New opportunities for NDE with air-coupled ultrasound

The area of NDE-applications of air-coupled ultrasound is expanded substantially for a weakly focused acoustic wave incident obliquely on a plate-like sample at an angle of ‘resonance’ transmission. Such a focused slanted transmission mode (FSTM) combines the benefits of plate wave excitation with a high spatial resolution and enables to characterise local elastic properties of a material. It was shown that the in-plain elastic anisotropy could be obtained by measurement of the ‘resonance’ transmission angle of the FSTM-output signal as a function of the azimuth angle. A striking contrast enhancement was demonstrated for the FSTM-C-scan imaging of cracked defects and delaminations. Non-contact FSTM-measurements of local variations of flexural wave velocity were used to detect fine structural changes and damage induced in polymer and composite materials during tensile tests.