Use of elastomers as pressure media for fatigue testing of cold forging tools

Cold forging has high potential as a sustainable production technology. With net-shape production, only the material needed for the produced part is used. By not heating up the workpiece, energy consumption is also low. However, this leads to high forming forces and consequently critical stresses in the tools. An accurate prediction of tool life is therefore necessary; both to find measures for its increase and to plan tool changes with minimal machine downtime. Currently there is little data about the fatigue properties of cold forging tool materials. Existing tests to determine this data strongly simplify the tools' stress state to a uniaxial load. Therefore, a new fatigue test is presented in this paper, which incorporates the actual load conditions of cyclically swelling inner pressures. For this purpose, a high-strength elastomer is used as pressure medium. Its low compressibility and high elasticity allow for cyclic testing with pressures up to at least 2000 MPa. This load causes wear on the elastomer, as it is pressed into the gap between the punch used to apply the load and the fatigue specimen. Therefore, the gap between punch and fatigue specimen should be as low as possible to ensure stable test conditions. Within the scope of the research, fatigue failure already occurred in one specimen. This shows that the compression of elastomers is able to generate very high loads comparable to the cold forging of steel.