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This research focused on developing a novel Friction Stir Riveting (FSR) technique, viz., Friction Stir Blind Riveting for pre-drilled holes (FSBR-pH), to improve the mechanical performance of hard-to-access aircraft joints. This objective is realized through a hierarchal four-pronged experimental and numerical analysis approach. Firstly, the existing FSBR process was evaluated where the existing process was found infeasible for pre-drilled holes. The existing process was then modified to develop good quality joints in case of pre-drilled holes. In the second step, a process window was created for FSBR-pH joints through (a) microscopic analysis of joint cross section to observe the presence of any defects/gaps and (b) numerical simulation. Microstructural and mechanical characterization (i.e., tensile strength and microhardness) of the optimized joints were performed in the third step where the presence of dynamic recrystallization was found responsible for the formation of submicron-sized grain in the stir region. The strain-hardening phenomenon was observed in the stir region which occurred due to extensive stirring at high spindle speeds. Finally, a comparison of the developed joints was drawn with existing Blind Riveted (BR) joints in terms of failure modes and tensile strength where the FSBR-pH joints performed better than their existing counterparts. [ABSTRACT FROM AUTHOR]

Abstract: Failure mode of laser welds in lap-shear specimens of high strength low alloy (HSLA) steel sheets is investigated in this paper. The experiments for laser welds in lap-shear specimens under quasi-static loading conditions are briefly reviewed first. The experimental results showed that the laser welds failed in a ductile necking/shear failure mode and the ductile failure was initiated at a distance away from the crack tip near the boundary of the base metal and heat affected zone. In order to understand the failure mode of these welds, finite element analyses under plane strain conditions were conducted to identify the effects of the different plastic behaviors of the base metal, heat affected zone, and weld zone as well as the weld geometry on the ductile failure. The results of the reference finite element analysis based on the homogenous material model show that the failure mode is most likely to be a middle surface shear failure mode in the weld. The results of the finite element analysis based on the multi-zone non-homogeneous material models show that the higher effective stress–plastic strain curves of the weld and heat affected zones and the geometry of the weld protrusion result in the necking/shear failure mode in the load carrying sheet. The results of another finite element analysis based on the non-homogeneous material model and the Gurson yield function for porous materials indicate that the consideration of void nucleation and growth is necessary to identify the ductile failure initiation site that matches well with the experimental observations. Finally, the results of this investigation indicate that the failure mode of the welds should be examined carefully and the necking/shear failure mode needs to be considered for development of failure or separation criteria for welds under more complex loading conditions. [ABSTRACT FROM AUTHOR]

Highlights This study presents a novel hybrid‐sandwich composite structure, customized for nose radomes applications, incorporating a foam core and distinct opposite face sheets composed of Kevlar and S‐Glass materials. Addressing in‐phase electromagnetic properties, UV protection and low velocity impact responses in our previously published work, this paper empirically and numerically investigates the damage tolerance response of the proposed structures through compression after impact (CAI) testing. The low velocity impacts (LVIs) on S‐Glass face sheets exhibited unique energy dispersion and absorption mechanisms, resulting in variations in indent damage depths and widths across all impact energy levels as compared with LVIs on Kevlar face sheets. After experimentally assessing the CAI behavior, a FE model is developed to predict CAI behavior, which closely aligned with experimental findings. This study, unprecedented in existing literature, proposes hybrid sandwich structures for nose radome aerospace applications, with superior specific impact and residual strength compared to various composite sandwich structures documented in the published literature expanding its utility beyond radomes. Innovative composites with superior low velocity impact response, tested for compression after impact performance. Designed for nose radomes found suitable for other impact prone applications. Experimental and numerical modeling showed comparable results. Major differences observed in damage mechanics, resistance, and tolerances. [ABSTRACT FROM AUTHOR]