Crack Propagation and Arrest Behavior under Thermal Striping in Liquid Sodium

As a part of the development of a rational structural design method and in order to formulate a strength evaluation method for high-cycle thermal shock (thermal striping) in the high temperature structure, we have developed an original experimental facility which can provide cyclic thermal load with liquid sodium at a frequency of ω=0.01 to 2.0Hz between a maximum temperature Tmax of 550°C and a minimum temperature Tmin of 250°C. The following results are obtained. (1) The thermal behavior test using this facility for Type 304 S.S. has revealed that the temperature fluctuating region is limited near the surface and within 0.8mm from the metallic surface under a cycling rate of ω=0.5Hz or greater. (2) With the thermal striping condition of Tmax=550°C, Tmin=250°C, and ω=0.5Hz, the spaces of the dry clay-like cracks vary from 1 to 2mm, and the cracks are arrested at a depth of approximately 1.6mm after 2.0×106 cycles. (3) The above mentioned test verifies that the depth of the arrested crack can be roughly estimated by the linear elastic fracture mechanics of edge cracks distributed by a certain pitch.