Structural health monitoring of defective single lap adhesive joints using graphene nanoplatelets

Adhesive joints are becoming increasingly important in engineering, especially in the field of mobile applications such as automotive and aircraft. As done in recent studies, Functionalized graphene nanoplatelets (f-GNPs) additives are a good candidate for improving the mechanical strength of the adhesives. Additionally, adhesives containing f-GNPs also exhibit a piezoresistive effect, i.e. the electrical resistance changes when subjected to mechanical stress. Consequently, they are ideally suited to be used for damage sensing in fibre-reinforced composite materials, and adhesive joints. In this work, the capability of f-GNPs for structural health monitoring and reinforcement of a defective single lap Al-Al adhesive joints under shear load is studied using the electrical impedance measurement. In order to study damage progression in the adhesive layer under shear stress, several artificial defects in circular and square shapes with different sizes are inserted into the adhesive joints. The effects of defects on the damage progression in the adhesive layer are studied by a correlation between fracture surface and impedance changes. The results showed that f-GNP adhesive is sensitive to deformation and damage extension in the adhesive layer. The ultimate strength has improved about 74 % by incorporating the 12 wt.% f-GNP into the adhesive joint without embedded defect. The maximum impedance changes (Delta R/R) were obtained almost 60 % close to the failure point for defective adhesive joint comprised of a square defect shape and 50 % defect area. This phenomenon was observed when the fracture occurred from the interface between the adhesive layer and the Al substrate.