Investigating wood under thermo-hydromechanical loading at the ring scale using full-field measurements

The thermo-visco-hydromechanical properties of Abies alba Mills (white fir of Massif Central) are investigated in this paper. These properties are generally complex to study and model because of various couplings that exist between underlying phenomena. In addition, wood is heterogeneous by species, but these properties are generally studied at the macroscopic scale, thus without having the possibility of discerning the contribution of the various levels of organization which exist in wood. In this context, the main contribution of this paper is to experimentally evidence that earlywood and latewood exhibit each a different visco-hydromechanical response and, more specifically, that humidity directly governs their viscoelastic properties. Three suitable experiments were carried out on three different specimens to highlight this phenomenon. These experiments consist of mechanically testing the specimens under natural drying conditions. The difference in response was observed at the scale of the annual ring by using a full-field measurement technique, namely the grid method. This technique provides the strain fields occurring on the surface of the specimens during the different steps of their hydromechanical loading. Its spatial resolution is such that the strain level in latewood and earlywood can be accurately measured, thus evidencing the significant difference in response of the rings. This measurement technique as well as the three experiments carried out during this study is first briefly described in this paper. The obtained results are then presented and discussed, with a special emphasis on the different responses observed in earlywood and latewood.