Tensile tests consist in qualifying and quantifying the elongation response of a sample when subjected to a stretching force. Indeed, solid samples can deform to a lesser extent depending on the force applied, and finally end up breaking under mechanical stress.
Elastic and plastic mechanical behavior
Solid samples often have an elastic mechanical behavior: their deformation is then proportional to the force applied, and it is above all reversible. It can be high in some cases (relentless example: rubber bands), or very low (metals). The Young’s Modulus is the parameter that characterizes the magnitude of the deformation with respect to the applied force.
Some materials have a plastic mechanical behavior: the deformation they undergo is no longer reversible. Often materials behave elastically up to their elastic limit, after which their behavior becomes plastic (unless their mechanical failure has taken place before).
Ductile, fragile materials, etc.
There is a kind of classification in the type of mechanical behavior of solid samples. Here are some examples:
The samples are said to be fragile when their mechanical rupture occurs in the elastic domain. Cast iron is one of these samples.
The so-called ductile samples are samples which are elastic up to their elastic limit, then acquire a plastic behavior. Many materials are in this class. One way to differentiate these samples is the type of mechanical response in the plastic zone.
How are tensile tests carried out?
Tensile tests are carried out with precision mechanical benches, capable of regulating and measuring tensile, compression or even torsion forces. They make it possible to measure and regulate displacements to measure variations in length, but also to regulate the temperature of test specimens.
The sample at rest is placed between two jaws of the device, the spacing of which is previously adjusted.
With this type of device allowing an infinity of different tests to be carried out, it is therefore necessary to choose the right test modes and parameters according to the need:
- Tensile force constant, or progressive, with measurement of stretching (length variation), up to breakage or not
- Constant or progressive elongation, with measurement of the tensile force applied, until rupture or not
- Constant temperature or controlled variation