No abstract

Repetitively Q-switching a Nd:YAG laser during a single flashlamp pulse has been used successfully to generate a train of acoustic pulses with a repetition rate as high as 53 kHz. The spectral content of this multiple-pulse ultrasonic signal is significantly narrower in bandwidth than that of a single pulse. A corresponding reduction in the detection system bandwidth results in a marked improvement in detection sensitivity.

The ultrasonic behavior of titanium alloy Ti6242 was determined as a function of ultrasonic frequency and material microstructure. The signal-to-noise ratio for synthetic flaws machined in the Ti6242 blocks was strongly influenced by microstructural condition, particularly as it was defined by electron backscatter diffraction pattern (EBDP) analysis. Ti6242 blocks with a microstructure consisting of uniform, fine, texture-free ␣Ti particles had signal-to-noise ratios about 20 dB greater than blocks with microstructures consisting of colonies of crystallographically aligned ␣Ti particles.

A passively mode-locked, flashlamp-pumped Nd:YAG laser with a cavity length of 11.19 m has been developed to study the noncontact generation of narrow-band ultrasound. The individual mode-locked pulses acted as separate sources of ultrasound, producing a train of acoustic pulses with a repetition rate of about 13.4 MHz. The ultrasound was generated in an aluminum sample and remotely detected with a path stabilized Michelson interferometer. The energy in the multiple pulse acoustic signal was confined to a considerably reduced spectral range compared with that in a single pulse.

A novel dual-beam interferometer has been designed and constructed that enables two beams from a He-Ne laser to probe remotely the surface of a material. The separation of the two He-Ne beams is adjustable in the 15-to- 40-mm range with a spatial resolution of 2 microm. Surface-acoustic-wave measurements have been performed with two different probe separations so that the travel time for the surface waves over a known distance can be determined accurately. With the aid of autocorrelation algorithms, the Rayleigh pulse velocity on 7075-T651 aluminum has been measured to be 2888 +/- 4 m/s. The current precision of the system is limited mainly by the 10-ns sampling rate of the digital oscilloscope used. Rayleigh pulse interactions with a surface-breaking slot, machined to a nominal depth of 0.5 mm, have also been examined and the depth estimated ultrasonically to be 0.49 +/- 0.02 mm. The system may also provide a technique for direct quantitative studies of surface-wave attenuation.