Adhesive bonding technology is rapidly gaining acceptance as an
alternative to spot welding. This technology is helping automobile
manufacturers reduce vehicle weight by letting them use lighter but
stronger advanced high strength steels (AHSS's). This can make
cars safer and more fuel efficient at the same time. The other
benefits of this technology include its flexibility, ability to
join dissimilar materials, distribute stress uniformly, provide
sealing characteristics and sound dampening, and provide a moisture
barrier, thus minimizing the chance for corrosion. The lap shear
work reported in the late 1980s and early 1990s has led to the
prevalent perception that the galvannealed (GA) coating can
delaminate from the steels, resulting in poor joint performance.
However, the above work was carried out on steels used primarily in
automobile outer body panels. The Dow Chemical Company and
ArcelorMittal collaborated on a joint project to carry out
comparative studies of bonding efficacies of fracture toughened
adhesives (FTA's) with conventional versus AHSS's. Three GA
steels, namely, extra deep drawing quality interstitial-free steel
(EDDS-IF) and two high strength steels, namely, DP780 and TRIP 780
were tested with two different adhesives, one FTA with a modulus of
~ 1500 MPa and another conventional adhesive with a modulus of ~
3500 MPa
The results of this joint study showed that the FTA, BETAMATE™
1496, exhibits significantly better energy absorption/impact load
than the conventional hem flange adhesive, BETAMATE™ 73305.
Whereas EDDS-IF steel led to coating delamination, the AHSS's
studied showed cohesive failure without any coating delamination.
This underscores the importance of an FTA during a crash
performance. Surface texture does not seem to be a factor in
bonding in these systems. The AHSS's can take advantage of the
adhesive bonding in automotive body structures to improve
structural integrity and crash performance. Results of lap shear,
impact peel, microscopy, as well as suitable analytical testing
will be presented and discussed.
This initial work primarily dealt with lap shear and impact peel
work carried out at ambient temperatures; it also included only a
limited number of steels and adhesives. Future work planned
includes GA steels covering a broader range of strengths as well as
adhesives of broader modulus range. Also in plans is high speed
impact work covering a broader temperature range following the
procedure outlined in ISO 11343.