Recovery of Mechanically Induced Residual Stresses in Densified Softwoods Created During a Densification Process
Keywords:Densified softwoods, mathematical model, stress recovery, residual stresses
AbstractMechanical densification technology has been used to increase density and mechanical properties of low-density wood. After the densification process, some internal stresses created during densification can be temporally "locked" in wood, which is defined as mechanically induced residual stresses. When the densified wood is exposed to wet conditions, these mechanically induced residual stresses along with swelling stresses can be released with time, which might result in dimensional instability causing warping. This study aimed at examining mechanically induced residual stresses in densified softwoods and simulating the stress-releasing process by means of a mathematical model. Balsam fir and eastern white pine were used for undensified wood specimens and densified wood specimens that were compressed at three compression ratios (CRs) of 0.25, 0.50, and 0.60. Specimens compressed at 0.50 and 0.60 CR plus one control group of undensifed specimens (ie CR = 0) was used to calculate model parameters, and ones at 0.25 CR were used to verify the model developed. Total residual stresses were directly measured by soaking softwood specimens in hot water of 60°C. It was found that 1) about 50% of maximum total residual stress in densified fir and pine specimens could be released in the first several minutes after soaking in the hot water; 2) the mechanically induced residual stresses increased with increasing CR; 3) the mechanically induced residual stresses released from pine were slightly larger than those from fir; and 4) the mathematical model developed in terms of CR could well simulate the release of mechanically induced residual stress with increasing time.
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