SOME PUBLISHED AND UNPUBLISHED ULTRASONIC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 SYSTEM BLOCK DIAGRAM

40 – 60 dB

0 – 40 dB

AE TRANSDUCER

25 MM EPOXY SHOE

PZT

AE EVENT WAVEFORM

LR, 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

INITIALSPRINGBACK

1: 0-1-2

2: 2-3-4

x x

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

175

30

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 1GOOD 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