Ductile polymers, plastics mechanical behavior

In terms of the mechanical behavior that has already been described in Sections 5.1 and Section 5.2, stress-strain diagrams for polymers can exhibit many of the same characteristics as brittle materials (Figure 5.58, curve A) and ductile materials (Figure 5.58, curve B). In general, highly crystalline polymers (curve A) behave in a brittle manner, whereas amorphous polymers can exhibit plastic deformation, as in... 

The mechanical behavior of polymers is quite different from metals and ceramics and depends greatly on their structure and operating temperature. Below their glass transition temperature, Tg (the temperature at which their covalent bonded chains can no longer move relative to one another), they are quite brittle and exhibit glass-like behavior. Above their Tg, they behave plastically. In this sense, they are similar to metals that exhibit ductile-to-brittle transitions, but for entirely different reasons. 

Despite the similarities in brittle and ductile behavior to ceramics and metals, respectively, the elastic and permanent deformation mechanisms in polymers are quite different, owing to the difference in structure and size scale of the entities undergoing movement. Whereas plastic deformation (or lack thereof) could be described in terms of dislocations and slip planes in metals and ceramics, the polymer chains that must be deformed are of a much larger size scale. Before discussing polymer mechanical properties in this context, however, we must first describe a phenomenon that is somewhat unique to polymers—one that imparts some astounding properties to these materials. That property is viscoelasticity, and it can be described in terms of fundamental processes that we have already introduced. 

Amorphous polymers exhibit two mechanisms of localized plasticity crazing and shear yielding. These are generally thought of separately, with crazing corresponding to a brittle response while shear yielding is associated with ductile behavior and the development of noticeable plastic deformation prior... 

Such a chain orientation in the direction of deformation is accompanied by their plastic deformation depending upon the proportion of entanglements, it occurs by a disentanglement process. The tensile test is the most convenient way to characterize the mechanical strength of a polymer. Before fracture which can be either fragile or ductile, four different scenarios of the stress-strain behavior can be contemplated, depending on the scale considered. 

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