Ultrasonic detection of fatigue cracks by thermoelastic modulation

Pulsed infrared laser irradiation was used to positively identify small fatigue cracks on the surface of fatigue damaged 2024 Al and Ti-6Al-4V specimens. The resulting transient thermoelastic deformation perceptibly changes the opening of 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 is very effective in 2024 aluminum alloy, must be modified in order to successfully adapt it to Ti-6A1-4V titanium alloy, where significant thermo-optical modulation occurs even from straight corners or open notches. This spurious modulation is caused by direct thermal modulation of the sound velocity in the intact material rather than thermal stresses via crack closure. It was found that the modified thermo-optical modulation method can increase the detectability of hidden fatigue cracks in 2024 Al and Ti-6Al-4V specimens by approximately one order of magnitude.