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A series of tests have been carried out on cast high strength ferritic steel rings 1090 mm internal diameter, with a central 20kg PE4 spherical charge, equivalent to 25 kg TNT. The rings have circular regions in them which have been thinned, some 16 mm wall thickness and some 26 mm thickness. The bulk of the ring undergoes limited cyclic plastic straining as the rings vibrate following the blast impact. The locally thinned regions undergo cyclic plastic strains of up to +/− 4%. The paper describes the experimental arrangements and results. Methods have been developed to measure high strains in the rings and the final plastic displacements of the windows have also been measured. It has proved remarkably difficult to obtain good finite element calculations of the strains measured. After the first blast, the thinned regions had some cracking and the extent of the cracking increased on further blast loads. Within the limits of measured/calculated strains, an assessment has been carried out to the new local tearing rules in ASME VIII Div 2 and 3 and compared to the observed damage.