eprintid: 1463979 rev_number: 48 eprint_status: archive userid: 608 dir: disk0/01/46/39/79 datestamp: 2015-05-08 10:50:33 lastmod: 2019-10-19 07:32:58 status_changed: 2015-05-08 10:50:33 type: thesis metadata_visibility: show item_issues_count: 0 creators_name: Ghandourah, EII title: Large plate monitoring using guided ultrasonic waves ispublished: unpub divisions: A01 divisions: B04 divisions: C05 divisions: F45 abstract: Areas of stress concentration around 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 might be 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 dispersive behaviour of the guided waves has been studied to ascertain a frequency thickness product that provides limited pulse distortion. The limitations of the plate geometry as well as the excitation and monitoring locations were discussed. The radial spreading of the incident, transmitted and reflected waves from a stiffener has been investigated. The efficient quantification of the transmitted and reflected waves from the stiffener for a wide range of angles has been obtained from a single Finite Element model containing two parallel lines of nodes in front of and past the stiffener. 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. The evaluation has shown that defect detection in inaccessible areas behind stiffeners is achievable if the signal-to-noise ratio is high enough. In experimental measurements the noise level was of similar magnitude to the observed reflections at the defect. Thus, there is necessity to enhance the signal-to-noise ratio in experimental measurements. date: 2015-03-28 vfaculties: VENG oa_status: green full_text_type: other thesis_class: doctoral_open language: eng thesis_view: UCL_Thesis primo: open primo_central: open_green verified: verified_manual elements_source: Manually entered elements_id: 1020548 lyricists_name: Ghandourah, Emad lyricists_id: EIGHA89 full_text_status: public pages: 170 institution: UCL (University College London) department: Mechanical Engineering thesis_type: Doctoral citation: Ghandourah, EII; (2015) Large plate monitoring using guided ultrasonic waves. Doctoral thesis , UCL (University College London). Green open access document_url: https://discovery.ucl.ac.uk/id/eprint/1463979/5/Thesis2015_to_Mansor.pdf