FRACTURE-TOUGHNESS OF THICK-SECTION WELD JOINT OF SUS-316 AT CRYOGENIC TEMPERATURE

A large superconducting magnet system for a fusion device requires lar ge support structures which must have enough stiffness and toughness i n 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 the se structures are assembled by welding, fracture toughness of the thic k-section weld joint of SUS 316 has been investigated at cryogenic tem perature using a large bend bar with a cross-section 87.5 x 175 mm. In three tests, unstable fracture (large pop-ins) occurred at maximum lo ad. The toughness at the first large pop-in showed a higher resistance to crack initiation than that obtained in a conventional compact tens ion specimen, however, the crack propagated rapidly and no stable tear ing 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 elast ic breaks. Published by Elsevier Science S.A. All rights reserved.