MEASUREMENT OF LOCKED-IN STRAINS IN A GRE EN CROSSCUT - THE CLOSING METHOD

Initiation and propagation of heart checks at log ends after tree fell ing lead to serious problems for wood exploitation. This phenomenon is caused by the release of strains locked-in during cell maturation and is triggered by steaming; its understanding requires an improved know ledge of residual strains in logs, particularly transversally to fiber s. A procedure completing the longitudinal measurements at tree periph ery is presented, based on the 'instantaneous' closing of green crossc uts caused by a radial opening, followed by the 'hygrothermal' closing provoked by boiling (fig 3). The relative displacement of two pins na iled near periphery is measured using a special device (fig 1) and div ided by girth to give a global transverse strain phi. Local measuremen ts of the radial strain ER and the tangential strain epsilon(T) perfor med using the same device (fig 2) showed that phi partially expressed the strain anisotropy epsilon(T) - epsilon(R) (fig 8). Instantaneous v alues of 0.2% obtained on 132 chestnuts (Castanea sativa Mill) were we akly correlated to longitudinal strains (fig 10). Hygrothermal strains reached 0.5% after 30 min at 80 degrees C, correctly expressing split ting trends: the more wood cracked tangentially, the less it did radia lly, the potiential transverse strain remaining for all rupture modes (fig 12).