The ultrasound agitator causes the wood to vibrate, which generates frictional heat wherever there are cracks. A thermal imaging camera show these defects up. Image: Fraunhofer WKI |
Is
there a hairline crack in the oak table? Was the window frame glued
badly? Ultrasound thermography can reliably identify material defects
during the production of wooden items. This allows rejects to be caught
quickly and eliminated, and faulty goods to be repaired in good time.
People
who buy an expensive solid wooden table or wardrobe want to be certain
that their new piece of furniture is absolutely faultless.
Pianos?whether upright or grand?can only produce an opulent tone if
their soundboard, bridge and keyboard are made 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 are able to pinpoint
defects in wood that cannot be seen with the naked eye. Using high-power
ultrasound thermography they can detect longitudinal and transverse
cracks, gluing errors, delaminations and black knots.
To
do this they 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. This heat at the defect’s
extremities is picked up by a thermal imaging camera connected to a
monitor; in the case of hairline cracks, frictional heat can be seen
along the length of the crack as well. High-power ultrasound
thermography even allows the researchers to probe beneath the surface to
uncover dowels that have not been glued and defects hidden under
coatings?something that today’s much less reliable testing methods, such
as mechanical materials testing or electrical measuring, are simply not
able to 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,” says 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 the wood can be
analyzed 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,” explains Meinlschmidt. The researchers
have even managed to 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,” says the research scientist. A demonstrator of
the ultrasound generator with thermal imaging camera has already been
built.
