The effect of temperature and strain level on the springback of wood compressed in the radial direction (Book 1 of 2)

Abstract

This report is a study of the influence of temperature and strain level of radial compression on the spring back of wood. Spring back has been defined as the lost work of compression. It is a major problem for the composite wood products industry. For composite products to perform as designed, the springback after pressing must be minimized. This study investigated both the initial springback which occurs immediately after pressing and extended springback which occurs from changes in the relative humidity and resulting moisture content of the wood. The study investigated responses across a ring.width for spruce and aspen. The real time video microscalar system designed by Dr. Tabarsa for his Ph.D. work at the University of New Brunswick was used to collect stress and strain information. This test method allowed the application of Dr. Tabarsa's findings on the nature of cell failure in earlywood and latewood to be applied. This level of study had never been conducted so there were no set testing standards that could be applied. The study produced the following qualitative findings: Initial Springback The first two minutes after the load has been removed is when rapid (clearly visible with microscope) spring back occurs. It takes more than 15 minutes for the elastic compression to recover once the load has been removed. Samples which are heated during compression will lose moisture and will continue to spring back after the initial period until they have equalized at a moisture content. Extended Springback There is no relationship between the temperature of the sample during compression and · the extended spring back. The higher the elastic to plastic deflection ratio the greater the amount of springback. When the samples were exposed to higher moisture levels the.amount of springback would increase. This means that the sample with the least plastic deflection would have the smallest permanent dimension changes from exposure to water. This study also produced useful information on the limitations of this test method. This information is very valuable for any researcher which would like to study this subject area in more detail or try to use a similar test method.