Compression

1. Experiment NO:6
COMPRESSION TEST FOR ANISOTROPIC MATERIAL
K.Vijay Simha, K.Sai Balaji, K.Anudeep, K.Praghna, Juili Gulhane, Jesna Rose K A, Jitendra
Aerospace Engineering, 3rd
sem
( Dated: 15/09/2015)
Abstract
In constructions most of the materials undergo compression rather than tension. So the com-
pressive properties of materials is very important to know. Wood is an anisotropic material. So its
properties depend on the directions. In a piece of wood, there are fibers (refer figure 1 [4]), going in
one direction and this direction is referred to as ”with the grain”. Compression test of wood gives
different result along the grain and perpendicular to grain.
I. INTRODUCTION
Compression is the reduction of the volume of the ob-
ject under applied stress on it. In the study of strength
of materials, the compressive strength is the capacity
of a material or structure to withstand loads tending
to reduce size. It can be measured by plotting applied
force against deformation in a Universal Testing Machine.
Some materials fracture at their compressive strength
limit; others deform irreversibly, so a given amount of
deformation may be considered as the limit for compres-
sive load. Compressive strength is a key value for de-
FIG. 1. Figure showing the fibers aligned in wood perpendic-
ular to the text
sign of structures. Compressive strength is measured on
a universal testing machine. Wood has unique, indepen-
dent properties in the three mutually perpendicular axes:
The compressive strength of lumber parallel to the grain
is different from the perpendicular to the grain.
II. THEORY
By definition, the ultimate compressive strength of
a material is that value of uniaxial compressive stress
reached when the material fractures. A loaddeformation
curve is plotted by the instrument and Stress-strain for
that would look similar to the following:
The compressive strength of the material is corre-
FIG. 2. True Stress-Strain curve for a typical specimen
sponding value at the red point shown on the curve. In a
compression test, there is a linear region where the ma-
terial follows Hooke’s Law. Hence for this region
σ = Eε (1)
where this time E refers to the Young’s Modulus for
compression. In this region, the material deforms elasti-
cally 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 mate-
rial 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 increas-
ing 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:
σ =
F
A
(2)
where:
F = Load applied [N]

2. 2
A = Area [m2
].
As stated, the area of the specimen varies on compres-
sion. 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,
σe =
F
Ao
(3)
A0=Original specimen area . Correspondingly, the engi-
neering strain would be defined by:
εe =
l − l0
l
(4)
But the force required to compress varies when it is ap-
plied in parallel from perpendicular to the fibers. Be-
cause when force applied in perpendicular to fibers, the
deformation is high, as one of the fibers fracture it has
to deform as shown in the following figure [4]. But in the
FIG. 3. Fracture of the fibres when force applied in the per-
pendicular direction to them
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 fibres when force applied in the par-
allel direction to them
III. PROCEDURE
• Firstly, the dimensions of the specimen was mea-
sured.
• Then the specimen was placed on the anvil.
• The machine was then started and the loading was
given by the means of hydraulic pressure gradually.
• Simultaneously, the readings of load and displace-
ment was noted.
• When the fracture point was reached, the dimen-
sions of the specimen were taken.
• Figure showing the machine used for the test [4].
FIG. 5. Figure showing the Universal Testing Machine avail-
able in Strength of Materials Lab, IIST
• Force was applied perpendicular to the fibers as
shown in the figure 3 [4].
• Force was applied parallel to the fibers as shown in
the figure 4 [4]
• The force was applied until the wooden block frac-
tures.
IV. OBSERVATIONS
The dimension of the given wooden block is
5cmx5cmx5cm.
With naked eye the fibers were observed and placed on
the anvil.

3. 3
FIG. 6. Wooden blocks used for compressive test
V. RESULTS
Ultimate Tensile Strength is equal to load applied per
unit area. Area of cross section of the wooden block is 25
cm2
. Fracture of the wooden block when force applied
in the perpendicular to fibers is shown in the following
figure [4].
Fracture of the wooden block when force applied in the
FIG. 7. Fracture of the wooden block when force applied in
the perpendicular to fibers
parallel to fibers is shown in the following figure [4].
FIG. 8. Fracture of the wooden block when force applied in
the parallel to fibers
The ultimate compressive strength of the block when
force applied parallel to the fibers is 1.204x107
N/m2
and
when force applied perpendicular to the fibers is 4.7712x
107
N/m2
.
TABLE I. Table showing values of Deformation for the cor-
responding load applied in the perpendicular to fibers
Deformation(mm) Load(KN)
0 0
0.1 0.04
0.3 0.3
0.4 1.2
0.4 1.6
0.5 1.88
0.5 2.2
0.5 2.7
0.6 3.2
0.7 3.3
0.7 4.2
0.8 5.08
0.8 6.5
0.8 7.5
0.9 7.8
0.9 9.1
1 11.08
1 12.5
1.1 13.4
1.1 15.6
1.2 16.8
1.3 19.9
1.4 23.05
1.5 25.2
1.6 26.9
1.7 28.7
1.7 29.6
1.8 29.8
1.9 30.5
2 31.1
2.1 31.66
2.2 32.6
2.3 32.87
2.4 33.07
2.5 33.31
2.6 33.72
2.7 33.88
2.8 33.95
2.9 34.05
3 34.2
3.2 34.6
3.3 34.76
3.4 34.98
3.5 35.12
3.6 35.22
3.7 35.29
3.8 35.36
3.9 35.41
4 35.46
4.2 36.17
4.4 36.28
4.6 36.98
4.8 37.17
5 37.29
5.2 37.35

