Modeling the fatigue crack growth behavior of Ti-6Al-4V by considering grain size and stress ratio

Ti-6Al-4V is a commonly used titanium base alloy in aerospace applications. The increasing demand for damage-tolerant designs of such components necessitates a detailed knowledge of its crack growth behavior. The aim of this research was the characterization and phenomenological modeling of long crack growth behavior with respect to microstructure and stress ratio. Therefore, the long crack propagation was characterized for eight different heat treatment conditions and four stress ratios. For comparison, physically short crack growth tests were also performed. The long crack growth threshold was found to be dominated by roughness-induced crack closure, and the fracture surface roughness is controlled by the primary α-grain size. The reason for this correlation is a near-threshold crack propagation mode, which is dominated by the transcrystalline fracture of α-grains. This correlation was used to model the crack growth threshold with respect to microstructure. A linear relation was determined between the stress ratio and the threshold value, which was also found in this approach. Further presented models cover the crack growth behavior in the near-threshold (Stage I) and mid-growth rate regions (Stage II).

► The crack growth thresholds correlate to both primary α-grain size and stress ratio.
► The long crack growth thresholds are controlled by roughness-induced crack closure.
► A near-threshold crack propagation mode causes increased fracture surface roughness.
► The near-threshold crack growth rates correlate with the threshold value.
► Long crack growth curves can be calculated based on the grain size and stress ratio.