Critical Reflections about Doel3 & Tihange2

Integrity reactor vessels Doel 3 and Tihange 2

Page: 17

5 Fracture mechanics.

The phenomenon of crack propagation has been subject to extended research in different mechanical applications, particularly in reactor and aviation applications. A crack in a metal does not propagate without reason, it needs energy to propagate. This energy is supplied as elastic energy due to the mechanical stresses as result of the reactor’s operation. The growth of the crack consumes a part of this elastic energy. As cousins phenomena, a part of the elastic energy will be dissipated in plastic deformation, a small part in surface energy and a small part in kinetic energy of the moving metal, generating sound waves in the metal. A brittle material uses the majority of the available elastic energy for crack growth and a small part for plastic deformation. A ductile material is the opposite, the majority of the available elastic energy is used for plastic deformation and only a small part for crack growth. For this reason, crack growth is slow in ductile materials. As criterion for crack tip stresses, the stress intensity factor (SIF) is introduced. In the SIF, the crack geometry and the bulk stress is taken into account. This SIF will then be evaluated to the critical SIF, which is determined as a material property at hand of tensile and Sharpy impact tests. There is a lot of literature available about this subject. However, most of the literature looks very specialized, but there is also some good non-specialist literature [ 4 ] , [ 5 ] , [ 6 ] , [ 7 ] . Three types of loading modes can occur: 1. Crack opening mode, mode I: The material tension is perpendicular to the crack and pulls the crack further open. The stress intensity factor of SIF is K I = βσ p π a , wherein β is a geometry factor, σ the bulk tensile stress and a is the half length of the crack. The SIF will be evaluated to K I C , which is a material parameter which is a measure for ductility. It is determined by tensile and Sharpy impact tests. 2. Crack shear mode, mode II: The material tension is parallel to the crack and and shears the crack further open. The stress intensity factor of SIF is K I I = βτ p π a , wherein β is a geometry factor, τ the bulk shear stress and a is the half length of the crack. In principle, this SIF will be evaluated to K I I C , but in most cases, it is converted to mode I and evaluated to K I C . 3. Crack tearing mode, mode III: In this mode the opening is torn sidewards. This mode does not appear in this investigation. This theory is based on the research of one single crack in the material, i.e. there is no interac- tion with other cracks. Studies about multiple cracks in the material are very recent and is not consolidated in a profound theory. In most of these studies, two cracks with interaction are under investigation. Some studies point out that this aspect is not fully covered in the ASME Boiler and Vessel Code and that the effects of multiple cracks on the strength of the vessel is not sufficiently conservative estimated [ 8 ] ,p363-364, [ 3 ] .

R.Boonen & J.Peirs

May 18, 2017