Understanding Tensile Testing: Stress, Strain, and Material Behavior

The purpose of tensile tests is to subject a standard cylinder to an axial traction, increasing the load until the specimen fails. This test measures a material's resistance to a static or slowly applied force. The strain rates in a tensile test are typically very small.

Stress-Strain Curve

The test measures the deformation of the specimen between two fixed points as the applied load increases, and this is plotted as a function of stress. Generally, this curve has four distinct areas:

Elastic Deformation

In this area, the deformation is distributed throughout the specimen, is of small magnitude, and if the applied load is removed, the specimen recovers its initial shape. The coefficient of proportionality between stress and strain is called the elastic modulus or Young's modulus, which is characteristic of the material. All steels have the same modulus of elasticity, but their resistance can vary greatly. The highest stress achieved in this region is known as the yield strength, which marks the onset of this phenomenon. There may be two zones of elastic deformation: the first linear and the second curved, with the yield limit marking the transition between them. Generally, the latter value has no practical interest, so a yield is defined as that for which there is a default extension in advance. This is obtained by drawing a line parallel to the proportional stretch (straight) with an initial deformation equal to the conventional.

Fluency

This is the sudden deformation of the specimen without an increase in the applied load. The creep phenomenon occurs when impurities or alloying elements block dislocations in the crystal lattice, preventing its sliding mechanism by which the material deforms plastically. Once the yield strength is reached, the dislocations are freed, producing a sharp deformation. The deformation in this case is also evenly distributed along the test tube but concentrates on areas where dislocations have been freed. Not all materials exhibit this phenomenon, in which case the transition between elastic and plastic deformation is not clearly seen.

Plastic Deformation

If the applied load is removed in this area, the specimen only partially recovers, remaining permanently deformed.

Pinch (Necking)

At one point in the test, the deformation concentrates in the central part of the specimen, exhibiting a marked reduction in the cross-section. After this point, the strains continue to accrue until the specimen fails. The necking is responsible for the decrease in the stress-strain curve. Brittle materials do not undergo significant necking or plastic deformation, breaking abruptly. The test determines the breaking load, ultimate load, and tensile strength.