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The use of acoustic emission in improving hardwood timber seasoning productivity

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Booker, James Derek (1994) The use of acoustic emission in improving hardwood timber seasoning productivity. PhD thesis, University of Tasmania.

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Abstract

The Tasmanian sawmilling industry typically does not season eucalypt
material specifically for furniture applications because of its highly refractory
seasoning characteristics. Boards cut with wide faces parallel to the growth
rings ('backsawn or 'flatsawn' boards), in particular, shrink significantly
with drying and often experience surface checking (seasoning 'degrade'). It
is standard practice to season timber batches under schedules designed to
produce relatively high-grade structural material and sell the small
proportion of boards that do not suffer seasoning degrade as furniture grade
material. The aim of this study was to investigate optimum seasoning of
backsawn material specifically for furniture applications to take advantage of
the premium prices paid by furniture manufacturers for degrade-free
material.
Acoustic emission is a well known non-destructive testing tool. Acoustic
emission results from stress waves generated by irreversible stress release
activity such as the growth of small cracks in a solid material under load.
Acoustic emission is typically used in homogeneous materials such as
plastics and metals to determine the structural integrity of components in
situ. Acoustic emission was first measured in timber subject to external load over
thirty years ago and has subsequently been measured in drying timber
specimens where differential drying rates between the surface and centre of a
sample may cause appreciable drying stresses. Acoustic emission appeared
to be related to the severity of the drying conditions and the development of
surface checking (Becker 1982, Noguchi et al. 1987). Under harsh drying
regimes, surface checking occurred and a significantly higher AE rate was
measured than in the same species subject to mild drying conditions. The
'AE rate' is a measure of the occurrence of high-energy bursts of acoustic
waves during a particular time-period - the higher the AE rate, the more
energetic bursts are measured. On this basis, it appeared feasible to measure
acoustic emission in sample boards in a kiln during drying and adjust the
drying conditions as the AE rate approached levels previously determined to
correspond to surface checking. Various AE-based kiln control systems were
reported in the literature (Honeycutt et al. 1985, Noguchi et al. 1987) but it
appears acoustic emission was not satisfactorily related to surface check development. These control systems controlled the drying conditions based
on arbitrary AE rates that were nominally selected as 'safe' without
optimising the drying process. The heterogeneous structure of timber posed
significant obstacles to forming a clear picture of the stress release processes
occurring in timber.
Acoustic emission measured in drying Tasmanian eucalypt boards was
employed in this project as an online measure of the magnitude of the
stresses during drying. Numerous batches of eucalypt boards were obtained
from a sawmill during the project and dried under harsh conditions to
induce surface checking. It was determined that the AE rate generated at the
onset of surface checking in backsawn and quartersawn boards was
effectively constant (within the variability of the material). This 'AE checking
threshold' was independent of basic density and clearly independent of bulk
stiffness (since the bulk tangential stiffness is effectively half the radial
stiffness in the species studied). It appeared that the acoustic emission
phenomenon was independent of the timber bulk material properties.
It is proposed that acoustic emission waves propagate from irreversible slips
or dislocations in the crystalline cellulose regions of cell walls under high
stress. These slips are sudden, energetic stress release events which
propagate stress waves from the local site. The amorphous regions of the cell
structure behave in a rather plastic manner with local stress expected to be
consumed by ductile flow processes that do not generate elastic waves.
Dinwoodie (1968) reported the existence of such crystalline slips in spruce
timber under external compression. Siau (1984) reported that the proportion
by mass of cellulose in normal wood (as opposed to tension and compression
wood) was remarkably constant and apparently independent of species.
Further, the proportion of crystalline cellulose is also relatively constant
(Siau 1984). This appears to be directly related to the remarkably constant
AE rate measured at the onset of surface checking in this study.
This finding led to the measurement of acoustic emission in other species. It
was hoped that the same AE 'rate would be measured at the onset of checking
in different woods. Acoustic emission was measured in backsawn radiata
pine (Pinus radiata) and myrtle (Nothofagus cunninghamii). The acoustic
emission measured in radiata pine was significantly different from the
acoustic emission measured in the eucalypt material, with relatively 'massive' AE rates detected in boards free of seasoning degrade. This
