Laser-Ultrasonic Inspection of the Composite Structure of an Aircraft in a Maintenance Hangar

Fort Benjamin Harrison Field Unit 1982

“…shows the resulting ultrasonic signal, which clearly shows the longitudinal wave detected after reflection from the backwall of the part. The detected signal had a smaller amplitude than expected based on previous work with CO 2 and Nd:YAG generating lasers [9] and thus averaging of 50 signals was required to obtain the displayed signal ( Figure 2). The signal amplitude is very low compared to the signal obtained from this same target using either the CO 2 or Nd:YAG generating lasers and a simple lens to couple light into the SFPI.…”

“…Although, the lengths of the PCS fibers could be tailored to reduce the NA, the 2.0 mm diameter fiber would always slightly exceed the required NA. The NAs required to obtain the appropriate étendue of 0.4856 mm 2 .sr are also given in Table 4. …”

“…This is a subjective measure unless the fiber positioner is equipped with a high-resolution digital readout. 2 Generation Laser A 1.5 m long hollow sapphire waveguide fiber, which had an internal diameter of 600 µm, was used to investigate transmission of the CO 2 generation laser to the Cassegrain subassembly. The fiber was not terminated, but a fiber chuck was used for interfacing with the Cassegrain optical subassembly.…”

“…Thus, a priori knowledge of the nominal part shape often does not provide sufficient accuracy to allow an automated conventional ultrasonic inspection. Many of the advantages of laser-based ultrasonics, including its noncontacting nature and applicability to rapid scanning of contoured and integrally stiffened aerospace structures, have been describ ed previously [1][2][3][4].…”

“…This leaves two possibilities. Perhaps the thermoelastic generation efficiency of the alexandrite laser is much less than that of the CO 2 and Nd:YAG generating lasers. Or perhaps the Cassegrain optical system is not effectively coupling light from the target to the SFPI through the fiber bundle.…”