ULTRASONIC METHODOLOGY TO CHARACTERIZE THE STATE
OF CURE
Wonsiri Punura^and Laurence J. Jacobs1
School of Civil and Environmental Engineering, Georgia Institute of Technology,
Atlanta, GA 30332-0355
ABSTRACT. As a polymer (like a thermoset resin) cures from liquid to solid, the cross-link density
increases and this change is accompanied by a significant production of heat and an increase in
material stiffness. The ability to predict the value of material stiffness at discrete times during the
process of curing would be helpful in process monitoring. This research examines an epoxy-amine
system during cure, and makes an in situ characterization using embedded piezoelectric (PZT) chip
sensors. A new concept of using ultrasonic-time cure-temperature measurement is proposed for
predicting the modulus at any given time during cure and at any given level of curing temperature.
The reliable results obtained with this method may enable the quantitative characterization of the
properties of the more complex state of cure in the cement-based materials.
INTRODUCTION
The cure process plays a key role in determining the properties of a thermoset
material. The way in which the property changes occur during the cure such as an increase
in stiffness, significant heat production and a decrease in volume, determines the
material's final structure. The properties of a material may be influenced by the cure cycle
through process-induced residual stresses or defects in the form of voids, micro cracks and
delamination.
A literature review reveals a considerable amount of work in the use of acoustic
waves in different polymer material applications. Among the most promising are
dielectric, spectroscopic, and ultrasonic techniques [1]. The primary advantage of using
ultrasound for process monitoring is based on the fact that the propagation of an acoustic
wave is sensitive to the macroscopic material structure as well as the mechanical properties
of the material. A number of researchers have correlated broadband ultrasonic
measurements (in the 2-20MHz range) of attenuation and phase velocity, to the viscosity
of the liquid, and the elastic moduli [2]. Some researchers have measured the degree of
cure, as an epoxy hardens [3]. Other researchers have shown the potential of using acoustic
techniques for defect detection (such as voids and porosity) in a polymer matrix composite
after processing [4]. Taken together, these studies show the utility of using piezoelectric
transducers for the generation and reception of ultrasound to monitor cure.
Some restrictions arise when applying piezoelectric transducers to monitor the
cure of epoxy. In particular, non-insulated transducers have an upper working temperature
range of about 60°C, which can present difficulties when trying to make continuous
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/$20.00
1119
ultrasonic measurements in the specimen's environment. Another factor that must be
considered when performing measurements using piezoelectric transducers is that the
resulting measurement is relatively localized, restricted to the thickness insonified by the
ultrasonic wave; this can be an issue when interrogating bulky composite components.
This research develops an innovative technique using an embedded ultrasonic
micro sensor. The sensor is made of a piezoelectric ceramic chip material to offer
temperature stability. This sensor is also thin enough, so that it can be embedded inside
the material, providing an interior measure of the state of cure. This embedded chip sensor
does not disrupt the structural properties or the cure of the material. A conventional
piezoelectric transducer is used to generate ultrasound while the embedded piezoelectric
chip sensor is used for detection. Changes in wave velocity, frequency-dependent
attenuation, temperature and amplitude of an elastic wave indicate in situ physical property
changes occurring in the material. This paper presents some preliminary results made with
this sensor configuration.
EXPERIMENTAL PROCEDURE
To explore the development of cross-link density as well as the performance of the
embedded sensor, a piezoelectric ceramic chip, 2 mm long, 3 mm wide and 0.5 mm in
thickness is pre-assembled to respond to displacements or stresses in the thickness
direction and placed inside the experimental cell (see Fig. 1). The process involves
mounting the lead zirconate titanate element (PZT) and making an electrical connection.
The electrical connection is a strip cut from a 3mil brass foil. The brass strip provides a flat
contact with the bar element, and is convenient for applying a thin layer of conductive
epoxy (ACE steel-filled epoxy) to form a solid connection. An electrical wire cable is then
soldered to the brass strip. A standard BNC adapter is assembled to the other end of the
cable for connection with the other electronics. Given the thickness selected, this PZT chip
covers a frequency range of 20 kHz to 2 MHZ.
