ABSTRACT The effects of the environment on adhesive bonded single lap joints formed using XMC-3 and SMC-R50 composites were investigated. Tests were performed at temperatures of 23C and 93C with test coupons immersed in air, water, and 5% NaClwater mixture. The weight changes of both bonded joints (XMC-3 to SMC-R50 and SMC-R50 to SMC-R50) and unbonded materials were measured. Data were also obtained showing the effects of moisture, temperature, and applied load on changes in weight, on creep deformation, lap shear strength, and separation modes of the joints

George S Springer

Michigan Ann Arbor

Tony K Wang

CiteSeerX

212
Creep, Strength and Moisture Absorption
of Adhesive Bonded FRP Joints*
GEORGE S. SPRINGER &dagger;
Department of Mechanical Engineering and Applied Mechanics
The University of Michigan
Ann Arbor, Michigan 48109
TONY K. WANG
GM Manufacturing Development
General Motors Technical Center
Warren, Michigan 48090
(Received May 25, 1984)
ABSTRACT
The effects of the environment on adhesive bonded single lap joints formed using
XMC-3 and SMC-R50 composites were investigated. Tests were performed at
temperatures of 23C and 93C with test coupons immersed in air, water, and 5% NaCl-
water mixture. The weight changes of both bonded joints (XMC-3 to SMC-R50 and
SMC-R50 to SMC-R50) and unbonded materials were measured. Data were also ob-
tained showing the effects of moisture, temperature, and applied load on changes in
weight, on creep deformation, lap shear strength, and separation modes of the joints.
1. INTRODUCTION
THE USE OF LIGHTWEIGHT FIBER REINFORCED PLASTICS (FRP) IN AUTO-motive applications offers a promising avenue towards achieving better
fuel economy through weight reduction. In order to take full advantage of
such materials, suitable methods must be found for joining them together.
Both mechanical fastening and adhesive bonding may be used to join two
FRP materials. Adhesive bonding is the often preferred method because it is
more economical, allows more design flexibility, and eliminates the need for
holes and cutouts which may result in undesirable stress concentrations.
Wide-spread application of adhesively bonded FRP joints requires a
knowledge of the response of such joints to automotive environments.
Recently, Wang, Sanders, and Lindholm [1] reported data on the lap shear
strength, lap shear modulus, and fatigue strength of single lap joints exposed
*Presented at the 29th SAMPE Meeting.
tPresent address: Dept. of Aeronautics and Astronautics, Stanford University, Stanford, CA
94305.
Journal Of REINFORCED PLASTICS AND COMPOSITES, Vol. 4-April 1985
0731-6844/85/02 0212-14 $4.50/0
@ 1985 Technomic Publishing Co., Inc.
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213
to automotive-related fluids. In the present investigation further data were
generated shedding light on the environmental response of adhesive bonded
single lap joints. Specifically, data were obtained on a) the moisture absorp-
tion characteristics of unbonded materials and single lap joints immersed in
different fluids, b) the creep behavior of such joints under environmental ex-
posure, and c) the changes in lap shear strength due to the combined effects
of load, moisture, and temperature. Tests were performed with XMC-3
bonded to SMC-R50 and with SMC-R50 bonded to SMC-R50. Brief descrip-
tions of these materials are given in Table 1. The adhesive is specified in the
next section. The following parameters were measured during the tests: a)
weight change as a function of time, b) creep deformation as a function of
time, and c) residual lap shear strength. The measurements were performed
with test coupons (specimens) exposed to air, water, and 5 % NaCI-water
mixture at 23C and 93C.
The experimental procedures are described in the next section.
2. EXPERIMENTAL
Two types of adhesive bonded coupons were used in the tests, the dif-
ferences in the coupons being only in their dimensions (Figure 1). The
Table 1. Material formulation.
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214
Figure 1. Descripubn of test coupons.
&dquo;small&dquo; coupons consisted of two 25.4 mm wide and 25.4 mm long
adherends bonded together over the entire 25.4 mm2 surface area. The
&dquo;large&dquo; coupons consisted of 25.4 mm wide and 102 mm long adherends
bonded with an overlap length of 25.4 mm. The thicknesses of the adherends
were nearly the same for both coupons, having the nominal values of 2.5 mm
and 2.9 mm for the XMC-3 and SMC-R50, respectively. The adhesive was
Goodyear Pliogrip 6040/6041, ranging in thickness from 0.6 to 1 mm. A
modified isocyanate base prepolymer (Pliogrip 6040) was mixed with a polyol
base curative in a 3:1 ratio by weight. Goodyear Pliogrip 6036 primer, con-
sisting of 2.5 percent isocyanate, 2.5 percent color dye, and 95 percent
methylene chloride, was wiped on the bonded areas before the adhesive was
applied. Additional information regarding the properties of this adhesive
may be found in Reference 1.
In addition to bonded coupons, simple unbonded (i.e. no adhesive) XMC-3
