Effect of Test Piece Size on Rheological Behavior of Wood Composites

The effect of size and the combined effect of the size and moisture sorption of test pieces on the long term creep behavior of wood composites were studied. Small-, wide-, and semisize test pieces from each of three commercial wood composites, particleboard (PB), oriented strand board (OSB), and medium density fiberboard (MDF) were tested. Three exposure regimes, constant 20°C/65%, a single change from 20°C/65% to 20°C/85%, and cyclic changes between 20°C/30% and 20°C/90% relative humidity (RH), were used. It was found that the width of test pieces had no effect while the length had a significant effect on long term behavior of wood composites, but the effect is in contrast to that of short term modulus of rupture (MOR) which ranged from 0.06 to 0.13 for the shape parameter and from 0.09 to 0.26 for the length parameter depending on the types of wood composites. The average ratio of the relative creep (kc) of small-:wide-:semisize was 1.14:1.13:1.00 for PB, 1.26:1.21:1.00 for MDF, and 1.24:1.24:1.00 for OSB, with the shape parameter ranging from 0.04 to 0.19 and the length parameter from 0.13 to 0.26. Change in RH significantly aggravated the size effect on kc with the most significant under cyclic RH, for which the ratio of kc small- to semisize was 1.45 for PB and 1.27 for OSB after 3 months’ exposure. Edge sealing on small test pieces efficiently prevented the effect of moisture sorption but the size effect on kc with a reduction of about 30 and 20%, respectively, for edge sealed PB and OSB in weekly changing climate between 20°C/30% and 20°C/90% RH compared to unsealed small-size test pieces. The findings elucidate the importance to take into account the size effects on short term strength, compensated size effect on long term creep, and the combined effects of size and moisture on long term behavior when predicting the long term load carrying capacity of wood composites in construction.