Laser ‘wand’ would look inside objects to detect hidden damage

US scientists have developed a technique which uses commercial lasers to detect hidden, early signs of damage. This could allow engineers to rapidly test aeroplanes and ships for safety with a laser ‘wand’.

Non-destructive testing is a growing multi-billion-dollar industry. Techniques such as ultrasound, liquid penetration and interferometry allow engineers to assess safety of objects without pulling them apart. In order to prevent catastrophic failure, expensive structures such as aeroplanes, ships and bridges require frequent testing.

“Metals are often subjected to mechanical stress or fatigue that can weaken them structurally, but you can’t tell that just by looking at them,” said Dr James Patterson, based at Brigham Young University, Utah. He describes a case of a multi-million-dollar US Air Force plane being scrapped after an accident during flight. Although it was uncertain whether any damage had been done, it could not undergo a safety check without being damaged.

Most techniques give uncertain results, however, and work by probing for microscopic cracks, which indicate fatigue. This means that current testing cannot detect the tiny deformations which cause cracking.

These deformations are caused by a build-up of irregularities within a material’s lattice; this covers its surface with distinctive pits and bulges. This new laser-based technique works by detecting these early indicators of damage.

The team of scientists behind the research, based at Brigham Young University, exploited second harmonic generation (frequency doubling) to develop this new technique. This is a process in which photons are combined to generate new photons with twice the energy and frequency. The researchers came to realise that second harmonic generation could be adapted to search for signs of internal damage in materials.

The researchers began by shining green lasers onto metal samples. Through second harmonic generation, the metal converted some of the green light into higher frequency ultraviolet light. This ultraviolet light is reflected back from the metal with the remaining green light.

“The amount of conversation depends on the properties of the metal, and if those properties have been changed by some form of stress, we can detect that in the converted light,” said Dr Patterson.

The technique could distinguish between parts that are intact, and those which need replacing.

According to the researchers, this method is more sensitive than existing non-destructive testing techniques in detecting early warnings of danger. It could prove useful in the aerospace industry, where aeroplane parts are replaced regularly for safety, rather than being replaced after damage. The research is partially funded by the Office of Naval Research, and has potential for testing structural integrity of Naval vessels.

“There are stories of someone walking along a metal deck and stepping in the wrong spot, and a big chunk falling through to the deck below,” said Dr Patterson. “Cracks also form in walls. And once visible cracks form, it’s often too late to reverse the damage.”

Eventually, the researchers hope to develop their technique into a mobile system that can scan objects and indicate whether they are in a good shape.

“In principle, you could go around with a wand and some fibre optics and scan large areas of a ship for hidden damage,” Patterson said.

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