PSI - Issue 28

Anja Gosch et al. / Procedia Structural Integrity 28 (2020) 1184–1192 Anja Gosch/ Structural Integrity Procedia 00 (2019) 000–000

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where d  a/dN is the crack extension a per cycle N,  K the stress intensity factor range, and A and m are the material parameters from the power law fit. Since these two parameters are directly used for lifetime estimation, the evaluation of A and m is a further indication of a successful application of the different crack length detection methods. The crack growth kinetics curve determined via the different testing set-ups (Microscope vs. Microscope, Microscope vs. IRT and Microscope vs. DIC) are shown in Figure 5 for PVC-U, POM and PMMA. Generally, both methods (IRT and DIC) showed good correlations with the commonly used microscope method. All three testing set-ups (“Microscope”, “IRT” and “DIC”) exhibit almost the same mean values for A and m for each investigated polymer. Hence, the chosen testing set-up for the measurement of the crack length displays only a small influence on the resulting material parameters A and m, which supports the use of the chosen methods for automation. Uneven crack growth is also expected to be the main influencing factor here. Thus, it is concluded, that all three testing methods are suitable to characterize the crack growth kinetics for lifetime estimation of polymer components. 4. Summary and Conclusion It is still state of the art to measure the crack growth in fracture mechanical fatigue tests of polymers via an optical microscope. To push these procedures towards automated crack length measurement, new methods for crack growth detection have to be developed. The aim of this study was to investigate two methods (infrared thermography IRT and digital image correlation DIC) for the evaluation of the crack advancement during fatigue testing. These methods were examined on three different polymers (PVC-U, POM and PMMA) and compared to the commonly used microscope method. Both testing set-ups (IRT and DIC) displayed promising results for an automated crack length detection, since similar results were determined as with the microscope method. Uneven crack growth during the fatigue experiment was identified as the main influencing factor on the results, which was even more significant than the chosen testing set-up. Generally, the IRT method is suitable for polymers with small plastic zones and linear elastic material behavior, whereby highly ductile polymers require further investigations and the influence of the plastic zone size has to be investigated. However, the IRT method requires less specimen preparation compared to the DIC method, which is especially beneficial for the use in automated crack length detection. The DIC method is influenced by the speckle size and also by the evaluation method used to determine the exact position of the crack tip. The chosen threshold value was based on a rather empirical approach. Therefore, also for this method, further research is necessary to verify the procedure chosen in this study. 5. Acknowledgements The research work of this paper was performed at the Materials Science and Testing of Polymers/Montanuniversitaet Leoben within the framework of the COMET-program of the Federal Ministry for Transport, Innovation and Technology and the Federal Ministry of Science, Research and Economy with contributions by the Polymer Competence Center Leoben GmbH. Anderson, T.L. (2005), Fracture Mechanics - Fundamentals and Applications , CRC Press, New York. ASTM International D5528-13 (2013), Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites , West Conshohocken, available at: www.astm.org. ASTM International E 647-11 (2011), Standard Test Method for Measurement of Fatigue Crack Growth Rates , West Conshohocken. Berer, M. and Pinter, G. (2013), “Determination of crack growth kinetics in non-reinforced semi-crystalline thermoplastics using the linear elastic fracture mechanics (LEFM) approach”, Polymer Testing , Vol. 32 No. 5, pp. 870–879. Berer, M., Pinter, G. and Feuchter, M. (2014), “Fracture mechanical analysis of two commercial polyoxymethylene homopolymer resins”, Journal of Applied Polymer Science , Vol. 131 No. 19, n/a-n/a. Hertzberg, R.W. and Manson, J.A. (1980), Fatigue of Engineering Plastics , Academic Press, Inc, New York. International Standard ISO 15024 (2001), Fibre-reinforced Plastic Composites Determination of Mode I Interlaminar Fracture Toughness, GIC, for Unidirectional Reinforced Materials , Ginebra. International Standard ISO 15850 (2002a), Plastics - Determination of tension-tension fatigue crack propagation - Linear elastic fracture mechanics (LEFM) approach No. 15850, Geneve. International Standard ISO 15850 (2002b), Plastics – Determination of Tension-tension Fatigue Crack Propagation – Linear Elastic Fracture 6. References