and SMC-R50 specimens were also used in the tests. The widths and lengths
of these were the same as those of the &dquo;small&dquo; bonded coupons (25.4 mm).
The &dquo;small&dquo; bonded and unbonded coupons were used only for measuring
moisture absorption characteristics. The &dquo;large&dquo; coupons were used in the
creep tests and in the lap shear strength measurements. Weight changes of
&dquo;large&dquo; coupons were also recorded after 30 days of environmental exposure.
Before placing the coupons into the environmental chambers for the moisture
absorption studies or before mounting them on the creep frames, the coupons
were dried in an oven at 65C. During drying, the weights of the specimens
were monitored. Drying was concluded when the weight loss became stabi-
lized and no more weight loss was observed.
Moisture absorption studies were conducted by exposing the small coupons
to the appropriate environmental conditions and by weighing them
periodically on a Mettler Analytical Balance. The coupons were exposed to a)
humid air, b) distilled water, and c) 5% NaCl-distilled water solutions. At
23C and 50 percent relative humidity the coupons were kept in an air condi-
tioned, temperature controlled room. At 93C (air) the coupons were placed in
a heated chamber with the temperature regulated within ± 1 C. The moisture
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content of the 93C air corresponded to 50 percent relative humidity at 23C.
Accordingly, at 93C the air was practically dry, the relative humidity being
about 0.1 percent. The distilled water and salt water were kept in temperature
controlled chambers. The temperatures of these chambers were maintained
within ± 1 C.
In addition to the small coupons, large coupons were also exposed to the
above fluids. The weights of the large coupons were not recorded period-
ically, but only after 30 days’ exposure.
The data are presented in terms of the percent weight change which is
defined as
Upon receiving the coupons, the &dquo;base line&dquo; (&dquo;as received&dquo;) lap shear
strength was measured with a Baldwin Universal Tester using a 5337 N/min
loading rate. During the 93C tests the test section was surrounded by a heater.
The baseline data thus obtained are given in Table 2.
Creep tests were performed as follows. After drying, the weight of each
coupon and its dimensions were recorded. The coupon was then mounted on
the creep frame. The applied load was either 10, 20, or 30 percent of the
baseline strength given in Table 2. During the creep tests the coupons were ex-
posed to the appropriate environment. For tests in 23C air at 50 percent
relative humidity the creep frames were located in an air conditioned,
temperature controlled room. For tests in 93C air, the coupons were sur-
rounded with a heater. At 93C, the relative humidity of the air around the
coupons was practically zero. Creep tests were also performed with coupons
immersed in distilled water and in 5 % NaCl-distilled water solutions. These
liquids were kept in a heated container surrounding the test coupons. The
temperature of the liquid was kept within ± 1 C.
During creep tests the deformation of the bonded joint was measured with
a modified Martens Optical Extensometer mounted at the two edges of the
joints. Thus, the extensometer recorded the deformation of the bonded sec-
tion of the coupon. The initial deformation on loading was recorded. The
deformation as a function of time was measured over a 30 day period, or
until the coupon failed, whichever came first.
Table 2. Baseline f&dquo;as received&dquo;) lap shear strengths of
adhesive bonded single lap joints.
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Coupons surviving the 30 day creep tests were weighed and their lap shear
strengths (residual strengths) were measured at 23C with a Baldwin Universal
Tester using a 5337 N/min loading rate. The residual strengths of large
coupons immersed for 30 days in different fluids (but not loaded) were also
measured.
3. RESULTS
The data on moisture absorption characteristics, creep behavior, and
residual strengths are presented in this section.
3.1 Moisture Absorption Characteristics
The results of the moisture absorption tests are shown in Figures 2-5. Each
point in these figures is the average of five data points. In general all five
points were within ± 15 percent.
At 23C both bonded (XMC-3 to SMC-R50 and SMC-R50 to SMC-R50)
and unbonded (XMC-3 and SMC-R50) coupons seemed to approach asymp-
totically the same maximum moisture content (Mm) when immersed in the
same fluid. During the two month test period Mm was reached only in air. In
water and in the 5 % NaCI-water mixture the maximum moisture contents
were not attained. However, the Mm values can be estimated by extrapolating
the data, giving 0.18, 1.5, and 2.0 percent for air, 5% NaCI-water, and water,
respectively.
flguie 2. Weight change as a function of immersion time of XMC and SMC-R50 (no adhesive).
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Figure 3. Weight change as a function of immersion time of adhesive bonded single lap joints.
