| Summary: | 碩士 === 國立臺灣大學 === 森林環境暨資源學研究所 === 106 === 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.
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