THE DEVELOPMENT OF SHEAR AND COMPRESSION ELASTIC
MODULI IN CURING EPOXY ADHESIVES MEASURED USING
NON-CONTACT ULTRASONIC TRANSDUCERS
S. Dixon, D. Jaques, C. Edwards and S.B.Palmer
University of Warwick, Department of Physics, Coventry CV4 7AL, UK
ABSTRACT. - Thin epoxy resin adhesive samples were ultrasonically measured using normal
incidence radially polarised shear wave ElectroMagnetic Acoustic Transducers (EMATs) that also
generate a significant compression wave. This allows us to make simultaneous measurement of the
shear and compression wave propagation through the polymer, although the signal to noise ratio is
significantly higher for the shear wave measurements. The adhesive thickness examined in the
experiments was 1mm, which was chosen to be the optimum thickness for experimental measurement
using our apparatus. The epoxy resin systems ('rapid' cure and a 'standard' cure time) described in
this paper were supplied in a 2-part cartridge form, mixed by through a special nozzle. Although the
number of samples investigated at this stage is small, there does appear to be a fundamental difference
in the way in which the elastic moduli develop as cure rate is increased.
INTRODUCTION
In addition to the potential industrial applications of monitoring adhesive cure for
process and quality control there is good reason to investigate the fundamental science
behind the cure process. Epoxy adhesives have been used for over SOyears, and our
understanding of the cure process itself has lagged behind the proliferation in the use of
such adhesives in joining technologies. Ultrasonic measurements are an attractive
measurement technique as they can be readily implemented into a production environment
to yield the elastic longitudinal and/or shear moduli [1-5]. The shear modulus is the most
sensitive to the physical changes within the adhesive and it is important to obtain an
accurate measurement of this. Using the technique that we describe here the shear modulus
can be measured to a higher accuracy than is available with alternative methods, whilst
compression wave measurements can simultaneously be made.
Normal incidence shear waves are difficult to generate and receive in a contacting setup, requiring piezoelectric transducers to be bonded to the sample. The extra complication
of another set of adhesive bonds (transducer-sample) makes calibration of the system more
difficult and adds another source of systematic measurement error. EMATs [6-10] however
can be used easily to generate normal incidence shear waves at the surface of an
aluminium sample and are non-contact devices which eliminates any potential problems
with coupling the transducers to the metal adherents.
The ultimate aim of this work is to try and relate ultrasonic measurements to structural
developments on a microscopic scale, which will require extensive experimentation in a
number of supporting techniques including NMR, DSC and X-ray analysis.
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
1049
EXPERIMENTAL DETAILS
Two identical radially polarised shear wave EMATs [4] were used in a through
transmission geometry. The EMATs were held directly opposite each other either side of
two 22mm thick aluminium plates as shown in figure 1. The magnets within each EMAT
were oppositely poled which lightly clamped them to the bonded sample, increased the
magnetic field normal to the plates and hence also increased the EMAT generation /
detection efficiency. The EMATs had been designed to yield a significant and measurable
compression wave component that is actually generated by the in-plane shear type force at
the sample surface within the electromagnetic skindepth. Figure 2 shows the a typical
EMAT waveform detected after the adhesive is fully cured. Note that the arrival that
occurs between the direct compression and shear wave is a compression wave
reverberation within the aluminium adherent.
The aluminium plates were selected for their relatively low acoustic birefringence [1113]. A simple computer model was used to predict the arrival times of the first few
significant bulk wave (and mode converted) arrivals through the joint for a range of
adhesive velocities that would typically be measured during cure. The adherent thickness
was chosen such that the shear wave signal that propagated directly from generator to
detector was clearly separated from other possible ultrasonic arrivals. Thermocouple were
used to take temperature measurements of the adhesive and the adherents and the entire
apparatus was placed into a thermally insulated box.
The system was initially calibrated so that the transit times of the shear and
compression waves through the epoxy layer could be measured in isolation as described in
a previous publication [14]. Using this data it was possible to determine the theoretical
arrival time of an ultrasonic bulk wave through an infinitesimal adhesive layer if we
consistently measure the same feature which in this case was the position of the bulk wave
pulse maximum. This approach takes account any trigger delays in the system and removes
the ambiguity of 'what feature on the ultrasonic signal corresponds to the arrival time1.
