FATIGUE TESTING AND PROPERTIES OF STRUCTURAL-MATERIALS AT CRYOGENIC TEMPERATURES

A new type of cryogenic fatigue test machine has been installed at the National Research Institute for Metals, Tsukuba Laboratories. A closed-loop helium recondensation system enables long-term fatigue tests of structural materials in liquid helium. Significant temperature rise occurs in a specimen during fatigue tests at liquid-helium temperature. We carried out measurements and an analysis of the specimen temperature rise to identify the appropriate testing conditions in liquid helium for typical austenitic stainless steels, titanium alloys and OFHC Cu. High cycle fatigue tests were performed for the titanium alloys and austenitic stainless steels. The S-N curves shifted to higher stress or longer life with decrease in test temperature. In cryogenic fatigue for titanium alloys and nitrogen-strengthed austenitic steels, subsurface crack initiation apparently occurs without the existence of any defects such as inclusions or pores. The micromechanism of subsurface crack initiation and deformation behavior of those alloys in liquid helium were investigated and discussed.