Lamb and SH Wave Transducer Arrays for the Inspection of Large Areas of Thick

摘要

The motivation for this work is to develop a guided wave system to inspect alarge area of a thick (5-25 mm) plate structure from a single test position.The immediate application area is in the testing of the floors and walls ofsteel storage tanks in the petrochemical industry. In order to test thefloor of a tank, the tank must be emptied and cleaned. The advantage ofusing a large area testing system is to reduce the inspection time andtherefore to reduce the amount of time for which the tank has to be out ofservice. A further application for which the proposed inspection systemcould prove advantageous is in the testing of structures clad withinsulating material such as rockwool. In such cases the cladding only needsto be removed at the test location, as the use of guided waves permits theinspection of the surrounding structure beneath the cladding. One approachto inspecting plates using guided ultrasonic waves, is to design atransduction system that uses monolithic devices such as wedge transducers,inter-digital transducers or meander coil electro-magnetic acoustictransducers (EMATs). Unfortunately, in order to achieve a given level ofperformance at a particular operating point on the dispersion curves, thephysical dimensions of a monolithic transducer must be scaled inproportional to the thickness of plate under inspection. This renders theuse of monolithic devices on thicker plates increasingly impractical forseveral reasons, not least because of the cost and difficulties involved inmanufacturing large devices. Also, particular monolithic devices are limitedto a small range of applications since they are generally designed to besensitive to a particular wavelength and to be either unfocused or have afixed focal length. A more attractive solution is to use an array devicecontaining a number of elements that behave individually as pointtransducers. By controlling the elements individually, wavelength selection,beam steering and focusing can all be performed by post processing the sameset of test data. This paper presents predicted and experimental resultsobtained from various different array geometries.