Plastic's Yield Strength

Taken from http://www.manterra.com/GE_Design_Guide.pdf:

The classical definition of the yield strength of a material is the stress level which will cause a small amount of permanent deformation, (standard is 0.2 % strain). This definition originates from the steel industry, but it also applies to plastics. However, the yield point of thermoplastics is difficult to determine because of their viscoelastic nature. The yield point for classical materials such as metals can easily be determined and tested by loading the sample to a point slightly beyond the proportional limit, (linear portion of the stress/strain curve), and then unloading it and measuring the deflection. Therefore, with some experience it is easy to predict. When plastic materials are tested in a similar manner, what originally appears to be permanent deformation can be recovered slowly. The amount of recovery is dependent on the temperature and the rate at which the sample is tested. The yield point of a thermoplastic cannot be determined from a stress/strain curve. For this reason, thermoplastic yield points are typically estimated or shown as a yield range, (see n FIGURE 13).

Due to the rheological behaviour of plastics the yield strength is depending on the speed with which the strain grows.

Also plastic usually stretch and become unusable way before they yield.

www.Matweb.com has lots of material properties.

Nearly all Material has a yield strength, although some are harder to measure than others, such as an elastomer, or glass.

an example of typical yield strength for extruded delrin plastic: 4.0 ksi

plastics can vary greatly depending upon their composition