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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4.1. Elastomer wear Fig. 3 shows an elastomer specimen before and after testing. It can be seen that the upper edge is missing material. The material loss through the gap between punch and die therefore occurs as expected.

5 mm

Cycle 0

Cycle 2000

Fig. 3: Wear on elastomer specimen

For a stable test setup, this wear should be minimized. Therefore, the parameters test force, frequency and gap size are analysed in a full factorial experimental design with three factor levels each. Forces are analysed between 75 kN and 150 kN, which correspond to hydrostatic pressures between 1000 MPa and 2000 MPa. The frequencies 1 Hz, 5 Hz and 10 Hz are tested. These frequencies allow for time-efficient testing. Higher frequencies are not used, since they might induce a high heat input into the elastomer material. The analyses are carried out for three different gap sizes between punch and die. The lowest gap size is 0.01. This means that in tool manufacturing the dimensions of the ellipse with 8 mm and 12 mm are tolerated with a maximum difference of 0.01 mm between die and punch. The maximum gap size in the experimental design is 0.03 mm. The analysed parameters are summarized in table 1.

Table 1. Analysed parameters Force (kN) 75 ( ≙ 1000 MPa) 113 ( ≙ 1500 MPa) 150 ( ≙ 2000 MPa)

Frequency (Hz)

Gap size (mm)

1 5

0.01 0.02 0.03

10

The elastomer wear is determined by weighing the specimens’ mass before and after 2000 test cycles using a scale Kern Aet 500-4. The difference between the measured mass and the original mass is then divided by the original mass to calculate the material loss or wear. The results are shown in Fig. 4. There are no results for 150 kN and a gap size of 0.03 mm, since in this parameter set the elastomer wear was too high to continue the test before reaching the specified 2000 test cycles. Firstly, it can be observed that the occurring wear is significantly higher with increasing gap size. For example, at a force of 150 kN and a frequency of 5 Hz the wear amounts to 10.5% for a gap size of 0.01 mm, 16.2% for 0.02 mm and 19.2% for 0.03 mm. This confirms that the wear occurs because the elastomer is pressed into the gap between punch and die. Since no detrimental signs of die wear were observed in the area critical for fatigue, the lowest gap size should be used for future tests. For the test frequency, no clear correlation can be seen. For most configurations, the lowest wear is reached for a frequency of 5 Hz. On the other hand, a clear dependency of the elastomer wear on the punch force can be seen. For a frequency of 1 Hz and a gap size of 0.01 mm it increases from 3.1% (75 kN) to 17.9% (150 kN). With higher forces, a higher pressure is exercised on the elastomer and more material is pressed into the gap. However, a higher pressure also leads to increasing tool loads and consequently an earlier fatigue failure. The optimal force to be used in future tests will therefore be a compromise between elastomer wear and die failure. This also depends on the influence of the wear on the die load, which is analysed in the following section.