The Effect of Film Thickness on Fatigue Fracture Properties in Rolled Copper Film.

Abstract

Using a new fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover an elliptical through-hole in a base plate subjected to push-pull cyclic loads, cold rolled copper films of 100 and 30 μm thickness were fatigued under various stress amplitudes with a stress ratio of R=0. The effects of the film thickness and the rolling direction on fatigue fracture properties were studied using two types of specimens that the rolling direction was parallel and perpendicular to the loading direction. As a result, for the case of films of 30 μm thickness, the crack initiation was caused at lower stress amplitude in the film with the loading direction perpendicular than parallel to the rolling direction, and the fatigue crack propagates faster toward the parallel direction to the rolling direction than toward the perpendicular direction. There was a difference between the two types of specimen in crack observation on the specimen surface owing to the difference in the shape of grains stretched by rolling. The thinner the film thickness was, the lower the fatigue limit was and the higher the fatigue crack propagation rate was against the same stress intensity factor range, ΔKest, evaluated from the crack opening displacement measured experimentally. The reason of the effects of thickness in the cold rolled pure copper film was probably that the crack propagates easier through the film thickness in the thin films than in the thick ones.