數位影像相關法之曲型單板層積材彎曲破壞行為分析

Abstract

本研究以柳杉、孟宗竹與桃花心木單板層積材（laminated veneer lumber, LVL）製作直梁與曲梁試體，探討曲率與材性對曲梁彎曲行為、中立軸位移及應力集中的影響，並兼顧膠合界面之耐久性與乾態膠合剪力表現。曲梁設計三種曲率半徑 R = 180、340、440 mm，於同一平面內施加端部集中力，使其承受典型曲梁彎曲（curved beam bending）；試驗過程結合數位影像相關法（digital image correlation, DIC）量測截面應變分佈與中立軸位置，同時建立有限元素模型（finite element method, FEM）進行比對。直梁彎曲試驗結果顯示，三樹種之彈性模數與彎曲強度整體呈現孟宗竹 > 桃花心木 > 柳杉之排序，並作為曲梁分析之材料輸入。 DIC 應變場與 FEM 模擬結果顯示，曲梁截面中立軸明顯偏離幾何中心並偏向受壓側，且偏移量隨曲率增大與材料剛性提高而增加，使拉壓區有效高度呈現不對稱分佈。最大拉應變及拉應力主要集中於內弧跨中附近與端部載重作用區的層間界面；外弧壓縮側則於曲率變化及靠近端部之位置出現局部壓縮集中。當曲率愈大時，中立軸向內弧側的偏移量隨之增加，內弧拉側最大應力與名義彎曲應力之比值亦同步升高，顯示中立軸位移與拉側應力集中程度之間具有明顯正相關關係，且與裂縫起始位置及破壞路徑高度一致。整體而言，DIC 換算之最大彎曲應力與 FEM 預測結果在量值與趨勢上具有良好一致性，而未考慮異向性的曲梁理論則普遍低估最大應力。 綜合彎曲行為、應變場與中立軸位移分析以及界面試驗結果，可知孟宗竹 LVL 在本研究條件下兼具較高彎曲承載力、較小拉側應力集中與穩定膠合強度，為高曲率 LVL 曲梁較具應用潛力之選項；柳杉與桃花心木系統則分別提供偏重耐水性與偏重強度之兩端設計參考。本研究針對曲梁之分析，可作為未來工程木構件曲率配置、膠合系統選擇與耐久性評估之實務依據。

This study used laminated veneer lumber (LVL) made from Japanese cedar (Cryptomeria japonica), moso bamboo (Phyllostachys edulis), and Japanese zelkova (Swietenia macrophylla) to fabricate straight and curved beam specimens, in order to investigate the effects of curvature and material properties on bending behavior, neutral axis shift, and stress concentration in curved beams, together with the durability and dry-state shear performance of glued joints. Curved beams with three radii of curvature (R = 180, 340, and 440 mm) were loaded by in-plane end forces to generate typical curved-beam bending. Digital image correlation (DIC) was employed to obtain full-field strain distributions and neutral-axis positions, and a finite element model (FEM) was established for comparison. Static bending tests on straight beams showed that the elastic modulus and bending strength of the three LVL species followed a consistent order of moso bamboo > Japanese zelkova > Japanese cedar; these properties were used as input parameters for the curved-beam analyses. DIC strain fields and FEM simulations revealed that the neutral axis of curved LVL sections was clearly displaced from the geometric centroid toward the compression side, and that the magnitude of this shift increased with increasing curvature and material stiffness, leading to a pronounced asymmetry between tensile and compressive zones. The maximum tensile strain and stress were concentrated along the inner arc near mid-span and in the interlayer regions around the loaded ends, while the outer arc exhibited local compressive stress concentrations near curvature changes and supports. As curvature increased, both the neutral-axis shift toward the inner arc and the ratio of maximum inner-tension stress to nominal bending stress increased, indicating a clear positive correlation between neutral-axis displacement and tensile-side stress concentration, consistent with observed crack initiation and failure paths. Overall, the maximum bending stresses converted from DIC agreed well in magnitude and trend with FEM predictions, whereas classical curved-beam theory without material anisotropy tended to underestimate peak stresses. Considering bending behavior, strain fields, neutral-axis shift, and interface test results together, moso bamboo LVL under the present conditions combines high bending load-carrying capacity, reduced tensile-side stress concentration, and stable dry-state glued joint strength, and thus represents a promising option for high-curvature LVL curved beams. The Japanese cedar and Japanese zelkova systems provide two design bounds that emphasize, respectively, better wet-durability performance and higher strength. The curved-beam analysis framework developed in this study—integrating LVL bending tests, DIC–FEM evaluation, and both water-soak delamination and dry glued joint shear tests—offers practical guidance for curvature configuration, adhesive system selection, and durability assessment in future engineered timber components.