Compression Test of Wood Parallel and Perpendicular to the Grain
Laboratory Experiment:
I.
Objectives:
II.
Apparatus:
Universal Testing Machine
Vernier Caliper
Timber specimen
III.
Set – Up:
IV.
Theory of Discussion:
When a material is loaded in compression, the maximum load it can withstand without crushing or
rupturing is called its compressive strength. For a wooden sample, anticipatingly, the failure will be
brittle, and the material will fail without prior warnings. However, the peak load that the wooden sample
bears, also depends on the orientation of its grains relative to the line of action of externally-applied
compressive load. The modulus of elasticity of wood is the ratio of stress to strain within the elastic
range of the material. The strain values can be obtained from deflection gauges attached to the wooden
samples during the test, whereas, the stress is calculated using applied load and the contact area of the
sample. Mathematically, the significance of elastic modulus is that it aids in determining the deformation
response of material under any applied loading in the elastic range. The stiffness of a material is the
energy it absorbs within the elastic range while being elastically deformed under the applied loading. In
other words, the resistance of a material to deformation caused by an externally-applied load within the
elastic range is an index of its stiffness. Mathematically, if the line of action of an applied load varies but
the strength or mechanical properties of material remain the same, the material is said to be isotropic.
Contrary to this is an anisotropic material, which shows different mechanical properties subject to
varying lines of action of external load. Wood is an anisotropic material. In the context of compressive
strength, when the load is applied parallel and perpendicular to the grains of the wooden sample, the
crushing load varies. Partial compression parallel to the grain was studied by Noguchi et al. Though they
proposed a theoretical evaluation method with a spreading effect, the impact of the spreading effect of
real wood specimens was not clear. Size effects of the tension strength are often explained by the
Weibull weakest-link theory. When the Weibull weakest-link theory is applied, the statistical distribution
of the test result should be Weibull distributed. By definition, the ultimate compressive strength of a
material is that value of uniaxial compressive stress reached when the material fractures. A load
deformation curve is plotted by the instrument and Stress-strain for that would look similar to the
following: The compressive strength of the material is corresponding value at the red point shown on the
curve. In a compression test, there is a linear region where the material follows Hooke’s Law. Hence for
this region where this time E refers to the Young’s Modulus for compression. In this region, the material
deforms elastically and returns to its original length when the stress is removed. This linear region
terminates at what is known as the yield point. Above this point the material behaves plastically and will
not return to its original length once the load is removed. On compression, the specimen will shorten.
The material will tend to spread in the lateral direction and hence increase the cross-sectional area. In a
compression test the specimen is clamped at the edges. For this reason, a frictional force arises which
will oppose the lateral spread. This means that work has to be done to oppose this frictional force hence
increasing the energy consumed during the process. There is a difference between the engineering stress
and the true stress. By its basic definition the uniaxial stress is given by: where F = Load applied [N] and A
= Area [m^2]. As stated, the area of the specimen varies on compression. In reality therefore the area is
some function of the applied load i.e. A = f(F). Indeed, stress is defined as the force divided by the area at
the start of the experiment. This is known as the engineering stress and is defined by, A0=Original
specimen area. Correspondingly, the engineering strain would be defined by but the force required to
compress varies when it is applied in parallel from perpendicular to the fibers. Because when force
applied in perpendicular to fibers, the deformation is high, as one of the fibers fractures it has to deform
as shown in the following figure [4]. But in the fracture of the fibers when force applied in the
perpendicular direction to case of parallel application of the force even if one of the fibers breaks it does
not deform easily as in the case of perpendicular application as shown in the figure. FIG. 4. Fracture of
the fibers when force applied in the parallel direction to them. A theoretical model based on equilibrium
and assuming a linear plastic behavior of the timber was developed by Van der Put in 2008. Elastic
properties in directions other than those along the orthotropic axes can be obtained from elastic theory.
Elastic properties in directions other than along the natural axes can be obtained from elastic theory.
V.
Procedure:
Parallel to the grains
Select a cube timber specimen for the test
Record any significant defects in the specimens.
Measure cross sectional area and size of each side of cube using a vernier caliper
Place the specimen in the universal testing machine and apply a small holdingload (under
100lb).
5. Continue loading until a peak load is reached or, if no peak is reached, until the load
approaches a constant value.
6. Record the result of the compression test
Perpendicular to the grains
1.
2.
3.
4.
Select a cube timber specimen for the test
Record any significant defects in the specimens.
Place the bearing plate on top of the specimen at the center of the specimen.
Make suretheplate is perpendicular to thespecimen, and preferablyagainsta radial surface,
if possible
5. Measure all sides of both specimens accurately, including area to be loaded.
6. On universal testing machine, place the specimen in the machine and apply a small holding
load (under 100lb).
7. Continue loading until failure, or just beyond 0.1 inch deformation, which we consider to
be “failure by excessive deformation.”
1.
2.
3.
4.
VI.
Data and Results:
Type of test
Parallel to the
grains
Perpendicular
to the grains
VII.
Dimensions
of the specimen
(mm)
Length
Width
Height
Length
Width
Height
Sample Computation:
Failure Load
(N)
Cross Sectional
Area of Cylinder
(mm2)
Ultimate
Compressive Strength
(MPa)
VIII.
Observation and Interpretation:
IX.
Conclusion: