Advances in resolution and sensitivity of ultrasonic guided waves for quantitative inspection of pipelines

Abstract

Ultrasonic guided wave (UGW) inspection has been commercially implemented by industry as a non-destructive testing technique. Compared to conventional ultrasonic testing, it is a relatively immature technique being used to detect corrosion in engineering structures i.e. pipelines since 1997. The main advantage of UGW is the ability to inspect tens of meters of pipeline length from a single test location. Guided waves are elastic stress which travels within the boundary of the waveguide and reflects back to the point of transmission after their interaction with a material discontinuity. It is a rapid screening technique with a relatively low resolution compared to the conventional ultrasonic technique due to the longer wavelength at the operating frequencies (20-100 kHz). There is the potential to increase the resolution and the sensitivity for the industrial examination of degrading pipelines which are subjected to wall-thinning due to corrosion. An additional need is for the technique to be able to provide quantitative information on corrosion flaws. The work presented in this thesis, begins with the development of a sound energy focusing technique to enhance the resolution and the sensitivity of flaws in pipe geometry. The proposed technique is a hybrid sound energy focusing technique which combines numerical/analytical simulation with active focusing and the time reversal concept. Performance of the proposed technique is studied against the sound energy focusing techniques described in the literature (i.e. active focusing, synthetic focusing and time reversal focusing). Subsequently, the first longitudinal guided wave mode is adopted in order to use it as an incident mode. This enhances the resolution and sensitivity as it inherits a higher number of associated flexural modes within the operating frequency. An alternative flaw sizing method is proposed by adopting the first longitudinal guided wave mode as the incident mode. During empirical investigations an anomalous signal arising after the guided wave interaction with the transmitter was observed. The origination and the behaviour of this phenomenon were studied. These signals contribute to the coherent noise and can be interpreted as a non-existing anomaly. The presence of this unexpected signal is simulated using finite element analysis and validated using laboratory experiments