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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wear occurring on the Vulkollan® specimen is analysed in the next step by carrying out 2000 cycles of the fatigue test and measuring the specimens ’ weight. The influence on elastomer wear of test frequency, load and the gap size between punch and die is analysed in a full-factor experimental design. Finally, the occurring die stresses are analysed regarding the influence of elastomer wear. This is achieved by measuring the strains occurring on the surface of the die with strain gauges. 3. Test setup The fatigue tests are carried out on a dynamic testing machine Hydropulser Amsler HA250 with maximum force of 250 kN. The part geometries are therefore designed so that pressures between 1000 and 2000 MPa can be easily reached considering the maximum force. The fatigue specimens are hollow to incorporate the inner pressure characteristic for cold forging dies. The cross-section is elliptical, since on the one hand non-circular symmetrical geometries are gaining in importance in cold forging and on the other hand this geometry provides a defined place of failure on major axis of the ellipse. The elastomer is inserted into the fatigue specimen and consequently has the same geometry with a major axis of 12 mm and a minor axis of 8 mm. With this geometry, a punch force of 150 kN corresponds to 2000 MPa of hydrostatic pressure. Fig. 2a illustrates the tool setup for the fatigue test, with the tool built into the testing machine as shown in Fig. 2b.

Ø20 Ø50

a

b

Hydro expansion chuck

8

Die / Fatigue specimen Punch

Punch

Elastomer

Strain measurement

20

Reinforcement

Hydro expansion chuck

Counterpunch

50 mm

Fig. 2: Test setup, a) tool system, b) setup in testing machine Hydropulser Amsler HA250

The tool materials are chosen in accordance with typical materials for cold forging tools. Die, punch and counterpunch are made of powder-metallurgy steel ASP2023. To impede too early fatigue failure, the die is prestressed by a reinforcement ring with the hot-working steel material 1.2344. For the elastomer, the high-strength polyurethane material Vulkollan® with a hardness of 92 Shore A is used. The load is applied by downward movement of the punch. Since the elastomer has the same shape as the inner die, it is subjected to a hydrostatic pressure and compressed. After releasing the load, the elastomer regains its original shape. This way, the die experiences an inner pressure with each compression of the elastomer. However, under the high pressure, with each cycle a part of the elastomer is pressed into the gap between punch and die. Therefore, there is wear occurring on the Vulkollan® specimen, which will be analysed in detail in the experimental results. 4. Experimental results In this section the fatigue test is carried out according to the setup described above. The focus of the analysis lies on the elastomer wear and its influence on the stresses occurring in the test. Within the experimental tests, one die already failed due to fatigue cracks. Therefore, the induced failure and possible termination criteria are also discussed.