Five different 2 part epoxy adhesives were examined, 2 'standard' cure time adhesives
from the araldite range (A2011 & A2013), 2 'rapid' cure time adhesives again from the
araldite range (A2012 & A2017) and another 'rapid' cure adhesive from the Permabond
range (E01) - denoted by PB01 here.
€m
EMAT
mm)
"Spacer
\alyminiuni
plate (22mm).
10
12
14
16
(|4$)
FIGURE 1. Set-up for ambient temperature
through transmission measurements.
FIGURE 2.
EMAT through transmission
waveform for a cured epoxy.
1050
RESULTS
After propagating through the cured adhesive the dominant frequency of the EMAT bulk
generated bulk waves is approximately 3.5MHz and there is significant frequency content
to around 10MHz. In the work presented here we have performed simple measurement of
bulk wave amplitude and velocity (transit time) ignoring any frequency dependence.
Attenuation coefficients were calculated using the measured velocities and assuming
constant density for the epoxy (negligible shrinkage).
Attenuation Measurements
As the epoxy cures it develops a shear rigidity and at some point is able to support shear
waves. The shear wave attenuation coefficients for the 'standard' (A2011 & A2013) and
'rapid' cure time adhesives (A2012, A2017 & E01) are shown in figures 3&4 respectively.
The 'rapid' cure adhesives appear to have an shear wave attenuation coefficient that
initially falls at a high rate, but then suddenly changes and continues to fall at a much
reduced rate. This is shown most clearly in the sample A2012 that is able to support shear
waves at the earliest time. The smoothed compression wave attenuation coefficients are
shown in figure 5 for samples A2011 & A2013, and in figure 6 for samples A2012,A2017
and PB01. The peak in the compression wave attenuation coefficients is clearly visible in
figure 5, and also appears to be present in figure 6, but is less convincing due to the poor
signal to noise ratio in the smoothed data.
^ 40-
'E
I
T~
A2013
A2011
30
40-
A2012
A2017
PB01
ol 30-|
| 20^o
o 201
u 10-
I 1
CO
O^r-^0.0
0-r
0
20
40
60
80
0.5
time (hours)
attenuation (dBmm" )
o-
i i i i i i i i 11 i i 1
40
1.5
2.0
FIGURE 4.
Shear wave attenuation
coefficients for 'rapid' cure time epoxies..
FIGURE 3.
Shear wave attenuation
coefficients for 'standard' cure time epoxies.
20
1.0
time (hours)
60
80
time (hours)
FIGURE 5. Compression wave attenuation
coefficients for 'standard' cure time epoxies.
A
6-
A2Q12
—— A2017
\
4••«........„„.„.„„.......' •«-H,,.^'«.^
2-
0.0
0.5
1.0
1.5
2.
time (hours)
FIGURE 6. Compression wave attenuation
coefficients for 'rapid' cure time epoxies..
1051
Velocity Measurements
In general it is generally more reliable to measure a velocity than an amplitude as
velocity measurements can be less prone to measurement error. The shear wave velocities
for the 'standard' (A2011 & A2013) and 'rapid' cure time adhesives (A2012, A2017 &
E01) are shown in figures 7&8 respectively. The compression wave velocities for the
'standard' (A2011 & A2013) and 'rapid' cure time adhesives (A2012, A2017 & E01) are
shown in figures 9&10 respectively. Note that the velocities shown in figure 8 show
sudden changes in gradient at the same times as similar features were observed in the shear
wave attenuation data of figure 4. The compression wave velocities of figure 10 also
exhibit rapid changes in gradient that are consistent in time with similar changes that are
seen in the shear wave data. This should not be too surprising as the bulk wave modulus is
linearly proportional to the shear wave modulus in isotropic solids. Close examination of
the ultrasonic data also shows that the epoxies start to support a shear wave some arbitrary
time after (but close to) the maximum that are observed in the compression wave
attenuation coefficients.
