Fatigue crack growth behavior of titanium alloys

Fatigue crack growth behavior of titanium alloys is examined using the Unified Approach developed by the authors. The approach is based on the realization that fatigue requires two load parameters for unambiguous description. This requirement manifests as two fracture mechanics parameters: maximum stress intensity factor, Kmax, and stress intensity amplitude, ΔK. The former essentially governs the breaking of the bonds contributing to crack growth, while the latter controls the extent of cyclic damage required ahead of the crack tip. The former can also be associated with the monotonic plastic zone, while the latter with the cyclic plastic zone. The crack growth behavior in Ti-alloys is complicated by several factors such as microstructure and its dependent flow properties, planarity and heterogeneity of slip, environmental sensitivity of the alloy, and changes in the flow behavior due to segregation of internal hydrogen contributing to localized creep and embrittlement. In the past, several anomalies in the behavior have been noted. The Unified Approach demonstrates that these observed anomalies are closely related to changes in the mechanisms of crack growth due to synergistic effects of mechanical and chemical driving forces. Crack growth trajectory maps developed under the Unified Approach document these changes in the mechanisms that occur with increasing crack growth rate or stress intensity.