Effect of Crack Closure on Fatigue Crack Propagation in Electrodeposited Nickel Thin Film using DIC

Abstract

Two types of nickel thin films with grain sizes of 384nm(UFG : Ultra Fine Grain) and 17nm(NCG : Nano Crystalline Grain) were created by electrodeposition using a nickel sulfamate bath. Nickel thin film of 10μm thickness was adhered to a through elliptical hole in a base plate and was fatigued in accordance with the displacement constraint along the hole circumference in the base plate subjected to a cyclic stress. Commonly, a stress intensity factor range was increased with a crack propagation under a constant stress amplitude but was decreased toward the hole edge because of the difference in thickness between the film and the base plate in this ΔK-decreasing test method. As a result, the figure crack propagation rate da/dN, is higher for NCG than for UFG in the relationship between the da/dN and stress intensity factor range, ΔK. The crack opening stress was evaluated by DIC(Digital Image Correlation) method and effective stress intensity factor, ΔK_eff was estimated. Relationships between da/dN and ΔK_eff are almost equal UFG and NCG.