Impedance analysis for condition monitoring of single lap CNT-epoxy adhesive joint

Multi-Walled Carbon NanoTubes (MWCNT) are ideally suited to be used for damage sensing in fiber reinforced composite materials, and are used for structural health monitoring in adhesive joints. In this work, the capability of MWCNTs for condition monitoring of a single lap Al-Al adhesive joints (SLJ) under shear load is studied using impedance measurements. Different weight percent of MWCNT, i.e. 2.5, 6 and 9 wt.% are added to epoxy adhesive. Epoxy adhesive containing 9 wt.% MWCNT is identified during shear loading to have the highest ultimate strength among the considered specimens and provide the best sensory properties. To provide a more concise evaluation of the damage progression in the adhesive layer under shear stress, artificial defects are inserted into the SLJ specimens. The effects of square and circular defects on the damage progression in the adhesive layer are studied. The results show that primary microcracks are initiated at and consequently develop from the defect boundary in the adhesive joints with larger size of defect. It was found that the shape of the defect does not have a significant effect on the impedance response and sharp resistance changes for specimens with circular and square defect areas of 70% overlap area were about 4.55 and 4.2% respectively. Additionally, when the crack grows entirely through the adhesive layer, a nonzero slope of the resistance change resulted even for low levels of shear displacement.