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概要

Developing a precise nondestructive estimation method of material degradation before crack initiation will contribute to enhancement of safety and economical efficiency of a fast breeder reactor plant. Magnetic property is expected to be an effective indicator. In this study, fatigue tests on SUS304 stainless steel at 650°C were conducted until transition hardening region, 25% and 50% of the number of cycles to fracture and the distribution of magnetic flux density from each sample was measured to reveal relationship between magnetic property and initial fatigue damage in high temperature environment. As the number of cycles increased, local changes appeared in the distribution of magnetic flux density and its maximum value increased linearly. In addition, magnetic Kerr effect microscopy observation showed that microscopic magnetic phases were induced in the samples. It was confirmed that the number of the magnetic phase increased and its size became bigger as the number of cycles increased. These results showed the possibility of nondestructive estimation of material degradation before crack initiation by a magnetic method.

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