Influence of Local Strain Distribution on Low-Cycle Fatigue Behavior of Thick-Walled Structures

An optimum design of thick-walled components such as high-pressure vessels and reactor vessels requires knowledge of the local strain distribution, the strain/stress state, and the material's behavior under elastic-plastic deformations. The multiaxial strain-state is transformed into a uniaxial strain-state according to Von Mises's yield criterion. Fatigue experiments on planar and cylindrical specimens with high strain concentrations and biaxial strain-states prove that low-cycle material properties as determined from unnotched specimens can be applied for design based on a defined initial crack length. A design including the stage of crack-propagation can be practiced only in special cases; in general it can not be recommended because of very high strain intensities and the resulting small critical crack depths, as service experience shows. For the functional reliability of the component it is also important that the local structural yield point, which can be a multiple of the material's yield point, is not exceeded by maximum loading. When designing such structures prestrains resulting from manufacturing or overloading must also be considered. Tests carried out with prestrained specimens show that fatigue life is decreased with increased prestrain due to cyclic softening.