Assessment of flexural properties of different grade dimension lumber by ultrasonic technique

The dimension lumber (45mm×90mm×3700mm) of plantation Chinese fir ( Cunninghamia lanceolata (Lamb.) Hook.) was graded to four different classes as SS, No.1, No.2 and No.3, according to national lumber grades authority (NLGA) for structure light framing and structure joists and planks. The properties of apparent density was determined at 15% moisture content, bending strength and stiffness were tested according to American Society for Testing and Materials (ASTM) D198-99, and dynamic modulus of elasticity (E usw ) was measured by ultrasonic technique, for predicting the flexural properties of different grade lumbers. The results showed that E usw was larger than the static MOE. The relationship between E usw and static MOE was significant at 0.01 level, and the determination coefficients (R 2 ) of the four grade lumbers followed the sequence as R 2 No.2 (0.616)> R 2 SS (0.567)> R 2 No.1 (0.366)> R 2 No.3 (0.137). The R 2 of E usw and MOR were lower than that of the E tru and MOR for each grade. The E usw of all the grade lumbers, except No.3-grade, had significant correlation with the static MOE and MOR, thus the bending strength of those grade lumbers can be estimated by the E usw . The E tru values of four grade lumbers followed a sequence as No.2-grade (10.701 GPa) > SS-grade (10.359 GPa) > No.1-grade (9.840 GPa) > No.3-grade (9.554 GPa). For the same grade dimension lumber, its E usw value was larger than static MOE. Mean values of MOR for four grade lumbers follow a sequence as No.2-grade (48.67 MPa) > SS-grade (48.16 MPa) > No.3-grade (46.55 MPa) > No.1-grade (43.39MPa).