Jong-Gu Lee

Sequential Folding using Light-activated Polystyrene Sheet

A pre-strained polystyrene (PS) polymer sheet is deformed when it approaches the glass transition state as a result of light absorption. By controlling the light absorption of the polymer sheet, non-contact sequential folding can be accomplished. Line patterns of different transparencies and shapes are used to control the light absorption. The line pattern shape is closely related to the folding angle and folding start time. The relation between the line pattern design and folding performance was evaluated experimentally to develop a technique for folding PS sheets. The results show that sequential folding of PS sheets can be accomplished by changing the degree of transparency of the line pattern. Using the technique developed in this study, self-folding origami structures with complicated shapes can be designed and manufactured.

Saddle-shaped, bistable morphing panel with shape memory alloy spring actuator

Bistable structures are attractive morphing components because they can transform from one stable state to another simply through the application of an external force or moment. Each stable state is preserved despite the absence of a continuous energy supply. However, its competency is quite limited because most bistable composites (unsymmetric laminated bistable composites) have limited equilibrium shape configurations after curing. Most of them are cylindrical in shape. In this paper, for the first time, we present a saddle-shaped bistable panel that can be used for various soft-morphing applications. The mechanics of the saddle-shaped, bistable panel are investigated using the Rayleigh–Ritz approximation. A simple analytical model is proposed to obtain saddle bistability and deformations. Finite element (FE) numerical analysis is also performed to validate the analytical model approach. The numerical FE analysis shows good correlation with simple analytical solutions. Furthermore, for practical, smart-structure applications of the saddle-shaped bistable panel, the FE method simulated a snap-through action induced by a shape memory alloy (SMA) spring actuator. A simple design criterion for the snap-through action of a saddle composite panel actuated by an SMA spring component is suggested, based on the FE analysis.

Dual-stiffness structures with reconfiguring mechanism: Design and investigation

To improve the multi-functionality of a structure, a foldable or deployable structure with variable stiffness is needed. This article presents dual-stiffness structures with two stiffness states: a stiff state and a flexible state for a multi-mission capability. This dual-stiffness structure is based on a hybrid structure that combines rigid and flexible segments; when the rigid segments are rearranged, the bending motion of the compliant material is constrained by the rigid segments, which varies the stiffness of the structure. Instead of continuously changing the stiffness, the dual-stiffness structure abruptly changes the stiffness state with a simple reconfiguring mechanism. We developed two reconfiguring mechanisms: a sliding mechanism and a folding mechanism. Using a layering process, the dual-stiffness structure with a two-dimensional multi-layer design was manufactured. To verify the behavior of the structure, a simplified structure with no sliding mechanism was designed and simulated using a finite element method. The ratio of the length of a rigid segment to the length of a compliant segment determined the stiffness of the structure. This dual-stiffness structure with the reconfiguring mechanism can be effective for applications that require a big change in stiffness, such as for a deployable solar panel, or flexible display.

A large-stroke shape memory alloy spring actuator using double-coil configuration

One way to increase the range of motion of shape memory alloy (SMA) actuators is to create displacements of the SMA associated with not only the deformation from straining but also rigid-body motion from translation and rotation. Rigid-body motion allows the SMA to create larger displacements without exceeding the maximum recovery strain so that the SMA actuators can have a larger shape recovery ratio. To improve the linear actuation stroke of SMA wire actuators, a novel SMA spring actuator is proposed that employs a double-coil geometry that allows the displacement of the SMA to be mainly induced by rigid-body motion. A double-coil SMA spring actuator is fabricated by coiling an SMA wire twice so that the double coiling results in a reduction of the initial length of the double-coil SMA spring actuator. The effects of the geometric parameters on the actuation characteristic of a double-coil SMA spring actuator are verified numerically by finite element analysis and experimentally according to a parametric study of the geometric parameters. The displacement-to-force profile of the double-coil SMA spring actuator is nonlinear, and the spring stiffness changes when the actuator transforms its configuration from a double-coil shape to a single-coil shape. According to the results of the parametric study, increasing the wire diameter increases both primary and secondary coil stiffness, and increasing the primary inner coil diameter decreases both primary and secondary coil stiffness, whereas increasing the secondary inner coil diameter decreases only the secondary coil stiffness. The result shows that one of the double-coil SMA spring actuators with an initial length of 8 mm has a recovery ratio of 1250%, while the recovery ratio of the single-coil SMA spring actuator with the same geometric parameters is 432%.

