Investigating the potential of FGMs through numerical minimization of thermal stresses

Functionally graded materials (FGMs) have the potential to reduce high stress concentrations near material interfaces. In the ITER divertor monoblock, this may be achieved by replacing the distinct W-Cu interface by an FGM, which gradually changes the coefficient of thermal expansion (CTE) from copper to tungsten. To assess the full potential of FGMs in improving the thermal and mechanical behavior, a first proof-of-principle study is performed in this paper to numerically optimize the tungsten volume fraction distribution of a W-Cu FGM based ITER monoblock for minimal thermal stresses using a simple thermal conduction model. We show that the optimizer can ensure that the temperature remains below the recrystallization temperature of tungsten and that it minimizes the chosen cost function, based on the fractional margin between the local temperature and melting temperature, in a computationally efficient way. Furthermore, the resulting design has a lower Von Mises stress measure at the original position of the W-Cu interface. However, a new material interface is introduced, which results in an additional undesired stress concentration. Future work should therefore aim at integrating a complete stress evaluation in the optimization method.