COMPARATIVE PERFORMANCE EVALUATION OF TYPICAL STRUCTURAL EPOXY AND METHACRYLATE ADHESIVES AT

COMPARATIVE PERFORMANCE EVALUATION OF TYPICAL
STRUCTURAL EPOXY AND METHACRYLATE ADHESIVES AT
DIFFERENT LOADING CONDITIONS
M. Eskandarian1*, and PRÉCICAD INC. 2
1
NRC-Automotive & Surface Transportation, National Research Council Canada,
501 boul. de l'Université, Saguenay, Québec, Canada G7H 8C3
[email protected]
2
PRÉCICAD INC.,
350, boul. Charest Est, QC G1K 3H4
Introduction
Lightweight materials and related concepts have been
progressively focused on as an important design topic in
automotive industries. Lightweighting is also considered as
one of the key solutions to assist the overcoming of barriers to electrification. Global trends toward reduction of
greenhouse gas emissions and fuel economy have also
significantly increased the importance of this topic over
the past years.
Several methods for weight reduction have been recently
used among them: three most important approaches are the
use of (a) low-density materials like aluminum; magnesium, thermoset / thermoplastic composites (b) high-strength
materials like ultra-high-strength steel (UHSS), and (c)
integrated multimaterial structures [1]. Although aluminum has been identified as one of the most viable materials
for lightweighting, replacing automotive parts with aluminum requires development of innovative multimaterial
joining technologies. One good example of this is adhesive
bonding. A proper choice of adhesive for any particular
application is always the first step toward an efficient joint
design. Different adhesive formulations can be considered
as having specific mechanical properties and specific advantages and limitations. Experimental data is required at
various loading and environmental conditions to identify
the influencing parameters and to determine the adhesive
bond strength.
used to fabricate compact-tension (CT), single-lap-shear
(SLS) and double-lap-shear (DLS) specimens by using
previously developed assembling jigs [2] as shown in Figure 1. The thickness of aluminum bars met the requirements of the ASTM standards D1062, D2002 and D3528,
which were 15/15 mm, 3.2/3.2 mm and 3.2/6.4 mm for
CT, SLS and DLS specimens, respectively. An overlap
length of about 25 mm was used for the three types of
joints.
Fig. 1: Fabrication of CT (left) and DLS (right) specimens
Results and Discussion
Figure 2 shows the variations of failure loads as a function
of bondline thickness for the CT and DLS joints bonded by
the epoxy adhesive. The maximum performance of this
adhesive was achieved at a thickness of about 0.5 mm and
0.1 mm under cleavage and shear loads, respectively.
Epoxy adhesive
Experimental
In the context of the present study, the mechanical
performance of two typical structural adhesives, selected
among the epoxy and methacrylate families, has been assessed under both static and dynamic loads. Both adhesives were bi-component and therefore have been cured at
an ambient temperature. The epoxy adhesive was almost
two times more rigid than the acrylic adhesive. A simple
and user-friendly surface treatment technique was adapted
for each adhesive system in order to ensure cohesive failure and also to minimize the environmental performance
loss in an aged condition. AA6061-T6 aluminum bars were
Fig. 2: Variation of failure load as a function of bondline
thickness for the epoxy adhesive
For the acrylic adhesive, on the other hand, the failure load
was less sensitive to the variation of bondline thickness
when tested under cleavage loading (CT). As shown in
Figure 3, the epoxy adhesive had a better mechanical performance under static loading, having a failure load of 3
and 1.5 times higher under cleavage and shear loads. After
careful observation of the failure surfaces, it noted that a
cohesive failure was occurred for all the tested specimens.
joint efficiency, this S-N curve is also presented in term of
the variation of substrate stress as a function of number of
load cycles as presented in Figure 5. As shown, some of
the specimens were failed in the substrate at a stress level
lower than the ultimate fatigue strength of aluminum 6061T6 that is about 100 MPa in a completely reverse cyclic
test up to 5×108 cycles (asterisk marked points). The stress
concentration at joint ends provokes this premature failure.
100
Stress variation in the substrates (MPa)
Acrylic adhesive
80
Acrylic
60
Epoxy
40
20
0
10,000
Acrylic - failed in substrate
Acrylic - failed in adhesive layer
Epoxy - failed in substrate
Epoxy - failed in adhesive layer
Epoxy - No failure
100,000
1,000,000
10,000,000
100,000,000
Number of cycle
Fig. 3: Variation of failure load as a function of bondline
thickness for the acrylic adhesive
Fig. 5: S-N Curves for the epoxy and the acrylic adhesives
in term of substrate stress variation
Following the monotonic tests, fatigue behavior of both
adhesives has also been evaluated under various levels of
sinusoidal loads. First, a series of SLS specimens were
fabricated from aluminum bars of 3.2 mm (T) x 25 mm
(W) and were tested under fatigue loads at pristine and
aged conditions. An overlap of 13 mm, a frequency of 60
Hz and a load ratio (minimum load divided by maximum
load in each load cycle) of -1 were used in this study. Figure 4 shows the S-N curves obtained for the tested specimens bonded by the acrylic and epoxy adhesives.
Conclusions
Sress variation in the adhesive layer (MPa)
25
20
It has been concluded from this study that the mechanical performance evaluation of an adhesive joint should not
be limited to monotonic tests. Depending on the application, other types of mechanical tests (fatigue, creep, relaxation and durability tests, etc) must also be done to ensure
an appropriate adhesive selection. In the case of this study,
the selected acrylic adhesive had a better fatigue strength
despite its inferior static strength when compared to the
selected epoxy adhesive. Furthermore, the acrylic adhesive
was significantly less sensitive to the variation of adhesive
bondline thickness under cleavage load, which is a critical
load case for an adhesive joint.
Acrylic
Acknowledgements
15
Epoxy
The authors wish to acknowledge Pierre Dion, General Manager of Précicad and all the other staff of Précicad
and the Aluminium Technology Centre who had a contribution to this study; in particular, Myriam Poliquin, Sandy
Laplante, Christian Savaria, Genevieve Simard, Michel
Perron and Amélie Ruest for their technical assistances.
10
Acrylic
5
Epoxy
Epoxy - No failure
0
10,000
100,000
1,000,000
10,000,000
100,000,000
Number of cycle
Fig. 4: S-N Curves for the epoxy and the acrylic adhesives
in term of adhesive stress variation
It can be seen from Figure 4 that the acrylic adhesive had a
better fatigue strength during the SLS tests despite its lower static strength. It seems that this adhesive, with a lower
elastic modulus, had a better capacity in redistributing
shear and cleavage stresses in the adhesive layer and in the
substrates. In order to have a better understanding of the
References
1.
2.
DOW Automotive Publication, “Structural Bonding
of Lightweight Cars; Crash durable, safe and economical”.
Eskandarian M., Jennings R.M. & Morneau M-A,
“Fatigue behavior of adhesively bonded aluminum
joints at different testing conditions”, Proceedings of
the Adhesion Society Meeting 2013, Daytona Beach
FL.