Deformation characteristics of CFRP bi-stable composites according to negative initial curvature

Deformation characteristic of CFRP bi-stable composites according to negative initial curvature is discussed, and twisted shape bi-stability of CFRP composites is introduced. Initial curvature of composites has effect on final curvature of bistable composites. Positive initial curvature tailor final curvature linearly. However, some negative initial curvature induces twisted shape bi-stability. CFRP laminated composites have three different final state (i.e. conventional shape bistability, mono-stability, and twisted shape bi-stability) according to initial curvature. Twisted shape bi-stable composites have different curvature changing characteristic from conventional shape bi-stable composites. In order to analysis effect of initial curvature, analytical model that include force and moment equilibrium is proposed. FE simulation results and analytical results are compared to verify the proposed analytical model. It is verified that some range of negative initial curvature induce losing of bi-stability or twisted shape bi-stability by analytical model. Final state shape of three different state is analyzed by FE simulation and analytical model. Final states shape from two different analyses are well matched each other in three different state.

Evaluation of Initial Curvature Effect on the Snap-through Load of Bi-stable Composites

The snap-through load which triggers off the snap-through of bi-stable composites with initial curvature is predicted in order to evaluate the effect of initial curvature on the snap-through load. To predict the snap-through load, snap-through process is simulated by minimizing the total potential energy of the bi-stable composites subjected to line moment by means of Rayleigh-Ritz technique. Additionally, FE analysis is carried out and experiment is performed for the verification of the initial curvature effect identified by analytical method. From this study, we show that the initial curvature increases or decreases the snap-through load linearly depending on the direction of snap-through.

The Prediction of Nonlinear behavior of Double Coil Shape Memory Alloy Spring

The recovery force and displacement occur due to the phase transformation from the martensite phase to the austenite phase induced by the mechanical loading or thermal loading. These recovery force and displacement depend on an initial geometrical configuration of SMAs and loading conditions. Although the SMAs generally generates large recovery forces, the sufficient recovery displacement cannot be expected without a proper design strategy. The functionality of SMAs is limited due to the unbalance between the large recovery force and the small recovery displacement. This study suggests the double coil SMA spring in order to amplifying the recovery displacement induced by the phase transformation. By predicting the recovery displacement of doble coil SMA springs and one coil SMA springs induced by thermal loading, we show that the double coil SMA spring not only mitigate the unbalance of performance but also have a large recovery displacement for its recovery force than one coil SMA spring.

Study on Bullet-Proof Performance of Multi-Layered Hybrid Armor Against 9mm FMJ Projectile

In order to prevent the high velocity bullet from penetration, aluminum alloy and RHA(Rolled Homogeneous Armour) steel, which have a high tensile and compressive strength, are usually used as the bullet-proof armor material. Although these materials have a good bullet proof performance, but not an area density which is a weight increasing factor of bullet-proof armor. Therefore, Mg(magnesium) alloy is a promising substitute for the traditional bullet-proof armor material due to the relatively low areal density. The spatial efficiency of Mg alloy, however, is inferior to the traditional material`s, which is a volume(thickness) increasing factor of bullet-proof armor. In this study, we select the multi-layered hybrid armor which consist of Ceramic, with a high strength; Mg alloy, with a low areal density; Kevlar, with a high tensile strength-to-weight ratio; in order to make up for the poor spatial efficiency of Mg alloy. By predicting V50 of the multi-layered armor against 9mm FMJ(Full Metal Jarket). we show that the multi-layered armor have the capability in improving bullet-proof performance in the respect of the areal density, but also the spatial efficiency.

Snap-through behavior of bi-stable composite structure using SMA spring actuator

Curvature tailoring and identifying loading condition which induces snap-through motion of bi-stable plate is important research issue in practical point of view. Curvature tailoring of bi-stable plate with curved tool-plate is investigated not only analytic simulation, but also experiments. New formulation, which is based on planestress constitutive equation and superposition principle, is proposed to figure out the initial curvature effect. FEM analysis with ABAQUS is adopted to figure out the loading condition which induces snap-through motion of bi-stable plate and effect of initial curvature to it. Based on the analysis results design guideline on initial length of SMA spring is presented. Nomenclature E1 = young’s modulus of along fiber direction E2 = young’s modulus of matrix G12 = in-plane shear modulus α1 = Thermal expansion coefficient along fiber direction α2 = Thermal expansion coefficient of matrix initial kl  = initial strain initial kl