Multiscale fatigue crack observations on Ti–6Al–4V

The fatigue process of dynamic loaded materials can generally be classified in four stages, namely crack initiation, short crack growth, long crack growth and rupture. A continuous fracture mechanical description of the crack growth stages under consideration of the crack initiation phase was the aim of this research. The fatigue behavior of forged Ti–6Al–4V with three different microstructures were thereby characterized on the basis of multiscale fatigue crack observations on electrolytically polished hourglass specimens loaded in the finite life region. The gathered crack propagation data was compared with single-edge bending long crack growth curves. It was determined that the crack initiation phase accounts for less than 5% of the total lifetime. Depending on the (α + β)-content, the fatigue behavior is dominated either by primary α-grain size (equiaxed-type microstructures) or colony length of the (α + β)-lamellae (bimodal-type microstructures). Transcrystalline crack propagation was observed for all microstructures. The comparison of the multiscale fatigue crack growth rates with single-edge bending long crack growth curves revealed that the short crack growth behavior is the extension of the Paris-region of the long crack growth curve, and can be fitted with a power law for lifetime estimation.