PSI - Issue 28

D. Weiß et al. / Procedia Structural Integrity 28 (2020) 2335–2341 D. Weiß et al. / Structural Integrity Procedia 00 (2019) 000–000

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The comparison between the R -ratios 0.1 and 0.3 shows that the average cyclic fracture toughness with 46.03 MPaꞏm 1/2 is significantly lower at an R -ratio of 0.3. This result is in line with theoretical expectations for ductile materials, see Richard and Sander (2012). But the results of the down tests deviate significantly from each other, especially in the area of the threshold region. Fig 5b shows the experimental parameter K max in the down test. First, a starter crack with a length of 1 mm is induced with constant Δ K before the lowering test is started. The experimental parameters in the down tests differ only in the K max set during the generation of the starter crack. In the tests highlighted in blue, the Δ K was reduced by a step from K max = 830 N/mm 3/2 to K max = 650 N/mm 3/2 before the actual lowering test was started. It can be seen that in this case (in blue color) the near threshold behavior is equal to the one at an R -ratio of 0.1. Whereas with the same K max (in green color) as for R = 0.1 without a sudden reduction of K max the threshold shifts to the left in accordance to the literature. A possible cause of this deviation at lower crack rate can be plastification of the material which might slow down the crack growth rate especially in the threshold region. The lower initial load used when taking the green curves is intended to reduce plastification and, hence, sequence effects at the beginning of the test. Under sole consideration of the results with lower initial load (in green color) the crack growth rate increases with increasing R -ratio and the threshold decreases. Furthermore, the influence of different R ratios is more pronounced in the low and the high crack growth rate range. 4. Conclusions A special specimen geometry was developed to experimentally determine fracture mechanical parameters of clinchable metal sheets. After the numerical and experimental investigations with the special specimen, it can be stated that the presented special specimen is suitable for crack growth tests. Furthermore, the experiments of the rolling direction show that there is no anisotropy arising during rolling which influences the fracture mechanical parameters. Finally, the crack growth rate increases and the threshold decreases with increasing R -ratio in accordance to literature if sudden reductions of the cyclic load are avoided. To validate these results, further experiments with an R -ratio of 0.3 and for example with an R -ratio of 0.5 have to be carried out to describe the mean stress sensitivity correctly. Acknowledgements This project was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – TRR 285 – Project-ID 418701707. References ASTM, 2008. Annual book of ASTM standards. Section 3: Metals test methods and analytical procedures, vol 03.01. Metals - Mechanical testing, elevated and low-temperature tests. Metallography, 2008: E 647-08. Barnes, T. A., Pashby, I. R., 2000. Joining techniques for aluminium spaceframes used in automobiles: Part II - adhesive bonding and mechanical fasteners. Journal of Material Processing Technology 99, 62-71. Böllhoff, 2013. Rivclinch: Devices and systems for joining sheets and profiles without fasteners. https://pdf.directindustry.com/pdf/boellhoff/rivclinch-devices-systems-joining-sheets-profiles-without-fasteners/9129-29986.html Accessed 4 July 2020. Böllhoff, 2015. Rivclinch: Ohne Schweißen oder Nieten https://media.boellhoff.com/files/pdf1/rivclinch-de.pdf Accessed 5 August 2020. DVS/EFB 3420, 2012: Merkblatt 3420: Clinchen – Überblick. Deutscher Verband für Schweißen und verwandte Verfahren e.V., Europäische Forschungsgesellschaft für Blechverarbeitung e.V. EC-European Commission, 2019: Regulation (EU) 2019/631 of the European parliament and of the council of 17 April 2019 setting CO2 emission performance standards for new passenger cars and for new light commercial vehicles, and repealing Regulations (EC) No 443/2009 and (EU) No 510/2011 (recast). Official Journal of the European Union L111, 13-53. Eshtayeh, M.M., Hrairi, M., Mohiuddin, A.K.M., 2015. Clinching process for joining dissimilar materials: state of the art. International Journal of Advanced Manufacturing Technology 82, 179-195. Gude, M., Lieberwirth, H., Meschut, G., Zäh, M., Tekkaya, E. (Hrsg.), 2018. Ressourceneffizienter Leichtbau für die Mobilität. FOREL-Studie. Kloster, V., Fulland, M., Richard, H.A., Wiedemeier, B., Niendorf, J., 2010. Risswachstum in Strukturen mit gradierten Materialeigenschaften. 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