Interfacial fatigue performance of hybrid titanium to composite joints reinforced with 3D printed pins

Titanium micro-pins have been reported to enhance the modes I and II interlaminar fracture toughness of titanium-to-composite joints. In this study, we examined the enhancement of modes I and II interlaminar fatigue crack growth resistance of titanium-to-composite joints using 3D printed titanium micro-pins created using selective laser melting (SLM). The joints were formed by printing an orthogonal array of thin (1.0 mm diameter) titanium micro-pins over the titanium substrate using SLM, which were then embedded into a carbon-epoxy composite substrate to create a micro-pin reinforced bondline interface. Interlaminar fatigue experiments were conducted under cyclic displacement control conditions using the Double Cantilever Beam (DCB) and End Notch Flexure (ENF) test methods. Under modes I and II cyclic loading, pinned joints exhibited significantly higher strain energy release rates for equivalent crack growth rates due to the micro-pins forming a crack bridging zone behind the crack tip, enhancing fatigue crack growth resistance compared to unpinned joints. The micro-pins increased the modes I and II cyclic critical strain energy release rate value by 18-fold and 4-fold, respectively. A single micro-pin cyclic testing was also performed to investigate the efficacy of the SLM Ti-pins for generating bridging traction loads under fatigue loading. The fatigue test results are presented together with fractographic evidence of the fatigue strengthening and toughening mechanisms.