An investigation of the effects of microstructure on dwell fatigue crack growth in Ti-6242

Abstract

This paper presents the results of a combined experimental and mechanistic modeling approach to the study of dwell fatigue in Ti-6242. Crack shape evolution, depth and surface crack growth rates are established using beachmarking, acoustic emission and scanning electron microscopy (SEM) techniques. The underlying crack nucleation and fatigue fracture modes are elucidated for three microstructures: equiaxed (microstructure 1), elongated (microstructure 2) and colony (microstructure 3) of Ti-6242. The dominant crack nucleation mode is shown to involve a Stroh-type dislocation mechanism, where sub-surface cracks are characterized by prominent facetted fracture modes in the near-threshold regime. Subsequent fatigue crack growth occurs by fatigue striations and ductile dimples or cleavage-like static modes at higher stress intensity factor ranges. The long crack growth data are similar for both dwell and pure fatigue. However, the dwell fatigue crack growth rates are shown to be much greater than those due to pure fatigue in the short crack growth regime. The differences between the dwell crack growth rates and the pure fatigue crack growth rates in the short regime are attributed to possible creep effects that give rise to a mean stress effect in the case of dwell fatigue. Subsequently, the measured crack growth rates are incorporated into a fracture mechanics framework for the estimation of fatigue life in the three microstructures. The implications of the predictions are discussed for the modeling of dwell fatigue.