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

Abstract

-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 FE model of a tensile test set up the influence of the test conditions is demonstrated on the observed shape of the stressstrain 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 4K can be judged with a pure mechanical analysis neglecting the thermal-mechanical interaction, using yield curves obtained at 77K instead.Index Terms -serrated yielding, intermittent yielding, discontinuous flow, cryogenic material properties, coupled thermal-mechanical FEA, W7-X

Technical challenges in the construction of the steady-state stellarator Wendelstein 7-X

“…A special conservative analysis procedure, taking into account the structural material degradation at 4K in the form of serration effects, has been developed and implemented to accept local plastification in the support elements [35,36,37].…”

“…All critical support elements were tested, using adequate mock-ups, and benchmarked with local FE models [26 ,32-34]. A special conservative analysis procedure, taking into account the structural material degradation at 4 K in the form of serration effects, was developed and implemented to accept local plastification in the support elements [35][36][37].…”

Numerical modelling in the construction of Wendelstein 7-X

“…Static, transient, b have to be performed, taking in plastic, and/or special rheologi effects and considering temper properties and geometry nonlinear d solvers asks to be solved require to types with suitable elements and iterative, implicit and buckling and dynamic tasks nto account elastic, elastoical models for serration rature dependent material rity [13][14][15][16][17][18].…”

“…In general, W7-X structural design criteria for the cryogenic magnet system follow the ones developed for ITER in 2004. However, criteria for critical components, welds, handmade insulation, and other unique elements were extended and elaborated on the basis of extensive test programs and FE limit analyses taking into account also material serration effects [14][15][16][17] …”