Viscoelasticity

Viscoelasticity is a very important behavior to understand for the designer. It is the relationship of stress with elastic strain in a plastic. The response to stress of all plastic structures is viscoelastic, meaning that it takes time for the strain to accommodate the applied stress field. Viscoelasticity can be viewed as a mechanical behavior in which the relationships between stress and strain are dme dependent that may be extremely short or long, as opposed to the classical elastic behavior in which deformadon and recovery both occur instantaneously on application and removal of stress, respectively.

The time constants for this response will vary with the specific characteristics of a type plastic and processing technique. In the rigid section of a plastic the response time is usually on the order of microseconds to milliseconds. With resilient, rubber sections of the structure the response time can be long such as from tenths of a second to seconds. This difference in response time is the cause of failure under rapid loading for certain plastics.

By stressing a viscoelastic plastic material there are three deformation behaviors to be observed. They are an initial elastic response, followed by a time-dependent delayed elasticity that may also be fully recoverable, and the last observation is a viscous, non-recoverable, flow component. Most plastic containing systems (solid plastics, melts, gels, dilute, and concentrated solutions) exhibit viscoelastic behavior due to the long-chain nature of the constituent basic polymer molecules (Chapter 1).

This viscoelastic behavior influences different properties such as britdeness. To understand why the possibility for brittie failure does exist for certain plastics when the response under high-speed stressing is transferred from resilient regions of a plastic, an analysis of the response of the two types of components in the structure is necessary. The elastomeric regions, which stay soft and rubbery at room temperature, will have a very low elastomeric modulus and a very large extension to failure. The rigid, virtually crosslinked regions, which harden together into a crystalline region on cooling, will be britde and have very high moduli and very low extension to failure, usually from 1 to 10%.

If the stress rate is a small fraction of the normal response time for the rubbery regions, they will not be able to strain quickly enough to accommodate the applied stress. As a consequence for the britde type plastics, virtually crosslinked regions take a large amount of the stress, and since they have limited elongation, they fail. The apparent effect is that of a high stretch, rubbery material undergoing britde failure at an elongation that is a small fraction of the possible values.

A fluid, which although exhibits predominandy viscous flow behavior, also exhibits some elastic recovery of the deformation on release of the stress. To emphasize that viscous effects predominate, the term elastico-viscous is sometimes preferred; the term viscoelastic is reserved for solids showing both elastic and viscous behavior. Most plastic systems, both melts and solutions, are viscoelastic due to the molecules becoming oriented due to the shear action of the fluid, but regaining their equilibrium randomly coiled configuration on release of the stress. Elastic effects are developed during processing such as in die swell, melt fracture, and frozen-in orientation.

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