A LABORATORY LASER-ULTRASONIC INSTRUMENT FOR
MEASURING THE MECHANICAL PROPERTIES OF PAPER WEBS
Emmanuel Lafond1, Paul Ridgway2, Ted Jackson1, Chuck Habeger3, Rick Russo2
Institute of Paper Science and Technology, 500 10th Street, N.W., Atlanta, GA 30318
Lawrence Berkeley National Laboratory / EETD, One Cyclotron Road, MS 70-108B,
Berkeley, CA 94720
Consultant for the Institute of Paper Science and Technology
2
ABSTRACT. For the paper industry, stiffness properties are an important parameter for producing more
efficiently a fibrous material like paper. Some stiffness properties of paper webs can be obtained in a
non-contact fashion using two lasers. The authors have developed an automated laboratory laserultrasonics instrument for paper, described here. The results of non-contact laser generation and
detection of ultrasound are also presented. The paper grades investigated were heavy grades like
linerboard, as well as copy paper.
INTRODUCTION
Applications of Laser Ultrasonic [1] Systems (LUS) have progressed from the
research laboratories to the factories. LUS are now penetrating little by little the mills in
both the areas of process control [2,3] and nondestructive testing [3,4,5]. Laser ultrasonics
has been successfully implemented for process control on sheets or tubes of steel or
aluminum. Laser Ultrasonics has also been used for process control of glass, silicon wafers,
optical crystal growth and real-time carbon-epoxy curing.
Experiments with industrial application to paper have just begun [6-10]. The
definition of the word "paper" in this application is very broad. It includes copy paper, as
well as "art paper" for drawing, tissue paper, newsprint, and coated paper. Heavier grades
of paper such as linerboard (generally brown color), and bleach board (white color) are
used for packaging. Whatever the grade of the paper is, paper production is a continuous
process. A pulp of wood fibers is deposited at the input of the paper machine on a moving
wire mesh, while rolls of paper exit at the output.
Because of variations in the operation of the paper machine and of variations of the
pulp, changes in paper mechanical properties occur [11]. Mass specific elastic stiffnesses as
determined from the velocity of ultrasonic pulses are direct measures of the mechanical
state of a material. For monitoring product quality and providing feedback for process
control, measuring the stiffness properties of the paper would be a valid method.
PRINCIPLE OF THE AUTOMATED LASER ULTRASONIC LABORATORY
INSTRUMENT
Mechanical Properties to be Measured
The mechanical properties of paper determined by LUS techniques are the flexural
rigidity and out-of plane shear rigidity. Figure 1 illustrates the deformations.
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
1665
t
l(a) Flexural rigidity
l(b) Shear rigidity
FIGURE 1: Schematic to explain flexural rigidity and shear rigidity of a paper plate
Machine
Direction
90-
Cross
Direction
FIGURE 2: A typical "polar plot" of the flexural rigidity polar distribution
Because of the preferred orientation of the fibers along the direction of movement
of the wire mesh of the paper machine, mechanical properties of paper webs are highly
anisotropic, and the characterization of this anisotropy (Figure 2) is highly valuable to
paper manufacturers. Machine direction (MD) is the direction in which the paper is moving
on the wire mesh and Cross Direction (CD) is perpendicular to that direction.
Needs Assessment for the Instrument
The instrument needs to measure quickly, in a reliable and automated way, flexural
rigidity and shear rigidity of paper and plot the flexural rigidity polar distribution. We need
the instrument to be able to provide statistics, about the spatial variability of paper. We
want the instrument to be able to test almost any paper grade. It must also be a class I laser
system, so that it can be operated safely by a technician not proficient with lasers.
Principle of the Measurements
The thickness of a paper web produced on a paper machine ranges from .06 to .5
mm depending on the paper grade. Paper is a thin plate and Lamb waves propagate well.
Because of the fibrous structure of paper [11] and of the scattering nature of paper (10 Jim
diameter fibers and 10 Jim holes typically), only low frequency ultrasonic waves (< 4
MHz) can propagate without being attenuated after a few mm travel. Hence even if the
acoustic source generates some high frequencies, only low frequency waves can be
detected. The power spectral density is very small over 1 MHz and the consequence is that
only the first order symmetric Lamb wave named So and the first order antisymmetric
Lamb wave (see Figure 3) named A0 are observed in a paper sheet [6,12] with a decent
signal to noise ratio. The second order symmetric Lamb wave Si is usually at too high
frequency to be observed in heavy paperboard.
