Development of a fatigue crack growth testing apparatus and its application to thin titanium foil

A low-cost experimental apparatus has been developed to investigate the mode I fatigue crack growth behaviour of thin metallic foils and sheets. The apparatus utilizes magnetic coupling between a ceramic magnet and a rotating steel disc to induce cyclic tensile loads in notched rectangular specimens. To illustrate the testing apparatus, mode I fatigue crack growth in 30-μm-thick high-purity titanium foils was studied. Experiments were performed at ambient temperature using a loading frequency of 2 Hz and a nominal stress ratio of 0.1. The cyclic crack growth data could be fit to a Paris relationship between crack growth rate and stress intensity range. The stress intensity factor exponent, m, in the Paris relationship was between 4 and 6, which is comparable with the relatively high values found in the literature for the tension-tension fatigue of other metallic bulk materials. Incomplete self-similarity analysis was used to explain the observed higher m values for thin metallic foils.