The macroscopic yielding behaviour of polymers in multiaxial stress fields

Experimental data on yielding of polyoxymethylene (POM) and polypropylene (PP) have been obtained in a wide range of complex triaxial stress conditions. These complex triaxial stress states have been produced by superimposition of simple stresses such as uniaxial compression, uniaxial tension and pure shear on hydrostatic pressure of various intensities. The actual yield surfaces of both polymers were constructed using the data. The yield surface of POM is a cone-shaped with a pointed apex and straight edges, while the yield surface of PP is cone-lime and non-linear, also with a pointed apex. A yield criterion is shown to very closely predict the observed behaviour of both polymers.