Technical Note: Experimental techniques for Impression Creep and Small Punch testing

Abstract

The test rig for Impression Creep testing was designed based on the application of resistance cable heating instead of a normal furnace. The thermal stability is crucial in Impression Creep testing because of small displacements, typically just some tens of micrometers. Good thermal stability was achieved by applying resistance heating cables with an integrated thermocouple inside. For frictionless displacement measurement, ceramic tube and rod are used, located inside the hollow loading bar, protected from direct heat radiation and thermal fluctuations, thus avoiding thermal strains. In an international Round Robin these innovative arrangements provided the best strain signal stability. For the Small Punch testing the aim was set to make the test rig gas tight in order to avoid ingress of oxygen. However, the combination of gas tightness and frictionless movement of the puncher are contradictory requirements. This was solved by using a metallic bellow. Only the spring load of the bellow needs to be compensated which is possible when the loading is provided by a servomechanical testing machine.

Keywords

• impression creep
• small punch
• thermal stability
• displacement measurement
• frictionless