Crack nucleation during mechanical fatigue in thin metal films on flexible substrates

The evolution of damage due to mechanical fatigue in thin metal films on flexible substrates was investigated by in situ electrical resistance measurements. A tensile fatigue load was applied to the metal films by subjecting a single edge of the curved samples to repeated linear motion. The change in the resistance of the metal films was monitored in situ. Upon the nucleation of a fatigue-induced crack, the electrical resistance of the metal film began to increase. The resistance subsequently continued to increase with crack propagation. Therefore, in situ electrical resistance measurements can be used to identify the fatigue-induced crack nucleation cycle. The number of cycles required for crack nucleation decreased with the increase in the fatigue-stressed area of the samples. This behavior is attributed to an increase in the crack nucleation probability with increasing sample size. The amount of strain applied also modified the number of cycles required for crack nucleation according to the Coffin-Manson relationship. The increase in the electrical resistivity with respect to the number of fatigue cycles can be accurately predicted when the fatigue cycle is normalized by the nucleation cycle. This indicates that the fatigue lifetime is determined by crack nucleation and not by crack propagation. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.