EFFICIENCY OF DIFFERENT ULTRASONIC SURFACE WAVES FOR SUBSURFACE FLAWS DETECTION

摘要

Plate-like structures, laminates and composites are becoming increasingly common in civil engineering, aerospace, ground transportation, etc. They are often over-designed to compensate for a lack of efficient techniques for testing the initial integrity of the mechanical structures and/or for a reliable monitoring of damage and ageing. Consequently the race for ever improving NDE techniques (in particular ultrasonic) is still open. Also, considerable interest has been recently devoted to the emerging science of damage assessment, i.e. the combination of ultrasonic excitation with embedded array detection and signature recognition. Established ultrasonic techniques for NDE take advantage of a wide variety of sources. Surface waves are extremely efficient in probing surfaces and thin plates, since they can propagate a long distance without appreciable attenuation and therefore a large specimen region can be interrogated with a single transducer. In particular, Rayleigh waves (RW) are used to detect subsurface flaws in thick plates, while Lamb waves (LW) can explore the entire thickness of a thin plate. However, LW testing is generally complicated by the coexistence of at least two modes at any given frequency and by the strongly dispersive nature of these modes at high frequencies, resulting in a deformation of the injected wave package along the propagation path. As shown by Cawley and coworkers, a single and pure Lamb mode may generate a variety of other modes either by interacting with a surface or subsurface flaw or by crossing to another region of different impedance. Thus the output signal becomes richer, but often difficult to interpret.