Effect of notch on fatigue strength of carbon steel with TiN coating
Sonobe Masaru; Shiozawa Kazuaki
Abstract:In order to investigate the effect of notch on the fatigue strength of carbon steel coated with TiN thin hard film, cantilever-type rotating-bending fatigue tests were carried out in air and in 3% saline solution by using U-shaped notch round-bar specimen of 0.35% carbon steel coated with TiN by physical vapor deposition (PVD) meyhod. From the experimental results of the fatigue tests in air, an increase in fatigue strength was observed for the TiN-coating specimens, as compared with those without coating. Improvement in fatigue strength depends on the stress concentration factor, and the maximum improvement of 50% was observed on the sharp notched specimen. The endurance limit was defined as the crack initiation limit for the notched specimen with coating, while it was defined as crack growth limit for the notched specimen without coating. It was clear from an elasto-plastic analysis by finite element method and the experimental results that a strain at the notch root for crack initiation limit was increased by the existence of hard thin film. From the corrosion fatigue test results, TiN-coating film effectively protects the substrate from the corrosive environment, and then the improvement of corrosion fatigue strength was about 87% fir the sull notched specimen, which was the same as that in the smooth specimen. Key Words:fatigue, corrosion fatigue, notch effect, surface improvement, TiN-coating, PVD, carbon steel