A STUDY ON GROWTH STRESSES, TENSION WOOD DISTRIBUTION AND OTHER RELATED WOOD DEFECTS IN POPLAR (POPULUS-EURAMERICANA CV I214) - END SPLITS, SPECIFIC-GRAVITY AND PULP YIELD

The development of radial shakes after felling the tree has been observed on transverse sections of 15 poplar logs. The importance of end splitting is related to the distribution of internal stresses in the stem (growth stress), the angular variations of wood structure (specific gravity and pulp yield), and the transverse mechanical resistance of wood. To investigate growth stresses, longitudinal displacements after stress release were estimated at the periphery of the stem using the single hole method. At least 4 measurements were necessary to estimate the maximum displacement value and the circumferential heterogeneity of the stress field. The position of this maximum was generally found on the upperside of the trees. To examine end splitting, the radial and longitudinal extension of splits were roughly estimated for all visible shakes occurring on cutting sections near stress measurements (breast height). Shakes were also measured for comparison at the felling section of the logs. The dimensions of the longest shake were used as an indicator of the severity of end splitting. A complete map of wood basic specific gravity was made at the breast height level for all trees. This is associated with pulp yield measurements, an increase in density and pulp yield being generally considered as an indicator of gelatinous fibres. Peak values of growth stresses in the stem were associated with a significant increase in pulp yield and specific gravity. The study was completed by a set of experiments on resistance to crack propagation via TR bending specimens. The critical stress intensity factor K(IC) was calculated. Quantitative measurements of end splitting have proved to be a useful tool for assessing the technological impact of growth stresses in trees; the importance of cracks is clearly related to the maximum value of displacement at stress release. However, crack propagation can also be explained by cell-wall properties and transverse cohesion of green wood. Further research should focus on this second' aspect, in order to determine structural properties of importance in crack propagation.