Today we are talking about the ultimate tensile and yield strength. In particular, about stainless steel.
Yield strength
The yield strength is the material property defined as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.
The yield point determines the limits of performance for mechanical components, since it represents the upper limit to forces that can be applied without permanent deformation. Up to this amount of stress, stress is proportional to strain, so the stress-strain graph is a straight line, and the gradient will be equal to the elastic modulus of the material.
High strength steel and aluminum alloys do not exhibit a yield point, so this offset yield point is used on these materials. When the yield point is not easily defined based on the shape of the stress-strain curve an offset yield point is arbitrarily defined. Commonly, its is value set at 0.2% plastic strain. (abbr. Rp0.2)
UTS – Ultimate tensile strength
The ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking. Some materials break very sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, experience some plastic deformation and possibly necking before fracture.
For non-deformable materials, it is the nominal stress at which a round bar of the material, loaded in tension, separates. For deformable materials, it occurs at the onset of necking at strains preceding breakage. (separation)
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