Yield (engineering)
Below the yield point, a material will deform elastically and will return to its original shape when the applied stress is removed.
The yield strength is often used to determine the maximum allowable load in a mechanical component, since it represents the upper limit to forces that can be applied without producing permanent deformation.
For most metals, such as aluminium and cold-worked steel, there is a gradual onset of non-linear behavior, and no precise yield point.
[1] In solid mechanics, the yield point can be specified in terms of the three-dimensional principal stresses (
It is often difficult to precisely define yielding due to the wide variety of stress–strain curves exhibited by real materials.
Yield strength testing involves taking a small sample with a fixed cross-section area and then pulling it with a controlled, gradually increasing force until the sample changes shape or breaks.
Longitudinal and/or transverse strain is recorded using mechanical or optical extensometers.
However, it is possible to obtain stress-strain curves from indentation-based procedures, provided certain conditions are met.
There are several ways in which crystalline materials can be engineered to increase their yield strength.
To move this defect (plastically deforming or yielding the material), a larger stress must be applied.
This increases the yield strength of the material since now more stress must be applied to move these dislocations through a crystal lattice.
is the yield stress, G is the shear elastic modulus, b is the magnitude of the Burgers vector, and
This relieves a tensile strain directly below the dislocation by filling that empty lattice space with the impurity atom.
Where the presence of a secondary phase will increase yield strength by blocking the motion of dislocations within the crystal.
A line defect that, while moving through the matrix, will be forced against a small particle or precipitate of the material.
[18] That experimentally measured yield strength is significantly lower than the expected theoretical value can be explained by the presence of dislocations and defects in the materials.
Indeed, whiskers with perfect single crystal structure and defect-free surfaces have been shown to demonstrate yield stress approaching the theoretical value.
The applied stress to overcome the resistance of a perfect lattice to shear is the theoretical yield strength, τmax.
During monotonic tensile testing, some metals such as annealed steel exhibit a distinct upper yield point or a delay in work hardening.
Yield Point Elongation (YPE) significantly impacts the usability of steel.
[20] The mechanism for YPE has been related to carbon diffusion, and more specifically to Cottrell atmospheres.
Coil and edge breaks may occur during either initial or subsequent customer processing, while fluting and stretcher strain arise during forming.
[20] When these conditions are undesirable, it is essential for suppliers to be informed to provide appropriate materials.
The presence of YPE is influenced by chemical composition and mill processing methods such as skin passing or temper rolling, which temporarily eliminate YPE and improve surface quality.
[20] Despite its drawbacks, YPE offers advantages in certain applications, such as roll forming, and reduces springback.
