Design of multifunctional architected cellular structures under dynamic loads

Abstract

Within the scope of the thesis, geometries that can be produced using additive manufacturing techniques were examined, and the selected geometries were manufactured under the most suitable production process conditions. Their responses under quasi-static and dynamic (impact load) conditions were investigated. In addition to the need for weight reduction in aerospace structures, the design and production of structural components with multiple functions are of critical importance. For instance, components that have tasks such as protection against impacts and load-bearing can also facilitate tasks such as vibration damping or heat absorption, which can be referred to as multifunctionality. In armored vehicles, the example of sandwich structures that effectively absorb impact energy while reducing in-cabin vibration during vehicle movement exemplifies this concept. Nowadays, it has become possible to produce high-strength and lightweight composite materials using continupus fiber based fused filament fabrication and alloys using metal additive manufacturing techniques. Those have made possible to produce architected materials with enhanced mechanical properties such as micro/macro level porous and lattice structures. Determining the efficient layering for fused filament fabrication, optimum laser or beam parameters in the powder bed production process and examining them under dynamic loads are necessary. One goal of the study is to investigate the applicability of new generation production techniques in obtaining lightweight structures. Another goal is to investigate the certain performance response (compression) of architectured structures that can be produced for multifunctional applications under crushing and shock loads. In the study, additive manufacturing techniques such as continuous fiber based fused filament fabrication, electron beam powder bed fusion and laser powder bed fusion were used to manufacture solid composite samples with a different architected base (different continuous fibers, different orientations and distributions of fibers), cellular lattice structures and sandwich structures, respectively. Productions in continuous fiber based fused filament fabrication technique were made to have various fiber material (glass fiber, carbon fiber, aramid fiber), fiber distrubution, fiber orientation in the architected material class. Productions in electron beam powder bed process were made for three different lattice blocks in the 2D lattice structures based metamaterials class (anti-tetrachiral, double arrow-headed, tree-like re-entrant). Electron beam powder bed fusion was used to produce these lattice blocks in two different production orientations (0° and 45°) and three different beam scanning speeds (speed function, low speed, high speed). The aim was to understand the differences in their crushing responses. Compression tests were conducted using compression test units to compare stress-strain curves and the energy absorption capabilities of the lattice structures, respectively. In the 2D lattice structures based metamaterials class, sandwiches with two different cores (anti-tetrachiral and tree-like re-entrant) and a sandwich with triangular type conventional core were manufactured using laser powder bed fusion process. The designs were in the form of sandwich structures with 2 mm surface plates and 20 mm core height. These sandwich beams were used to measure their responses localized impact experiments under the low velocty impact conditions using a CEAST low velocity test system. As the sandwich beams with auxetic topologies exhibited stiff response, the sandwich with conventional triangular topology exhibited higher energy absorption and did not broke due to failures of both facesheets. As a result, solid blocks with different continuous fiber reinforcements were succesfully produced using continuous fiber based fused filament fabrication technique and optimum architectured composite materials were obtained within the design of experiment set. Then, Ti64 lattice blocks were successfully produced using PBF-EB and exhibited different energy absorption capabilities based on different orientations and production process characteristics. Those were employed to observe their compression response. Lastly, sandwich structures with lattice core were produced without any deformation in PBF-L and their responses under low velocity impact loads were experimentally observed. The tests revealed that when same material stock was employed to build sandwich beams in powder bed fusion process, stiff/thick facesheets are required for stiff core topologies while less stiff cores enable higher energy absorbent response due to importance of plastic deformations during the impact loads.