PSI - Issue 25

Tiago Bento et al. / Procedia Structural Integrity 25 (2020) 234–245 Tiago Bento/ Structural Integrity Procedia 00 (2019) 000 – 000

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4. Conclusions

This work proved that Bragg sensors could be incorporated in hybrid FSW welds. Structural integrity in critical structures is essential so that it can resist the possibly harsh external conditions of its work-life. So, this ability to monitor the connections’ condi tion is of paramount importance. That is one of the main reasons why in the aerospace industry this technique is not widely spread, because unlike riveted and screwed joints which form differential structures, weld integrity can not be easily monitored as the welding forms integral structures. To increase the adoption of this technique, new monitoring technologies like the one developed are essential. Therefore, the integration of Bragg fibers in the weld production and service is a big step towards those goals. In the lap joint the strain variations during the welding process were measured. It was observed that the loading direction imposed on the sensors changed as the tool progressed, and that the efforts in the fiber were null when the tool was crossing the sensor’s measuring length. Those results helped to understand the formation of the residual stresses observed on the final specimen. This joint also proved it self to be fully functional in the tensile test conditions, measuring the localized strain increment as the applied load increased. We could then produce functional self-monitoring T-joints and place them under bending or tensile load in which they could fully monitor the strain variations as the loading conditions intensified. In the T-joint bending test the strain values measured were very close to the prediction given by the FEM results. The slight variation present between both curves could be explained by the existence of two continuous lines of weld defects, hook and cold lap defects, which were not modeled or taken into account in any way in the numerical model. The presence of these defects on the real specimen imposes a bigger deformation than expected by the simulation. In addition to the conclusion that is possible to monitor T-joints produced with this technology, we can conclude that a variety of loads applied in this kind of geometry can be accurately monitored. However, in the tensile test the approximation given by the FEM model was not as accurate, as the strain evolution was not as steep in the FEM as in the real fiber. This does not mean that the fiber’s readings in the tensile test were inaccurate or that the model was poorly designed, but rather that the defects present in the real joint changed its elastic behavior at some point during the test (around an applied displacement of 1 mm) leading to a higher deformation being present. It was demonstrated that embedding strain sensors in FSWB joints is possible, creating the ability to make smart joints to constantly monitor the structural integrity of a joint. These sensors could also be proven very useful in determining the presence of defects in welded panels since theses defects would alter the measured deformation fields. The production of this joints can be simplified and is suitable for big scale automation conferring this technology a huge potential in the aerospace industry. Acknowledgements This work was supported by FCT, through IDMEC, under LAETA, project UID/EMS/50022/2019. Funding provided from NORTE - 01 - 0145 - FEDER - 000022 and LISBOA - 01 - 0145 - FEDER - 029339 SciTech – Science and Technology for Competitive and Sustainable Industries is acknowledged. The authors acknowledge the funding provided by FCT project PTDC/EME - EME/29340/2017 – DisFri. References Braga, Daniel F.O. et al. 2015. “Aluminum Friction Stir Weldbonding.” Procedia Engineering 114: 223 – 31. http://dx.doi.org/10.1016/j.proeng.2015.08.062. Braga, Daniel F O, and L T I December. 2017. “Innovative Structural Joining for Lightweight Design.” Chao, Yuh J., X. Qi, and W. Tang. 2003. “Heat Transfer in Friction Stir Welding - Experimenta l and Numerical Studies.” Journal of Manufacturing Science and Engineering, Transactions of the ASME 125(1): 138 – 45. Ericsson, M., and R. Sandström. 2003. “Influence of Welding Speed on the Fatigue of Friction Stir Welds, and Comparison with MIG and TIG.” International Journal of Fatigue 25(12): 1379 – 87. “Fiber Optics Sensors & Systems Monthly Newsletter July 2010” Hunstsman Advanced Materials. 2004. “Aerospace Adhesives Araldite 420 A/B Two Component Epoxy Adhesive.” (August): 1 – 4. www.araldite.com. Introduction to Friction Stir Welding (FSW) . https://ntrs.nasa.gov/search.jsp?R=20150009520 (November 9, 2019).