Firstly, a manufacturing plan was developed. It was determined that all coupons will be manufactured using the TC1225 prepreg with T700 fiber provided by Toray. Even though the original plan included welding of individual coupons, prior to the DAF installation, out of other welding programs it was found that the TC1225 with T700 fibers did not respond well to inductive heating, leading to poor weld quality. Thus, the decision was made to produce the coupons via co-consolidation rather than to induction welding.
Once the test plan was finalized, the consolidation of the laminates that would be used to produce the coupons was initiated. Plates of 300 mm x 300 mm dimensions were consolidated with a 0.025 mm-thick polyimide (Kapton) insert that was previously coated with release agent LOCTITE® FREKOTE 700NC, in the appropriate position to produce the DCB, ENF, SLS, and CLS coupon configurations. The co-consolidation procedure was undertaken in a hot platen press at 350 ℃ at 6 bars for 20 min, 7 ℃/min heating and cooling rate. Finally, the individual coupons were cut to the final dimensions table with a water-cooled circular diamond saw, with their longitudinal direction parallel to the 0 ° fibers.
A series of preliminary numerical simulations were performed for quasi-static tests to select the proper modeling techniques for the 2 DAFs. Manufacturing of sample coupons for testing with the two types of DAF was carried out. In parallel, the industrialized ILSFSR installation method was being further developed. The Refill Friction Stir Spot Welding (RFSSW) process was assessed as well as a DAF approach.
The ILSFSR feature was installed in 4 CLS specimens for evaluation purposes, while four different metallic rivet geometries were considered. The final design of the ILSFSR rivet to be used as DAF was selected upon testing and numerical evaluation for its crack arresting efficiency and out-of-plane loading capabilities. Regarding the RFSSW approach, a rotating tool with a 9 mm diameter was utilized, resulting in a 9 mm spot. Another significant parameter was the distance of the potential DAF's edge from the debonding initiation point, which was set at 10 mm.
Selection of all the parameters for the coupon scale testing was held. For DAF testing and validation numerous tests were performed on specimens in accordance with the test matrix. The data obtained with these tests were used for the numerical simulations.
Moreover, fatigue testing was held for the same type of specimens to study the crack growth behavior along the co-consolidated interfaces and evaluate the DAFs’ efficiency in more realistic operating conditions.
The fatigue loading tests were simulated with a numerical model, which was developed within the framework of the TORNADO project. The simulations were fully validated both for the reference coupons’ cases and the DAF containing coupons.
Finally, an upscaling campaign for the evaluation of the DAFs was carried out. After a series of preliminary trial simulations, it was decided to incorporate and test the two DAFs in a mono-stiffener panel (omega stinger – flat panel), representative of an aeronautical structural element.
Overall, the findings of the TORNADO project show that there is significant potential for embedding the two novel features into thermoplastic laminates in order to increase the safety levels of aviation components and minimize operating and manufacturing costs. In the framework of TORNADO, a series of results and knowledge dissemination actions were taken including 6 publications in scientific journals and 3 participations in international conferences.
