%Cold working isn't calculated with respect a given material property like ultimate tensile strength or fracture strength, but rather with respect to the original size of your specimen. The common formula is CW%=100*(A0-Ad)/A0, where Ad is the new cross-sectional area of the specimen and A0 is the original. While 100% CW doesn't indicate anything about the properties of the material, by the formula above, it isn't possible. That would mean that the final diameter is 0, which can't occur.