Epoxy 301 is selected as a matrix for composite materials for this study. Epoxy 301
is based on the mixture of epichlorohydrin and Bisphenol A. Trimethyl Hexanediamine is
selected as the curing agent. The mixing ratio is four parts of epoxy to one part of curing
agent by weight. Both materials are manufactured by Epoxy Technology, Inc., Billerica,
Massachusetts.
Figure 1 shows a diagram of an ultrasonic system. The testing device consists of a
steel frame in which the rectangular tubing test cell is placed inside, and axially aligned
with respect to the transducers. A standard Wavetek 50 MHz Pulse/Function generator is
used to generate a 1 MHz transient pulse. Two wideband ultrasonic transducers are
clamped and aligned parallel to the 5-mm sidewalls of the test cell. One emits the
compressional pulse, which is sent through the epoxy where it is detected by the embedded
micro sensor. Another transducer is employed as a receiver and is input to a digital
oscilloscope controlled by the system personal computer (PC) using a general-purpose
interface bus (GPffi).
The experimental cell was filled with the prepared epoxy mixture. Special care was
taken to eliminate possible void formation in the prepared epoxy mixture. The
measurements were performed. The typical duration of measurement is completed after
three hours with a data acquisition interval of 1 minute.
1120
L1
L2
Propagation
Distance:
Propagation
L1Distance:
=
11.50
L2 =
mm.
11.50 mm.
mm.
50.00
PZT Chip
50.00
mm.
Wave
Generator
Thermocouple
Data Logger
Oscilloscope
Personal Computer
Signal
Processing
FIGURE
1. The
setupsetup
of the
measurement
FIGURE
1. schematic
The schematic
of ultrasonic-temperature
the ultrasonic-temperature
measurementofofthe
theepoxy
epoxy cure
cure specimen.
specimen.
For For
the temperature
measurements,
a small
the temperature
measurements,
a smallthermocouple
thermocouplewire
wire isis inserted
inserted through
through
the of
topthe
of test
the test
connected
a datalogger
loggersosothat
thatany
anytemperature
temperature changes
the top
cellcell
andand
connected
to to
a data
changes due
due
to the
exothermic
reaction
duringthethecuring
curingprocess
process are
are continuously
continuously monitored.
monitored.
to the
exothermic
reaction
during
Temperature
is taken
simultaneously
with
equalintervals
intervalsofof1-min
1-minusing
using the
the computer
computer
Temperature
datadata
is taken
simultaneously
with
equal
a time
reference.
experimentsareareconducted
conductedatat room
room temperature
temperature and
and at
clockclock
as aastime
reference.
AllAll
experiments
at
atmospheric
pressure.
atmospheric pressure.
RESULTS
RESULTS
The exothermic chemical reaction of the epoxy-matrix results in the change of
The exothermic
reaction of
theto epoxy-matrix
in the changethe
of
material
state from a chemical
viscous suspension
fluid
a solid. Duringresults
this phase-transition,
material
state
from
a
viscous
suspension
fluid
to
a
solid.
During
this
phase-transition,
the
wave velocity, the amplitude, and the frequency spectrum of an elastic wave also change.
waveThese
velocity,
the amplitude,
and the correlated
frequencytospectrum
of an
elastic
wave also change.
parameters
can be indirectly
the stiffness
of the
material.
These parameters
indirectly correlated
of the
Duringcan
thebe
experiment,
the changestointhethestiffness
amplitude
as material.
well as the drift of the
During
theis experiment,
theoscilloscope
changes inscreen
the amplitude
as well
as theminutes;
drift of this
the
arrival
signal
observed on the
on a time scale
of several
arrival
signal is
on the
oscilloscope
a time
scale of
minutes;
this
is shown
in observed
Figure 2. The
amount
of signalscreen
drift ison
also
calculated
by several
computing
the crossis shown
in Figure
2. The
amountthe
of generated
signal drift
is also
calculated
by computing
the crosscorrelation
function
between
and
detected
ultrasonic
signals captured
over
the curing
age [5].
The value
the shift for
thedetected
peak (positive
or negative
correlation
function
between
theofgenerated
and
ultrasonic
signalsdepending
captured upon
over
the polarity
of The
the value
signal of
and
replica)
in peak
cross-correlation
delay ofupon
the
the curing
age [5].
theitsshift
for the
(positive or indicates
negative the
depending
travel time.