Fgure 4. Weight change as a function of immersion time of XMC- and SMC-R50 (no adhesive).
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Figure 5. Weight change as a function of immersion time of adhesive bonded single lap joints.
At 93C (immersion in water) the maximum moisture levels were not ap-
proached asymptotically (Figures 4,5). Here the weight increased for about
the first 100 hours, and then decreased at a rapid rate. After 20 to 30 days the
weight of the coupons became actually less than the dry weight. This indicates
that the material deteriorated and resin was lost during the exposure. Similar
behavior has been observed previously with different types of SMC materials
[2,3]. The fact that both the bonded and unbonded coupons behaved
similarly at 93C suggests that the degradation was mostly in the material
(XMC-3 and SMC-R50) and was not caused by the adhesive.
The rate of weight change depends on the thickness of the material.
Although the thicknesses of unbonded XMC-3 and SMC-R50 materials
were slightly different, the data in Figure 2 indicate that, under the same con-
ditions, moisture was gained by both materials at nearly the same rates. This
observation, and the fact that the maximum moisture levels were also the
same, implies that the moisture transport characteristics of unbonded XMC-3
and SMC-R50 materials are similar.
The bonded coupons were thicker than the unbonded ones. Therefore, the
weight changes in Figures 2 and 3 can not be compared directly.
The effects of load, fluid, and temperature on the moisture transport in
adhesive bonded single lap joints are illustrated in Table 3. The results in this
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219
table again show that the types of adherends forming the bonds did not cause
appreciable differences in the weight change. Furthermore, up to 20 percent
(of baseline) load, the applied load did not significantly increase the moisture
uptake. Even with an applied load of 30 percent above the baseline value the
weight increase was small. More significant are the weight changes caused by
the different types of fluids and by differences in temperature. The data in
Table 3 are consistent with those in Figures 2-5. After 715 hours of exposure,
at 23C the weight change is lowest in air and highest in water. At 93C the
weight changes in air were small, manifesting materials degradation during
the 715 hours of exposure, in agreement with the trend observed with water at
93C (Figure 5). The data for immersion of SMC-R50 to SMC-R50 bonded
joints in water at 93C do not show such low values of weight change. The
limited data do not offer a safisfactory explanation for this phenomenon.
3.2 Creep and Residual Strength
The creep deformations are presented in Figures 6-10. The deformations
upon loading and the numerical values of the deformations over a 715 hour
period are listed in Reference [4].
Table 3. The effects of load, fluid, and temperature on moisture
transport in adhesive bonded single lap joints.
N.S. No coupon survived 715 hours.
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220
Rgure 6. Creep deformation as a function of time. Load levels indicated are percent of baseline
value. Solid circle indicates test coupon failed
Hgru~ 7. Creep deformation as a function of time. Load levels indicated as percent of baseline
value. ,
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221
Figures Creep deformation as a function of time. Load levels indicated are percent of baseline
value. Solid circles indicate test coupon failed.
fi8ur~e 9. Creep deformation as a function of time. Load levels indicated are percent of baseline
value Solid circles indicate test coupon failed.
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222
Figure 10. Creep deformation as a function of time. Load levels indicated are percent of baseline
value.
Figures 6-10 illustrate the effects of material, fluid, temperature, and ap-
plied load on the creep behavior. The type of adherends used in forming the
bonded joints (XMC-3 to SMC-R50 or SMC-R50 to SMC-R50) did not have
a significant influence on the creep. The type of fluid, the temperature, and
the applied load strongly affected the creep. The creep was lowest in air, and
was higher in water and in salt water. The creep also increased with
temperature and with applied load. At 23C none of the joints separated in
any of the fluids. At 93C only one of the joints separated. This occurred at a
30 percent load level. However, during water immersion (93C) all but 3 joints
separated before the end of 715 hours. The smallest deformation at which
separation occurred was about 0.8 percent.
The results in Figures 6-10 also show that there was a sharp increase in the
deformation rate after about 100 hours. The increase occurred somewhat
sooner at higher loads.
The changes in lap shear strength due to load and environmental exposure
are summarized in Table 4 and in Figures 11 and 12. The lap shear strength
