AbstractThe JET toroidal field (TF) coils are made of two 12-turn copper pancakes. The tails are designed to strengthen the inter-pancake joint and to transfer the tension between the tail and the bulk of the coil through shear in the inter-turn insulation. Stresses in the tail region, due to the combination of thermal and mechanical loads, have been found to be close to the measured material strength. In the high tensile stress region at the truncated tip of the tail, the computed tensile strain release energy is close to the critical value typical for glass-fibre/epoxy insulation. In contrast, the computed strain release energy for a crack extending to the region of the peak shear stress is smaller than the critical shear strain release energy measured on specimens taken from a used coil. This indicates that even if a crack opens at the tail tip, it would propagate slowly, if at all.