To Perform Compression Test on Wooden Sample, when load is applied

To Perform Compression Test on Wooden Sample, when load is applied

  1. Parallel to Grains
  2. Perpendicular to Grains

Objectives:

  1. To determine compressive strength of the wooden sample
  2. To observe the anisotropic behavior of the wood sample.
  3. To determine the modulus of Elasticity

Apparatus:

5000 KN Compression Testing Machine, Wooden Sample, Vernier Caliper, Deflection Gauges.

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.

Modulus of Stiffness (K)

It is defined as force required producing unit deformation.

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.

Failure Mechanism in Different Directions:

Failure of Wooden Cube when load is applied 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 the rest of the fibers will keep on taking the load.

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 why we see a crack in between  the grains. See the sketch below of wooden cube after fracture.

C:\Users\Administrator\Desktop\abc\load.PNG

Failure of Wooden Cube when load is applied Perpendicular to Grains

When the load is applied perpendicular to the grains, the wooden sample takes 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 compared to when the load is applied perpendicular to grains.

C:\Users\Administrator\Desktop\abc\per.PNG

Procedure:

Before testing (Parallel to the Grains):

  1. Select two 4″ x 4″ x 4″ specimens for test.
  2. Record any significant defects in the specimens.
  3. Measure cross sectional area and size of each side of cube minutely.

On the Compression Testing Machine:

wood test perpedicular to grain Verify load application speed of 0.15 N/mm2/sec.

  1. Place the specimen in the machine and apply a small holding load (under 100lb).
  2. Continue loading until a peak load is reached or, if no peak is reached, until the load approaches a constant value.
  3. Repeat to test for other specimen.

Perpendicular to the Grains:

  1. Select two 4″ x 4″ x 4″ specimens
  2. Record any significant defects in the specimens.
  3. Place the bearing plate on top of the specimen at the center of the specimen. Make sure the plate is perpendicular to the specimen, and preferably against a radial surface, if possible
  4. Measure all sides of both specimens accurately, including area to be loaded.

On the Compression Testing Machine:

  1. Verify load application rate of 0.15 to 0.35 N/mm2 /sec.
  2. Place the specimen in the machine and apply a small holding load (under 100lb).
  3. Continue loading until failure, or just beyond 0.1 inch deformation, which we consider to be “failure by excessive deformation.”

Calculations and Observations

Students should measure the wooden samples and record the data as given in the table

Wooden Specimen Test type Length (mm) Width

(mm)

Height

(mm)

Weight(gm)
Parallel to grains Before Test
After Test
Perpendicular to grains Before Test
After Test

Readings of Experiment when load is applied Parallel to Grains

Sr # Load (KN) Compressive Strength (MPa) Deflection Gauge reading Deformation (mm) Stress (MPa) E (MPa) Remarks
1
2
3

Readings of Experiment when load is applied Perpendicular to Grains

Sr # Load (KN) Compressive Strength (MPa) Deflection Gauge reading Deformation (mm)/Dimensions after loading Stress (MPa) E (MPa) Remarks
1
2
3

Students should measure the dimensions of the samples after load and work out the strain, both lateral and axial. They should work out the poissons’ ratio and E also

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 fibers slide one over another. If we are designing any wood structure, we should keep in mind that load should be applied on wood, parallel to its grains because doing so strength is approximately ten times more than when the load is applied perpendicular to grains

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