Effect of post-cure on adhesively bonded functionally graded joints by induction heating

Functionally graded joints with an adhesive functionally modified by induction heating confer a more uniform stress distribution along the overlap and reduce the stress concentrations located at the ends of the overlap. The adhesive stiffness varies gradually along the overlap, being maximum in the middle and minimum at the ends of the overlap. This work studies the effect of post-curing on functionally graded joints obtained by induction heating. The performance of functionally graded joints, when submitted to different post-cure temperatures, was experimentally tested. Three different post-curing conditions were considered, with temperatures above and below the glass transition temperature of the fully cured network, T-g. The functionally graded joints (with and without post-cure) were compared with joints cured isothermally (with and without post-cure). The cure temperature values applied to the ends and to the middle of the graded joint are the same temperatures used to cure the isothermally cured joints. Analytical modelling was used to predict the failure load and to assess the effectiveness of the graded joint concept. The functionally graded joints subjected to post-cure at low temperatures (below T-g) show a slight decrease of the strength and the joints cured isothermally show a slight increase of the strength. With increase of the post-cure temperature (above T-g) the functionally graded joints exhibit strength similar to that of the joints cured isothermally. However, even for the highest post-cure temperatures, the functionally graded joints have a slightly higher strength.