Thermo-optical modulation for improved ultrasonic fatigue crack detection in Ti-6Al-4V

Pulsed infrared laser irradiation was used to positively identify small fat igue cracks on the surface of fatigue damaged Ti-6Al-4V specimens. The resu lting transient thermoelastic deformation perceptibly changes the opening o f partially closed surface cracks without affecting other scatterers, such as surface grooves, corrosion pits, coarse grains, etc. that might hide the fatigue crack from ultrasonic detection. We found that this method, which was previously shown to be very effective in 2024 aluminum alloy, must be m odified in order to successfully adapt it to Ti-6Al-4V titanium alloy, wher e significant thermo-optical modulation was found even from straight corner s or open notches. This spurious modulation is caused by direct thermal mod ulation of the sound velocity in the intact material rather than thermal st resses via crack closure. Different methods have been developed to distingu ish direct thermal modulation from crack-closure modulation due to thermoel astic stresses. It was found that the modified thermo-optical modulation me thod can increase the detectability of hidden fatigue cracks in Ti-6Al-4V s pecimens by approximately one order of magnitude. (C) 2000 Elsevier Science Ltd. All rights reserved.