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A kHz Rate Non-contact Compact Laser-ultrasound Scanner for Advanced Evaluation of Aircraft Composites

A very compact, inexpensive, fiber-optic laser-ultrasound scanner for NDT&E of composite materials is demonstrated at more than 1000 A-scans per second imaging rate combined with a fast 2D translator allowing sample movement with a scanning speed of 100 mm/s. A 2.5 mJ, 12 ns, 1053nm diode-pumped laser capable of operating at kHz repetition rates is used to generate wide-band optoacoustic signals at the surface of a composite sample. Detection of ultrasound signals back reflected from the composite structure is performed with a modified balanced fiber-optic Sagnac interferometer, the key component of this new system. Both interfering beams are formed by reflection of the probe beam from the same point on the sample surface (no reference arm is employed), making the scheme extremely stable to environmental noise, robust to surface roughness and possible to operate at kHz scan rates. A position-synchronized output of the 2D translator triggers laser firing, referencing all measurements to the current sample coordinate and providing a constant precise step between A-scans even for regions of large acceleration/deceleration. The performance of the system is demonstrated for robust flaw detection, material porosity evaluation, assessment of heat damages and visualization of wrinkles in real composite samples provided by Boeing Research & Technology.

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