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Title: Guided wave structural health monitoring
Author: Ward, Jacob Thomas Elliott
ISNI:       0000 0004 5923 3730
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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Routine airframe Non-Destructive Testing (NDT) procedures are costly and prone to human error. Guided wave structural health monitoring (GWSHM) shows great promise to in future assist these carefully regulated aerospace NDT practices. Using automatic GWSHM to both detect and localise damage can better focus the human NDT effort and ultimately lead to safer operation of airframes. The thesis presents structural health monitoring techniques for airframes using measurements of guided waves. Work is presented on both metal plates and carbon fibre reinforced plastic panels. An active GWSHM method is considered in its capability to detect and localise damage by measurements of scattered Lamb waves from artificially placed damage. The contribution to knowledge on active GWSHM has been towards effective and practical strategies for placing a low number of transducers into arrays suitable for global coverage. Much early active GWSHM studies often adopted a uniformly sparse distribution of transducer elements, perhaps in an attempt to gain the best possible global coverage. In this thesis, active GWSHM performance has been evaluated for arrays of different geometry and has shown that a uniformly sparse distribution of transducer elements may not be the most effective strategy when using a minimal number of sensors. Simulated and artificial damage, placed with different orientations over a large area, has been used to test candidate array layouts. It finds the layout optimal for damage detection is not necessarily the layout optimal for damage localisation. The zeroth order anti-symmetric Lamb wave mode has been used at low frequency-thickness. The mode, referred to as the flexural mode when propagating with low frequency-thickness, is favoured for its short wave length and long range. At low frequency-thickness this mode is quickly outrun by its symmetric counterpart, causing coherent noise in the signals recorded. Baseline subtraction is used to suppress the coherent noise before imaging. Benign structural features, that would usually hinder damage-localisation from an image, are actually found to assist damage localisation for some array layouts when using the reference baseline signal subtraction technique. A passive GWSHM method is considered in its capability to localise impacts. Impact events on carbon fibre panels are localised using a low frequency passive array. The technique is suggested for evaluating damage from tyre-burst or propeller debris impacts to airframe surfaces. It is particularly relevant to new airframe designs that have significant usage of composite materials on their outer surface. Historically the aerospace sector has readily adopted time of arrival estimation methods similar to those found on a standard oscilloscope. As an example, acoustic emission monitoring, in recent decades has routinely used threshold-crossing as a means of time of arrival measurement. An alternative is presented requiring the whole time series to be post-processed. It extracts an alternative arrival time from propagating waves resulting from the impact, which can be used in time-difference of arrival algorithms. This method is shown to be more reliable and accurate for impact localisation than historical techniques.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available