Pressure-modified criteria, yield stresses

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] ... 

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). 

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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