Acoustical linear and nonlinear behavior of fatigued titanium alloys

Nondestructive evaluation (NDE) methodology based on linear and nonlinear acoustics has been developed for monitoring the accumulation of damage under cyclic loading. An experimental measurement system using piezoelectric transducers for generation and detection of fundamental and second harmonic acoustic waves in a sample has been developed. The technique has been utilized for characterizing through interrupted fatigue measurements. A few modifications have lead to the development of a system capable of monitoring while the material is being fatigued. Measurements on Ti-6Al-4V dog bone samples under different loading conditions have been conducted. Results of both linear and nonlinear acoustic measurements will be presented. Difficulties of using linear acoustic properties of elastic modulus and attenuation to characterize fatigue are presented. Observation of large changes (up to 200%) in the nonlinear acoustic parameter of the material during fatigue is presented. Changes in the nonlinear acoustic parameter due to fatigue are related to continuous gradual changes in the microstructure of the material. This is supported by transmission electron microscopic imaging of microstructure in the material fatigued to different levels. The reliability, robustness and limitation of utilizing the nonlinear acoustic parameter for characterization of fatigue damage are discussed.