Air-coupled Ultrasound - A Millennial Review

摘要

During the last decade Air coupled Ultrasonics has advanced from being a laboratorycuriosity, of little practical application, to a point where it is a viable techniquefor many manufacturing inspections.With appropriate equipment it is possible to perform sensitive inspections for defectssuch as voids, cracks and dis-bonds in a wide variety of water-incompatible materials.Originally used primarily for Aerospace composites, the applications have been extendedto cover a variety of materials where conventional NDT methods are not normallyconsidered suitable.Diverse applications of the technique include wood, foams and missile propellants. Avariety of frequencies can be used, allowing optimization for Resolution or penetrationas with 'conventional' Ultrasonics.Most applications of Air coupled Ultrasound have been for Single channel C-scansystems, testing flat panels. In this context it is normally used as a directreplacement for water-coupled 'squirter' probes, and installing Air-coupled probes andinstrumentation on an existing system so as to inspect a wider range of materials isgenerally a simple procedure. The results are often indistinguishable from thoseobtained with the water coupling, Figure 1 shows results from inspection of a 'Knot' intimberMore recently the use has been broadened. In particular multi-channel systems have beendeveloped allowing very rapid inspection of large composite panels at a rateapproaching 1m~2 per minuteThe current challenge, hopefully completed by the time of presentation, is to overcomethe requirements for high transmission power, thus allowing a portable instrument ofuncompromised performance to be developed.This technology shows considerable promise as a solution to a number of currentlyimpractical testing problems.