The mode I fatigue behaviours of notched freestanding Al-3Mg foils

The effect of the thickness on fatigue crack propagation behaviour of rolled and annealed Al-3Mg alloy foils was investigated using notched rectangular specimens with thicknesses of 30 µm and 100 µm and elastic finite element method analysis. A specialized apparatus in combination with a force sensor and a video graphics array digital camera to record the crack growth was employed to investigate the mode I fatigue behaviour of freestanding Al-3Mg alloy foils. Tests were conducted at ambient temperature, with a loading frequency of 2 Hz and a loading ratio R of 0.1. The foil thickness was found to play a distinct role in the fatigue properties of micron-foil metals. Compared to the 100-μm-thick foil, the 30-μm-thick foil exhibited higher average yield stress (σ_0.2), but lower elongation, fracture toughness and fatigue life. The crack propagation rate da/dN of both foils as a function of the stress intensity factor ΔK followed the Paris relationship. The Paris constant m, increased slightly, from 4.6 to 5.6, as the foil thickness decreased from 100 µm to 30 µm. A smaller plastic zone size at the crack tip and a higher value for the self-similarity parameter z were found in the thinner foils, resulting in the higher Paris constant m value. With typical fatigue striations, the 30-μm-thick foils demonstrated failure in a brittle mode during the stable fatigue crack growth stage. However, the 100-μm-thick foils had abnormal striations and dimples in the stable fatigue crack growth stage, they failed in a mixed ductile and brittle mode.