Crack sensing mechanisms of Mode-II and skin-stringer joints between dissimilar materials by using carbon nanotubes

Abstract The mechanical and crack sensing capabilities of CFRP-metal Mode-II coupons and skin-stringer T-joints under bending conditions with carbon nanotube (CNT) doped adhesive films are explored. Different surface treatments of the aluminium substrate are tested (grit blasting and sandpaper brushing). Mode-II energy fracture slightly increases with CNT addition due to the nanoparticle toughening effect when the surface treatment is adequate. However, in the case of brushed specimens, the enhanced lubricant properties of CNTs leads to the promotion of weak interfaces between adhesive and metal substrate. The electrical resistance increases with the load nose displacement with a good correspondence to the crack length. A crack opening effect is herein noticed in the last stages of the Mode-II tests due to the difference between the stiffness of metal and CFRP adherents, which is reflected in a sudden increase of the electrical resistance. These results have been also validated by theoretical analysis. Electromechanical response of T-joints shows a similar behaviour with an increasing sensitivity when placing a CFRP stringer, due to a more prevalent crack opening observed in this type of specimens. Therefore, crack sensing capabilities are demonstrated at both coupon and sub-element level.