Amorphous hydrostatic stress

Tphe literature is replete with examples showing that the application of hydrostatic pressure enhances the ductile behavior of strained amorphous polymers. In this paper we present a possible explanation of this effect and two experiments demonstrating the enhanced ductility of polymers under compressive shear stresses applied orthogonally to the plane of shear. 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

Important information on the atomic properties of impurity centres and defects are obtained by recording the transmission of a sample while subjected to an external pressure. The pressure can be hydrostatic and it can be applied to amorphous as well as monocrystalline samples. This is usually performed by inserting the sample in a diamond anvil cell (DAC). When the samples are monocrystalline, the stress can be applied along one symmetry axis of the crystal. In the following section, the set-ups with which a uniaxial stress can be applied to a sample are described. 

As noted, the field of molecular simulation is relatively new, and a detailed review of it is beyond the scope of this text and we introduce here a few of the more relevant references. One of the first applications of molecular mechanics to polymers was by Theodorou and Suter (93,94), who modeled atactic polypropylene as an amorphous cell subjected to a range of stress conditions (hydrostatic pressure, pure strain, and uniaxial strain). Such modeling generally gives reasonable estimates of the elastic constants of a material [within 15% (79)], providing the density of the glass is correctly modeled. 

Fig. 8.18. Yield surface of an amorphous thermoplastic that can fail by crazing or formation of shear bands. If the hydrostatic tensile stress is sufficiently large, crazing occurs before shear bands (after [92,132])...

Amorphous PA Cooling below T Thermodynamic long-term aging, T>Tj-50 = C Hydrostatic, internal stresses All types of internal stresses Volume relaxation. Non-equilibrium Viscoelastic relaxation... 

---