Variations in Physical and Mechanical Properties Between Clear and Knotty Wood of Chinese Fir

Significant market value discrepancies exist between clear and knotty Chinese fir (Cunninghamia lanceolata) wood, distinguished not only by their aesthetic variations but also by their distinct material properties. This study aimed to explore the differences in physical and mechanical properties between clear and knotty Chinese fir wood. Nine standard trees were chosen from a 26-year-old Chinese fir plantation for the experiment. Subsequent to felling, trunk segments below 7 m in length were transported to the laboratory. For each tree, detailed preparations were made to obtain clear and knotty wood specimens, and these distinct wood specimens were subjected to thorough physical and mechanical assessments. The results revealed significant variations in properties between clear and knotty Chinese fir wood. The shrinkage and swelling coefficients of knotty wood were generally lower than those of clear wood, except for higher radial and tangential air-dry shrinkage. Specifically, the swelling ratio of knotty wood was at least 0.40% lower, and the oven-dry shrinkage was at least 0.58% lower than that of clear wood. Knotty wood exhibited higher air-dry and oven-dry densities, with its density being at least 0.15 g cm−3 higher than that of clear wood. However, its mechanical properties, including tensile strength, compression strength, impact bending strength, bending strength, and modulus of elasticity, were lower than those of clear wood. For instance, the tensile strength parallel to the grain of clear wood was 40.63 MPa higher, the modulus of elasticity was 1595 MPa higher, and the impact bending strength was 27.12 kJ m−2 greater than that of knotty wood. Although the tangential and radial surface hardness of knotty wood increased significantly compared to clear wood, the end hardness remained relatively lower. Overall, knotty Chinese fir wood displayed enhanced physical properties, whereas clear wood showcased superior mechanical properties. Careful selection between clear and knotty wood is recommended based on the specific requirements of wooden structural elements to optimize timber resource utilization.