Additively manufactured integrated slit mask for laser ultrasonic guided wave inspection

Guided ultrasonic waves are attractive for inspection of additively manufactured plate-like components. Illumination of a slit mask by a pulsed laser is one method by which guided ultrasonic waves can be generated. This work proposes a method for generating narrowband ultrasonic guided waves using an additively manufactured slit mask that is integrated onto the component during selective laser melting (SLM) process. Multiple guided wave modes with a dominant wavelength but with different frequencies were generated using the slit mask fabricated using AlSi12 material. The generated modes were identified using the time frequency response of the received signals and dispersion plots. Identifying the modes and its characteristics (frequency, wavelength, phase and group velocity) beforehand facilitates material and defect characterization. A multiphysics numerical model was developed to simulate laser generation of ultrasound and the model was validated using experimental results. The numerical model developed aided in understanding the physics of line arrayed laser ultrasonic generation and was used as a tool to optimize laser parameters. The developed model was used to study the effect of pulse width of the laser on Lamb wave mode generation. It was observed that a pulse width of 100 ns reduced the overall ultrasonic bandwidth to 4.5 MHz thereby limiting the modes to the fundamental modes A0 and S0 for the given wavelength of 0.8 mm. Rayleigh wave studies using a slit mask showed that the rate of decay of the fundamental frequency component was steeper than the rate of decay of the second harmonic component.