Yield Strength As we apply a low level of stress to a material; the material initially exhibits elastic deformation. In this region the strain that develops is completely and quickly recovered when the applied stress is removed. However, as we continue to increase the applied stress the material begins to exhibit both elastic and plastic deformation. The critical stress value needed to initiate plastic deformation is defined as the elastic limit of the material. In metallic materials, this is usually the stress required for dislocation motion, or slip to be initiated. In polymeric materials, this stress will correspond to disentanglement of polymer molecule chains or sliding of chains past each other.
The proportional limit is defined as the level of stress above which the relationship between stress and strain is not linear. In most materials the elastic limit and proportional limit are quite close. However, neither the elastic limit nor the proportional limit values can be determined precisely. Measured values depend on the sensitivity of the equipment used. We, therefore, define them at an offset strain value (typically, but not always, 0.002 or 0.2%). We then draw a line starting with this offset value of strain and draw a line parallel to the linear portion of the engineering stress-strain curve. The stress value corresponding to the intersection of this line and the engineering stress-strain curve is defined as the offset yield strength, also often stated as the yield strength. For some materials the transition from elastic deformation to plastic flow is rather abrupt.
This transition is known as the yield point phenomenon. When we design parts for load-bearing applications we prefer little or no plastic deformation. As a result we must select a material such that the design stress is considerably lower than the yield strength at the temperature at which the material will be used. We can also make the component cross-section larger so that the applied...
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