Earlywood
and Latewood Longitudinal Shrinkage in Loblolly Pine
Research by
Seth Pfeil, Chad Mertz/ Prof.
Steve Cramer
Dept of Civil & Environmental Engineering
Prof. Rod Lakes
Dept of Engineering Mechanics
Dave Kretschmann
Forest Products Laboratory
Wood
tends to shrink in all directions as it dries. Ideally, this shrinkage
would be uniform throughout a structural member such that it could
be accurately predicted. In reality, warping caused by differential
shrinkages is a common occurrence in wood members. The variation
of shrinkage properties throughout the cross section of a tree
may account for these warp-causing differentials.
Shrinkage
properties vary greatly from earlywood to latewood. Earlywood
and latewood bands together form one growth ring in a cross-section
of a tree. Earlywood is produced at the beginning of the growing
season while latewood develops in the later months. The lighter
colored earlywood has larger lumens and thinner cell walls as
compared to the darker colored latewood (See Figure 1). These
physical differences lead to a specific gravity in latewood that
is roughly twice that of earlywood. Specific gravity is believed
to be a large contributor to the amount of longitudinal shrinkage.
Shrinkage
properties vary not only within a single growth ring but also
throughout the cross-section and along the longitudinal axis of
a tree. The variance in properties by location can largely be
attributed to changes in both specific gravity and microfibril
angle. Microfibril angle is a measure of the orientation of the
fibrils in the thickest layer of the cell wall. The fibrils act
as reinforcement for the cell wall and have the largest single
influence on shrinkage behavior in southern pine.
Trends
as discussed above will be determined by measuring the longitudinal
shrinkage from green to oven dry in 1 x 1 x 30 mm specimens created
from three loblolly pine trees. Microfibril angle will be determined
using x-ray diffraction techniques. The specimens are classified
as strictly earlywood or latewood and come from four different
heights and up to seven different ring numbers in four cardinal
directions. The testing will provide a large amount of data that
will be used to determine how earlywood and latewood longitudinal
shrinkage varies by cardinal direction, ring number, and height.
