Yield point measurement
What is theyield point?
Some materials have an elastic domain which is a place of stress for which their deformation remains proportional to the applied mechanical stress. Outside this zone, there is either plastic deformation (irreversible) of the material (ductile material), or total mechanical rupture of the latter (brittle material). The yield point is therefore the stress for which the material passes from the elastic to plastic domain.
On a stress-strain diagram, the elastic zone is in the form of a straight line whose guiding coefficient is the Young’s modulus E of the material. The elastic limit is the stress at which the line ends.
It is also often called “elastic limit”. Its unit is the Pascal, but it is most often expressed in MPa or GPa.
How to measure the yield point of a sample?
It is conventionally determined during tensile tests. An increasing tensile stress is applied to the material, during which the strain in the tensile direction is measured. The curve of tension obtained then makes it possible to delimit the range of stress of the elastic field (right), and thus to deduce the yield point from it.
It can also be determined during compression tests. It is then noted differently, because its value is often different from the value measured during tensile tests (with the exception of certain metals).