0
20
1.0
time (hours)
40
time (hours)
1.5
FIGURE 8. Shear wave velocities of the
'rapid' cure time epoxies.
FIGURE 7. Shear wave velocities of the
'standard' cure time epoxies.
20
30
time (hours)
40
60
time (hours)
FIGURE 10. Compression wave velocities of
the 'rapid' cure time epoxies.
FIGURE 9. Shear wave velocities of the
'standard' cure time epoxies.
1052
Temperature Variation and Stability During Cure
The experimental set-up can have a significant effect on the results of the ultrasonic
measurements, particularly on the temperature and thickness of adhesive. As these
thermoset adhesives cure they release heat as the cure process is an exothermic reaction.
This heat can increase the reaction rate of the cure process reducing cure time and also
dynamically changing the nature of the reaction kinetics taking place. Thus the way in
which the adhesive is contained has a significant effect, and we have used large aluminum
discs that have a relatively large thermal mass to try and remove heat from the epoxy and
maintain a relatively uniform temperature. This works most effectively for thinner epoxy
layers, and in general thicker layers will tend to cure more quickly as the rate of heat loss
per unit volume is reduced in line with the surface area to volume ratio of the sample. To
illustrate the effect we cured Icm3 of epoxy in a spherical latex membrane in air and the
same volume between the aluminium adherents at 1mm thick. The temperature of the
epoxy was measured using a thermocouple in the centre of each sample and the results are
shown in figure 11. Note that the 'air-cooled' sample reaches a much higher temperature
and reaches its peak temperature much more quickly than the sample between the
aluminium adherents. The rapid decrease in temperature after the peak indicates that the
bulk of the exothermic reactions have taken place, but the adhesive does still undergo
further reactions at a much reduced rate after the peak.
In addition to changing the reaction kinetics the temperature change in the epoxy can
have significant effects on the elastic properties of the adhesive. If we consider the small
variation in temperature seen in our case (0.6 °C), we can show that even temperature
changes of this magnitude are likely to effect the ultrasonic data. To illustrate this the
through transmitted shear wave amplitude recorded on sample A2017 is shown together
with the time varying temperature of the adherent in figure 12. The temperature of the
adherent is cyclical on a 24hr cycle, but does not change by more than a degree of so.
Examining the cure times between 20-40 hours, the through transmitted amplitude is
observed to vary by over 15% for a temperature variation of less than 0.5 °C. This
illustrates that it is important to accurately control the temperature, and consider the effect
of how the adhesive is contained during cure as this also has an effect of the temperature of
the epoxy.
C°C)
r25.5
— between Al
air' cooled 80
5
10
15
20
10
time (mins)
20
30
40
50
80
time (hours)
FIGURE 11. Temperature of curing epoxy
in 'air' cooled membrane and between
alumiunium adherents.
FIGURE 12. Through transmitted shear
wave amplitude of curing epoxy (A2017) and
ambient temperature of adherents.
1053
DISCUSSION AND CONCLUSIONS
The most important result in this paper is that we have demonstrated a system in which
normal incidence broadband shear and compression waves can be generated and detected
simultaneously using EMATs. The compression wave component is smaller in amplitude
than the shear wave components and thus the compression wave measurements have a
lower signahnoise ratio. Nevertheless the amplitude is still sufficient to permit reasonably
accurate attenuation measurements and highly accurate velocity measurements.
We have previously shown [14] that the adhesive from a single cartridge can have final
cured state mechanical properties dependent on order of extrusion. In this paper we have
also demonstrated that small variations in temperature of the epoxy system can lead to
relatively large variations in the elastic properties of the epoxy. Thus it is not only
important to have stable temperature control during the later stages of cure, but the way in
which the adhesive is contained (including its thickness) can dramatically effect the
measured elastic properties during the initial stages of cure. In this respect 'standard' cure
time adhesives that do not exhibit significant initial exothermic rises in temperature are
likely to yield less variation in measured properties between different experimental set-ups.
ACKNOWLEDGEMENTS
We would like to acknowledge the EPSRC (UK) for funding this work through an
Advanced Fellowship.
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