Stress concentration polymers

Controlled stress viscometers are useful for determining the presence and the value of a yield stress. The stmcture can be estabUshed from creep measurements, and the elasticity from the amount of recovery after creep. The viscosity can be determined at very low shear rates, often ia a Newtonian region. This 2ero-shear viscosity, T q, is related directly to the molecular weight of polymer melts and concentrated polymer solutions. 

The presence of notches or sharp angles or of a few holes, voids, particle inclusions or small inserts tends to concentrate the stress. Different polymers vary in their notch sensitivity and this is presumably a reflection of how close they are to their tough-brittle transitions. The aim of the designer and processor must be to reduce such stress concentration to a minimum. 

Whilst the aliphatic nylons are generally classified as being impact resistant, they are affected by stress concentrators like sharp comers which may lead to brittle failures. Incorporation of mbbers which are not soluble in the nylons and hence form dispersions of rubber droplets in the polyamide matrix but which nevertheless can have some interaction between mbber and polyamide can be most effective. Materials described in the literature include the ethylene-propylene rubbers, ionomers (q.v.), polyurethanes, acrylates and methacrylates, ABS polymers and polyamides from dimer acid. 

N. Polyblends, block, and graft polymers IJ. Brittle Fracture and Stress Concentrators... 

In the previous section we discussed the ultimate strength of a polymer fibre o0. This value corresponds to the stress at which all secondary bonds in the fibre are broken. Due to the presence of the chain orientation distribution alone, it follows that even the strength of a polymer fibre without any flaws will never attain this value. Yet, fracture in a real fibre may not always initiate in the most disoriented domains. If there are inhomogeneities that lead to stress concentrations, fracture can also occur in domains at a smaller angle to the fibre axis. 

Host irradiated polymers show a continuing change in properties for a long period after irradiation. These post-irradiation effects may be attributed to (1) trapped radicals which react slowly with the polymer molecules and with oxygen which diffuses into the polymer (2) peroxides formed by irradiation in the presence of air or trapped within polymers irradiated in vacuum or an inert atmosphere) and slowly decompose with formation of reactive radicals, usually leading to scission, (3) trapped gases in glassy and crystalline polymers which cause localized stress concentrations. 

Apparently two opposing effects play a part On the one hand the glass fibres take over a part of the stress, thus partially releasing the stress on the polymer. On the other hand, fibres cause, in particular at their ends, stress concentrations, which can initiate crack formation. 

The explanation of the effect of secondary inclusions on the delocalization of shear banding is based on the concept of modification of the local stress fields and achieving favorable distribution of stress concentrations in the matrix due to presence of inclusions. This leads to a reduction in the external load needed to initiate plastic deformation over a large volume of the polymer. As a result, plastically deformed matter is formed at the crack tip effectively reducing the crack driving force. Above approximately 20 vol% of the elastomer inclusions. 

Incorporation of hard particles into the polymer matrix creates stress concentration, which induces local micromechanical deformation processes. Occasionally these might be advantageous for increasing plastic deformation and impact resistance, but usually they cause deterioration in the properties of the composite. Encapsulation of the filler particles by an elastomer layer changes the stress distribution around the particles and modifies the local deformation processes. Encapsulation can take place spontaneously, it can be promoted by the use of functionalized elastomers (see Sect. 6.3) or the filler can be treated in advance. 

Selective bond rupture at entanglement points, or other such sites of stress concentration, could magnify the effect of a chain scission in the presence of an external stress, but it seems unlikely that this is occurring since the sol-gel data actually indicated a (slightly) lower ratio of scissions to crosslinks with an imposed stress. It also is difficult to visualize how the formation of free radicals, scissions, and crosslinks could directly cause the radiation expansion noted under no stress. Therefore, the mechanism of accelerated creep is probably not caused by the formation and reaction of macromolecular free radicals in the polymer specimens. 

Lodge, A. S. A network theory of flow birefringence and stress in concentrated polymer solutions. Trans. Faraday Soc. 52,120-130 (1956). 

Williams,M.C. Concentrated polymer solutions. Part III. Normal stresses in simple shear flow. A.I.Ch.E. J. 13,955-961 (1967). 

---