This study was conducted to support warm prestress experiments on unclad and sub-clad flawed beams loaded in pure bending. Two cladding yield strengths were investigated: 0.6 Sy and 0.8 Sy, where Sy is the yield strength of the base metal. Cladding and base metal were assumed to be stress free at the stress relief temperature for the 3D elastic-plastic finite element analysis. The model results indicated that when cooled from the stress relief temperature tensile residual stresses were generated in the cladding due to its greater coefficient of thermal expansion. The magnitude of the residual stresses depended on the amount of cooling and the strength of the cladding relative to that of the base metal. During loading, the sub-clad flaw elastic-plastic stress intensity factor, KI(J), was at first dominated by crack closing force due to tensile residual stresses in the cladding. After the cladding residual stress were overcome by the applied bending stresses, KI(J) gradually increased as if it were an unclad beam. A combination of elastic stress intensity factor solutions was used to approximate the effect of cladding in reducing the crack driving force along the flaw. This approximation was quite in keeping with the 3D elastic-plastic finite element solution for the sub-clad flaw.
Finally, a number of sub-clad flaw specimens not subjected to warm prestressing were thought to have suffered degraded toughness caused by locally intensified strain ageing embrittlement due to welding over the preexisting flaw.