Hybrid laser/broadband EMAT ultrasonic system for characterizing cracks in metals

Detection and characterization of defects in metal parts in industrial and commercial settings has typically been carried out by nondestructive ultrasonic inspection systems. Correct measurement of crack size is critical for lifetime prediction inspections. Normally, measurements are made based on far-field ultrasonic diffraction models and time-of-flight reflection signals making accurate measurements for parts less than approximately 25 mm in thickness impossible. In this work a hybrid noncontacting laser generation/broadband electromagnetic acoustic transducer (EMAT) detection system is used to characterize ideal cracks in aluminum in which the far-field condition for ultrasonic diffraction cannot be met. Time domain signals show that diffracted energy is measured in the geometrical shadow zone of the crack. Fourier transform methods are used to show that the frequency content of the diffracted signals is different than those from the waves that do not interact with the crack. Crack size measurements are made by using the frequency content of the ultrasonic signal rather than time-of-flight information.