Fourier-transform vibrothermography with frequency sweep excitation utilizing local defect resonances

The defect activation and detection efficiency of low-energy vibrothermography can be further improved by periodically sweeping the ultrasonic excitation frequency and Fourier-transforming the temperature data at the modulation frequency. This is due to the resulting periodic heat production. While typical local defect resonance investigations make use of known resonance frequencies, this technique instead employs a wideband ultrasonic excitation signal. It can therefore be used to detect defects quickly without knowing the LDR frequency in advance, while still keeping the efficient defect activation effects provided by LDR. Otherwise undetectably low thermal defect signals are enhanced by means of Fourier filtering and the defects' resonance frequencies are characterized by their phase values.