Shear Stress Monitoring of a Single-Lap Joint Using Inkjet-Printed Carbon Nanotube Strain Distribution Sensor

Adhesive bonding is a major structural joining technique used for both metallic and fiber-reinforced (FRP) composite structures in the aerospace and automotive industries. Instead of mechanical fasteners, which induce stress concentration near the bolting holes (in FRPs higher than in isotropic materials), adhesive bonds offer distributed integration of different structural parts. Adhesive bonds are particularly inclined by lightweight design due to their reduced weight and resistance to corrosion. However, the failure mechanism of an adhesive bond in practice can be complex due to various sources of influence, such as the quality of adhesives, surface preparation of adherends, process parameters and geometry of the joints, etc. Moreover, it is challenging to interrogate over the bonding area due to its sandwiched structure. In this study an inkjet-printed carbon nanotube (CNT) strain distribution sensing film is embedded at the interface between an adherend and the adhesive in a single-lap joint configuration, measuring the in-plane strain distribution over the area during a shear loading test. The conductivity distribution reconstructed by an algorithm of electrical impedance tomography (EIT) results in similar trend to the strain distribution from the stress analysis model. Moreover, the EIT result of a defect-embedded specimen shows a different conductivity distribution comparing to a “healthy” one, indicating the existence and approximate location of the debonded spot.