Weakness finder

Press, 12 April 1988, Page 21

Weakness finder

By

MAVIS AIREY

Hedged in by a maze of I mirrors, a motor-cycle helmet glows eerily in the brilliant green light of an argon laser! Before the laser was; switched on, it had looked perfect from the outside, but,{now, a D.S.I.R. scientist, Peter Boyd, can see a tell-tale pattern j of stress contours, like al (weather map, around the damaged area. The technique he is using is an industrial application of the hologram, called holographic inter-! ferometry; which is so successful! at- detecting hidden flaws it has 1 been dubbed I “the j Sherlock Holmes of high tech.” It can be used to visualise: otherwise invisible flawslin anything from computer disc drives to car tyres, and aircraft parts to tennis racquets. * | | ■ The moior-cycle helmet testing is being done in Christchurch by the D.S.I.R.’s Southern Industrial Development Division. At present,! the Accident Compensation Corporation has to. replace aIP helmets involved in even minor accidents, because there is ho way of) telling how seriously damaged they are. By correlating a||lot of tests with helmets; passed on by the A.C.C., the divis on hopes to be able to find a level lof stress patterns that indicates when a helmet isistill safe to use. With motor-cycle helmets ’costing $2OO and more to replace, I a simple, .! ! : di Hi

speedy test could save a lot of; waste. i

Unlike the other methods of stress analysis the division uses, holograms have the advantage of not damaging the object in any way, or even doing anything directly to it. I ' Holograms are so sensitive, they can pick up fractions of a wavelength of stress. The touch of a finger, or the vibration of a footfall send out shock

waves. To guard against unintentional

stress upsetting the holograms, the laser has to be set up on a; special anti-vibration table made of concrete over air bags.

The light from the laser is split) into two beams. Using I mirrors,! one beam is reflected bn to the helmet, illuminating it. Lightwaves from the helmet are, in' turn, reflected on to a photo-, graphic plate. j

At the same time, the other beam is bounced directly on to the plate, without touching the helmet. Where the iltwb beams intersect, they disturb each other’s light waves and create a hologram. •

To analyse the stress, two holograms are taken: the first, when the helmet is just sitting on

the table, “unloaded”; the second, “loaded,” with weight or heat to stress it. The light coming off the loaded helmet interferes with the light from the unloaded helmet and creates “interference fringes.”

By measuring these wavelengths, the scientist can measure the amount of stress and the amount of damage to the helmet.

“When you look at the plate in ordinary light, you can’t see anything, but when you shine laser light on it again, the interference fringes split up the light) into the image of the original object,” Peter Boyd explains.

“And what’s fascinating is that any part of the plate contains the complete pattern. If you separate any part of the plate and reilluminate it, it will give the full image — although it may be a bit hazier than the image from the full plate.” Peter Boyd believes the division is the only place in New Zealand doing holographic interferometry because it is so expensive. The type of laser used now costs about $50,000. Just to recondition the tube two years ago cost $lO,OOO. Nor is it cheap to run. To produce up to five watts of laser light takes a 30-kilowatt power source. ;

Cost and lack of staff mean it may be a long time before the project is completed.