Design and Fabrication of Self-folding Polymer Structure using Light-absorption Folding Behavior : 광흡수 접힘 거동을 이용한 자가 접힘 고분자 구조체의 설계 및 제작

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Lee Yonghee

공과대학 기계항공공학부
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서울대학교 대학원
folding behaviorpolymer sheetcohesive line elementlight-activated deformationself-folding structure
학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2017. 2. 조맹효.
In this dissertation, the light-absorption folding deformation of polymer sheet is introduced, and the self-folding polymer structure using light-activated folding deformation is designed and manufactured. Through the concentration of the thermal absorption to the specific narrow area in the polymer sheet, the unusual behavior as folding deformation can be realized. The localized thermal absorption control is achieved by the employment of the light and the black-color printed line pattern. While the light almost passes the transparent polymer sheet, the area with the black-colored line pattern blocks the penetration of the light. The infrared rays included in the light provides thermal energy to the polymer sheet, and the area near the black-colored line pattern is folded by the localized thermal absorption. Because the polymer sheet is easily folded by the light and the printed black-colored line patterns, the various shape using the polymer sheet and the line pattern can be considered. However, the folding behavior of polymer sheet is based on the thermal contraction phenomenon, so the numerous line patterns in the small area may induce the unintended deformation due to the excessive thermal absorption in a limited area.
For the stable folding behavior design of polymer sheet using the light, the analysis of the folding deformation is required, and the shell/cohesive line element based on the continuum theory is employed. The shell element shows the good performance in the case of the bending deformation of the thin object, but it may miss the folding deformation of polymer sheet. When the polymer sheet is folded, the normal vectors at the both sides of the folded line are not matched. For this reason, the cohesive line element is considered. In the cohesive line element for folding deformation, additional nodes are defined at the folded lines, and these nodes allow another displacements and rotation angles at the folded line. Then the discontinued displacements and rotation angles at the folded line can be identified, and the folding deformation is analyzed by the cohesive line element. Also, the folding deformation of polymer sheet is induced from the thermal contraction at the glass transition state. Therefore, the constitutive model for thermal absorption of the polymer sheet is required for the folding behavior of polymer sheet, and this constitutive model is obtained from molecular dynamic simulation. The shape memory thermomechanical cycle of polymer sheet is simulated by using polystyrene unit cell in nano-scale, and the constitutive model for polystyrene polymer sheet is calculated.
The folding behavior of polymer sheet is varied by the amount of the absorbed light, and the line pattern printed in the sheet influences the light-absorption. Accordingly, the relation between the folding deformation and the line pattern is identified from the actual measurement of the polymer sheet, and it is applied to the self-folding structure design and fabrication. The folding angles and the folding start time are considered as the main design variables for the light-absorption self-folding structure design.
Through analysis of the folding deformation of polymer sheet, the self-folding structure with complex shape, such as hexahedron, icosahedron and lotus flower shape, can be designed and manufactured. Also, the periodically patterned structure as negative Poissons ratio structure can be considered. As the structure including sub-cells are manufactured, it is proved that the patterned structure by light-absorption folding deformation of polymer sheet can be realized. The mechanical properties of the periodically patterned structure are compared to the equivalent stiffness obtained from the well-established homogenization method.
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Mechanical Aerospace Engineering (기계항공공학부)Theses (Ph.D. / Sc.D._기계항공공학부)
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