Acoustic harmonic generation at unbonded interfaces and fatigue cracks

Abstract

The generation of a second-harmonic signal at a flat unbonded interface between two solids has been observed. This signal is caused by passage of a longitudinal acoustic wave across the interface. The harmonic amplitude depends upon the pressure applied normal to the interface and is largest close to zero pressure, as expected theoretically. The effect has also been used to detect the presence of microcracks on the surface of Al 2024, developed during fatigue.

Observation of the “Luxemburg–Gorky effect” for elastic waves

“…This nonlinearity results in the violation of the superposition principle. Therefore, nonlinearityinduced components of elastic excitations may be used as a very sensitive indicator of structural damage in the material [1][2][3][4][5]. It is essential that apparently similar effects could be caused by different particular nonlinear mechanisms, e.g., hysteresis [5], or dissipative nonlinearity [6], or purely elastic material's nonlinearity.…”

Acoustic Nonlinearities Produced by a Single Macroscopic Fracture in Granite

“…The nonlinear effects we observe are analogous to those caused by partially open fatigue cracks in metals, which generate harmonics when insonified at ultrasonic frequencies (2).…”

Cheap optical transducers (CHOTs) for narrowband ultrasonic applications

“…NEWS techniques have shown to be more sensitive in detecting microscale fractures and predicting early damage [22] than traditional linear acoustical methods. The concept of NEWSbased methods is that internal damage can be measured directly with the instantaneous detection of an increase in the nonlinearity parameters.…”