Yielding means the start of engineering stress and strain pdf of fibres. 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. Yield load can be taken as the load applied to the centre of a carriage spring to straighten its leaves.
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. 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. 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. 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.
Yield occurs when the largest principal stress exceeds the uniaxial tensile yield strength. Although this criterion allows for a quick and easy comparison with experimental data it is rarely suitable for design purposes. This theory gives good predictions for brittle materials. This theory assumes that the stored energy associated with elastic deformation at the point of yield is independent of the specific stress tensor. Thus yield occurs when the strain energy per unit volume is greater than the strain energy at the elastic limit in simple tension.
It is proposed that yield occurs when the distortion component exceeds that at the yield point for a simple tensile test. When a metal is subjected to large plastic deformations the grain sizes and orientations change in the direction of deformation. As a result, the plastic yield behavior of the material shows directional dependency. Under such circumstances, the isotropic yield criteria such as the von Mises yield criterion are unable to predict the yield behavior accurately. Several anisotropic yield criteria have been developed to deal with such situations. In general, the yield strength increases with strain rate and decreases with temperature. C is a constant and m is the strain rate sensitivity.