Nonlinear Ultrasonics: Signal Processing Considerations and a Nonlinear Parameter for Rayleigh Waves
An effective way to describe changes in the microstructure of a material or
to assess fatigue damage at an early stage in fatigue life is by measuring the acoustic
nonlinearity parameter beta. The nonlinearity parameter is defined for harmonic
longitudinal plane waves and it depends on the ratio of the amplitudes of the first
harmonic of the exciting signal and the second harmonic. A reliable measurement
of the amplitudes of these harmonics is crucial since their amplitude of the second
(higher) harmonic is much smaller than the amplitude of the first harmonic.
This research investigates the influence of the apparent nonlinearity that can occur
due to the signal processing and shows how this influence can be quantified and
minimized to enable a more accurate evaluation of the acoustic nonlinearity parameter.
Furthermore the concept of the nonlinear parameter is extended to Rayleigh surface
waves by developing a connection between the harmonic amplitudes and the third
order elastic constants, using the approximate model by Zabolotskaya.
Finally the problem of modeling the influence of fatigue damage on the nonlinear
parameter and the elastic constants is discussed.
The reduction of the processing nonlinearity combined with Rayleigh surface waves
- Rayleigh surface waves are more efficient in the detection of fatigue damage initiated
and concentrated at the surface - helps improve the prediction of fatigue damage and
the remaining life of a sample.
Advisor:Kim, Jin-Yeon; Qu, Jianmin; Jacobs, Laurence
School:Georgia Institute of Technology
School Location:USA - Georgia
Source Type:Master's Thesis
Keywords:civil environmental engineering
Date of Publication:09/28/2005