液体ヘリウム温度での低サイクル疲労試験法の標準化

抄録

Since 1991, co-operative studies have been conducted toward the standardization of the low-cycle fatigue testing method under fully reversed axial-strain control at the liquid helium temperature (4K), Nine organizations were involved, and the low-cycle fatigue tests have been shared to determine whether or not rules for 4K-testing methods were needed. Materials used were SUS316L, the strain hardening type stainless steel, and JN-1, the strain softening type high strength stainless steel.
The first viewpoints were the strain rate dependency of the serration behavior and the maximum degree of the strain control error due to such serrations at the given strain range, which were assumed to result from the typical response of stress-strain curves at 4K. The second viewpoints were the strain rate dependency of fatigue lives and the specimen temperature, the variations of which were considered to be the sources of the serrations. The third viewpoints were to verify the general definition of fatigue lives such as N₀ and N₂₅, which were used for high-temperature testing based on relations between the tensile peak stress and the crack size.
Results obtained so far indicated that the typical rules needed for 4K-testing were strain control error within ±4% and a strain rate of 0.4%/s or less at which the minimum temperature rise was assured to be 1K or less. Also it was indicated that N₀ and N₂₅ were applicable for 4K-testing. This year, supplementary studies have been planned and a draft for JIS-code will be prepared in 1995.