Abstract
The long term durability of CFRP strengthened steel structures is a key parameter for
their safe use and effective design. Strengthened members can be subjected to different
environmental conditions and loading scenarios during their service life, the effect of
which on the failure mechanism of the strengthened members require fundamental
investigations.
This research presents experimental and theoretical investigations on the effects of
freeze-thaw cycling and wet thermo-mechanical loading on the bond strength and
failure mode of steel-CFRP adhesive joints. The influence of these conditions on the
mechanical properties of pure epoxy is also investigated to give insight into the
structural behaviour and also provide data for the theoretical model. The results show
that the freeze-thaw cycling decreases the bond strength of the joint by about 28% and
leads to variations in the failure mode. A reduction in the initial elastic modulus of the
pure epoxy was also observed.
The results of the wet thermo-cyclic exposure combined with sustained loading show
that these conditions have little impact on the bonding strength when applied separately.
However, when applied simultaneously, the mechanical properties of the epoxy and the
bond strength of the joints are significantly reduced with failure observed at less than
30% of the static strength and at the temperature range well below the glass transition
temperature of the adhesive.
A simple shear stress-slip model is developed to analyse single-lap joints, in which the
effects of freeze-thaw and thermal cycling are introduced in terms of reduced elastic
modulus of the adhesive. The predicted failure load correlates well with the
experimental data. A more accurate high order model is also developed that is combined
with fracture mechanics and can account for the shear deformability of the adhesive
layer and considers normal interfacial stresses. A new methodology to evaluate the
effects of freeze-thaw cycling is proposed, which considers the effect of moisture
swelling, thermal expansion and change in material properties. Finally, parametric
studies are conducted and the results are presented in terms of bond strength, interfacial
stresses and deformations.