Engineering Approach for Low-Cycle Fatigue Assessment of Porous Alloys

Most components used in gas and steam turbines are metallic parts produced by either casting or forging processes. Although process control works to eliminate defects, there can be variation in microporosity from component to component. Previously, this microporosity was only able to be detected destructively using metallography. Using computer tomography (CT), one can find voids in the range of a few tenths of a millimeter and know the location of the voids with high precision. This allows one to map the defects present in each component onto the stress and temperature fields for that component. However, there is not yet universal agreement upon a consistent method to evaluate the effect of these small porosities on a components lifetime. Having a robust analysis tool to understand the impact of microporosity would decrease development costs, decrease the time to bring a product to market, and increase the likelihood of failure-free operation. This paper presents an approach using equivalent low-cycle fatigue (LCF) material properties which avoids the need to explicitly model the morphology of the microstructure in the region of the microporosity. The homogenization methodology calculates new LCF curves depending on porosity ratios in material. This approach uses Morrows correlation factor of LCF cycles to crack initiation regarding energy amount dissipated in stable cycling (shakedown) and ultimate strain energy under monotonic loading. The paper generalizes Morrows postulate and formulates the hypothesis that energy stored and dissipated in the material under shakedown conditions corresponds directly to the number of LCF cycles to crack initiation. The paper demonstrates that the reduction of LCF life based on the porosity ratio agrees well with the experimental results. These results also show that the methodology is very sensitive to the void orientation and loading direction.