1666
FIGURE 3: A0 antisymmetric (left) and S0 symmetric (right) mode shapes as viewed from the edge of a
sheet of paper.
The AO and So waves in paper are different from many points of view. First the AO
wave at low frequencies is mainly an out-of-plane wave, i.e. generates displacements in a
direction perpendicular to the surface of the web. The phase velocity of the A0 mode
depends on the frequency, flexural rigidity, shear rigidity and basis weight. The So wave is
almost exclusively an in-plane wave, i.e. generates displacements in a direction parallel to
the surface of the web. Second, the A0 wave is dispersive at low frequency whereas the So
wave is non-dispersive over the full LU frequency range. Third, the LU generated
amplitude of the So wave is much smaller than the amplitude of the A0 wave. Fourth,
usually the speed of the So wave is a several times larger than the one of the A0 wave,
depending of the frequency. We decided to record only A0 because we can extract from it
the properties of the web, represented by flexural rigidity and shear rigidity. Thus, only the
AO wave is analyzed and flexural rigidity and shear rigidity are measured.
Processing of the Measurements
The calculation of flexural rigidity and shear rigidity is performed as follows:
The instrument digitizes and saves two A0 waveforms at two different emitter to
receiver separations. The phase difference between the two waveforms is unwrapped. The
phase velocity of A0 is determined as function of frequency. Then the phase velocity of A0
divided by the square root of frequency is also determined as a function of frequency. The
flexural rigidity and shear rigidity are determined by as least square of two parameters fit of
this function. A typical frequency range for those calculations is 20 kHz to 600 kHz. The
data acquisition, phase unwrapping, mechanical constants determination are performed by
a LabView program developed in-house.
SCHEMATIC AND EXPERIMENTAL LAYOUT OF THE INSTRUMENT
General Schematic of Instrument
The schematic of the instrument is displayed in Figure 4. The instrument is fully
computer-controlled. The computer controls the movement of the paper sample and the
generation and detection probe separations. It also controls the firing of the laser and data
acquisition. Finally the computer also processes the data, and extracts the flexural rigidity
and shear rigidity.
Schematic of the End-Effector Part of the Instrument
The measurement head and paper sample holder are shown schematically in Figure
5(a) and on a photo in Figure 5(b). The arrows indicate the different sample movements.
This portion of the instrument is separated from the interferometer and generation laser,
and from the computer. The bottom part translates the frame holding the paper sample
along two perpendicular directions in the horizontal plane, and also rotates the paper frame
in the horizontal plane. The "C" frame above the paper sample carries the two probes: one
for the generation of elastic waves and one for their detection. A vertical translation stage
brings both the generation and detection optic fibers and optics to the proper distance from
the paper surface. Finally a horizontal translation stage moves the generation probe to two
1667
interlock
Fiber optics
Computer:
+A/D board
+ robot movement
+laser firing
+signal processing
+ data display & storage
Robot: Translation
& rotation stages
Robot: Translatioi
Robot power
supplies
& rotation stages
Power
supplyÆ Interferometer
interlock
trigger
Generation
laser
Power
Power
unit
supplyÆ
umbilical
Generation laser
head
attenuator
f.o.
launch
wheels
FIGURE 4:
General schematic of the automated laser-ultrasonic laboratory instrument.
FIGURE 4: General schematic of the automated laser-ultrasonic laboratory instrument.
FIGURE 5:
(a) Schematic and
(b) Photography of the measurement part of the instrument.
FIGURE
5:
(a)
Schematic
and
(b)
Photography
the measurement
of the instrument.
pre-determined locations, so that two signals
with oftwo
different part
generation–detection
separations
can belocations,
recorded.so that two signals with two different generation-detection
pre-determined
separations can be recorded.
Ultrasound Detection
Ultrasound Detection
For demodulating the phase shift of the laser light produced by the ultrasonic
Forthedemodulating
thewe
phase
of the laser
light (TWM)
producedphotorefractive
by the ultrasonic
motion of
paper surface,
usedshift
a Two-Wave
Mixing
[13]
motion of the paper surface, we used a Two-Wave Mixing (TWM) photorefractive [13]
1668
interferometer
interferometer
[14].
uses
8mmxxx8mm
8mmxx66mm
mm(in
(inthe
thethickness
thickness direction)
direction) crystal
crystal of
of
interferometer[14].
