Damage monitoring of pinned hybrid composite–titanium joints using direct current electrical resistance measurement

The present research addresses structural health monitoring of pinned, composite–titanium (i.e.: hybrid) joints with the aim of using their lightweight potential and damage tolerance in future aircraft designs. Together with additively manufactured titanium pins, protruding into the carbon-fiber composite, a single-lap shear joint specimen is monitored with direct current electrical resistance measurements (DC ERM) across the overlap, without conductivity-enhancing additives (e.g., carbon nanotubes), but rather with the pins’ complex electrical network that forms with the carbon-fiber composite. For a proof-of-concept demonstration, a structural test with quasi-static, tension–tension loading and unloading is performed. Using digital image correlation, degradation of the joint is monitored. Results are validated by a 2-dimensional finite element model, considering multiple damage states. For DC ERM, a damage indicator is proposed to evaluate the joint’s structural condition. It is shown that typical damage for this joint type reported literature (i.e., cracks occurring at the overlap ends) could be reproduced and detected by the electrical property change across the overlap. Under the given laboratory conditions, the proposed DC ERM damage indicator clearly shows a non-reversible increase in resistance by 3.8% due to damage, starting at first damage initiation and also reflecting further damage growth. Thereby, the method’s capability for damage detection and monitoring is demonstrated.