Characterization of accumulated fatigue damage in titanium-(6)aluminum-(4)vanadium plate material using transmission electron microscopy and nonlinear acoustics

Internal damage accumulation of Ti-6Al-4V specimens subjected to controlled levels of low cycle fatigue (LCF) and high cycle fatigue (HCF) was studied for this dissertation. The objective of this work was to characterize controlled damage in the crack precursor stage in Ti-6Al-4V forged plate material for two microstructural conditions using transmission electron microscopy (TEM) and nonlinear acoustics (NLA). The purpose of the controlled-damage samples is to create calibration standards for damage characterization and quantification for the nondestructive technique of nonlinear acoustics. In addition, TEM identified the amount and types of accumulated damage present in the samples as a result of fatigue at various stages of partial life. The online, in-situ nonlinear acoustic curve generated during LCF experiments of the duplex microstructure showed sensitivity of the nonlinearity parameter to the early stages of fatigue damage. The localized measurements of the nonlinearity parameter on fractured LCF and HCF specimens with a duplex microstructure correlated with dislocation density measurements of samples with similar fatigue history, indicating that the NLA technique can detect accumulated damage at individual locations. The dislocation density was shown to increase steadily throughout the fatigue life with a drastic increase past 90% of life. Dislocations in LCF samples were found to accumulate in dense networks, while in HCF samples well-defined slip in slip bands was prevalent. Pyramidal slip was found to be the most common slip active in both microstructures of this alloy.