advertisement

Compression testing wood Wood has unique, independent properties in the three mutually perpendicular axes: longitudinal, radial, and tangential. The longitudinal axis is parallel to the grain, the tangential axis is perpendicular to the grain but tangent to the annual rings, and the radial axis is normal to the annual rings (and perpendicular to the grain direction). Modulus of elasticity The modulus of elasticity in the longitudinal direction is the highest, and is between 1-2 x 106 psi (6.9 – 13.8 x 103 MPa) at 12% moisture content, depending on the species. The modulus of elasticity of many hardwoods (e.g. Birch, Hickory, and Oak) is closer to or higher than the high range, but that of many softwoods (e.g. Douglas Fir, Hemlock, Redwood, and Loblolly Pine) is closer to the low range. The modulus of elasticity (both parallel and perpendicular to the grain) depends on the moisture content, and decreases (from dry condition) by 1-3% for every 1% increase in the moisture content. For example, the modulus of elasticity (parallel to the grain) of Western Hemlock at 12% moisture content is 1.63 x 106 psi, whereas it is 1.31 x 106 psi in green condition. The average modulus of elasticity of higher grade lumber (Construction, No. 1, etc.) is nearly the same as that of clear wood. In a particular species, the modulus of elasticity (as well as other mechanical properties) increases with the amount of summerwood. The modulus of elasticity in the radial direction is about 10% of that in the longitudinal direction, and that in the tangential direction is about one-half of that in the radial direction. Compressive strength The compressive strength of lumber parallel to the grain is much higher than that perpendicular to the grain. Columns, posts and members of a truss are subjected to axial loads parallel to the grain of the wood. When a column rests on a beam, the load from the column creates compressive (bearing) stress on the beam that is perpendicular to the grain of the wood. The compressive strength of most softwood lumber parallel to the grain is in the range of 2000-4000 psi. The stress at the proportional limit is about 80% of this strength. The compressive strength perpendicular to the grain varies between 12-18% of that parallel to the grain. The compressive stress at the proportional limit, for loading perpendicular to the grain, is about 12-25% of that for loading parallel to the grain. In dry condition, lumber can possess significantly higher compressive strength. For example, the compressive strength (parallel to the grain) of Douglas fir in green condition is about 3700 psi, while that at 12% moisture content is about 7000 psi. Seasoning increases the compressive strength. Typically, the compressive strength (parallel and perpendicular to the grain) increases an average 4-6% for every 1% decrease in moisture content. Species such as Oak, Maple, Douglas Fir, Southern Yellow Pine, and Western Larch possess very high compressive strengths. TEST Purpose: To determine the parallel-to-grain compressive strength of a clear wood specimens. Related Standard: ASTM D143 Equipment: Universal testing machine, compressometer. Sample: Clear wood specimen of Alder, Walnut, Maple, Pine and Fir will be tested. Six samples of each, with nominal dimensions ¾” x ¾” x 3” will be tested. Procedure: 1. 2. 3. 4. 5. The ends of the specimen should be level Measure the distance between grain, and the angle of the grain. Position the specimen under the crosshead of the testing machine, and apply load continuously at 0.024 in. /min. Continue the loading until the proportional limit is well passed. Get the deformation readings to the nearest 0.001 in. Report: 1. Calculate the compressive strength and elastic modulus of each wood specimen. 2. Calculate the average compressive strength and elastic modulus, and the standard deviation of each. 3. Make a table listing the compressive strength and elastic modulus for each type of wood tested. 4. For each type of wood (Maple, Oak, Fir, Pine), make a graph of breaking strength vs. thickness of the grain, and breaking strength vs. angle of the grain with respect to the longitudinal axis. Discuss how the strength varies with each of these variables. 5. Compare the failure strength of each wood, and comment on the variation of compressive strength with respect to wood type (e.g., compare the strength of all the fir samples, and compare differences between fir and walnut and…). Comment on how variation of properties should be allowed for in the design process.