The effect of knots in tension face on bending properties of Cunninghamia lanceolate juvenile wood
This work was aimed at revealing the effect of knots'features and distribution locations on wood bending strength and investigating the variation of this effect on tree growth direction.In this study,the specimens made from Cunninghamia lanceolate juvenile wood were obtained from three different heights of tree stems and divided into two groups,including specimens with and without knots.The locations of knots at the bottom of the specimens were examined.The modulus of rupture(MOR)and modulus of elasticity(MOE)of the specimens were measured using the three-point bending test.After the bending test,the images of the failure regions were collected.Knot information,including knot to specimen thickness ratio,knot area and knot to loading distance was recorded.The influence of knot features on MOR and MOE was statistically analyzed.The results showed that the size of knots increased as the increase in height of tree stem.MOR of the specimens with knots decreased as the increase in height of tree stem.The influence of knots on MOE of the specimens was trivial and knots significantly decreased MOR of the specimens.The knot to specimen thickness ratio and knot area negatively related to MOR,and knot to loading distance and MOE posi-tively related to MOR.Knot to loading distance had extremely significant influence on MOR and MOE.The knot to specimen thickness ratio,knot area,knot to loading distance and MOE can explain MOR with R2=0.580.Knot to specimen thickness ratio,knot area,knot to loading distance and MOE can explain MOR with R2=0.658 when the specimens are failure only at knots.The linear relationship between MOE and MOR of the specimens with knots was R2=0.580.Knot to specimen thickness ratio,knot area,knot to loading distance and MOE can explain MOR with R2=0.851 when the specimens with knots are not failure at knots.Because there is a strong linear relationship between MOE and MOR(R2=0.856),the influence of knot to specimen thickness ratio,knot area,knot to loading distance on MOR was slight when the specimens with knots were not failure at knots.The structural failure of specimens with-out knots was tearing of fibers.Shear strain can effectively transfer along fiber direction of wood without knots.The structural failure of specimens was sharp fractures or rupture around knots.Shear strain accumulation was prone to occur in knots when the specimens fail at knots.The effect of knots of bending properties of Cunninghamia lanceolate juvenile wood was statistically analyzed.The key knot parameters affecting bending properties of the specimens were found.
Cunninghamia lanceolate juvenile woodknotknot to specimen thickness ratioknot areaknot to loading distancemodulus of rupturemodulus of elasticity
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