Knowing
travel
path of the
the wave velocity
results
theof
onset
the polarity
of the
signalthe
and
its replica)
in waves,
cross-correlation
indicates
thefrom
delay
the
of Knowing
the signalsthe
cantravel
also bepath
computed.
traveltime
time.
of the waves, the wave velocity results from the onset
usingcan
the also
Fast-Fourier
Transform (FFT) technique, the frequency content of the
time of theBy
signals
be computed.
signal
can
also
be
extracted.
The
phase velocity
attenuation
at selected
By using the Fast-Fourier Transform
(FFT) and
technique,
thecoefficient
frequencydata
content
of the
frequencies
are
evaluated
from
the
phase
spectrum
of
the
transmitted
signal.
This
method
signal can also be extracted. The phase velocity and attenuation coefficient data at selected
follows the technique proposed by Sache and Pao [6]. Figure 3 shows a typical phase
frequencies are evaluated from the phase spectrum of the transmitted signal. This method
velocity plot at 1MHZ versus the temperature variation as a function of curing time. The
follows
theplot
technique
by Sache
and Pao
[6]. of
Figure
3 shows
typical phase
same
could be proposed
obtained after
converting
the delay
the travel
time acomputed
from
velocity
plot
at
1MHZ
versus
the
temperature
variation
as
a
function
of
curing
time. The
the cross correlation procedure.
same plot could be obtained after converting the delay of the travel time computed from
the cross correlation procedure.
1121
175
175
150
150
(12
(12
.
Curing
Curing
TimeTime
(min)(min)
125
125
100
100
75
75
point)
point)
50
50
25
.
25
0
0
0
0
0.2
0.2
0.4
0.4
0.6
0.6
0.8
1
0.8Time
1
(sec)
1.2
1.4
1.6
1.8
1.2
1.4
1.6
1.8
-5
Ax I10
U
Time (sec)
(sec)
Time
_.
x 10
.
-5
FIGURE 2. Waterfall plot of the compressional waves at increasing curing times captured by embedded PZT
chip.
FIGURE
at increasing
increasing curing
curing times
times captured
captured by
byembedded
embeddedPZT
PZT
FIGURE 22.. Waterfall
Waterfall plot
plot of
of the
the compressional
compressional waves
waves at
chip.
chip.
1800
140
1800
140
1600
120
120
120
1?
o
1400
100
1400
100
1200
80
1200
1200
80
80
1000
Temperature
(celcius)
Temperature
(celcius)
Phase
Velocity
(m/s)
Phase
Velocity
(m/s)
1600
*
o
1000
60 1
60
60 2.
800
40
800
600
0
600 0
0
40
20
20
20
40
40
40
60
60
60
80
80
80
100
100
100
120
120
120
Curing Tim e (m in)
Curing Tim
Time
(min)
Curing
e (m
in)
140
140
140
160
160
160
180
180
180
20
200
20
200
200
FIGURE 3. The phase velocity of the compressional waves at 1MHz versus temperature and time of curing.
FIGURE 33.. The
Thephase
phase velocity
velocity of
of the
the compressional
compressional waves
waves at
at 1MHz
1MHz versus
FIGURE
versus temperature
temperature and
and time
time of
of curing.
curing.
As
parameter that
that
Ascan
canbe
beseen
seenin
inFigures
Figures222and
and3,
3,the
thephase
phase velocity
velocity is
is an
an ultrasonic
ultrasonic
As
can
be
seen
in
Figures
and
3,
the
phase
velocity
is
an
ultrasonic parameter
parameter
that
isisquite
sensitive
to
the
cure
state
of
thermosetting
polymers.
