Modeling of small fatigue cracks in low SFE microstructures
McDowell, David L.
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Nuclear reactor pressure vessels are manufactured from high strength steels like A302B. The inner surface of the pressure vessel is exposed to extremely corrosive environments at high temperatures. The pressure vessels are therefore clad with corrosion resistant materials such as austenitic stainless steel. However, the manufacturing process results in defects in the clad material and the clad/base material interface. These defects are subject to fatigue loading due to the pressurization/shutdown cycles of the pressure vessel during service resulting in fatigue crack formation and growth from the defects. Here, we focus on the formation and growth of a fatigue crack from a surface defect in the cladding material to the base metal that could lead to exposure of the base metal to the corrosive environment in the pressure vessel and subsequent failure. To quantify the crack propagation rate from these defects under cyclic loading conditions, a Short crack growth (ShCGr) model has been developed by Dr. Clint Geller of the Bettis Atomic Power Laboratory (BAPL). The ShCGr model employs the concept of an accumulating debris field associated with the irreversibility attributed to dislocations encountering obstacles to their motion over a number of loading cycles; this contributes to stress intensification at the crack tip.The model uses an irreversibility factor to account for the damage accumulation. The number of cycles required for crack extension is found from energy balance between the free energy change associated with creating a crack and the energy released by a dislocation pile-up when a crack is created. The model is able to correlate crack initiation lives in smooth specimens through certain assumptions.