The quasi-harmonic ultrasonic polar scan for material characterization: experiment and numerical modeling

The conventional ultrasonic polar scan (UPS) records the transmitted amplitude of an acoustic pulse for a certain solid angle, resulting into a unique fingerprint of the local critical bulk wave angles, which relate to the local elasticity tensor at the insonified material spot. Numerical plane wave computations reveal that the use of harmonic sound, instead of sound pulses, provides a picture that exposes the leaky Lamb wave angles. For the first time, UPS experiments have been obtained with the use of quasi-harmonic pulses to check this premise. To achieve good correspondence with the experimental recordings, a more advanced numerical model (based on the partial wave technique and the stiffness matrix method) has been implemented. Both temporal and spatial beam dependency is accounted for by application of the Fourier transform. Besides a traditional amplitude measurement, the analysis has also been extended, both numerically and experimentally, to the phase of the transmitted sound, hence introducing the concept of a 'phase ultrasonic polar scan'. The results clearly indicate that a phase UPS offers very defined and detailed results, with high sensitivity to small disturbances. Even more, combination of both the amplitude and phase UPS yields complete knowledge about the complex transmission coefficient. To demonstrate the potential of the quasi-harmonic UPS for NDT and material characterization, it has been applied to a unidirectional (UD) carbon/epoxy laminate which contains an artificially generated delamination. It is demonstrated that, contrary to the pulsed UPS, the quasi-harmonic UPS show great sensitivity to the presence of the delamination.The conventional ultrasonic polar scan (UPS) records the transmitted amplitude of an acoustic pulse for a certain solid angle, resulting into a unique fingerprint of the local critical bulk wave angles, which relate to the local elasticity tensor at the insonified material spot. Numerical plane wave computations reveal that the use of harmonic sound, instead of sound pulses, provides a picture that exposes the leaky Lamb wave angles. For the first time, UPS experiments have been obtained with the use of quasi-harmonic pulses to check this premise. To achieve good correspondence with the experimental recordings, a more advanced numerical model (based on the partial wave technique and the stiffness matrix method) has been implemented. Both temporal and spatial beam dependency is accounted for by application of the Fourier transform. Besides a traditional amplitude measurement, the analysis has also been extended, both numerically and experimentally, to the phase of the transmitted sound, hence introducing the concept of a 'phase ultrasonic polar scan'. The results clearly indicate that a phase UPS offers very defined and detailed results, with high sensitivity to small disturbances. Even more, combination of both the amplitude and phase UPS yields complete knowledge about the complex transmission coefficient. To demonstrate the potential of the quasi-harmonic UPS for NDT and material characterization, it has been applied to a unidirectional (UD) carbon/epoxy laminate which contains an artificially generated delamination. It is demonstrated that, contrary to the pulsed UPS, the quasi-harmonic UPS show great sensitivity to the presence of the delamination.C