[14].ItItItuses
usesaaa8mm
Gallium
Arsenide
(GaAs)
to
which
no
voltage
is
applied.
This
interferometer
Gallium
was
Gallium Arsenide
Arsenide (GaAs)
(GaAs) to
to which
which no
no voltage
voltage is
is applied.
applied. This
This interferometer
interferometer was
was
developed
developed
in-house
and
the
optical
setup
very
similar
to
the
one
described
in
detail
in
the
developedin-house
in-houseand
andthe
theoptical
opticalsetup
setupisis
isvery
verysimilar
similarto
tothe
theone
onedescribed
describedin
indetail
detailin
inthe
the
Ultrasonics
journal
[15].
A
noticeable
improvement
however,
is
that
two
fiber
optics
bring
journal
Ultrasonics [15].
journal Ultrasonics
[15]. A
A noticeable
noticeable improvement
improvement however,
however, is that
that two
two fiber
fiber optics bring
the
the
incoming
beam
the
objective
above
the
paper
surface
and
collect
the
backscattered
theincoming
incomingbeam
beamtoto
tothe
theobjective
objectiveabove
abovethe
thepaper
papersurface
surfaceand
andcollect
collectthe
thebackscattered
backscattered
light
light
for
return
the
interferometer.
lightfor
forreturn
returntoto
tothe
theinterferometer.
interferometer.
EXPERIMENTAL
EXPERIMENTAL
RESULTS
EXPERIMENTALRESULTS
RESULTS
Individual
Individual
Results
with
Different
PaperGrades
Grades
IndividualResults
Resultswith
withDifferent
DifferentPaper
A
Some
Some
results
with
white
copy
paper
(grammage:
78.3
g/m
2) are
are presented
presented inin
Someresults
resultswith
withwhite
whitecopy
copy paper
paper (grammage:
(grammage: 78.3
78.3 g/m^2)
g/m^2)
figures
6
and
7.
The
energy
for
the
generation
source
on
the
paper
was
figures
2.8
mJ/pulseand
andthe
the
figures 66 and
and 7.
7. The
The energy
energy for
for the
the generation
generation source
source on
on the
the paper
paper was
was2.8
2.8mJ/pulse
waveforms
were
averages
of
4
different
shots
at
the
same
location
on
the
paper.
waveforms
waveforms were
were averages
averages of
of 44 different
different shots
shots at
at the
the same location on the paper. The
The
separation
separation
distance
along
MD
was
6.5
mmfor
forthe
theupper
upperwaveform
waveform
and10.5
separationdistance
distancealong
alongMD
MDwas
was6.5
6.5mm
waveformand
10.5mm
mmfor
forthe
the
lower
(red)
one.
Signal
processing
is
used
routinely
to
enhance
the
signal
to
noise
ratio.
lower
(red)
one.
Signal
processing
is
used
routinely
to
enhance
the
signal
to
noise
ratio.
lower (red) one. Signal processing is used routinely to enhance the signal to noise
The
The
Phase
velocity
and
the
Phase
velocity divided
divided
by the
the
square
root
of the
the
ThePhase
Phasevelocity
velocity and
and the
the Phase
Phase velocity
divided by
the square
square root
root of
frequency
frequency
are
plotted
function
frequency
onFigure
Figure7.7.They
Theywere
wereextracted
extractedfrom
from the
the
frequencyare
areplotted
plottedasas
asfunction
functionofoffrequency
frequencyon
from
phase
phase
unwrapping
between
the
two
time
signals
Figure
Onthe
theupper
uppergraph
graphofoffigure
figure
phaseunwrapping
unwrappingbetween
betweenthe
thetwo
twotime
timesignals
signalsininFigure
Figure6.6.On
7,7,
the
line
represents
the
fitted
equation,from
from
whichflexural
flexuralrigidity
rigidityand
andshear
rigidityare
7,the
theline
linerepresents
representsthe
thefitted
fittedequation,
fromwhich
shearrigidity
are
calculated.
calculated.
