Properties of Wood
The objectives of this experiment are:
1. to determine the strength of wood under compressive loading applied at different angles with respect
to the orientation of the wood grains
2. to determine the modulus of rupture and the modulus of elasticity of wood subjected to bending
3. to study the failure characteristics of wood under compression and bending
4. to determine the moisture content of wood.
Essential Apparatus
A Universal Testing Machine
Micrometer or ruler
PART I. Compression Test
Materials and Specimens
Wood to be used in the experiment is Douglas Fir, or as specified, with clear grains and free from defects.
The specimens consist of three blocks, nominally 1.5 in. by 1.5 in. by 3 in., with wood grains oriented at
0o, 45o, and 90o with respect to the longitudinal axis of the specimen.
Procedure
1.
Measure the cross-section and length of the specimen and record the dimensions on the data
sheet. The ends of the specimens should be plane and at right angles to the axis of the specimen.
2.
Place the specimen in the machine.
3.
Apply the load continuously until the specimen fails. Record the maximum load.
4.
Draw a sketch, in perspective, indicating the grain of the wood and the manner of failure.
5.
For the specimen with the grains oriented at 90o, plot the load vs. deformation and determine the
proportional limit from the curve.
6.
Compute the compressive strength (for the specimen with 0o and 45 o grain orientation) or the
proportional limit (for the specimen with 90o grain orientation).
PART II: Flexure Test
Materials and Specimens
The type of wood to be used is Douglas Fir or as specified. The beam specimen nominally measures 1.5
in. x 3.5 in. by 36 in. or as specified and has clear wood grains parallel to the longitudinal axis of the
beam. The wood shall be free from defects.
Procedure
1.
Mark the center and end points of the specimen for a 32 in. span.
2.
Place the beam in the machine with the ends supported by rollers. Place the loading block at the
center of the beam. Position the whole assembly such that the loading block is at the center of the
machine's loading head.
3.
Estimate the maximum failure load from the equation (A) given below by assuming a reasonable
value for the modulus of rupture of the wood (ref: Table 16-3, Materials of Construction by Lai).
4.
Lower the loading head until a small compressive load is applied to the beam. Apply the load
continuously at the rate of approximately 500 pounds per minute until the maximum load is
reached.
5.
Sketch the appearance of the failure.
6.
Plot the load-deflection curve and compute the modulus of rupture and the elastic modulus of the
wood using the following formula:
Modulus of Rupture:
MR = (3PmaxL)/(2bh2)
(A)
Elastic Modulus:
E = (P/)(L3/4bh3)
(B)
Where
Pmax = max. load
L = span length (not specimen length)
P/ = slope of the linear portion of the load-defection curve
b = width
h = height
PART III: Moisture Content
Material and Specimen
The type of wood to be used is Douglas Fir or as specified. The wood shall be free from defects.
Procedure
1. Weigh the wood sample.
2. Place in a 110C oven for 24 hours. Re-weigh.
3. Calculate moisture content as a percentage of the oven dry mass.
Results
PART I - Compression Test of Wood
1. Tabulate the compression test results including: sample dimensions, cross-sectional area, grain
orientation, maximum load, compressive strength, and proportional limit (for the specimen with 90
grain orientation).
2. For the 90 grain orientation, plot load vs. deformation and stress vs. strain.
3. Include a sketch of each specimen at failure.
PART II - Flexure Test of Wood
1. Plot load vs. beam deflection and show the slope and load at rupture
2. Report the elastic modulus and the modulus of rupture
3. Include a sketch of the specimen at failure.
PART III – Moisture Content of Wood
1.
Report the moisture content of the wood sample.
PART IV – Comparing Stress-Strain Data
1. On one plot, plot stress-strain data for: concrete (from data in homework #2), steel, HDPE, and wood
at 90. Use the convention that compressive stress and strain are negative and tensile stress and strain are
positive.
Discussion
PART I - Compression Test of Wood
1. Discuss the types of failure observed for the specimens at different grain orientations
2. Estimate the compressive strength of the wood with the direction of loading at 10o and 30o with
respect to the grain orientation.
3. If the specimen were fully saturated with water how would the compressive strength parallel to the
grain and the proportional limit perpendicular to the grain compare to the test results obtained in this
lab? Why?
PART II - Flexure Test of Wood
1. Discuss the types of failure observed and identify the cause or causes of each failure observed on the
specimen.
2. Estimate the modulus of elasticity, and the modulus of rupture. If the specimen were fully saturated
with water, how would these properties compare to the test results obtained in this lab?
3. Predict the failure mode if the specimen were tested at an orientation of 90o to that at which it was
tested in this lab.
PART III – Moisture Content of Wood
1. Compare the moisture content of the wood to typical values for fiber saturation point (fsp), and
characterize the sample as green, at/near fsp, air dried, kiln dried, or oven dried, based on criteria
given in class notes.
PART IV – Comparing Stress-Strain Data
1.
Compare the stress-strain behavior of concrete, steel, HDPE, and wood.
WOOD COMPRESSION TEST RESULTS
Type of wood: __________________________
Sample No.
Sample Dimensions (in.)
Width
Depth
Angle
(degree)
Load
(lb.)
Strength
(psi)
Height
1
2
3
Sketch of Failure
45
0
90
WOOD FLEXURE TEST RESULTS
Sample Dimensions: Width (in.) =
Length (in.) =
Depth (in.) =
Span (in.) =
Maximum Load ______________
Sketch of Failure
WOOD MOISTURE CONTENT RESULTS
Mas received = ____________
Moven dry = ___________
Laboratory Instructor: __________________________________________________