Serrated Yielding at Cryogenic Temperatures in Structural Components of Wendelstein 7-X

Structural materials like stainless steels are prone to serrated yielding at cryogenic temperatures. The serration effect is the unstable yielding resulting in a zigzag shape of the stress-strain curve observed in displacement-controlled tensile tests. Moreover, a local temperature rise in the plastic zone is observed. In the current paper, the theories explaining the serration effect are briefly discussed. With a coupled thermal-mechanical Finite element (FE) model of a tensile test set-up the influence of the test conditions is demonstrated on the observed shape of the stress-strain curve. The FE model is extended to quantify the temperature rise and significance of the serration effect in critical support structures of the magnet system of W7-X. For W7-X loading rates the temperature rise due to yielding is shown to be moderate and yielding does not localize. As a simple and safe approach, plastic collapse of critical components to be operated at 4 K can be judged with a pure mechanical analysis using yield curves obtained at 77 K. In doing so, the thermal-mechanical interaction may be neglected.