• To determine the compressive strength of the wooden cubes • To observe the anisotropic behavior of the wood. • To determine the modulus of elasticit...
Related Theory Compressive Strength Maximum stress that a material can bear in compression is termed as compressive strength.
Modulus of Elasticity (E) It is the ratio of unit stress to unit strain and is determined by slop of straight line from zero to proportional limit in a stress strain diagram.
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→
=
→
=
→
=
∝
Modulus of Stiffness (K) It is defined as force f orce required producing unit deformation.
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Isotropic Materials Isotropic materials are those materials which exhibits same properties in different direction for example steel.
Anisotropic Materials Anisotropic materials are those materials which do not show same properties in different directions.
1 Compression Test on Wooden Cubes
07Civil73
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Failure Mechanism in Different Directions
Failure of Wooden Cube when load is applied o
Parallel to Grains
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 fiber act as column to the applied load and even after the failure of the single fiber f iber the rest of the fibers will keep on taking the load.
Failure of Wooden Cube when load is applied o
Perpendicular to Grains
When the load is applied perpendicular to the grains, the wooden sample takes comparatively comparatively less load. This is because the failure of the single fiber will lead to the failure of the whole sample. 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.
Procedure
First of all determine the dimension of all three sides of the wooden cube by the Vernier caliper.
Then fix the cube in the machine as shown in figure.
We increased the load in increments.
We noted the load and deflection readings on machine and deflection gauges respectively.
Calculations and Observations Cube Dimensions S pe pecimen
Comparison between Two Graphs %age strain on x-axis and Stress (MPa) on y axis 50.0000 45.0000 40.0000 35.0000 30.0000 25.0000
Parallel Perpendicular
20.0000 15.0000 10.0000 5.0000 0.0000 0 .0 0 0 0
1 .0 0 0 0
2 .0 0 0 0
3 .0 0 0 0
4 .0 0 0 0
5 .0 0 0 0
6 .0 0 0 0
Cube after Fracture
Figure 2 when the load is applied parallel to grains
Figure 1 when the load is applied perpendicular to grains
5 Compression Test on Wooden Cubes
07Civil73
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Comments When the load is applied parallel to grains, the failure of the sample is due to shear failure, which results in cracks at edges, and during load applied perpendicular to grains, the failure is also shear failure where fiber have slide over one another. If we are designing any wood structure, we should keep in mind that load should be applied on wood to the parallel to its grains because doing so strength is approximately ten times more than when the load is applied perpendicular to grains.
When the Load is Applied Parallel to Grains
In this type of loading shear failure cracks start developing near the edges at about 45 degrees.
But due to Platen Effect and Eccentricity, failure is not purely shear failure, so that’s way we see a in between crack in the sketch of wooden cube after fracture.
6 Compression Test on Wooden Cubes
07Civil73
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When the Load is Applied Perpendicular to Grains
In this type of loading, failure is also due to shear failure because in this thi s layers of wood have slides over each other.
