Experimental technique for elevated temperature mode i fatigue crack growth testing of Ni-base metal foils

Details are provided for an experimental approach to study the tensile fatigue crack growth behavior of very thin metallic foils. The technique utilizes a center-notched specimen and a hemispherical bearing alignment system to minimize bending strains. To illustrate the technique, the constant amplitude fatigue crack growth behavior of a Nibase superalloy foil was studied at temperatures from 20°C to 760°C. The constant amplitude fatigue tests were performed at a frequency of 2 Hz and stress ratio of 0.2. The crack growth rate versus stress intensity range data followed a Paris relation with a stress intensity range exponent m between 5 and 6; this exponent is significantly higher than what is commonly observed for thicker materials and indicates very rapid fatigue crack propagation rates can occur in thin metallic foils.