Adaptive morphing structures using multistable laminates have significant potential application due to their ability to provide continuous and smooth shape transitions through multiple equilibrium configurations. The simplest candidate for morphing structures is the bistable cured shapes of unsymmetrical laminates, where the residual thermal stresses generated during the curing process result in two distinct equilibrium configurations. However, they often do not meet the requirements for continuous shape transition. To increase the degree of morphing, one option could be to connect multiple bistable unsymmetrical laminates in series generating multistability, which has been explored in the literature. However, when making series connections, the connecting regions often influence the degree of multistability, and triggering snap-through becomes more complex as the snap-through energy needs to be applied selectively to trigger selective shape transitions. In this study, an efficient semi-analytical and finite element frameworks are employed to understand the behavior of series-connected multistable laminates, where the snap-through is triggered using surface-bonded piezoelectric macro fiber composite (MFC) actuators. Since the geometry involves multiple laminates, several MFC patches are required to trigger the snap-through. Such a design needs to be investigated in detail, as multiple patches can further reduce the degree of multistability. In the quest of finding an efficient laminate-MFC assembly for the preliminary designs, systematic parametric studies have been performed to determine the optimal size and location of the MFC actuators. In the later stage of the parametric study, the opportunity of tailoring fiber alignments through the option of variable stiffness laminates is explored to identify a best suitable multistable laminate-MFC assembly. During each step of the parametric study, detailed comparisons of out-of-plane displacements and snap-through voltages of each stable shape have been reported to identify the optimum configurations. The study will contribute to the development of active multistable structures across a wide range of morphing applications.

A carbide-free bainitic microstructure is suitable for achieving a combination of ultra high strength and high ductility. In this work, a steel containing nominally 0.34C-2Mn-1.5Si-1Cr (wt.%) was produced via industrial hot rolling and laboratory heat treatments. The austenitization (900°C, 30 min.) and austempering (300-400°C, 3 h) treatments were done in salt bath furnaces. The austempering treatments were designed to approximately simulate the coiling step, following hot rolling and run-out-table cooling, when the bainitic transformation would take place and certain amount of austenite would be stabilized due to suppression of carbide precipitation. The microstructures and various mechanical properties (tensile properties, bendability, flangeability, and room and subzero temperature impact toughness) relevant for applications were characterized. It was found that the mechanical properties were highly dependent on the stability of the retained austenite, presence of martensite in the microstructure and the size of the microstructural constituents. The highest amount of retained austenite (∼ 27 wt.%) was obtained in the sample austempered at 375°C but due to lower austenite stability and coarser overall microstructure, the sample exhibited lower tensile ductility, bendability, flangeability and impact toughness. The sample austempered at 400°C also showed poor properties due to the presence of initial martensite and coarse microstructure. The best combination of mechanical properties was achieved for the samples austempered at 325-350°C with a lower amount of retained austenite but with the highest mechanical stability.