Comparative investigation on viscoelastic and mechanosorptive creep behavior of two tropical hardwoods and one temperate softwood

Two series of 3-point bending tests under constant mechanical load and temperature of 20 degrees C have been performed on two tropical hardwoods, Aucoumea klaineana Pierre (Okoume) and Pterocarpus soyauxii Taub (Padauk), and one temperate softwood, Abies alba (Silver fir). A first series of 14-day tests was performed at 10% stress level and 65% relative humidity (RH), with moderate step variations at 45 and 75% RH. A second series of 5-day tests was performed at 35% stress level and 75% RH with a stronger step variation at 45% RH. Deflection and transverse expansion were measured using linear variable displacement transducers. The results show that the first variation of RH always increases the deflection and that subsequent changes decrease (humidification) or increase (drying) it. It was also observed that Padauk has a higher viscoelastic response and a lower mechanosorptive response than Silver fir and Okoume. As an indication of interaction between viscoelastic and mechanosorptive behaviour, in 14-day test the latter influenced the former, with small increments of relative compliance at each cycle. The analysis of the mechanosorptive trajectories suggested the existence of a creep limit and confirmed that of pseudo-creep/recovery.