Ultrasonic Infrared Thermography of Aluminium Thin Plates for Crack Inspection in Friction Stir Welded Joints

Ultrasonic infrared thermography, thanks to its defect selective, dark-field damage detection capability, is a powerful tool for the inspection of cracks, but the ultrasound coupling is difficult for thin-walled or low-rigidity specimens if an external stimulation is employed, and the crack signature is poor for metals with high thermal conductivities. In order to detect cracks in friction stir welded joints of aluminium alloy thin plates, the constraint-excitation schemes and post image processing methods were investigated. A base excitation combined with unsymmetrical constraint condition of the thin plate were proposed and analyzed by using experiment and vibration modal analysis; a wavelet image enhancement based on symlets or biorthogonal wavelets was presented. The base excitation skill with unsymmetrical support condition and the post image processing with wavelet image enhancement were high-lighted. The results showed that the new constraint-excitation scheme was a feasible approach to detect kissing cracks in friction stir welded joints of thin plates; the wavelet image enhancement method was effective. The presented measures can greatly improve the detectability of ultrasonic thermography for thin plates.