SOME PUBLISHED AND UNPUBLISHED ULTRASONIC
RESEARCH ON WOODFrank Beall
University of California, Berkeley
Background
• Ultrasonic technology: acoustic emission (AE)• Advantages and disadvantages with wood• Examples of several studies in 1986, 1987
while at Weyerhaeuser
AE Characteristics/Challenges
• Need active fractures to generate waves• Placement/coupling are issues• Attenuation/anisotropy of wood
Radial Compressive Strength (RCS)
• 1% mass loss from decay can result in up to 90% loss in dynamic mechanical properties
• Is AE sensitive to strength loss?
RCS Results
• AE very sensitive to decay• Separation at <1% elastic compression (NDE)• Curve shape as indication of failure mode
SPRINGBACK
• “Springback” occurs from stress relieval of pressed boards (6 - 10% increase in thickness)
• Industrial particleboard• 20 (thick) x 50 x 50 mm• Exposure: 50/90/50/90/50 %RH; 22 C• One month at each exposure
Causes of Springback
• Shape recovery of compressed particles• Failure of adhesive bonds between particles• Swelling from moisture adsorption• Which comes when??
SPRINGBACK
• Initial springback (6% recovery) has AE, indicating combined shape recovery, swelling, and bond failure
• After this point (6 - 9% recovery), there are no AE, indicating largely shape recovery and particle swelling in equal amounts
• Most bond failures occurs very early
FISSURING OF WOOD
• Hard maple• 3 (thick) x 25 x 165 mm• Exposed to pyrolysis and flaming combustion• Monitoring of AE for micro- and macro-
fissuring
FISSURING
• Can mcro- and macro-fissuring be separately detected?
• If so, does microfissuring precede macrofissuring (like earthquakes)??
FISSURING
• Microfissuring precedes macrofissuring• Process is activated• AE could be used to track char front during
combustion• Airborne AE similar in numbers to visible
macrofissures
TREE WATER STRESS
• A number of studies using AE conducted in mid-1980s
• Most done with excised material• None done with intact controls• Embolism or “capillary snap” most likely
source of AE
TREE WATER STRESS
• Two Pinus contorta; 2-m potted• Good and poor form• In uncontrolled greenhouse• AE transducers attached to stems
TREE WATER STRESS
• Could AE output be used to manage drought-sensitive loblolly pine plantations in the US south?
• Such an approach would monitor a sampling of trees and selectively harvest when stress is detected
TREE WATER STRESS
• “Good” tree watered on day 1• “Poor” tree not watered• Greenhouse floor flooded on day 5• Data printed daily (but not saved)
TREE WATER STRESS
• AE followed diurnal cycle, in phase with presumed stem dia changes
• AE characteristics were remarkably similar between trees
• Min AE at sunset• Tree AE reacted to ΔRH within minutes• Poor tree had much greater indication of
stress from AE level and range
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