Experimental Study on Properties of High Cycle Thermal Fatigue, 3; Results of sinusoidal temperature fluctuation test at 20 second cycle

Hasebe, Shinichi; Kobayashi, Sumio; Tanaka, Hiroshi*; lbaraki, Koichi*; Fukasaku, Hiroshi*

In a nuclear power plant, it is necessary to be attentive to fatigue fracture of the structural material caused by cyclic thermal stress due to the mixing of temperature different fluids. The purpose of this study is to obtain data to demonstrate high cycle thermal fatigue evaluation methods by applying the effects of the frequency of temperature fluctuation. A sinusoidal temperature fluctuation test of with a 20 second period was conducted using high cycle fatigue test equipment (SPECTRA). A SUS304 steel pipe was used as the test sample, at an average sodium temperature of 425 deg-C, fluctuation amplitude of 200deg-C and a sodium flow rate of 300 l/min in the test pipe. The results obtained are as follows: (1)valid strength data to verify evaluation methods could be obtained by applying a 20 second cycle temperature fluctuation to the test sample with SPECTRA. A Crack penetrated at about 157,150 cycles. (2)Numerous cracks in an axial direction were observed on the jnner surface of the test sample in the upper flow area. An air fatigue test demonstrated the difference in the strength of the test sample between axial direction and circumferential direction, revealing that cracks were distributed in an axial direction since anisotropic influences easily appear on the hjgh cycle side. (3)An approximated curve obtained by the common relation of crack and axial direction distance indicates that the boundary of a crack would be located about 430 mm downstream from the tapered end of the test sample with the upper now. (4)Crack occurring on the inner surface progressed to a depth of 1 to 2 mm in thecrystal grain, then progressed along the crystal grain boundary. Striations were formed on areas of the fracture surface in the grain, but were not found on the fracture surface of the grain boundary. Sinusoidal temperature fluctuation tests at the periods of 2,5,10,and 40 seconds are planned to confirm the influence of fluctuation frequency responsiveness on structural material