The
simultaneous
increase
in
quite sensitive
sensitive to
to the
the cure
cure state
state of
of thermosetting
thermosetting polymers.
polymers. The
The simultaneous
simultaneous increase
in
is quite
increase
in
the
phase
velocity
does
not
occur
in
the
epoxy-matrix
as
it
hardens
from
a
viscous
fluid
to
the phase
phase velocity
velocity does
does not
not occur
occur in
in the
the epoxy-matrix
epoxy-matrix as
as itit hardens
hardens from
aa viscous
fluid
to
from
viscous
fluid
to
athe
glassy
solid
as
found
in
other
studies,
instead
there
exists
a
gradual
linear
decrease
in
the
aglassy
glassy solid
solid as
asfound
found in
inother
other studies,
studies, instead
instead there
there exists
exists aa gradual
gradual linear
linear decrease
decrease in
the
a
in
the
velocity,
to
velocity,followed
followed by
by the
the sudden
sudden dip
dip during
during the
the middle
middle of
of the
the cure
cure corresponding
corresponding
to
velocity,
followed
by
the
sudden
dip
during
the
middle
of
the
cure
corresponding
to
temperature
variation
peak,
and
finally
increasing
to
the
very
much
higher
value.
temperaturevariation
variation peak,
peak, and
and finally
finally increasing
increasing to
to the
the very
very much
higher
value.
temperature
much
higher
value.
The
is
Theattenuation
attenuationcoefficient
coefficient at
at1MHz
1MHzisisisalso
alsocomputed
computed from
from the
the same
same data.
data. This
This
is
The
attenuation
coefficient
at
1MHz
also
computed
from
the
same
data.
This is
shown
in
Figure
5.
Again,
the
peak
is
correspondent
to
the
highest
temperature
of
120°C.
shown
in
Figure
5.
Again,
the
peak
is
correspondent
to
the
highest
temperature
of
120°C.
shown in Figure 5. Again, the peak is correspondent to the highest temperature of 120°C.
1122
Attenuation
AttenuationCoefficient
Coefficient at
at11MHz
MHz
1 .3
1 .2 5
Attenuation, NEP / mm
1 .2
1 .1 5
UJ
z
c1 . 11.1
o
**
*
1
1£. 01.05
5
1
0 .9 5
0 .9
0
20
20
40
40
60
60
80
80
100
100
120
120
C u r i n g Time (min)
140
140
160
160
180
180
200
200
C u rin g T im e (m in )
FIGURE 4. The epoxy attenuation plot as a function of the curing time represented at 1 MHz.
FIGURE 4. The epoxy attenuation plot as a function of the curing time represented at 1 MHz.
DISCUSSION
DISCUSSION
So far quantitative results are obtained by the use of an innovative embedded
So far quantitative results are obtained by the use of an innovative embedded
sensor technique, however their interpretation is quite complicated by two factors: (a) the
sensor
technique,
however
their interpretation
is quite
complicated
by two
factors:
(a) the
exothermic
nature
of the polymerization
and the
evolution
of cross-link
density
in relation
exothermic
nature
of
the
polymerization
and
the
evolution
of
cross-link
density
in
relation
to the change from liquid to solid reactions which raises the temperature in the epoxy; and
to (b)
the the
change
fromthe
liquid
to resin
solid isreactions
which
raises
the temperature
in the epoxy;
and
fact that
epoxy
viscoelastic
while
hardening,
and this causes
the change
(b)inthe
that the epoxy
resin
is viscoelastic
while hardening,
this causes
the change
thefact
mechanical
motion.
This
forms the connection
that theand
stiffening
process
of the
in polymer
the mechanical
motion.
the parameters:
connection ultrasonic
that the stiffening
process of and
the
is sensitive
to the This
three forms
important
velocity, attenuation,
polymer
is
sensitive
to
the
three
important
parameters:
ultrasonic
velocity,
attenuation,
and
temperature. The laboratory experiments provide the direct measurements of velocity and
temperature.
thethat
direct
measurements
of velocity
and
attenuation The
as alaboratory
function ofexperiments
temperature;provide
however
cannot
quantify what
mechanisms
attenuation
as a function
temperature;
however
cannot
quantify
what mechanisms
are responsible
for the of
observed
changes.
In thisthat
regard,
a wave
propagation
theory is
arebeing
responsible
forinthe
developed
thisobserved
study. changes. In this regard, a wave propagation theory is
being developed
thisthe
study.
Inspiredinby
work of Biot in 1956 [7] where the simplest model for wave
Inspired inbythe
the fluid-filled
work of Biot
in 1956
the simplest
modelconsiders
for wavea
propagation
porous
media[7]iswhere
developed,
this study
propagation
in
the
fluid-filled
porous
media
is
developed,
this
study
considers
a
viscoelastic medium such as the cured epoxy as a two-component system consisting of
viscoelastic
such as
thecoupling
cured epoxy
as athe
two-component
liquid andmedium
solid phases.
