Plastic constraint polymers

The mechanism for craze nucleation and growth describai here is essentially possible in semicrystalline polymers since the criterion is only related with a stress field due to plastic constraint. Therefore, the size and geometry of a local plastic zone at the notch root is responsible for the formation of crazes (sometimes named internal crazes by the authors). 

The stearic acid treated CaCOj nanoparticles significantly improve the tensile modulus of PP, and the modulus increases with an increase in coating thickness [40]. The improvement in modulus is attributed to the presence of stiff particles (Table 9.15). The debonding between nanoparticle and the polymer matrix occurred prior to yielding and therefore released the plastic constraint and allowed shearing to occur. 

The dependence k on G, is presented in Fig. 5.7 and shows that for polymer, as and for metals [31], Gj enhancement ineieases plastic constraint. At Gj = 0 plastic constraint is absent, that is quite obvious, and at the greatest value of sharp notch the value reaches 2.74, that is close enough... 

Hence, the stated above results have shown that plastic constraint factor is dependent on structiual eharaeteristics of polymer and their change in deformation process and influenees essentially on its meehanical properties in impact tests. Growth k results to reduction of both plasticity and strength of polymer samples [32]. 

Kozlov, G. V., Novikov, V. U. (1997). The Physical Significance of Dissipation Processes in Impact Tests of Semicrystalline Polymers. Prikladnaya Fizika, 1,77-84. Kozlov, G. V., Serdyuk, V. D., Beloshenko, V. A. (1994). Plastic Constraint Factor and Mechanical Properties of High Density Polyethylene at Impact Loading. Mekhanika... 

The stresses near the root of a notch are extremely complex and the stress analysis becomes exceedingly difficult when the strain is large, as is the case when yield or failure is imminent. A sharp notch causes constraints and introduces a state of triaxial tension behind the root of the notch (5). This state of stress is consistent with LeGrand s observation of the growth of a flaw behind a notch in a bar of polycarbonate (4). A blunt notch causes constraints when the thickness of the specimen is large. Such a notch can also introduce a state of triaxial tension. While it is desirable to investigate the behavior of polymers in a well-defined state of triaxial tension, it is difficult to accomplish experimentally. However, as we demonstate below, a state of plane strain is relatively easy to produce. The relationship between plane strain and brittleness of plastics is the subject of our investigation. 

Fig. 2a—c. Schematic drawing of several postulated microscopic steps in craze nucleation a Formation of a localized surface plastic zone and buildup of significant lateral stresses, b Nucleation of voids in the zone to relieve the triazial constraints, c Further deformation of polymer ligaments between voids and coalescence of individual voids to form a void network... 

Common defects encountered with extrusion include effects associated with the viscoelastic nature of plastic melts. As the melt is extruded from the die for example, it may exhibit sharkskin melt fracture and extrudate (die) swell. Diagrams of these defects are shown in Fig. 1.16. Sharkskin melt fracture occurs when the stresses being applied to the plastic melt exceed its tensile strength. Extrudate swell occurs due to the elastic component of the polymer melt s response to stress and is the result of the elastic rebound of the polymer as it leaves the constraints of the die channel prior to cooling. 

Since a blend containing high concentration of cavitated rubber particles becomes cellular solid (porous) rather than continuous material, Eq. 11.15 does not apply to it any longer and any analysis of the plastic zone size must be based on yield criteria appropriate for the porous solid. Free from the constraints of continuum mechanics, the cavitated plastic zones formed in polymer blends are able to increase substantially in radius even under plane-strain conditions (Bucknall and Paul 2009). 

Flat tape, whether forced flat by mechanical constraints or held flat by gravity or good chemistry, experiences Level 2 stress and may or may not undergo Level 3 plastic deformation. We have never seen a tape that did not store residual tensile stress in the polymer matrix. This stored stress will be discussed in detail in later sections, along with methods for alleviating the stress. Plastic deformation cannot really be measured, but can be assumed when the same slip without a Type II plasticizer shows multiple Level 4 behaviors, but with the Type II plasticizer lays flat. 

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