Yielding means the start of breaking of fibres. Engineering materials and their applications flinn trojan pdf 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.
In structural engineering, this is a soft failure mode which does not normally cause catastrophic failure or ultimate failure unless it accelerates buckling. Yield load can be taken as the load applied to the centre of a carriage spring to straighten its leaves.
The lowest stress at which dislocations move. This definition is rarely used, since dislocations move at very low stresses, and detecting such movement is very difficult. Beyond the elastic limit, permanent deformation will occur. The elastic limit is therefore the lowest stress at which permanent deformation can be measured.
This requires a manual load-unload procedure, and the accuracy is critically dependent on the equipment used and operator skill. For elastomers, such as rubber, the elastic limit is much larger than the proportionality limit. Also, precise strain measurements have shown that plastic strain begins at low stresses. The value for this is commonly set at 0.
The offset value is given as a subscript, e. High strength steel and aluminum alloys do not exhibit a yield point, so this offset yield point is used on these materials. Some metals, such as mild steel, reach an upper yield point before dropping rapidly to a lower yield point. The material response is linear up until the upper yield point, but the lower yield point is used in structural engineering as a conservative value.
If a metal is only stressed to the upper yield point, and beyond, Lüders bands can develop. This section does not cite any sources. Please help improve this section by adding citations to reliable sources.
Unsourced material may be challenged and removed. A yield criterion, often expressed as yield surface, or yield locus, is a hypothesis concerning the limit of elasticity under any combination of stresses. There are two interpretations of yield criterion: one is purely mathematical in taking a statistical approach while other models attempt to provide a justification based on established physical principles. Other equations have been proposed or are used in specialist situations.