decreased appreciably (by about 50%) only in water and in 5 % NaCI-water at
93C. At all other conditions the lap shear strength was unchanged, or even in-
creased slightly. Improvements in lap shear strength of SMC materials ex-
posed to various fluids were also observed by Wang et. al [1]. They attributed
the beneficial effects of fluids to plasticization.
An inspection of the coupons revealed that the joints separated either due
to delamination of the composite, due to separation along the interface, or
due to a combination of both (Table 5). Generally, delamination was the
major cause of separation when the coupons were exposed to 23C and 93C
humid air. At 23C delamination was also the cause of separation for coupons
immersed in water and in 5% NaCI-water. At 93C the separation along the
interface occurred first when the coupons were immersed in water and in 5 %
NaCI-water.
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223
Table 4. Changes in lap shear strengths of loaded and unloaded, adhesive
bonded single lap joints after 715 hours of environmental exposures.
Data shown are the ratio of the average residual strength (S) to the
baseline strength (SBJ given in Table 2.
N.S. No coupon survived 715 hours.
4. SUMMARY
The following general conclusions can be reached on the basis of the data
generated in this program.
1. The moisture absorption characteristics of XMC-3 and SMC-R50
materials are nearly the same.
2. At 23C both bonded joints and the unbonded coupons asymptotically ap-
proached maximum saturation levels, the values of these levels being 0.18,
1.6, and 2.0 percent, respectively for air (50% r.h.), water, and 5% NaCl-
water.
3. At 93C the weight changes of both bonded joints and unbonded coupons
increased and were followed by a decrease, indicating material degrada-
tion and subsequent material loss.
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224
Figure 11. The residual strength (S) of loaded and unloaded, adhesive bonded single lap joints
after 715 hours of environmental exposures. SB is the baseline strength given in Table 2. Shaded
areas represent spread in data. N. S. indicates no coupon survived 715 hours.
4. The applied load had only a small and in most cases negligible effect upon
the weight changes of adhesive bonded single lap joints.
5. The creep deformation of adhesive bonded single lap joints was influenced
by the applied load, the fluid, and the temperature, but was independent
of the adherends used in forming the joints.
6. The rate of deformation increased significantly after 100 hours.
7. The coupons survived 715 hours of creep testing under all conditions,
Figure 12. The residual strength (S) of loaded and unloaded, adhesive bonded single lap joints
after 715 hours of environmental exposures. SB is the baseline strength given in Table 2. Shaded
areas represent spread in data. N. S. indicates no coupon survived 715 hours.
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225
Table 5. Separation modes of adhesive bonded single lap joints
at various test conditions.
except in water at 93C, where nearly all joints separated before the end of
the 715 hours creep test. The lowest deformation at which separation
occurred was about 0.8 percent.
8. The lap shear strength of the adhesive bonded single lap joints did not
change appreciably during the 715 hours of load and environmental ex-
posure, except in 93C water and in 93C 5% NaCI-water mixture. Under
these two conditions the lap shear strength decreased by about 50 percent.
9. In general, joints separated due to delamination when they were exposed
to humid air at 23C and 93C or immersed in water and in 5% NaCI-water
at 23C. Separation occurred along the interface for coupons immersed in
93C water and in 93C NaCI-water.
ACKNOWLEDGEMENT
A version of this paper was presented at the 29th meeting of the Society for
the Advancement of Material and Process Engineering.
REFERENCES
1. Wang, T. K., Sanders, B. A. and Lindholm, U. S., "A Loading Rate and Environmental Ef-
fects Study of Adhesive Bonded Single Lap Joints," GMMD Report 80-044, GM Manufactur-
ing Development, General Motors Technical Center, Warren, Michigan 48090.
2. Loos, A. C., Springer, G. S., Sanders, B. A. and Tung, R. W., "Moisture Absorption of
Polyester-E Glass Composites," Journal of Composite Materials, 14, pp. 142-154 (1980).
3. Springer, G. S., Sanders, B. A. and Tung, R. W., "Environmental Effects on Glass Fiber
Reinforced Polyester and Vinylester Composites," Journal of Composite Materials, 14, pp.
213-232 (1980).
4. Springer, G. S., "Basic Properties Characterization of Adhesive Bonded Single Lap Joints,"
Final Report to GM Manufacturing Development, General Motors Technical Center, Warren,
MI 48090 (February 1981).
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Creep, Strength and Moisture Absorption of Adhesive Bonded FRP Joints*

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Creep, Strength and Moisture Absorption of Adhesive Bonded FRP Joints*

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