Oblique excitation of nearly-longitudinal waves in thick plates

Abstract

Our earlier research has studied the generation of nearly-longitudinal waves in thick plates by edge excitation at relatively high frequency-thickness products. These nearly-longitudinal waves, also known as trailing pulses, are promising for flaw detection due to their shorter wavelength and the capability of retaining the pulse characteristics after scattering from defects. However, in reality, the edges of the structures may not be accessible. This paper explores exciting ultrasonic waves using a wedge transducer at oblique incidence. We first describe a simple model for the formation of trailing pulses by oblique excitation. We then use simulations to examine the creation of nearly-longitudinal waves in a thick plate, study the effect of incidence angles and discuss the energy distribution through the thickness. Next we provide experimental results from both pitch-catch and pulse-echo tests to validate the characteristics of the nearly-longitudinal waves excited by oblique incidence. The good agreement between the simulation and experiments shows oblique excitation can also produce nearly-longitudinal waves with uniformly-spaced trailing pulses over a range of incidence angles. The amplitude level of such pulses reaches its maximum when the incidence angle approaches to the critical angle at the plexiglass-steel interface. The responses by oblique excitation are weaker than those by edge excitation, but can still illuminate the plate through the thickness when the incidence angle is close to the critical angle. The results show that the nearly-longitudinal waves by oblique excitation are a good alternative for infrastructure inspection, especially for plates limiting edge access or permitting only surface access.