A frequency agile approach to air-coupled Lamb wave inspection
This Thesis describes the design, manufacture and evaluation of a single sided through-air Lamb wave scanner for Non-Destructive Evaluation (NDE). The new scanning system utilises novel wideband piezocomposite transducers and specialised receive electronics to detect and monitor the thickness of faults within the structure under investigation using a frequency agile approach. Based upon the superheterodyne principle, this frequency agility dispenses with the costly requirement for precision manipulation of the transducer alignment, by effectively tuning the system for the thickness of the sample. Several important applications are identified, including defect detection on samples with tapered thicknesses and defect depth characterisation on plates with thickness erosion, in addition to conventional defect evaluation. Through the application of finite element modelling and practical analysis, a new range of wideband composite transducer have been developed which offer a considerable improvement in operational bandwidth in comparison to conventional piezocomposites. Moreover, a specialised ultra low noise heterodyning amplifier was designed and constructed to provide sufficient gain and selectivity to detect the Lamb waves generated in the various structures under examination. Experimental results obtained from the prototype system illustrate a capability to distinguish defects within test specimens of differing materials and determine the thickness of the given defect. The current system is capable of resolving defects down to less than 2mm in diameter or 20% thinning in a 1mm thick aluminium plate for a Lamb wave with a wavelength of 3mm. Moreover, the handheld nature of the scanning head employed within this system has facilitated the examination of practical NDE examples, such as disbonds between vehicle support structures and outer panelling over curved structures.