Ductile fracture and free surface roughening behaviors of pure copper foils for micro/meso-scale forming

Abstract

To clarify the fracture mechanism of metal foils, the ductile fracture and the free surface roughening behaviors for pure copper foils are focused on in this study. Pure copper foils and sheets with various initial thicknesses of 0.05, 0.1, 0.3, and 0.5mm are used. Fracture strain decreases with decreasing thickness under the uniaxial tensile state. In particular, the fracture strain decreases dramatically from a thickness of 0.3mm to 0.1mm. Dimples cannot be observed on the fracture surface of metal foils with thicknesses of 0.05 and 0.1mm, whereas metal sheets with thicknesses of 0.3 and 0.5mm had some dimples on the fracture surface. Free surface roughening can be observed for metal foils as well as metal sheets. The rate of increase in the ratio of surface roughness to thickness dramatically increases from a thickness of 0.3mm to 0.1mm because of the decreasing number of grains in the thickness, which is inversely proportional to the rate of increase in the ratio of surface roughness to thickness. In particular, when the number of grains in the thickness becomes less than approximately 5, the rate of increase in the ratio of surface roughness to thickness significantly increases in inverse proportion, leading to low fracture strain. Additionally, the fracture strain and the number of dimples on the fracture surface decreases for a metal sheet with a simulated surface roughness prepared by machining, which has nearly the same ratio of surface roughness and pitch to thickness as those of metal foil. Furthermore, ductile fracture criteria could not be used to predict the fracture of metal foil in stretch forming. Referring to these results, we discuss the ductile fracture mechanism of metal foils. Firstly, the surface roughness of metal foils increases with plastic deformation owing to free surface roughening phenomenon. Secondly, the ratio of surface roughness to thickness increases with plastic deformation. At the concave region formed by free surface roughening, local deformation occurs. Thirdly, it can be considered that the local deformation caused by free surface roughening leads to fracture. On the basis of this mechanism of ductile fracture resulting from the free surface roughening of metal foils, the metal foils have low fracture strain compared with metal sheets.