One
can
observe
that
the
workson
onaavery
verywide
wide frequency
frequency
range, from
from
51
calculated.One
Onecan
canobserve
observethat
thatthe
thefitfit
fitworks
frequencyrange,
from51
kHz
kHz
986
kHz
this
case(generally
(generallyfrom
from
fewtens
tensofofkHz
kHztotoabout
kHz), which
kHztoto
to986
986kHz
kHzininthis
thiscase
fromaafew
about600
600kHz),
kHz),
whichisis
good
goodfor
forthe
thestability
stabilityof
ofthe
theelastic
elasticconstants
constantsdetermination.
determination.
good
for
the
stability
of
the
elastic
constants
determination.
itiko
150:0: 200.0
2500
3000
;250:0
3000
3500 4doM8 TOO ;5QD;Q isdio efio.o 6§0:0 Idfed
JSii^^
700:0
ilMJllgig^ig^
JJiSjParSial
FIGURE
paper
obtained
with
the
laboratory
instrument.
Time
µs.
FIGURE6:6:
6: Laser-generated
Laser-generatedA
wavesin
Copypaper
paperobtained
obtainedwith
withthe
thelaboratory
laboratory
instrument.
Time
µs.
00 waves
FIGURE
Laser-generated
AA
ininCopy
Copy
instrument.
Time
ininin
|is.
0waves
..;....;......................,............................:..........................................................:...................................:..........;
N':*!:i'!:%^S*;;*3;*;*
FIGURE7:7:
7:Reconstructed
ReconstructedPhase
Phasevelocity
velocitycurve
curveof
functionof
frequency(from
(from000to
1MHz)on
on
lower
FIGURE
function
ofoffrequency
frequency
(from
toto1MHz)
1MHz)
on
lower
FIGURE
Reconstructed
Phase
velocity
curve
ofofA
AA
as
lower
0 asfunction
00 as
graphand
andphase
phasevelocity
velocitydivided
dividedby
√frequency,as
functionof
frequencyfor
forthe
thefit
onthe
the
upper
graph.
graph
ofoffrequency
frequency
for
the
fitfiton
on
the
upper
graph.
graph
and
phase
velocity
divided
byby√frequency,
Vfrequency,
asasfunction
function
upper
graph.
1669
,M. diO
1-d^!
50,0
100:0
DQ.Q
; 500.0 mm 6oo;o
350:0 4oo,d
700;
i%o 350.0 400.0 450,0 500,0 spo ;spp.:o 650,0 ?oo;;o
•••••••••••••••-••••••••••••••••••••••••••••••••"
'''T""';r"^!tiBsyir:;
FIGURE
AA
ininuncoated
uncoated
magazine
(5(5and
and
0 0waves
FIGURE8:
Top:Laser-generated
Laser-generatedA
wavesin
uncoatedmagazine
magazine(5
and10
mmseparation).
separation).Time
Timein
µs.
FIGURE
8:8:Top:
Top:
Laser-generated
1010mm
mm
separation).
Time
ininµs.
jis.
0 waves
Bottom:
plots
of
the
phase
velocity
and
phase
velocity
/√f,
obtained
from
the
time
signals.
Bottom:
plots
of
the
phase
velocity
and
phase
velocity
/√f,
obtained
from
the
time
signals.
Bottom: plots of the phase velocity and phase velocity /Vf, obtained from the time signals.
700.0
mMi 2&Q ;3oo.p; mm 4bo!o 4itea Soao 550:0 ebati
450,0
0:70
5pP.d
nM^M^
0;80
0;90
65fl.O
FdO;p
0.99
FIGURE
AA
inin42-Lb
42-Lb
Linerboard
(10
Time
ininµs.
0 waves
FIGURE9:
Top:Laser-generated
Laser-generatedA
42-LbLinerboard
Linerboard(10
(10and
and15
mmseparation).
separation).Time
Timein
µs.
0 waves
FIGURE
9:9: Top:
Top:
Laser-generated
in
and
1515mm
mm
separation).
jis.
0 waves
Bottom:
plots
ofof
the
phase
velocity
and
phase
velocity
/√f,
obtained
from
the
time
signals.
Bottom:
plots
the
phase
velocity
and
phase
velocity
/√f,
obtained
from
the
time
signals.
Bottom:
plots
of
the
phase
velocity
and
phase
velocity
/Vf,
obtained
from
the
time
signals.
On
OnFigure
Figure888we
weshow
showsignals
signalsobtained
obtainedfrom
froman
uncoatedwhite
whitemagazine
magazinepaper
paper
On
Figure
we
show
signals
obtained
from
ananuncoated
uncoated
white
magazine
paper
A
(grammage:
68.9
g/m^2).
