The arrest properties of A533B steel are derived from measurements of the crack velocity and crack length at arrest using: (a) 140 by 400-mm duplex double-cantilever-beam (DCB) specimens with hardened and welded-on AISI 4340 steel starter sections, (b) a wedge-loading procedure, and (c) the Timoshenko beam-on-a-generalized-foundation dynamic analysis. Propagation and arrest events were studied at temperatures from −18 to 33°C (nil ductility temperature + 61°C) and involved fractures propagating in the A533B at velocities up to 710 ms⁻¹ and for distances up to 159 mm. The arrest capability of the steel is expressed in terms of the propagating crack fracture energy for the corresponding toughness, K_D, and the velocity dependence of these quantities. The thickness, crack velocity, and temperature dependences of K_D and its relation to K_Ic and K_Ia are examined. At −18°C, K_D = 129 MNm^−3|2, and K_a = 76 MNm^−3|2. This method of evaluation promises to provide access to the full range of arrest properties of low and medium strength structural steels.