Safety Check: Finding Weakness in Wood

Monday, August 8, 2011 @ 04:08 PM gHale

Ultrasound thermography can now identify material defects during the production of wooden items. This allows the manufacturer to catch rejects, which could end up saving money over the long haul.

Manufacturers that sell expensive solid wooden tables want to be certain their new piece of furniture is absolutely faultless. That is where ultrasound thermography comes into play.

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Pianos – whether upright or grand – can only produce an opulent tone if their soundboard, bridge and keyboard consist of high-quality materials. And wood that is free of imperfections is also essential in house building and window construction: Load-bearing wooden beams need to be of the highest quality, as even the smallest crack can cause them to fail.

Research scientists from the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI in Braunschweig, Germany, are able to pinpoint defects in wood that cannot be seen with the naked eye.

Ultrasound thermography can identify defects during the production of wood products.

Ultrasound thermography can identify defects during the production of wood products.

Using high-power ultrasound thermography they can detect longitudinal and transverse cracks, gluing errors, delaminations and black knots.

To do this the researchers vibrate the wooden item using a sonotrode, or ultrasound agitator, at a frequency of 20 kHz – in other words, 20,000 times a second. Where there are defects, the different parts of the material rub against each other and produce heat. A thermal imaging camera connected to a monitor detects the heat at the defect’s extremities; in the case of hairline cracks, the manufacturer can see the frictional heat along the length of the crack as well.

High-power ultrasound thermography even allows the researchers to probe beneath the surface to uncover dowels not glued and defects hidden under coatings – something that today’s much less reliable testing methods, such as mechanical materials testing or electrical measuring, cannot do.

“We can spot the imperfections in raw timber. That is crucial for rejecting defective wood before time and money have been invested in processing it,” said physicist Peter Meinlschmidt at the WKI.

Whether the wood in question is oak, walnut or beech is not important, and neither is the condition of the wood; defects in damp parts show up on the thermal imaging camera too. The depth to which a manufacturer can analyze the wood depends on its thermal conductivity, but up to 20 millimeters are possible.

“Our process is especially suited for finding defects in high-quality solid wooden parts and window frame squares and to detect badly glued joints. It’s a non-destructive testing method. Applying the ultrasound agitator does leave small pressure marks though – but these aren’t an issue when you’re dealing with raw timber,” Meinlschmidt said.

Researchers also use high-energy ultrasound thermography to detect cracks in ceramics and glass. In laboratory tests, they were able to pinpoint defects in ceramic floor tiles and in glass mouthwash bottles.

“In ceramics and glass we can spot defects that are up to 30 centimeters away from the sonotrode,” Meinlschmidt said.

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