. DOI: 10.1177/096739111702500602. Deutsches Institut für Normung (2016). Load controlled fatigue testing – Execution and evaluation of cyclic tests at constant load amplitudes on metallic specimens and components, DIN 50100:2016-12 Häusler S., Fink R., Benz C., Sander M. (2021). Quasi-static and cyclic study of a fibre reinforced plastic with digital image correlation. In: DVM Bericht 253, 53. Tagung des Arbeitskreises Bruchmechanik und Bauteilsicherheit. Deutscher Verband für Materialforschung und -prüfung e.V., Berlin, pp. 137-146. https://doi.org/10.48447/Br-2021-016 (in German) International Organization for Standardization (2003). Fibre-reinforced plastics - Determination of fatigue properties under cyclic loading conditions. ISO 13003:2003-12. Liu, F. T., Ting, K. M., Zhou, Z.-H. (2008): Isolation Forest. In: 2008 Eighth IEEE International Conference on Data Mining. 2008 Eighth IEEE International Conference on Data Mining (ICDM). Pisa, Italy, pp. 413 – 422. DOI: 10.1109/ICDM.2008.17. Nijssen, R. (2006): Fatigue life prediction and strength degradation of wind turbine reactor blade composites. Wieringerwerf: Knowledge Centre Wind Turbine Materials and Constructions. Roundi, W., El Mahi, A., El Gharad, A., Rebiere, J.-L. (2019): Experimental investigation of the fatigue behavior of glass/epoxy composites evaluated by the stiffness degradation and damage accumulation. In: Journal of Composite Materials 53 (6), S. 731 – 740. DOI: 10.1177/0021998318790341. Wedel-Heinen, J., Tadich, J., K., Brokopf, C., Janssen, L. G. J., van Wingerde, A. M., Delft, D. R. V., Kensche, C. W., Philippidis, T. P., Brøndsted, P., Dutton, A. G., Nijssen, R. P. L. (2006): Implementation of OPTIMAT in Technical Standards. OB_TG6_R002 rev. 8. Hg. v. Optimat Blades Zangenberg, J., Brøndsted, P., Gillespie, J. W. (2014): Fatigue damage propagation in unidirectional glass fibre reinforced composites made of a non-crimp fabric. In: Journal of Composite Materials 48 (22), S. 2711 – 2727. DOI: 10.1177/0021998313502062.