PSI - Issue 42

Martin Killmann et al. / Procedia Structural Integrity 42 (2022) 66–71 Killmann, Merklein / Structural Integrity Procedia 00 (2022) 000 – 000

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4.3. Fatigue failure Within the scope of this research, over 150,000 test cycles with different parameters were carried out. This caused one die to fail due to fatigue as shown in Fig. 6. During the test, the die fatigue coincides with an abrupt failure of the elastomer specimen. Due to the crack the elastomer has another opening to flow into. This changes the previously solely compressive stress in the Vulkollan® specimen, which causes it to collapse. As a consequence, the punch position changes and triggers an abort criterion. This provides an easy point of termination for the test. Furthermore, the die failure shows that the loads applied via the elastomer are critical for cold forging tools and in a comparable area with the forming of steel. An exact tool life cannot be specified, since the loads applied to the failed die were not constant. The tool life for different configurations will be analysed in future research.

Fatigue crack

2 mm

Fig. 6: Fatigue crack in die

5. Summary and Outlook Tool failure through fatigue is a main challenge in cold forging today. Currently, there is no fatigue test that incorporates the characteristic stress state caused by high and cyclically swelling inner pressures. Therefore, a fatigue test using elastomers as pressure medium was used and analysed in this research. As fatigue failure occurred within the investigations, this concept is able to generate the same kind of loads as in cold forging of metal materials. A challenge in the test concept is wear on the elastomer specimens. With each cycle, some material is pressed into the gap between punch and die. Consequently, high sizes of this gap and high forces increase the wear. As the lowest material loss was achieved for a gap of 0.01 mm and a frequency of 5 Hz, these parameters should be used in future applications. A higher test force increases elastomer wear, but leads to faster fatigue failure. Suitable forces should be analysed in future research by investigating the influence of the elastomer wear on the die stress in the middle of the pressure chamber. Furthermore, the test should be used for different reinforcement setups to show its full potential. References Hirschvogel, M., Dommelen, H., 1992. Some applications of cold and warm forging. Journal of materials processing technology 35(3-4), 343-356. International Cold Forging Group, 2002. Tool Life & Tool Quality in Cold Forging Part 1: General Aspects of Tool Life. ICFG Document No. 14/02. Bamberg, Meisenbach. International Cold Forging Group, 1992. Calculation methods for cold forging tools. International Cold Forging Group 1967-1992 - Objectives, History Published Documents. Bamberg, Meisenbach: 59-72. Lee, H.-S., Park, S.-G., Hong, M.-P., Kim, Y.-S., 2022. Process design of multi-stage cold forging with small size for ESC solenoid valve parts. Journal of Mechanical Science and Technology, 1-12. Meidert, M., 2006. Beitrag zur deterministischen Lebensdauerabschätzung von Werkzeugen der Kaltmassivumformung. In: Geiger, M. (Ed.). Fertigungstechnik – Erlangen 172. Bamberg, Meisenbach. Pilz, F., D. Gröbel, M. Merklein, 2018. Investigation of Fatigue Strength of Tool Steels in Sheet-Bulk Metal Forming. In: L. Fratini, Lorenzo, R. D., Buffa, G., Ingarao, G. (Eds.): AIP Conference Proceedings - The 21st International ESAFORM Conference on Material Forming. Skov-Hansen, P., Bay, N., Groenbaek, J., Brondsted, P., 1998. Fatigue in cold-forging dies: tool life analysis. Journal of Materials Processing Technology 95(1-3), 40-48. Tekkaya, A., Sonsöz, A., 1996. Life Estimation of Extrusion Dies. International Journal of Mechanical Sciences 38(5), 527-538. Zhang, Y., Hu, C. L., Zhao, Z., Li, A. P., Xu, X. L., Shi, W.B., 2013. Low cycle fatigue behaviour of a Cr – Mo – V matrix-type high-speed steel used for cold forging. Materials & Design 44, 612-621. Buckley, C. P., Prisacariu, C., Martin, C, 2010. Elasticity and inelasticity of thermoplastic polyurethane elastomers: Sensitivity to chemical and physical structure. Polymer 51(14), 3213-3224.