SOME PUBLISHED AND UNPUBLISHED ULTRASONIC RESEARCH ON WOOD Frank Beall University of California, Berkeley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 SYSTEM BLOCK DIAGRAM 40 – 60 dB 0 – 40 dB

AE TRANSDUCER 25 MM EPOXY SHOE PZT

AE EVENT WAVEFORM

L R, T

AE Characteristics/Challenges Need active fractures to generate waves Placement/coupling are issues Attenuation/anisotropy of wood

AE SYSTEM

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 White fir 10 x 10 x 20 mm Brown rot decay (Poria placenta) 0, 1.3, 13, 28% mass loss

RADIAL COMPRESSION TRANSDUCER WOOD SPECIMEN LOAD CELL

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??

PARTICLEBOARD SPRINGBACK

INITIAL SPRINGBACK 1: : 2-3-4

xx

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

MACRO-FISSURING OF WOOD DURING COMBUSTION CO O2 Flaming Glowing Shrinkage

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)??

WOOD PYROLYSIS

96 kJ/mol

350ºC

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 STRESS

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)

POOR TREE DAY 1

GOOD TREE DAY 1

POOR TREE DAY 1

POOR TREE DAY 2

POOR TREE DAY 5

GOOD TREE DAY 1

TREE 1 DAY 1 GOOD TREE DAY 2

GOOD TREE DAY 5

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

Water Stress Lesson Document everything, even if presumed to be preliminary; it may not be…

Sensor Hardboard Preamp Ampl Filter Saw blade

AE OUTPUT FROM SAWING HARDBOARD PANEL RAW AE OUTPUT RMS