Doctrate Degree

Doctorate degree from Mechanical Engineering Department, Faculty of Engineering, University College London, March 2015

welded structures are likely to lead to fatigue cracks and corrosion pitting during the life time of technical machinery. Performing periodical non-destructive testing of the critical area is crucial for the maintenance of structural integrity and the prevention of unforeseen shutdowns of the system. Low frequency guided ultrasonic waves can propagate along thin structures and allow for the efficient testing of large components. Structural damage can be localized using a distributed array of guided ultrasonic wave sensors. Guided waves were employed to overcome the accessibility problem for stiffened plate structures where access to some parts of the inspected structure is not possible. The transmission and reflection of the A0 Lamb wave mode for a variation of the stiffener geometry and excitation frequency was investigated numerically and verified experimentally.

The research outcomes have shown the dependency of the scattered wave on the incident angle and stiffener dimensions. Reasonably good A0 wave mode transmission was obtained from the oblique wave propagation (up to an angle of 45o) across realistic stiffener geometries. The choice of an optimum excitation frequency, which can ensure maximum transmission across the stiffener for specific plate geometry, was recommended. The ability for defect detection in inaccessible areas has been investigated numerically and validated experimentally. The possibility of detecting and characterizing the reflection of a guided wave pulse (A0 mode) from a through-thickness notch located behind the stiffener has been discussed. Two different approaches, based on the access to the sides of the stiffener on the plate, were employed. The limitations of the detectable defect size and location behind the stiffener have been investigated. The energy of the transmitted wave across the stiffener was adequate to detect simulated damage behind the stiffener.