JEC COMPOSITES MAGAZINE - Issue #91 - August/September 2014 - 54

hybrid structure

Fig.4: Top view images and cross sections of AP1, AP2, BP1 and BP2 hybrid
joints

Fig.5: Cross section of two types of CFRP sheets, adhesive-bonded specimen and three different placements

in Figure 5. The cross sections perpendicular and parallel to
the direction of the tensile shear test are respectively named
T and L sections. In the optical micrographs, the dark round
zones between the A2017P and CFRP sheets are microvoids generated in the adhesive bonding process. Delamination between different layers can be observed in the T
section of the AP3 joint. In the case of the BP3 hybrid joint,
in addition to the occurrence of delamination, the CFRP
laminates were severely damaged. Therefore, the cross-sectional observation confirms that the use of a dummy sheet
in the embossing process improves the formability of the
CFRP sheet effectively.

The results of tensile shear tests on the adhesive-bonded joints
and the AP1, AP2, AP3, BP1, BP2 and BP3 adhesive/embossed
hybrid joints are shown in Figure 6(a) and (b). During the
adhesive bonding process, some micro-voids were randomly
generated in the epoxy adhesive and located between the CFRP
and A2017P sheets, which led to a slight instability of the
adhesive-bonding tensile shear load. Therefore, the experiment
was conducted five times on each joint to mitigate the inevitable
instability of adhesive bonding. From Figure 6(a) and (b), it is
clear that the AP2 and BP2 hybrid joints have higher maximum
tensile shear load, maximum displacement and absorption
energy than the other joints. The joint efficiency factors of the
AP2 and BP2 hybrid joints are 34.4% and 43.4%, respectively.
In contrast to the results of the AP2 and BP2 joints, the tensile
shear load and displacement of the AP3 and BP3 joints did
not show a significant increase compared to the case of normal
adhesive bonding. In addition, after reaching the peak load, the
tensile shear load of the BP3 joint exhibited a gradual decrease,
which is a characteristic of fibre de-bonding, fibre fracture and
fibre pullout. The improved joining quality after the optimal
warm embossing processes is mainly attributed to the mechanical anchoring effect of the embossed pit, the expansion of the
adhesive area, the concentration of adhesive at the edge of the
pit and the heating procedure, which has been experimentally
verified in an article published by the authors (http://dx.doi.
org/10.1016/j.jmatprotec.2013.04.015).
The failure modes of CFRP/metal single-lap joints were
categorized as: a) metal and adhesive interfacial debonding, b)
cohesive failure in the adhesive layer, c) CFRP and adhesive
interfacial debonding, d) CFRP delamination, e) metal yielding
and f) CFRP tensile rupture. After the AP2 and BP2 embossing processes, the failure of CFRP/A2017P single-lap joints
changed from the c mode to the a-b-c coexisting mode. For the
BP3 hybrid joint, the fracture of the CFRP sheet occurred earlier than the complete adhesive failure. In contrast to the BP2
hybrid joint, during the tensile shear test, the damage did not
propagate along the interface of the two adherends but mainly
occurred in the CFRP adherend. The failure mode of the BP2
joint was interfacial debonding-cohesive failure coexistence,
while the failure mode of the BP3 joint was dominant CFRP
tensile rupture fracture.
Through the optimization of the embossing experimental sets
discussed above, the joining quality of the AP2 and BP2 hybrid
joints was verified to be the highest. The tensile shear loads of
the optimal adhesive/embossed hybrid joints were compared
with those recorded in the case of adhesive bonding, rivet
joining and adhesive/rivet hybrid joining for A2017P/CFRP
(A-type) and A2017P/CFRP (B-type), as respectively shown
in Figure7 (a) and (b). The diameter of the applied rivet was 4.0
mm, and the adhesive area of the adhesive/rivet hybrid joint
was the same as that of the adhesive-bonded joint. Figure 7 indi-

54 jec composites magazine / No91 August - September 2014

http://www.dx.doi.org/10.1016/j.jmatprotec.2013.04.015 http://www.dx.doi.org/10.1016/j.jmatprotec.2013.04.015

JEC COMPOSITES MAGAZINE - Issue #91 - August/September 2014

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #91 - August/September 2014