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WISCONSIN STRUCTURES & MATERIALS TESTING LABORATORY (WSMTL)

Figure 1. Micrograph showing earlywood and latewood differences in softwood

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Earlywood and Latewood Longitudinal Shrinkage in Loblolly Pine
Research by
Seth Pfeil, Chad Mertz/ wsmtl@engr.wisc.edu
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.

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