Health Monitoring of Aircraft by Nonlinear Elastic Wave Spectroscopy
Nonlinear Elastic Wave Spectroscopy (NEWS) and related acoustic and ultrasonic methods comprise a new class of innovative non-destructive techniques that provide extreme sensitivity in detecting and imaging incipient damage in the form of microcracks or delaminations, weakening of adhesive bonds, thermal and chemical damage, etc. The sensitivity and applicability of nonlinear methods to damage are superior to those obtained by currently used technologies. NEWS methods are in various stages of development and have not yet been applied to aircraft health monitoring. The project’s goal was to examine, confirm and exploit the successful results of these techniques, using fundamental materials research on fatigue loading, and to apply them to the particular field of aeronautics.
The project joined the efforts of 19 different partners from the industry and the academic world, spread in 8 different European countries. Our role in this project had a dual aspect:
- Development of novel optical transducer suitable for NEWS and aerospace applications that would allow generation of narrowband ultrasonic signals and filtering of harmonics.
- Experiments with dual sources (low and high frequency) for non linear mixing
For the first task we developed CHOTs (Cheap Optical Transducers). These are structures that are printed, deposited or in someway attached to the surface of the sample and are optically excited to generate and detect ultrasound. They can be broken down in to two parts, one for generation and one for detection and each part can work independently from the other for example generating ultrasound that will be detected by other means or detect ultrasound that is generated by other means. Most importantly, the user has full control of the generated/detected wavemode, the directivity of the wave and its frequency content.
One of the fields that CHOTs are most useful for is nonlinear acoustics. For such experiments we need narrowband ultrasound for generation and appropriate filtering for detection. This brings us to the second task within this project. The experiment is based on the interaction between a Low Frequency (LF) Surface Acoustic Wave (in our case a 1MHz SAW generated by a transducer) and a High Frequency (HF) SAW (the laser generated 82MHz SAW). By varying the delay between the 2 waves we can observe a phase modulation of the HF. This modulation is dependant on the degree of stress that the transducer is putting on the sample. The concept is that when the HF interacts with for example with the peak of the LF, the degree of phase modulation is different than when they interact at (for example) a trough. It can be shown that this phase variation is proportional to the non-linear change of the material's elastic constants and the detection of the change can be used to detect early stage defects. A pair of CHOTs have been successfully incorporated in this experiment for generation of the HF SAW and detection of the ultrasonic signal on a fused silica sample to measure the phase modulation of the HF.
The project was funded by the EU and finished in March 2008. Teti Stratoudaki was work package leader and lead researcher. The work on the nonlinear experiment was conducted by Ian Collison as part of his PhD study. Matt Clark and Mike Somekh held the direction for our participation in AERONEWS.