Thermal shock tests have been investigated on a fused mullite refractory brick, the DURITAL from RHI Magnesita, according to the EN 993-11 standard. The co-simulations heat transfer - stress show a full convergence for all the investigated mesh sizes and seem to be independent of the cohesive element thickness. The crack modelling with a mesh size of 0,0005 m is characterised by a flexural frequency close to the experimental one (3959 Hz vs. 3415 Hz).