模擬圓筒板結構銲接變形與殘餘應力及在水壓情況下的銲後挫曲強度分析

MODELING OF BUCKLING STRENGTH OF CYLINDRICAL BARREL UNDER HYDRO-STATIC PRESSURE WITH CONSIDERATION OF WELDING INDUCED DEFORMATION AND RESIDUAL STRESS

銲接為造船界廣泛使用來連接部件的技術，銲接種類可以分為固態銲接及融態銲接，船廠較常使用的是屬於融態銲接的金屬遮蔽銲接（SMAW）及氣體保護銲接（GMAW），由於融態銲接需要對銲件輸入巨大熱量，這將會導致銲件變形，並且導致銲件內部產生殘餘應力以及銲接變形，影響部件銲接後的幾何與強度。本文以有限元素分析軟體模擬板材銲接過程以及造成的殘餘應力、銲接變形，並討論板材在銲接後的挫曲強度。有限元素模擬分成兩個部分：第一步使用實體單元模擬板材銲接加熱的過程，以不同溫度下的材料性質如楊氏模量、降伏強度、柏松比等，模擬材料加熱過程材料性質的改變，並研究圓筒殼結構銲接後的變形，找出銲道附近的位移及應力的分佈模式；第二步將銲接導致的位移分佈及殘餘應力分佈加載至圓筒殼結構上，研究受到銲接變形及殘餘應力影響的圓筒殼挫曲強度將如何改變。我們藉由分別以實體單元與板單元進行挫曲強度分析，知道以板單元簡化模型趨向於高估銲後挫曲強度，其高估幅度約莫在1％；藉由模擬不同銲接溫度造成的影響，我們確認了銲接溫度會增加銲後變形，並擴大殘餘應力分佈的範圍，進而影響銲後挫曲強度，其行為趨向隨銲接溫度增加而挫曲強度下降，其下降幅度從0.02％至0.06％；藉由分析不同程度縮短的圓筒殼模型，發現在僅考慮銲後變形的簡化模型，傾向低估具有較低波數模態的挫曲強度，而高估具有較高波數模態的挫曲強度。

Welding is one of the widely used technique in the ship building industry to connect parts during assembling processes. Weld-ing is categorized as melting type metal-joining method which requires heating welding material to melt and to form a bounding layer. Among all welding methods, shipyards usually use shielded metal arc welding (SMAW) or gas metal arc welding (GMAW) methods. However, due to the large heat input during the welding processes, the surrounding material will suffer plastic deformation, which leads to residual stress and extra deformation after cooling is complete. In this research, we use the finite element method (FEM) to model the complete welding processes to obtain the residual stress, welding deformation. Then we use the model with the complete welding information to perform the buckling analysis on a cylindrical panel structure under hydrostatic pressure. The FEM analysis contains two steps: 1. Use the solid FEM element to simulate the welding processes with different welding tempera-ture. All mechanical properties of the metal are temperature dependent to capture the nonlinear material behavior when heated. Then extract the stress and displacement information around the welding path; 2. Load the detail information from step 1 into the cylinder model and perform buckling analysis. The goal is to find the effect of welding deformation ad residual stress on the buckling strength. Through the simulation results, we can find that using plate element in the second step will lead to 1% higher strength estimation when comparing to solid element. When the welding temperature increases, the range of residual stress distribution increases and buckling strength decreases up to 0.02% to 0.06%. Model with shorter axial length will underestimate buckling strength of modes with low wave number while overestimate buckling strength with high wave number.