behaviour was attributed to brittle failure in the resin canals, structural
elements not present in the eucalypt genus. The characteristic AE responses
measured in drying myrtle boards resembled the behaviour measured in the
eucalypt boards, apparently due to the more similar structure. Surface
checking was detected at approximately the same AE rate as that measured
in the eucalypt boards.
Much of the improved understanding of the acoustic emission phenomenon
developed in this study was facilitated by the existence of a one-dimensional
nonlinear drying model developed by Oliver (1991). Oliver wrote
KILNSCHED (KILN SCHEDULING PROGRAM), a computer program
based on this model, which simulates the drying behaviour of a single board,
with arbitrary bulk material properties, subject to arbitrary drying
conditions. KILNSCHED is particularly suited to low temperature drying of
eucalypt materials. Kiln drying trials quickly revealed that 'green'
Tasmanian eucalypt boards invariably suffered surface checking within 24
hours when subject to drying temperatures as low as 23°C dry bulb
temperature and 21°C wet bulb temperature (at 0.5 m/s airspeed). Such
temperatures are significantly lower than those often employed in drying
material direct off-saw in the timber industry but none-the-less are
considered 'harsh' in this study. At the commencement of this study, KILNSCHED was used in a purely
predictive mode to assess drying schedules prior to drying. The author of
this thesis modified KILNSCHED to simulate drying using the real-time
drying temperatures measured in the experimental kiln. This allowed
measured acoustic emission during drying to be compared with the drying
behaviour calculated with KILNSCHED. This modification revealed that
Tasmanian eucalypt timber is far more sensitive to small temperature
fluctuations than was previously expected. The reader must keep this
material sensitivity to temperature and temperature change in mind at all
times when reading this thesis. The AE rate measured during drying was successfully related to the
instantaneous strain at a board surface calculated with KILNSCHED using
the measured drying conditions. Instantaneous strain is the strain
component employed as the failure criterion in the drying model. This enabled the author to place considerable confidence in behaviour calculated
with KILNSCHED and the 'optimum drying' program SMARTKILN
discussed below.
This author modified KILNSCHED to incorporate an optimisation algorithm
that determined the optimum drying conditions required to dry the timber in
the minimum time at a preset arbritrary maximum surface instantaneous
strain. The resulting program, SMARTKILN, develops drying schedules to
maintain the calculated surface instantaneous strain at a preset 'Control
Strain' below the ultimate surface instantaneous strain. Together,
SMARTKILN and acoustic emission measurement form the basis of Clever
Kiln Controller®, a kiln control system to dry Tasmanian eucalypt timber in
the minimum time with minimum degrade. In Clever Kiln Controller, SMARTKILN uses datalogged real time drying
conditions to simulate the drying behaviour of a sample board in the kiln.
Calculated drying behaviour is continuously compared with measured
drying behaviour (measured AE rate and moisture profiles measured by
regularly slicing sample boards). Provided the calculated and measured
drying behaviour are satisfactorily matched, the optimum drying schedule
developed by SMARTKILN is applied to the kiln. When the AE rate
approaches the AE checking threshold, the drying conditions are
automatically ameliorated to prevent surface checking. Subsequently,
SMARTKILN develops a refined optimum schedule to incorporate the new
datalogged drying conditions.
This study has developed the understanding of acoustic emission from the
existing (often misleading) information recorded in the literature to a level
that could be incorporated with current knowledge of the behaviour of
drying timber. The thesis describes how this was achieved. Various aspects
of the understanding have been reported in papers published, in press or
under review.
The final result is the development of a commercial kiln controller which is
described and already implemented in some selected experimental kilns at
present restricted to eucalypt materials.

Item Type: Thesis (PhD)
Copyright Holders: The Author
Copyright Information:

Copyright 1994 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Acoustic emission measured in drying Tasmanian eucalypt boards was employed as an online measure of the magnitude of the stresses during drying. A computer program, KILNSCHED, based on a one-dimensional drying model, was used, and modified as SMARTKILN, which uses datalogged real time drying conditions to simulate the drying behaviour of a sample board in the kiln. Thesis (Ph.D.)--University of Tasmania, 1996. Includes bibliographical references. Acoustic emission measured in drying Tasmanian eucalypt boards was employed as an online measure of the magnitude of the stresses during drying. A computer program, KILNSCHED, based on a one-dimensional drying model, was used, and modified as SMARTKILN, which uses datalogged real time drying conditions to simulate the drying behaviour of a sample board in the kiln

Date Deposited: 25 Nov 2014 00:46
Last Modified: 20 Jun 2016 23:00
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