High and Low-Cycle Fatigue Damage Evaluation of Multilayer Thin Film Structure

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Abstract
A five-layered (Cu/Cr/Kapton® /Cr/Cu) metallic thin film structure was subjected to a completely reversed bending fatigue cycling with a wide ranges of applied strain amplitudes from 0.25 percent to about 30 percent. A new model for determination of the number of cycles to fatigue failure NF was proposed for single-crack and multi-crack formations. Within a strain amplitude ranging from 1 to 10 percent, a Manson-Coffin’s relationship was recognized for both the number of cycles to crack initiation NC and NF with exponents of 0.39 and 0.51, respectively. Selected fatigued test samples were further subjected to X-ray diffraction line analysis for dislocation density (ρ) calculation, which was related to the number of fatigue cycles N and strain amplitude (Δ εT ) in an empirical formula. It was also found that dislocation densities accumulated up to both Nc and NF were related to applied strain amplitudes. Consequently, if applied strain amplitude is known and progressive change in dislocation density is measured, one can predict the remaining fatigue life as well as fatigue cycles which were already consumed.

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