Compression test how it is done, properties, examples

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Egbert Haynes

The compression test is an experiment carried out by progressively compressing a sample of material, for example concrete, wood or stone, known as test tube and observing the deformation produced by the applied compressive stress or load.

A compressive stress is produced by two forces applied to the ends of a body in order to reduce its length when compressing it..

Figure 1. Compression stress. Source: Wikimedia Commons. Adre-es / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

At the same time, its cross-sectional area widens, as can be seen in figure 1. As greater and greater stresses are applied, the mechanical properties of the material become apparent..

Article index

  • 1 How is compressive stress applied?
  • 2 Properties and data obtained
    • 2.1 Reliable results
    • 2.2 Stress-strain curve
  • 3 Examples of comprehension efforts
    • 3.1 Concrete
    • 3.2 Gray cast iron
  • 4 References

How is compressive stress applied?

To apply the compressive stress, the sample, preferably in the shape of a cylinder of circular cross-section, is placed in a machine, known as universal testing machine, which compresses the specimen progressively in previously established pressure increments.

The points of the stress curve (in newton / mtwo) versus the strain ε are graphed as they are generated. The stress is the ratio between the applied force and the cross-sectional area, while the strain is the quotient between the shortening ΔL and the original length of the specimen Lor:

ε = ΔL / Lor

From the analysis of the graph, the mechanical properties of the material under compression are deduced.

As the experiment progresses, the sample shortens and widens, as seen in Figure 1, or it also twists or bends, depending on the initial length of the specimen. The experiment ends when a failure or fracture occurs in the sample.

Figure 2. Compression test on a concrete specimen. Source: Wikimedia Commons.

Properties and data obtained

From the compression test, the mechanical properties of the material before compression are obtained, for example the modulus of elasticity and the compressive strength, very important in the materials used in construction.

If the material to be tested is brittle, it will eventually fracture, so ultimate strength is easily found. In this case, the critical load, the type of failure presented by the material and the shape of the fracture are noted..

But if the material is not brittle but ductile, this ultimate resistance is not going to manifest easily, so the test is not prolonged indefinitely, since as the stress increases, the state of internal stress of the specimen stops be uniform. At that point the validity of the test is lost.

Reliable results

For the results to be reliable, it is necessary that the internal fibers of the material remain parallel, but the internal friction causes the fibers to bend and the tension to be homogeneous.

The first thing to do is consider the initial size of the specimen before starting the test. The shorter test tubes, called compression specimen, tend to take the shape of a barrel, while the longer test tubes, called column specimens, they bulge.

There is a criterion known as reason for slenderness, which is the quotient between the initial length Lor and the radius of gyration Rg:

r = Lor / Rg

Turn Rg = √ (I / A) Where I is the moment of inertia and A is the cross-sectional area.

If the slenderness ratio is less than 40, it works as a compression specimen, and if it is greater than 60 it works as a column. Between 40 and 60 the specimen would have an intermediate behavior that is preferable to avoid, working with ratios less than 40 or greater than 60.

Stress-strain curve

The compression test is analogous to the tension or tensile test, only instead of stretching the specimen to rupture, it is the compressive strength that is tested this time..

The behavior of the material tends to differ in compression and tension, and another important difference is that the forces in the compression test are greater than in the tension test..

Figure 3. Tensile or tensile stress and compression stress. Source: F. Zapata.

In a compression test, for example of an aluminum sample, the stress-strain curve is ascending, while in the tension test it rises and then falls. Each material has its own behavior curve.

Figure 4. Curve of the compression test for aluminum (left) and that of the corresponding tensile test (right). The specimen fractures at point 4. Source: F. Zapata / Wikimedia Commons

In compression, the stress is considered negative by convention, as well as the deformation produced, which is the difference between the final and initial length. For this reason, a stress-strain curve would be in the third quadrant of the plane, however the graph is taken to the first quadrant without problem.

In general, there are two well differentiated zones: the elastic deformation zone and the plastic deformation zone..

Figure 5. Compression test curve for a ductile material. Source: Beer, F. Mechanics of materials.

Elastic deformation

It is the linear region of the figure, in which the stress and strain are proportional, the constant of proportionality being modulus of elasticity of the material, denoted as Y:

σ = Y. ε

Since ε is the strain ΔL / Lor, has no dimensions and the units of Y are the same as those of effort.

When the material works in this zone, if the load is removed, the dimensions of the specimen return to the original ones.

Plastic deformation

It comprises the non-linear part of the curve in figure 5, although the load is removed, the specimen does not recover its original dimensions, being permanently deformed. Two important regions are distinguished in the plastic behavior of the material:

-Yield: deformation increases without increasing applied load.

-Deformation: if the load continues to increase, eventually the specimen ruptures.

Examples of comprehension efforts

Concrete

The figure shows the response of concrete in a compression test (third quadrant) and in a tension test (first quadrant). It is a material with a compression response different from that of tension..

The range of linear elastic response of concrete to compression is greater than to tension, and from the extension of the curve it is seen that the concrete is much more resistant to compression. The breaking value of concrete before compression is 20 × 10N / mtwo.

Figure 6. Compression and tension test curve for concrete The numerical values ​​depend on the proportion of the mixture. Source: Beer, F. Mechanics of materials.

For this reason, concrete is suitable for building vertical columns that must withstand compression, but not for beams. Concrete can be reinforced by steel rods or metal mesh held under tension while the concrete dries.

Gray cast iron

It is another material with good compression behavior (AC curve in the third quadrant), but fragile when subjected to tension (AB curve in the first quadrant)..

Figure 7. Compression and tension test curve for gray cast iron. Source: Hibbeler, R. Mechanics of materials.

References

  1. Beer, F. 2010. Mechanics of materials. McGraw Hill. 5th. Edition.
  2. Cavazos, J.L. Materials mechanics. Recovered from: youtube.com.
  3. Giancoli, D. 2006. Physics: Principles with Applications. 6th. Ed prentice hall.
  4. Hibbeler, R. 2011. Mechanics of materials. 8th edition. Pearson.
  5. Valera Negrete, J. 2005. Notes on General Physics. UNAM.

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