Yield ductile polymers

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

In an initially ductile polymer, failure properties (ultimate elongation, fracture toughness, impact resistance) decrease rapidly during a chain-scission aging process, whereas elastic and yield properties are practically unaffected at the embrittlement point. 

Shear yielding in polymers has much in common with ductility in metals. In polymers, the yielding may be localised into shear bands, which are regions of high shear strain less than 1 m in thickness or the yield zones may be much more diffuse " Under a general state of stress, defined by the three principal stresses Gi, 02 and 03, the condition for yielding is given by a modified von Mises crite-rion l ... 

The fall in yield stress that occurs in ductile polymers as the temperature increases or the... 

Figures 13.16 and 13.17 are plots of the compressive stress-strain data for two amorphous and two crystalline polymers, respectively, while Figure 13.18 shows tensile and compressive stress-strain behavior of a normally brittle polymer (polystyrene). The stress-strain curves for the amorphous polymers are characteristic of the yield behavior of polymers. On the other hand, there are no clearly defined yield points for the crystalline polymers. In tension, polystyrene exhibited brittle failure, whereas in compression it behaved as a ductile polymer. The behavior of polystyrene typifies the general behavior of polymers. Tensile and compressive tests do not, as would normally be expected, give the same results. Strength and yield stress are generally higher in compression than in tension.

Polymers are very sensitive to the rate of testing. As the strain rate increases, polymers in general show a decrease in ductility while the modulus and the yield or tensile strength increase. Figure 13.32 illustrates this schematically. The sensitivity of polymers to strain rate depends on the type of polymer for brittle polymers the effect is relatively small, whereas for rigid, ductile polymers and elastomers, the effects can be quite substantial if the strain rate covers several decades. 

The deformation characteristic of more ductile polymer materials at ambient temperatures like most thermoplastics or all elastomers is highly non-linear, e.g., either mostly viscoelastic or entropy-elastic or a combination of both. Compared to concepts of LEFM relatively rarely used for polymer materials different concepts of non-linear elastic firacture mechanics such as elastic-plastic fracture mechanics (EPFM) or post-yield fracture mechanics (PYFM) are somewhat widely applied, therefore. One of the most important concepts of EPFM is the J integral concept. Notwithstanding the J integral is primarily defined to be valid... 

The normative strain determined by a strain or clip gauge is used for brittle materials up to the break of the specimen or for ductile polymers up to the yielding point. In all other cases the traverse path is used to calculate the nominal strain according to (Eq. 4.3). 

Because of the complex geometries involved, especially in notched specimens, it is virtually impossible in most cases to measure strain rates directly during impact It is however known that yield strains in ductile polymers are usually between 0.05 and 0.1. Data sheets on PC give a figure... 

The fall in yield stress that occurs in ductile polymers as the temperature increases or the strain rate reduces. This fall results in a plane-strain to plane-stress transition, as indicated in Eq. 10.7 and therefore an increase in fracture resistance. 

Taking the two polymers discussed in the previous section as an example, PVC fracture toughness should be ATjc = 0.5—1 MPam and PE between 2 and 3 MPam. In fact, PVC is a brittle polymer, and its typical stress—strain curve is more or less like curve 1 in Fig. 1.7 with fractures stress (Black and Hastings, 1988) up to 75 MPa. On the other hand, PE is very ductile polymer with a stress—strain similar to curve 2 in the same Fig. 1.7. PE never reaches brittle failure but yield at various stresses between 10 and 30 MPa. 

When a particulate filler is introduced into a ductile polymer compound, the yield stress is normally decreased but, where the filler is a silica and is treated with a coupling agent it can in fact improve the yield stress. 

Yielding in Amorphous Ductile Polymers. Typical amorphous ductile polymers are PVC and PC. Deformation at room temperature results in the... 

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