Fundamental considerations and application of acoustics as a nondestructive evaluation technique of wood quality properties

Abstract

The growing global demand for wood and wood products has resulted in a tremendous investment in tree improvement programs with the aim of improving growth, form, and disease tolerance. There has been significant success in achieving this overarching objective in Pinus taeda L (loblolly pine) over the past few decades in the southern United States, but at the expense of wood quality. Currently, about 58% of the total United States timber is extracted from 13 million hectare of southern pine plantations in the southeastern US. Of all the seedling established plantations in the southeastern United States, loblolly pine (Pinus taeda) constitutes about 84% making loblolly pine the most dominant tree species in the southern United States. Fast growing trees produce wood with a considerable amount of juvenile wood which has reduced modulus of elasticity (stiffness), modulus of rupture (strength), high microfibril angle and therefore poses utilization challenges. Consequently, the design values for visually graded southern pine lumber (including loblolly pine) were generally reduced in 2013 to reflect the material quality on today’s market. As a future direction for improvement of loblolly pine, stakeholders want to select and deploy elite families with superior wood quality properties to meet future demands. Acoustic techniques have been found to be one of the nondestructive tools for rapid characterization of seedlings and trees in tree breeding programs. This study focused on understanding some fundamental concepts of acoustic techniques as a nondestructive evaluation tool for rapid characterization of trees in tree breeding and forest management operations with special reference to fifteen selected 14- year old elite loblolly pine families grown on two sites in the southern United States. The effect of distance between probes of the tree acoustic tool on velocity was investigated as well as the sensitivity to variation in equilibrium moisture content of the two main types of acoustic tools. Fundamentally, cellulose, hemicellulose, and density are important drivers of strength, stiffness, and velocity. It was found that cellulose and lignin are the highest and lowest respectively contributor to acoustic stress wave propagation at the molecular level with cellulose being the most important conductor of the stress wave while lignin acts as a stress wave dispersant and hemicellulose acts as a special coupling agent between these components. These polymeric constituents are thus important drivers of sound wave propagation at the molecular level while density played a co-subsequent role at the macro scale. With respect to the distance between probes, for distances below 60 cm, the waveform is dominated by the fundamental frequency of the transmitter probe hence the velocities determined within 10 to 60cm are not statistically different. Furthermore, velocity determined at distances below 60 cm is significantly higher than that determined at 120 cm suggesting velocity is dependent on the between probes distance. Using 120 cm as a standard distance, the dynamic stiffness of the tree is overestimated by 13, 50, 102, and 197 MPa respectively for 100, 80, 60, and 40 cm. Consequently, distances from 80 to 120 cm constitute the optimum range for velocity determination for this tree (time - of – flight) acoustic tool. Finally, microfibril angle and the fiber wall thickness are the main anatomical properties driving the signal at the micro-level. The two main types of acoustic tools used for stiffness characterization in the forest and wood industry are the resonance (harvested wood products) and time – of – flight (used for tree). The equilibrium moisture content of loblolly pine significantly affects the velocity measured below and above the hygroscopic region. The acoustic velocity decreased by 33.9 m/s and 28.8 m/s for time – of – flight (TOF) and the resonance tools respectively for a unit increase in EMC below fiber saturation point (FSP). The change was lower for EMC above FSP - 5.4 m/s for TOF and 6.1 m/s for resonance. This statistically nonsignificant slope in velocity above the FSP coupled with better accuracy in predicting green static MOE using velocity with oven-dry density supports the hypothesis that the cell wall material controls the acoustic velocity while the water in the cell lumen plays a nonsignificant role in stress wave propagation. It was demonstrated that using the density at test (green density) to predict the dynamic MOE of trees and freshly harvested logs with the moisture content above the FSP, the dynamic MOE will be least 40% higher than similar prediction using oven-dry density. Based on this observation, oven – dry or basic density is recommended for dynamic MOE computation for tree or products with moisture content above FSP. The effects of site and genetic families on morphological, anatomical and wood quality properties of fifteen 14-year-old loblolly pines stock were studied. The dynamic MOEs of the trees were determined using the velocity with either basic density (DMOEB) or green density (DMOEG). There were significant site and genetic family effects on diameter, microfibril angle, fiber length, and dynamic MOE. While there was a nonsignificant difference in DMOEB between sites; velocity2 for site 1 was significantly higher than site 2, but DMOEG was higher for site 2 than site 1. This suggests that depending on the type of density used in computing the dynamic MOE, a difference decision can be made. Therefore, practitioners should take care when extrapolation velocity reading of trees from different locations. Additionally, about half of the selected genetic families had statistically similar morphological and wood quality properties between sites suggesting that half of the planting stock exhibit stability and homogeneity in the southeastern USA. Therefore, foresters and landowners have the opportunity to select genetic families (T26 and T 18 – proprietary code) which are superior in both the morphological and wood quality traits for plantation development across sites. Also, the landowners have the opportunity to match some elite planting stocks to a specific site for greater productivity and quality outturn due to the significant effect of site or family or site by family interaction.