A novel ultrasonic vibration analysis technique for discontinuity detection

A novel ultrasonic vibration analysis (VA) technique was developed that is capable of damage detection in complex structures. This technique was developed with the intention of combining the most beneficial aspects of low-frequency (that is, non-ultrasonic) modal analysis and ultrasonic transient wave inspection techniques. A guided wave vibration decomposition technique was derived to demonstrate that steady-state structural vibrations could be described as a superposition of guided waves propagating within the structure. It was also shown that, by generating specific guided wave modes and frequencies over a large number of cycles, the structural vibration modes that arise from these guided waves could be efficiently excited. Based on this premise, phased annular array piezoelectric transducers can be used to excite different ultrasonic vibration modes in a structure at ultrasonic frequencies by varying the input frequency and phasing between actuator elements; separate receivers are used to sample the resulting vibration field. Since these ultrasonic vibration modes vary in sensitivity to different damage types and locations, varying the ultrasonic loading function (that is, actuator frequency and phasing) allows for a greater range of discontinuity sensitivity. Key experiments were conducted on a number of test samples, including realistic aircraft composite structures, to demonstrate the high damage detection sensitivity of the ultrasonic VA technique.