University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

COMPRESSION OF WOOD.

General

The results of mechanical tests performed on wood specimens frequently show large scatter because wood is neither homogeneous nor isotropic. It is extremely difficult to perform tension tests on wood due to physical problems of gripping the specimen in a testing device. For this reason, mechanical tests on wood are usually limited to compression and bending tests.

Compression tests of wood are generally performed with the load applied both parallel to, and perpendicular to the grain. The parallel-load tests may be used to determine a modulus of elasticity, a highly variable property for construction lumber, with an upper range of approximately 1.8 x 10 6  psi, which is about six percent that of ferrous metals. Wood members loaded in the direction perpendicular to the grain include joists bearing on beam and other similar situations), hence, this test is performed mainly for the purpose of establishing minimum standards and specifications for this type of loading. The test, also, demonstrates the very small tolerance to loading in this direction compared to that parallel to the grain.

The modulus of elasticity determination from the parallel compression tests is done by a successive accumulation of data of simultaneous load and deformation in the same manner it is done for the tensile test of steel. Other mechanical properties such as the yield stress and the ultimate stress are selected from the stress-strain curve, although this may not provide a clearly defined yield point.

One of the most important mechanical properties to be determined is the ultimate stress of wood in compression, because working stresses defined by building codes and other standards usually employ a factor of safety based on this ultimate stress. This results in working stresses that are fractional values of the ultimate stress, and not the yield stress. The yield stress, therefore, is not a highly important property.

Objectives

In these tests, the behavior of wood under axial compression parallel to and perpendicular to the grain will be evaluated with the determination of certain mechanical properties of wood.

Equipment

•  Universal Testing Machine (UTM)

•  A mechanical compressometer (parallel test)

•  1-inch dial gage (perpendicular test)

•  Mechanical calipers

•  Tape measure

Specimen

1. Nominal 2” x 2” x 8” clear wood specimen for compression parallel to the grain.

2. Nominal 2” x 2” x 8” clear wood specimen for compression perpendicular to the grain.

Procedure

Click here to download the compression of wood procedures

•  Measure and record the actual dimensions of the specimens.

•  Attach the compressometer to the specimen for parallel testing. Place the specimen in the testing machine with its long dimension parallel to the direction of the load. Be careful to position the specimen in such a way that the load will be a truly centric load.

•  Select a load range on the testing machine that will supply a minimum of 20 kips. Apply the load slowly until failure occurs, obtaining simultaneous readings of load and axial deformation for every 0.001 inch of specimen deformation. Describe the type of failure (see Figure 1).

•  Place the perpendicular-to-the-grain specimen in the testing machine with its long dimension perpendicular to the direction of the load. See Figure 2 for the orientation of the cross grain with respect to the direction of the load.

•  Place a two-inch wide steel plate on top of the specimen to concentrate the applied load to the center two inches of the specimen.

•  Apply the load slowly until a total deformation of 0.10 in. is recorded on the dial gage. Record load and deformation every 0.001 inch to 0.040 inch and every 0.010 inch thereafter till at total deformation of 0.10 inch is obtained.

Required

1a. Plot the stress-strain curve for the parallel test.

1b. Plot the load-deformation curve for the perpendicular test.

•  Calculate the following for the parallel test:

•  Proportional limit stress,

•  Modulus of elasticity,

•  Ultimate compressive stress.

•  Calculate the following for the perpendicular test:

•  Proportional limit stress,

•  Compressive stress at 0.04 inch deformation,

•  Ultimate compressive stress (at 0.1 inch deformation).

•  Compare the experimentally observed values above with established values for clear wood. Calculate percent differences.

References

1. ASTM D143, Standard Methods of Testing Small Clear Specimens of Timber , Vol 4.10.

1. ASTM D2555, Standard Test Methods for Establishing Clear Wood Strength Values , Vol 4.10.

Figure 1. Types of Failure in Compression Parallel to the Grain.

Sample Data

Data obtained in CE 305 Fall 2003