RATCHETTING OF FIRST-WALL ELEMENTS UNDER COMBINED THERMOMECHANICAL AND ELECTROMECHANICAL STRESSES

During the operation of ITER a temperature difference in the range of some 100 degrees C occurs between the heat-loaded outer surface and th e wall of the cooling channels. Due to this inhomogeneous temperature field secondary stresses are produced. In addition high electromagneti c forces can occur caused by disruptions or vertical plasma movements. The resulting stresses may lie beyond the yield stress of the structu ral material. From the interaction of thermal and mechanical forces, a complex stress state is achieved which may lead to premature failure or to progressive plastic deformations (ratchetting). In the present p aper the conditions for the occurrence of ratchetting are studied usin g simplified models of the first wall. For the experimental study of r atchetting phenomena an testing rig has been erected. In this rig, ben ding forces can be achieved on first-wall mockups with rise times of s imilar to 30 ms combined with cyclic temperature gradients.