Lucy Dougill 

Supervisors: Prof. Anthony Croxford & Prof. Paul Wilcox 

The project focuses on the field of visual vibrometry and its application to ultrasonic non-destructive testing (UNDT) through the use of various video and signal processing techniques. The concept of visual vibrometry as a method of NDE is explored by utilising regular consumer cameras to recover signals from standard frame-rate videos. Visual vibrometry has significant potential to be a major method for rapid large area monitoring within NDE. As visual vibrometry works on the basis that in-service damage alters vibrational modes in a way that is imperceptible to the human eye, but visible through image processing. A common form of NDT involves the use of ultrasonic sound waves to extract underlying material and dynamic properties of a system. One aim of the project is to combine the ability of ultrasonic guided waves, in order to penetrate a part at depth, with the ability of visual systems. In particular, to explore how large area visual monitoring approaches may be employed to monitor the state of structures. For the video data collected, forced excitation is utilised as opposed to simply recording the passive response of the structure 

One goal is to successfully image structural vibrations at frequencies significantly greater than the advertised frame-rate of the camera and extend the use of the camera beyond its hardware specification. This may be achieved by exploiting short exposure times and periodic excitation common in UNDT, even when the excitation frequency exceeds the Nyquist sampling limit. This will involve using stroboscopic effects and exploiting camera exposure times in order to achieve this and essentially ‘cheat’ the camera system in to sampling at a  significantly higher frame-rate than normally possibly. These cases are intended to be analysed by taking into account stroboscopic effects produced, where natural frequencies and input excitations are either calculable or known, respectively. The final challenge will be whether these developments may be pushed far enough to allow the measurement of displacement from useful ultrasonic frequency. In order to provide the basis for visual vibrometry as a method to monitor and detect damage in parts. 

To achieve these goals a simulation and post processing tool has been developed as a means to validate experimental testing and to predict the behaviour of experimental testing. Where, the simulated video was designed to imitate real data and used a noise model defined on results from experimental testing performed to investigate the relationship between noise and intensity levels as shown in figure 1.  

The simulated test video provides a means to test experimental set ups prior to physical testing and may be used to explore the effects of key camera parameters on the results. As the dynamics of the camera are highly influential on the performance of the testing, each of the camera testing parameters will be investigated and quantified. A relationship between image quality, frame-rate and exposure time is required and will likely be quantified by using a signal to noise ratio relationship as well as the accuracy of the identified output displacement amplitude and phase. By exploring the effect of each camera and test parameter using the simulation, the limitations of the testing may be explored. Such as, identifying the smallest possible pixel displacement that may be reliably and accurately extracted from video data.