Ultrasonic Rayleigh wave enhancements from angled defects in aluminium

Non-linear enhancements of ultrasonic surface wave amplitude and frequency have been observed when an incident wave interacts with a surface defect. Previous measurements of surface wave interactions with defects have considered only those that are inclined normal to the surface. Here, the enhancement effects have been studied in aluminium samples with machined slots of fixed length and of varying angle to the horizontal; the degree of enhancement was studied as a function of defect angle using both a scanning laser source, and a scanning laser detector. An automated scanning system has been developed for use with the detector, an IOS two-wave mixer interferometer, capable of measuring the out-of-plane surface displacement on rough surfaces. B-scans, consisting of many A-scans stacked together, were used to identify wave modes present in the near field, the arrival times of which are dependent on the angle of the defect. The observed enhancement is caused by superposition of the incident Rayleigh wave with reflected and mode converted waves, thereby making it angle dependent