Yield criteria of polymers

In contrast to metals, the yield strength of polymers is different in compression and tension. Frequently, the yield strength in uniaxial compression is 20% to 30% larger than in uniaxial tension (see also section 8.4). To account for this, the von Mises yield criterion is augmented by terms that depend on the hydrostatic stress state. We will discuss two possible approaches. 

The parabolically modified yield criterion assumes that the yield surface is a paraboloid centred on the hydrostatic axis (figure 3.24(a)). Its radius depends on the hydrostatic stress Thyd according to 

The yield strengths in compression and tension are denoted Rc and Rp. If a negative number results in the square root, the radius is taken to be zero the material yields. 

If we insert the radius of the yield surface, R Thyd) into equation (3.29) 

The yield criterion Ueq pM = Rp is used for compressive and tensile loads. 

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The yield criteria of polymers have been reviewed by Ward (7) and more recently by Raghava et al. (8). Except for the craze yield criteria of Sternstein and Ongchin (9) and Bowden and Oxborough (10), most of the yield data can be described by a pressure-modified, von Mises-yield criterion. The corresponding yield surface is everywhere convex. A typical yield locus on the [Pg.103]

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