數位影像相關法分析直交集成材之滾動剪力性質

Abstract

直交集成材 (Cross-Laminated Timber, CLT) 是一種利用日益增加的木質工程材料，由多層直交排列的集成元 (Laminate) 單板膠合組成，利用性廣泛，可在牆、樓板與屋頂多處場合中使用，更是許多高層木質建築的組成構件。 在面外彎曲的場合中，直交集成材的滾動剪力性質為一重要的弱點，滾動剪力定義為弦徑面 (Radical-Tangential) 上垂直纖維的剪應力。木材側向的剪力剛性與強度較纖維長軸方向的力學性質來的弱，因此會導致具有直交層之CLT在這彎曲時產生較大的撓曲，且影響整體的強度與可服務性。滾動剪力性質不同於木材小尺度RT面之性質，為大片材料受剪力之影響，其彈性性質除受木材本身之剪力性質影響亦會因其生長輪之分布不同而不同。 本研究為研究柳杉 (Cryptomeria japonica) CLT之滾動剪力性質與CLT在受彎曲時試材表面受力之影響，採用數位影像相關法 (Digital image correlation, DIC) 使用試驗時隨時攝影所得之影像數據進行比對分析應變。本次試驗採用短跨深比 (span to depth ratio) 3點荷重之試驗進行，主要試驗厚度為45 mm之CLT試材。 小型試材之剪力強度介於1.0 - 1.6 MPa之間，平均為1.4 MPa，經由DIC所得之應變在線性區間內計算所得之滾動剪力模數平均為115.3MPa，大小介於50 -200 MPa左右，且依不同之生長輪排列型態、秋材率而有所不同。彎曲性質方面，因剪力造成之較大撓曲，試材之視彈性模數僅有約2 – 3 GPa。 剪應變最高者多為生長輪之春材位置，在差異較大者春材有秋材部分的5 – 10倍之差異。除春秋材外，試材內部節的缺點造成之應力集中亦會顯示在應變分布圖中。在較大荷重時，水平方向之正應變 (exx) 會交替出現拉伸與壓縮之區間，試材破壞的位置多在此位置。春材與試材膠合面之高應變在試驗之早期即可觀察到，若出現在膠合面時，試材有高比例會在此位置產生初始之裂縫，或是滾動剪力破壞發生在此位置附近，顯示此方法有發現試材之缺點的潛力。

In recent years, the utilization of engineering wood panel, Cross-Laminated Timber (CLT), has become popular. It is suitable for floor, roof, and wall applications and has been used in many tall timber buildings. However, the rolling shear properties of CLT is the critical mechanical properties in out-of-plane bending situation. Rolling shear is defined as the shear stress in the radical-tangential (rt) plane perpendicular to the grain. Comparing to the longitudinal properties of wood, transversal properties is much lower. Because of the lower mechanical properties of the cross layer, CLT will has a larger deflection when it is subjected to bending and affect its strength and serviceability. Rolling shear is different from the smaller scale shear properties of wood. It is the reaction of the panel subjected to shear. It depends on the primary shear properties of wood, growth ring orientation and so on. In order to understand the reaction of rolling shear properties in CLT, Japan cedar (Cryptomeria japonica) were used in CLT manufacturing. Digital image correlation method (DIC) was used to analysis the strain distribution on the surface of specimens. The central concentrated (3-point) bending test was carried on the CLT beam with a span-to-depth-ratio of eight and 45mm in thickness. Shear strength of the specimens is ranged from 1.0 to 1.6 MPa with average of 1.4 MPa. Rolling shear modulus is ranged from 50 to 200 MPa with average of 115.3MPa by DIC method in the elastic range. The results are vary from growth-ring pattern and the latewood ratio. The apparent modulus of elastic shows a larger deflection due to shear distortion ranged from 2 - 3 GPa. The largest shear strain occurred mostly in earlywood of growth ring, with significant 5 - 10 times difference in some specimens. Beside above, the stress concentration caused by knots, for example, is also shown in the strain distribution contour. In addition, the horizontal normal-strain (exx) distribution under higher load will show higher strain in tension and compression zones alternatively. Rolling shear failure usually occurred at the large tension strain area. The large shear strain of earlywood area and bonding line were observed early at elastic range. If the large strain is observed near the bonding line, the initial crack or rolling shear failure would also occurred around it. It shows that DIC method could to find the defect of the material potentially.