Evaluating damage in adhesive joints of intraply hybrid composites under quasi-static tension

An experimental study is conducted to evaluate damage in adhesively bonded joints of intra-ply hybrid carbon/glass epoxy composites using their electro-mechanical response under quasi-static tensile loading. Three joint methods are examined: tapered lap (30°, 45°, and 60°), stepped lap (1-step and 3-step), and scarf joints (45° and 60°). All joints are bonded with a carbon-nanotube-enhanced epoxy adhesive. The electro-mechanical behavior measured using the four-circumferential-probe method is compared with that of a control composite without an adhesive joint. The control composite exhibited a distinct constant-stress stage accompanied by a sharp increase in piezo-resistance due to ply delamination under tensile loading. Among all joints, the tapered lap method showed the highest piezo-resistance sensitivity, with a peak resistance change of about 250%. The 45° scarf joint showed the lowest sensitivity, with only about an 8% change, but achieved the highest tensile strength of approximately 100 MPa. Scanning electron microscopy revealed that all joint types failed through a combination of adhesive and cohesive mechanisms.