The
between
heat equation system
and the consisting
equations of
of
liquid
andfor
solid
The
between
the heat
equationIt and
the equations
of
motion
the phases.
fluid and
thecoupling
elastic solid
are being
considered.
is noted
here that the
motion
for the is
fluid
and the
solid are being
considered.
noted here
thatwave
the
temperature
allowed
to elastic
vary throughout
the medium
as It
a is
variable
of the
description.isThe
appropriate
boundaries
are also
temperature
allowed
to vary
throughout
theapplied.
medium as a variable of the wave
the appropriate
developmentboundaries
to follow, are
the constraint
of linearity in both displacements and
description.InThe
also applied.
velocities
play an important
meansofthat
any motions
are assumed toand
be
In thewill
development
to follow,role.
the This
constraint
linearity
in both displacements
small sowill
thatplay
the linear
theory can
applied.
If the
solution
is assumed,
velocities
an important
role.beThis
means
thatplane
any wave
motions
are assumed
to bea
solution
is possible
in form
dispersion
relations
forplane
compression
and shearis waves.
This
small
so that
the linear
theoryofcan
be applied.
If the
wave solution
assumed,
a
relationship
will enable
of the relations
mechanicsformechanisms
such
phasewaves.
velocityThis
and
solution
is possible
in forminclusion
of dispersion
compression
andasshear
attenuation,
asenable
a function
of frequency
or temperature,
to be included
in such
a wayand
not
relationship
will
inclusion
of the mechanics
mechanisms
such as phase
velocity
currently available.
attenuation,
as a function of frequency or temperature, to be included in such a way not
currently available.
1123
From this study, it is also interesting to compare these results with the observed
quantities found in some other references [8,9] where the ultrasonic measurements are
performed on the three oil field hydrating cement slurries over a period of 24 hours. In
these studies, an increase in the compression wave speed that accompanies the setting of
the cement paste is observed. In fact, their findings are consistent with the results
discussed earlier in connection with Figure 2, 3 and 4. Some fundamental questions that
need explanation: How could the two materials, having different material composition
however involving with the same vitrication process, give similar responses when they
subjected to ultrasound? Is it because of the evolution of the cross-link density only results
in the lag of the wave velocity? Or is it because of the change of the temperature upon the
kinetic of phase change that reflects the true mechanism of phase change?
CONCLUSIONS
The process of cure inside the epoxy resin has been monitored using an embedded
piezoelectric chip sensor. The ultrasonic signal phase velocity, attenuation as well as
temperature are monitored as the epoxy cured from a viscous fluid to a glassy solid state.
The high-temperature resistance of the embedded PZT chip provides an advantage when
used to determine the state of cure over a confined region in the interior of the structure.
ACKNOWLEDGEMENTS
We would like to thank Epoxy Technology, Inc. and Veriteq Instruments, Inc. for
their equipment donations.
REFERENCES
1. Marvin, R.S. and McKinney, J.E., Physical Acoustics, Vol. 2, Academic Press, Inc.,
New York, Pt. B, 165(1965).
2. Challis, R.E. and Freemantle, R.J., Meas.Sci.Technol. 9, 1291(1998).
3. Winfree, W.P. and Parker, R.F., Review of Progressive in Quantitative Nondestructive
Evaluation, Vol.5B, Plenum Press, New York, 1055-1061(1985).
4. Heller, K., Jacobs, L.J., NDT&E International, 8, Vol.33, 555 (2000).
5. Aussel, J.D. and Monchlin, J. P., Ultrasonics, 27, Vol. 5, 165 (1989).
6. Sachse, W. and Pao, Y.H., /. AppL Phys. 49, 4320 (1978).
7. Biot, M.A., /. Acoustic. Soc. Am. 28, 168 (1956).
8. Boumiz, A. Vernet, C. and Tenoudji, F.C., Adv. Cem. Bas. Mat. 3, 94 (1996)
9. Sayers, C.M and Dahlin, A., Adv. Cem. Bas. Mat. 1, 12 (1993)
1124