The 'size effect' on the stress-strain, fatigue and fracture properties of thin metallic foils

In this investigation a microtensile machine in combination with a non-contacting laser-optical speckle correlation sensor to determine strain with high resolution was used to study the stress-strain behavior of thin metallic foils of Cu and Al with varying thickness ranging between 10 and up to 250 μm. The grain sizes varied between 2 and up to 250 μm. A size effect was detected resulting in an influence mainly on the fracture strain. This effect will be explained on the basis of texture differences, the number of activated gliding systems as a dependence on the ratio of grain size to foil thickness. To support these experimental findings the fracture topography has also been investigated. In addition, the fatigue crack propagation properties of the above mentioned Cu foils were studied as function of thickness. Using a specially designed fatigue testing set-up it was feasible to determine crack growth curves from free-standing foils. Depending on thickness, an unexpected crack growth behavior was detected. Using the ECCI technique it was feasible to study the interaction of the global dislocation arrangement with the crack tip.