Real-Time Monitoring of Acoustic Linear and Nonlinear Behavior of Titanium Alloys During Cyclic Loading

Abstract

Variation in acoustic nonlinearity has been monitored in real time during fatigue, on four dogbone specimens of Ti-6A1-4V, under low cycle fatigue conditions, from the virgin state all the way to fracture. The results of these experiments show that the acoustic nonlinearity undergoes large changes during the fatigue and follows a similar trend for the material under given fatigue test conditions. Transmission electron microscopic (TEM) examination of the samples with similar composition fatigued to different stages indicates a gradual change in the microstructure and dislocation density, which correlates with the changes in acoustic nonlinearity.

New Trends in Materials Nondestructive Characterization Using Surface Acoustic Wave Methodologies

“…This implies that the harmonic signal is very sensitive to the microstructural changes in the material. The variation of nonlinearity continues due to the generation of additional dislocation dipoles by the fatigue process and their interaction with the acoustic waves, as predicted by relevant models and experimental work [38][39][40]53]. Fig.…”

“…Nonlinear acoustics enables real-time monitoring of material degradation in aerospace structures [39]. When a sinusoidal ultrasonic wave of a given frequency and of sufficient amplitude is introduced into a non-harmonic solid the fundamental wave distorts as it propagates, and therefore the second and higher harmonics of the fundamental frequency are generated.…”

“…(19) and normalized by the value o (nonlinear parameter of the material at the virgin state) is experimentally measured. In this sense, in order to assess the level of fatigue damage using nonlinear acoustics measurements a determination of the absolute value of the material's nonlinearity is not required, enabling real-time experiments [38][39].…”

“…Other studies were carried out [38,39], where two piezoelectric crystals were placed on the two opposite ends of a dog-bone titanium alloy specimen, and nonlinear measurements were performed in real time on the same specimen while undergoing cyclic loading. These studies utilized bulk acoustic waves and was a first step in the direction of developing a methodology for continuing monitoring of fatigue damage in the laboratory.…”

A Review Paper on Looseness Detection Methods in Bolted Structures

“…If the axial force of the bolted structure is inadequate and the bolts are loose, the contact surfaces are not bolted firmly or flawlessly and the acoustic impedance is increased; hence, the second harmonic will appear in the response signal in the Fourier space. The bigger the range of the appeared harmonic, the worse the flaw in the joint (Hikata, et al, 1963;Kim, et al, 2006;Cantrell & Yost, 20001;Frouin, et al, 1999).…”