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Large Strrain Analysis for P.U. Strucures in Seat Cushions via Experimental Verification
於座椅設計中需要考量的要件之一為背墊與座墊之撓性。一般而言，背墊與座墊均使用多孔性泡棉材料製造，其孔隙率大於0.9以上。這類泡棉材料深具彈性及高度的非線性，其機械性質十分複雜且難以決定的。因此，本研究首先進行材料之壓縮試驗，其次為數值分析模擬。在材料試驗方面，設計一實驗設備以取得泡棉材料受力與變形之數據資料，並計算其應力-應變、楊氏係數-應變與波松比-應變的關係，再利用最小平方法以求得上述關係之曲線方程式。數值模擬方面，則依據試件規格構建其三維有限元素模型，並配合Newton-Raphson Method 與Incremental Method 進行數值模擬與驗證。其結果顯示，泡棉材料屬於多孔性材料，具高度非線彈性（階數達9次方以上）之特性，而有限元素之模擬亦具有相當高之準確性。
Seat is one of the most common and important devices in the world which is directly related to the health of the human spine. The structural behaviors of the spine are correlated to the mechanical properties and geometrical shapes of the seat. Correct seat design is absolutely required. One of the considerations in seat design is the flexibility of seat cushion and the back cushion. As we know, cushions are made of P.U. materials and porous with porosity higher than 0.9. The cushions may behave elastically with high, specific nonlinearity. The mechanical properties of P.U. cushion are complicate and hard to define. In this study, material tests are performed first and numerical schemes are then considered. In the material tests, an experimental apparatus is designed to obtain the load-deflection datum on either seat or back cushions. Constitutive relationships, such as the stress-strain, Young's modulus-strain, and the Poisson ratio-strain, are then derived. Proper fitting results are also derived. A three-dimensional finite element model based on the geometry of experimental specimen is constructed and the Newton-Raphson method and stiffness incremental method both are implemented for the verification via experiments. Numerical results show that even the material behaviors of the P.U. are porous and highly nonlinear, usually exponential function or polynomial higher than order, the finite element procedures adopted here may give significant predictions.
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