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Stress-Strain Relationships In Engineering Materials


Stress-strain is one of the major phenomenon in engineering materials, and this relationsip is generally used in various kinds of mechanical engineering disciplines. Understanding of stress-strain relationship in engineering is very important thing. So in this article, we eill explain;

  • Tensile tests of material specimens,
  • Stress-strain relationship and its formula,
  • Stress-strain charts of materials and interpretation of them.

What Is Stress-Strain Relationships In Engineering Materials?

Tensile test system of material specimens.

Tensile test specimens are produced and prepared according to various standards. So the specimens and the tensile test system must have standards. Tensile test specimen has a constant cross sectional area as shown above. The specimen is fitted to tensile test fitting from its threaded edges. These threaded edges of specimen are assemblied to the crossheads of test device.

One of these crossheads is moving and one of them is fixed. With this movement, there is a axial force application on crosssectional area of test specimen. We can calculate the stress of the specimen with applied axial force and the cross sectional area such as;

σ= F/A;

F = applied axial force on test specimen

A = Cross-sectional area of test specimen

Also the strain is calculated according to axial deformation of test specimen with a strain gauge that attached on the surface of test specimen.

Typical stress-strain graph of a material.

According to the Stress and Strain values that calculated through time, a graph called Stress-Strain graph can be obtained. This Stress-Strain graphs are specific for every material that are tested with standard specimen sizes. We can read lots of informations about materials from these stress-strain graphs.

Specimen shape change in tensile test.

In the tensile test, the general behaviour of test specimen is like above illustrations. In the fist phase, there is no application of force. After the force application starts by the test device, the constant elongation starts. After a point, a necking occurs then a abrupt breaking occurs.

If we take a look to the stress-strain graph of this phenomenon, the constant elongation take place at the elastic region that shown on the graph above. Where the necking begins, the plastic region starts. Tensile graph ends at where the breakaway occurs.

We can read the some of informations about material on tensile graph that;

Yield Strength: This is the stress value that where 0.2% ofset is taken from elastic region. If we draw a line that parallel to the elastic region, the intersection point of this line will give us the tensile strenght of material.

Tensile Strength:  This is the top point of stress-strain graph as shown above at tensile graph. Maximum stress value that is observed on test specimen.

These values about an engineering material, frequently used in engineering strength calculations.

So the stress-strain relationship of materials can be explained like that. Leave your comments and questions about Stress-Strain relationships of engineering materials!

Sources of images: Fundamentals of modern manufacturing

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