Detection of early stages of wood decay with ultrasonic pulse velocity
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Solid wood Douglas fir beams (6x14 inches, 10-14 feet long) that had been decayed by brown rot in service were subjected to ultrasonic pulses, the velocities of which were calculated in longitudinal transmission either from end to end or between transducers on the same beam face in direct transmission through the thickness of the beam and in direct transmission from edge to edge. Then 10 beams were selected for more intensive study on the basis of initial testing that indicated a gradual transition from decayed to sound wood. Transducers with frequencies of 37, 54, 150 and 500 kHz were used. After ultrasound testing, segments of the beams were examined to assess whether there was any correlation between diminished pulse velocity readings and observed decay. Direct transmission through the 6 inch thickness of the beam appeared to be a method that could be used to delineate advanced internal decay and, in some cases, detect early stages of decay at the boundary of decay pockets. The use of 500 kHz transducers, however, was not successful. Testing on small wood wafers in known stages of decay showed that the technique was capable of detecting the early stages of decay in 3 of the 4 wood/fungus combinations tested, namely Douglas fir or white fir ( Abies concolor ) and Gloeophyllum trabeum or Poria ( Oligoporus) placenta ; the exception was Douglas fir decayed by G. trabeum .