Abstract
Adhesive joints have many advantages over joining methods involving fasteners
and are getting more and more attention in the high-performance sports cars
segment. This work investigated two structural, safety-critical bond applications
in sports cars. While the focus was on automotive applications, the findings apply
to other high-volume CFRP transport applications, such as marine and rail.
Part I investigated adhesion between aluminium inserts and CFRP epoxy
structures, which can be impacted by an internal mould release (IMR) agent used
to accelerate the manufacturing process of the CFRP component and a high
temperature (HT) processing step exceeding 200°C to which the inserts are
exposed during manufacturing. Three coatings, silane-based primer (SP),
cataphoretic epoxy-based electrocoat (CC & CCB (blasted)) and epoxy-based
primer (EPB (blasted)), were investigated to provide strong adhesion between
the insert and the epoxy resin. Mechanical properties were tested with single lap
joint (SLJ), double cantilever beam (DCB) and wedge tests (WT). Loci of failure
were investigated by optical microscopy and X-ray photoelectron spectroscopy.
SP was found to be sensitive to IMR and HT, resulting in decreased joint strength.
The IMR did not impact CC, CCB and EPB. However, HT reduced the joint
strength of CC and CCB samples but increased it in the EPB samples.
Part II investigated the load rate sensitivity of fracture toughness (Mode I) of a
crash-resistant adhesive, from quasi-static rates (double cantilever beam (DCB)
and wedge double cantilever beam (WDCB) tests) on a screw-driven machine to
high loading rates (WDCB tests) prepared on a new, cost-effective, pendulum-based
testing rig. Compared to traditional DCB, the quasi-static WDCB samples
showed more sensitivity to geometrical imperfections, resulting in what appeared
as stiffness variation between the samples. The critical energy release rate
dropped with an increased loading rate. Over the entire loading rate from
1.7 x 10-5 m/s to 12 m/s, the critical energy release rate decreased by 80%. The
work shows the importance of high-loading-rate testing in designing safe
structures and illustrates the use of a cost-effective, high-speed testing rig for
DCB samples.