Measurement of locked-in strains in a green crosscut. The closing method.

Initiation and propagation of heart checks at log ends after tree felling 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 knowledge of residual strains in logs, particularly transversally to fibers. A procedure completing the longitudinal measurements at tree periphery is presented, based on the 'instantaneous' closing of green crosscuts caused by a radial opening, followed by the 'hygrothermal' closing provoked by boiling (fig 3). The relative displacement of two pins nailed near periphery is measured using a special device (fig 1) and divided by girth to give a global transverse strain phi. Local measurements of the radial strain ER and the tangential strain epsilon(T) performed using the same device (fig 2) showed that phi partially expressed the strain anisotropy epsilon(T) - epsilon(R) (fig 8). Instantaneous values of 0.2% obtained on 132 chestnuts (Castanea sativa Mill) were weakly correlated to longitudinal strains (fig 10). Hygrothermal strains reached 0.5% after 30 min at 80 degrees C, correctly expressing splitting trends: the more wood cracked tangentially, the less it did radially, the potiential transverse strain remaining for all rupture modes (fig 12).