As to accurately ascertain the value of dynamic rotation factor r*, strain analysis‐based measurement was performed on the bent low‐blow specimens by means of two approaches. (i) Surface strain analysis on the ligament indicates a critical transition zone from the necking to swelling, where the total strain reaches the minimum as undeformed material. (ii) Hardness measurement on the thickness‐reduction specimen also reflects a typical V‐shape distribution of interior plasticity along the ligament as observed from the surface, suggesting the location of true rotation centre relative to the neutral strain axis where the hardness is nearly invariable without any work hardening. As a result, the obtained value of r* maintains nearly 0.45 for (B × 4B) size drop‐weight tear test‐type specimens after their crack initiation and propagation, well consistent with the values calculated from the slip‐line field theory.

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