4. 4
Deformation(mm) Load(KN)
5.4 37.47
5.6 37.64
5.7 37.75
5.9 37.82
6 37.91
6.1 38.02
6.2 38.11
6.4 38.23
6.6 38.4
6.8 38.5
6.9 38.62
7 38.7
7.2 38.89
7.4 38.98
7.6 39.17
7.8 39.32
8 39.51
8.1 39.67
8.2 39.73
8.4 39.65
8.6 39.81
8.7 39.89
8.8 39.97
8.9 40.09
9 40.21
9.2 40.39
9.4 40.59
9.6 40.79
9.7 40.89
9.8 40.97
9.9 41.13
10 41.23
10.2 41.43
10.4 41.63
10.6 41.83
10.8 42.03
11 42.29
11.2 42.49
11.4 42.75
11.6 43.03
11.8 43.29
12 43.99
12.2 43.83
12.4 44.07
12.5 44.25
12.6 44.39
12.8 44.63
13 44.91
13.2 45.25
13.3 45.43
13.5 45.59
13.6 45.81
13.7 45.99
13.8 46.13
14 46.33
14.1 46.61
14.3 46.85
Deformation(mm) Load(KN)
14.4 47.05
14.5 47.33
14.6 47.49
14.8 47.75
14.9 47.93
15.1 48.13
15.2 48.41
15.3 48.69
15.5 49.09
15.7 49.37
15.9 49.65
16 49.93
16.2 50.37
16.4 50.73
16.6 51.27
16.7 51.51
16.8 51.73
17 52.15
17.1 52.37
17.3 52.87
17.5 53.29
17.6 53.79
17.7 54.03
17.8 54.39
18 54.97
18.2 55.49
18.3 55.83
18.4 56.13
18.7 56.77
18.9 57.61
19.1 58.31
19.3 59.17
19.4 59.55
19.5 59.93
19.6 60.35
19.7 60.89
19.9 61.27
20 61.47
20.2 62.41
20.3 62.93
20.5 63.83
20.7 64.79
20.8 65.23
21 66.23
21.2 67.25
21.4 68.09
21.5 68.89
21.7 69.93
21.8 70.43
21.9 70.97
22 71.45
22.2 72.51
22.4 73.39
22.5 74.55
22.7 75.21
22.8 76.23
23 77.67

5. 5
Deformation(mm) Load(KN)
23.2 78.63
23.3 80.28
23.5 81.53
23.7 82.43
23.8 83.98
23.9 84.98
24 85.08
24.1 85.68
24.3 87.33
24.4 88.58
24.5 89.28
24.5 89.93
24.7 90.07
24.8 91.23
24.9 92.28
25 92.98
25.1 93.73
25.2 94.43
25.3 95.93
25.4 96.28
25.5 96.83
25.6 97.53
25.7 98.53
25.8 99.28
25.9 99.98
26.1 101.08
26.2 101.98
26.3 102.98
26.4 103.13
26.5 104.58
26.5 105.23
26.6 105.93
26.7 106.63
26.8 107.98
26.9 108.33
27 109.98
27.1 110.98
27.2 111.88
27.3 112.78
27.4 114.53
27.5 115.38
27.5 116.43
27.6 117.13
27.7 117.88
27.8 118.98
27.9 118.53
27.9 119.28
VI. DISCUSSION
When the load is applied parallel to grains, the wooden
sample will take more load to fail, the ability of wood to
take more load parallel to grains before failure is because
each fibre act as column to the applied load and even
after the failure of the single fibre the rest of the fibres
will keep on taking the load.When the load is applied
perpendicular to the grains, the wooden sample takes
comparatively less load. This is because the failure of
the single fibre will lead to the failure of the whole sam-
TABLE II. Table showing values of Deformation for the cor-
responding load applied in the parallel to fibers
Deformation(mm) Load(KN)
0 0.06
0 0.1
0 0.14
0 0.2
0 0.25
0 0.3
0 0.35
0 0.4
0 0.45
0 0.5
0 1
0 1.25
0 1.5
0.1 1.51
0.1 1.61
0.1 1.71
0.1 1.81
0.1 1.91
0.1 2.01
0.2 2.04
0.2 2.1
0.2 2.14
0.2 2.24
0.2 2.34
0.2 2.4
0.2 2.44
0.2 2.5
0.3 2.51
0.3 2.82
0.3 3.1
0.3 3.3
0.3 3.6
0.3 4.1
0.3 4.32
0.4 4.34
0.4 5.34
0.4 6.34
0.4 7.34
0.4 8.34
0.4 9.1
0.4 9.27
0.5 9.34
0.5 10.34
0.5 11.34
0.5 12.34
0.5 13.34
0.5 14.34
0.5 15.85
0.6 15.89
0.6 16.89
0.6 17.89
0.6 18.89
0.6 19.89
0.6 20.63
0.6 21.83
0.7 22.83
0.7 23.84
0.7 24.84
0.7 25.86
0.7 26.9
0.7 27.84
0.7 29.74
0.8 30.1

6. 6
ple. The strength of the wooden sample when the load
is applied parallel to the grains is about ten times more
as compare to when the load is applied perpendicular to
grains [3].
REFERENCES
[1] https : //en.wikipedia.org
[2] https : //www.kstr.lth.se
[3] http : //www.scribd.com
[4] ImagesfromStrengthofMaterialsLab, IIST