Effects of roundness of laser formed film cooling holes on fatigue life of nickel based single crystal

A form of plate specimen with a laser formed film cooling hole was designed to simulate Ni based single crystal cooling blade. The roundness error of the holes was obtained by image measuring instrument and low cycle fatigue (LCF) experiments were carried out at 900°C. The profile of real film cooling hole was assumed to be an ellipse and the roundness error was expressed as the difference of the major and minor radii of the ellipse. A group of elliptical holes with different roundness errors and a real hole were selected for finite element modelling. A crystallographic plastic finite element method (FEM) was used to analyse the resolved shear stress, damage distribution and LCF life of the models. Experimental and numerical results show that the fatigue crack growth path observed by optical microscopey is consistent with FEM analysis. A slip damage model based on crystallographic plastic theory is confirmed to have the ability to predict the LCF life of nickel based single crystal with consideration of roundness. Moreover, the roundness error can lead to a significant decreasing of the life of specimen with cooling hole and the larger the roundness error, the shorter the LCF life. An approximate quadratic curve is found to fit the relationship between the roundness error of hole and the logarithmic fatigue life.