Plastic Amplification β of the Stress Intensity Factor for Underclad Defect in a Vessel Submitted to a Pressurised Thermal Shock
For the assessment of an under-clad defect in a vessel submitted to a pressurized thermal shock, the plasticity is considered through the amplification β of the elastic stress intensity factor KI in the ferritic part of the vessel. The current solution in the French RSE-M Code has been developed from a fitting of F.E. calculation results and specifies to keep constant the amplification (deduced from its value at the maximum loading point) when the stress intensity factor is decreasing. A more physical solution is proposed in this paper to deal with the initial increasing loading phase but also with the unloading phase. It takes into account two phenomena: the amplification of the elastic KI due to the plasticity in the cladding and a plastic zone correction in the ferritic part. The first amplification is determined assuming that the plasticity in the cladding could be represented as an imposed opening displacement on the crack lips. When the loading is decreasing, the model takes into account the initial elastic unloading of the cladding but also the plastic compression which leads to reduce the cladding influence on the loading of the crack tip located in the ferritic part.