Microscopic examination of wood fiber reveals the fibril angle in the S2 layer. The smaller microfibril angle (MFA) in "mature" wood gives it a higher stiffness and strength than that of juvenile or compression wood. [1]
Sensitivity to Fundamental Wood Properties
in the
The Metriguard Model 7200 High Capacity Lumber Tester (HCLT) and the CLT Continuous Lumber Tester use a direct bending measurement that properly responds to the effects of fundamental wood properties in lumber, and therefor the visual graders do not have to consider these effects when they make their final grade determination.
Microscopic examination of wood fiber reveals the fibril angle in the S2 layer. The
smaller microfibril angle (MFA) in "mature" wood gives it a higher stiffness
and strength than that of juvenile or compression wood. [1]
Mechanical properties of stiffness and strength in wood are related to species, moisture content, and the wood fiber characteristics. Wood fiber characteristics vary between the low specific gravity earlywood and higher specific gravity latewood that make up the ring structure, and between higher microfibril angle juvenile wood and the low microfibril angle wood found in the more mature portions of the tree stem. Compression wood has other undesirable characteristics including high microfibril angle, poor dimensional stability and short grain. The use of specific gravity as a single indicator of wood stiffness and strength can be misleading because wood of the same specific gravity can have a wide range of mechanical properties due to fibril angle and grain length.
I recently visited the Forintek Canada laboratory in Vancouver BC, where I saw a demonstration of two 2x4's of the same species, with identical specific gravity and very similar small tight knots. One of these had a stiffness that was at least twice that of the other. The difference, we were told, was the fibril angle. Direct measurement of E (Modulus of Elasticity) properly sorts these two pieces. Density measurements do not.
WOOD STRUCTURE
Wood fiber is composed of layers of crystalline cellulose (fibrils) wrapped in a cylindrical shape with an open center, or "lumen". Wood fiber is broken down into five distinct layers and are referred to as layers ML, P, S1, S2, and S3. The fibrils in the "S2" layer form the major strength-producing portion of the wood fiber. The fibril angle varies from over 35 degrees in juvenile and compression wood to 10 degrees or less in mature wood. Juvenile wood will be found in a relatively small part of a tree stem which grew in close proximity to other trees, and will be in as much as 100 percent of the stem in low-density stands. So, the portion of juvenile wood is not necessarily correlated with specific gravity in the wood.
Normal juvenile and mature wood fibers are approximately rectangular in cross-section with the wall thickness varying from 10% of the total cross section in earlywood to about 80% of the cross section in latewood. The gross wood structure is composed of fibers in the longitudinal direction bound together by a lignin "glue" and bundles of ray cells oriented in the radial direction in the wood which act as reinforcement rods to increase the radial strength.
COMPRESSION WOOD
Compression wood fibers have a circular cross section with the S3 layer missing, have extreme fibril angle in the S2 layer and have short grain length. Dimensional stability is poor and the E and strength are low in compression wood, making it very undesirable as a structural element.
Compression wood can be found visually from color changes if it appears on the surface of a cut piece and if the visual grader knows what to look for. The effects of microfibril angle and grain length are generally hidden from the visual grader. A measurement of bending stiffness reveals the structural effects of both juvenile and compression wood which can be present at any specific gravity or density.
MOISTURE CONTENT
In the living tree the walls of the wood fiber are saturated with moisture and will have free water partially or fully filling the cavity (lumen) inside the fiber. As the moisture content is reduced to the "fiber saturation" level the free water is removed and the fiber walls remain saturated. No change is observed in the mechanical properties of the wood, but the apparent specific gravity decreases. Between fiber saturation and the 15-19% level where the wood is processed and marketed, the wood shrinks and the modulus of elasticity and strength both increase. In service the wood will undergo further changes in moisture content and eventually wind up at an equilibrium moisture content of 6-12% depending upon climatic conditions. From 15-19% and 6-12% there is some increase in E and strength, but not a major amount of shrinkage, so we have improved the dimensional stability and mechanical properties.
Mechanically graded lumber (MSR, or MEL) can be produced at any moisture content. The moisture content is really a separate specification from the mechanical grade. If green lumber is produced using a bending test, we simply adjust the stiffness to the estimated dry value. Moisture above the fiber saturation level affects the specific gravity but has no significant effect on the bending measurement.
If moisture varies in a density based grading process, the change is reflected as a higher specific gravity as the moisture content increases (including above the fiber saturation level), and because higher specific gravity correlates to higher strength, the higher moisture content piece is graded to a higher estimated stiffness and strength, just the opposite direction the mechanical properties change.
CONCLUSIONS
The CLT and HCLT properly respond to the effects of the fundamental wood structure and moisture content, through the direct measurement of lumber stiffness. Use a Metriguard 7200 HCLT or CLT for machine processing MSR, MEL or E-rated lam grades to ensure that the vital strength reducing characteristics are properly accounted for in your grading.
X-ray testing misses these essential strength-determining characteristics whenever these properties are not directly reflected in specific gravity so it cannot be relied upon to make an accurate strength estimation when juvenile wood or compression wood are present. X-ray measurements respond in the wrong direction to changes in moisture content.
1. Josza, L.A., and G.R. Middleton (1994) "Wood Quality Attributes and their Practical Implications", Forintek Canada Corp., 2655 East Mall, Vancouver BC Canada V6T 1W5-- [Limited supply of copies available from Metriguard.]
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