Double cantilever beam (DCB) experiments

DCB specimens of different material systems were prepared by 3D printing. The material systems included epoxy-epoxy, epoxy-carbon fiber (CF) composites, and polymer-silicone interfaces. We 3D printed epoxy and its composites into DCB specimens using direct ink writing (DIW) method. The inks were prepared based on our previously reported literature⁵³. Briefly, epoxide resin (Epon 826®) was mixed with cross-linkers (Epikure W®, 25 wt%, and Epikure 3140®, 5 wt%), and silica nanoparticles (EH-5®, 13 wt%) using a planetary mixer. Carbon fibers (10 wt%) were added to the mixture to prepare the epoxy-CF composite ink. The as-prepared inks were 3D printed by DIW technique using a Hyrel 3D SR Engine® printer with a nozzle size of 0.3 mm, layer height of 0.15 mm, and a printing speed of 10 mm/s. After printing, the materials were thermally cured at 150 °C for 10 h. For polymer-silicone samples, we used stereolithography (SLA) to 3D print polymer beams (Formlab® Tough resin), which were then washed by isopropyl alcohol (IPA) and cured at 60 °C for 60 min. Then, a layer of silicone glue was applied at interface and cured at room temperature for at least 2 h. The polymer-silicone samples had a beam thickness of 5 mm, a beam width of 20 mm, a length of 100 mm and an initial crack length of 25 mm (ASTM D5528 standard). A scaling factor of 0.8 was used for epoxy and epoxy-CF samples to reduce the 3D printing time. Different material properties were considered in FEA validations, where E_pol = 1.5 GPa for SLA printed polymer beams, E_epoxy = 1.6 GPa and E_epoxy-CF = 2.2 GPa for DIW printed epoxy and epoxy-CF samples, respectively. DCB tests were performed via a universal tensile machine under displacement control (0.1 mm/s) up to 10 mm. F–D curves are collected at 10 pts/s.