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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1. Introduction Cold forging is gaining in importance for the mass production of high-quality technical parts, since it has a higher material efficiency than machining (Lee, Park et al. 2022) and offers better surface quality and geometrical accuracy compared to hot forging (Hirschvogel and Dommelen 1992). Therefore, there is a demand for increasingly complex cold forged parts. This leads to a challenging stress state in forging tools, since they have to withstand high contact pressures in intricate geometries. The resulting alternating cyclic load makes fatigue the dominating failure mechanism in forging dies for complex part geometries (Tekkaya and Sonsöz 1996). Since fatigue failure not only causes costs for the production of new tools, but also machine downtime and production stops (International Cold Forging Group 2002), an accurate prediction of tool life is necessary in order to better coordinate tool changes. Attempts to calculate the service life of cold forging dies face the problem of limited availability of material data concerning fatigue (Skov-Hansen, Bay et al. 1998). Additionally, the tests used to generate this data strongly simplify the tool’s multiaxial stress state, which is generated by cyclically sw elling inner pressures. For example, the rotary bending test (Pilz, Gröbel et al. 2018) and the tension-compressive test (Zhang, Hu et al. 2013) have been used in cold forging applications, both of which use a uniaxial stress state. Additionally, reinforcements are often used to prestress forging dies (International Cold Forging Group ICFG 1992), so that the tool load alternates between a multiaxial compressive and a multiaxial tensile/compressive stress state. To accurately model this and enable the experimental analysis of different prestressing systems, a new fatigue test is necessary. For this purpose, a test that incorporates cyclically swelling inner pressures on hollow specimen is analysed within this research. To model this kind of load, a pressure medium in hollow fatigue specimens is necessary. Since the use of fluids necessitates the seals, which have limited the applicable inner pressure in past tests (Meidert 2006), a solid is to be used as pressure medium. Being nearly incompressible (Buckley, Prisacariu et al. 2010), elastomers have similar properties as liquids in that area. The high-strength elastomer Vulkollan® of the company Covestro AG is therefore suitable for the intended purpose. Apart from the incompressibility, the elastomer’s elasticity will allow the use of cyclic testing without changing the specimen each cycle, as would be necessary in steel forming. This paper aims to show the basic concept of fatigue testing by elastomer compression as well as outline challenges and give recommendations for test parameters. 2. Methodology Cold forging tools are subjected to loads of several thousand MPa of inner pressure. The use of reinforcements is recommended for a pressure range of 1000 MPa to 2000 MPa. Within this research, a fatigue test that is able to model this kind of load is designed. The methodology used for the analysis of the test is shown in Fig. 1.

Test setup

Analysis of elastomer wear

Analysis of die stress

Frequency Load Gap size

Elastomer

10 mm

10 mm

Concept for the fatigue testing of hollow specimens under high inner pressure

Fig. 1: Methodology

First, the test setup along with relevant parameters is introduced. The high loads necessary for the comparability to cold forging processes are expected to have an influence on the elastomers used as pressure media. Therefore, the