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Laser heating detects cracks

14 Jan 2004

French technology could benefit nuclear, automotive and aerospace industries.

A laser-based system for detecting tiny cracks in mechanical parts could soon be spotting invisible flaws in pressure vessels in a nuclear reactor or components of a car engine.

Framatome, the French maker of nuclear plants, has patented the technique and is working with CEDIP Infrared Systems, an infrared camera specialist, in order to turn the concept into reality.

“The technique can detect very small cracks that have a depth of less than 1 mm and are not visible to the eye,” said Pierre Potet, CEDIP’s chief executive officer. “Our goal for 2004 is to make several versions to suit different applications such as checking components in automobile engines or high performance materials.”

The so-called photothermal or “flying-spot” technique works by irradiating a target surface with a laser beam in order to raise the temperature of a small region by a few degrees. An infrared camera then captures a map of the heat flow over the irradiated region. Any flaws or cracks in the surface interrupt the heat flow and show up clearly on the infrared image. The process is repeated over the entire surface in order to perform a complete inspection of the part.

Typical operating parameters are a temperature rise of up to 5°C and a laser beam of up to 20 cm wide. The duration and power of the laser irradiation is dictated by the properties of the target material.

According to CEDIP, the technique is compatible with any kind of laser and material providing that it’s possible to perform the laser-heating. However, tests to date have centred on the use of Nd:YAG and CO2 lasers and metal parts.

CEDIP believes that its technique could become a promising alternative to dye penetration inspection which uses a fluorescent dye that gets trapped in cracks to reveal tiny flaws in surfaces. “We started this work about 10 years ago with the goal of replacing dye penetration which is messy and uses nasty liquids that are pollutants,” explained Potet.

The biggest drawback to the technique is undoubtedly its expense. CEDIP says that photothermal systems are expected to have a price-tag of around EURO 200,000 ($157,000).

Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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