The
signals
were
not
averaged
and
the
generation
was
(grammage:68.9
68.9g/m
g/m^2).
Thesignals
signalswere
werenot
notaveraged
averagedand
andthe
thegeneration
generationenergy
energywas
was
(grammage:
2). The
energy
A
5mJ/pulse.
Figure
9
shows
signals
obtained
on
brown
paperboard
(grammage:
205
g/m^2),
5mJ/pulse.Figure
Figure99shows
showssignals
signalsobtained
obtainedon
onbrown
brownpaperboard
paperboard(grammage:
(grammage:205
205g/m
g/m^2),
5mJ/pulse.
2),
aa avery
verystiff
stiffpaper
papertypically
typicallyused
usedto
manufacturecorrugated
corrugatedboxes.
boxes.Although
Althoughthe
thefrequency
frequency
very
stiff
paper
typically
used
totomanufacture
manufacture
corrugated
boxes.
Although
the
frequency
1670
range on which the fit is sufficiently good to be reliable, is a little narrower, the frequency
range still extends from 14 kHz till 673 kHz.
Statistical Results on Copy Paper
The statistical results were obtained with the laboratory instrument on the same magazine
paper mentioned earlier. Nine different locations along MD and 10 along CD across a
sample of 18 cm x 18 cm were investigated. The separation distances were 5 and 10 mm;
hence local variations were averaged over a 5 mm span. The energy was about 5 mJ per
pulse and the generation beam was focused to a point that was leaving visible marks on the
paper sample. The results are presented in Table 1. The coefficient of variation is
significantly higher for the shear rigidity than for the flexural rigidity. That is caused by the
fact this paper is relatively thin (<100 |iim) and not much of the overall compliance comes
from shear rigidity over the frequency range of the experiment. Hence a large variation of
the shear rigidity will not have much influence on the phase velocity curve. A similar study
is presented for copy paper in Table 2. In that case 11 different locations were tested along
CD and 14 along MD. The energy per pulse was 2.8 mJ and the signals resulted from 4
averages (4 laser shots) per location. CD and MD in-plane PZT are results coming from
narrowband contact ultrasonic transducers. They should not be exactly the same as LUS
results because the principle of measurements is different but should be close to the LUS
results and that is what we observed. For both paper grades above, experiments showed
that this is (within a few %) the true spatial variability of paper and not the variability of
the LUS method, we observed.
TABLE 1. Statistical results obtained with the laboratory instrument on Magazine paper.
Grade:
Magazine grade
68.9 g/mA2
Flexural
rigidity
(10-4N.m)
Orientation MD
Orientation CD
5.52
2.405
Flexural
rigidity
coef. of
variation
13.0 %
18.0%
Shear
Rigidity
(N/m)
22424
15979
Shear
Rigidity
coef. of
variation
23.1 %
38.2 %
MD/CD
flexural
rigidity
ratio
2.30
TABLE 2. Statistical results obtained with the laboratory instrument on copy paper.
Grade:
Copy paper
78.3 g/mA2
CD
in-plane PZT CD
MD
in-plane PZT MD
Flexural
rigidity
10'4Nm
2.80
2.96
5.57
5.92
Flexural
rigidity
coef. of var.
9.7 %
14.8%
Shear
Rigidity
N/m
19170
22800
22770
na
Shear
MD/CD
Flex. Rig.
Rigidity
coef. of var ratio
15%
14%
1.99
2.00
CONCLUSION
In this publication we have described a laboratory instrument utilizing a generation
laser and a laser interferometer for non-contact and broadband ultrasonic generation and
detection on materials. The instrument is used for the determination of flexural rigidity and
out of plane shear rigidity of paper and paperboard samples. It is fully automated. It uses
fiber optics for the delivery of generation and detection beams so as to allow more
1671
flexibility and versatility in the design and use. Preliminary results obtained with very
different grades of paper showed that the signal to noise ratio is sufficient. Moreover the
spatial variability of the mechanical properties of paper produced on a paper machine were
found to be within the expected values over a span of a few mm. Testing of the instrument
and software for instrument controls and end-user interface is proceeding. Future plans
include enclosing the laser areas of the instrument so as to make it a class I laser system.
ACKNOWLEDGEMENTS
This work is supported by the U.S. Dept. of Energy under contract DE-FC-07-97ID13578.
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1672