Yield criterion, pressure-modified

Lower values of the yield stress measured in tension compared to those measured in compression suggest that the effect of pressure, which is important for polymers, is not accounted for in this criterion. Hence, appropriate correction has to be made in order to account for the effect from external pressure. The most frequent version of pressure-dependent yield criterion is the modified von Mises criterion [20] ... 

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]

The commonly used criterion for shear yielding in cavity-free rigid polymers is a pressure-modified von Mises criterion (Ward 1983) ... 

The presence of voids increases markedly the pressure sensitivity of the material. Gurson (1977a, b) modified the von Mises criterion to be used for porous solid that contains well-distributed small voids. He applied a continuum treatment to a cavitated ductile material containing a volume fraction of voids and obtained the following yield criterion ... 

AGS may be expressed as proportional to compressive yield stress), yt (fracture strain), the plastic zone size, and the square of the concentration factor, K. The influence of hydrostatic pressure was taken into account with a modified von Mises criterion (Chapter 12). 

A first stage in the analysis consisted in determining the elastic or plastic nature of the contact loading using the known yield properties of the bulk DGEBA/IPD system. The latter were established experimentally assuming that they obey a modified Von Mises criterion taking into account the effect of the hydrostatic pressure. This criterion may be written as ... 

ModifiGd Tr SCa CritGrion. A simple way to modify the Tresca criterion to allow for a pressure dependence is to make the critical shear yield stress a linear function of hydrostatic pressure ... 

Annealing at 160 C promoted the enlargement of the yielding envelopes of PP homopolymer and PP modified with 10%POE and did not modify the pressure dependence of the yield stress of these materials. Von Mises criterion does not yield to the same fitting quality when applied to the annealed polymers, i.e. increased crystallinity. Further work is in progress to find a more suitable criterion for annealed PP polymers. 

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