Fracture toughness of thick-section weld joint of SUS 316 at cryogenic temperature

A large superconducting magnet system for a fusion device requires large support structures which must have enough stiffness and toughness in cryogenic service. In the large helical device (LHD), an austenitic stainless steel SUS 316 with a thickness of up to 100 mm has been used for a supporting shell, shell arms and helical coil covers. Since these structures are assembled by welding, fracture toughness of the thick-section weld joint of SUS 316 has been investigated at cryogenic temperature using a large bend bar with a cross-section 87.5 × 175 mm. In three tests, unstable fracture (large pop-ins) occurred at maximum load. The toughness at the first large pop-in showed a higher resistance to crack initiation than that obtained in a conventional compact tension specimen, however, the crack propagated rapidly and no stable tearing was observed. When a linear elastic failure concept is employed to estimate the critical section thickness, it is confirmed that the LHD cryogenic structures have enough safety margin to resist linear elastic breaks.