Anisotropy and grain: why direction decides everything

Three directions

Wood behaves differently along three directions. The longitudinal direction runs along the stem and the cell axes. Across the grain, the radial direction runs from the pith across the rings, and the tangential direction runs along a ring. A Canadian Wood Council lecture (North America) gives stiffness in the three directions as roughly 20 : 1.6 : 1 and says strength parallel to the grain is about 20 times that perpendicular to it.

Shrinkage in three directions

Shrinkage starts at about the fibre saturation point. The Wood Handbook gives longitudinal shrinkage of only 0.1 to 0.2 % for most species. Tangential is roughly twice radial; the portal's anisotropy article puts the usual ratio at 1.5 to 2.5. LWF Bayern (Germany) gives, green to oven-dry:

For service changes LWF gives shrinkage per 1 % MC change between 5 and 20 %: oak 0.16 % radial and 0.36 % tangential.

Worked example (own arithmetic, linear estimate). An oak board 200 mm wide dries from 20 to 12 %, 8 points. Flat-sawn (width tangential): 8 x 0.36 = 2.88 %, about 5.8 mm. Quarter-sawn (width radial): 8 x 0.16 = 1.28 %, about 2.6 mm. The flat-sawn board moves 2.25 times as much.

Why the ring angle matters

Where a board sits in the log decides its ring angle. In a flat-sawn board the rings run roughly parallel to the wide faces. Wengert and Meyer (University of Wisconsin, North America) state that flat-sawn lumber cups toward the bark side. Flat-sawn boards move most in width, cup most and check first on the face. Quarter-sawn boards move about half as much in width (Conners, University of Kentucky) but dry more slowly and are more sensitive to crook.

All warp is differential shrinkage. Cup comes from the tangential-radial difference across the width. Bow and crook come from longitudinal differences (juvenile wood, reaction wood, sloping grain). Twist is linked to spiral or interlocked grain, which Conners says makes gums, sycamore and beech hard to dry flat. Frühwald (Lund University, Sweden) found that restrained drying or top loading reduced twist in spruce.

Fibre direction: slope of grain

Strength is highest when the load follows the fibres. The Canadian lecture lists the remaining fraction of straight-grain strength as 93 % at a slope of 1 in 20, 81 % at 1 in 10 and 55 % at 1 in 5. The lecture does not name the property and the portal could not confirm the figures in the Wood Handbook, so treat them as indicative. At face value, 1 in 10 costs 19 % and 1 in 5 costs 45 %.

Two kinds of cross grain

• Diagonal grain comes from sawing a tapered or crooked log parallel to the bark rather than the pith. It is a sawing issue, and the sawmill can correct it.
• Spiral grain winds around the stem and is a growth feature. In Swedish progeny trials of Norway spruce (Hallingbäck and others, 2008), mean grain angle under bark near breast height was 1.76 degrees, but about 7 % of progeny trees had 4 degrees or more. In Swedish Norway spruce studs, grain angle and ring curvature explained 73 % of the variation in twist (Johansson and Kliger, 2002).

Spiral grain is hard to judge by eye on a rough face. Teknos (Europe) warns that it causes severe deformation in spruce, so spiral logs belong in other product lines.

What it means at the saw and in the kiln

• At the saw. Choose the pattern for the product: quarter- or rift-sawn where stability matters, flat-sawn where movement is acceptable. Keep grain parallel to board edges.
• In the kiln. Dry flat-sawn and quarter-sawn stock in separate charges where possible (the portal's inference, since they dry at different rates), restrain the stack, and expect surface checks first on flat-sawn faces.

Key points

• Strength and stiffness are far higher along the grain than across it (about 20 : 1).
• Longitudinal shrinkage is 0.1 to 0.2 %; tangential is about twice radial (spruce 7.8 against 3.6 %, Germany).
• A flat-sawn board moves about twice as much in width as a quarter-sawn one and cups toward the bark side.
• Sloping grain cuts strength sharply (1 in 10 leaves about 81 %, property unnamed in the source).
• Spiral grain is a growth feature that causes twist; diagonal grain is a sawing fault.