Nondestructive evaluation of modulus of elasticity of southern pine LVL: Effect of veneer grade and relative humidity

Nondestructive testing (NDT) methods, stress-wave propagation, and transverse vibration were used to evaluate the modulus of elasticity (MOE) of laminated veneer lumber (LVL). Five types of LVL, fabricated with southern pine veneers of B, C, and D grades and liquid phenolic formaldehyde adhesive, were tested flatwise at environmental conditions of 65% and 95% relative humidity (RH) and 75 degrees F (23.9 degrees C) to examine the influence of veneer grade and RH on some nondestructive mechanical properties of LVL. All LVLs, 1.5 in. (3.81 cm) thick x 3.5 in. (8.89 cm) high x 96 in. (243.84 cm) long, consisted of 13 plies of southern pine veneer, and their structural designs were: (I) all B grade veneers, (II) 2 plies of B grade veneer on both faces and all C grade veneers in the core plies, (III) 2 plies of B grade veneer on both faces and all D grade veneer in the core plies, (IV) all C grade veneers, and (V) all D grade veneers. Results indicated that MOE of LVL predicted by NDT was influenced by the veneer grade, and specimens fabricated with better grade veneers showed a higher value of MOE. A significant decrease in the MOE determined by both NDT methods was found when RH increased from 65% to 95% at 23.9 degrees C (75 degrees F). The MOE measured by the stress-wave method was found to be more sensitive to the RH change than that determined by the transverse-vibration method. A lognormal distribution accurately described the distributions of MOEs determined by both nondestructive methods at both RH levels. As expected, a significant increase in moisture content (MC) in the LVL resulted from increasing RH levels. However, changes in densities of the tested materials due to the RH changes were found to be smaller. Results also indicated that regardless of the RH level, MOE determined from the stress-wave test was consistently higher than that obtained from the transverse-vibration test. For comparison, the results of tests on southern pine No. 1 and No. 2 grade lumber, commonly used in light-frame construction, are also presented. Analysis of the correlation between the static bending and NDT MOEs was made and results suggested that edgewise static bending MOE of LVL can be predicted with reasonable accuracy by the stress-wave testing. Good correlations were not observed between the edgewise static bending MOE and the nondestructive MOE evaluated by flatwise transverse vibration. However, excellent correlations between static bending and both NDT MOEs were observed in southern pine dimension lumber. Correlations between the MOEs evaluated by both nondestructive methods were found to be fair for LVL specimens.