Viscoelastic Behavior of Plastics

For those not familiar with this type information recognize that the viscoelastic behavior of plastics shows that their deformations are dependent on such factors as the time under load and temperature conditions. Therefore, when structural (load bearing) plastic products are to be designed, it must be remembered that the standard equations that have been historically available for designing steel springs, beams, plates, cylinders, etc. have all been derived under the assumptions that (1) the strains are small, (2) the modulus is constant, (3) the strains are independent of the loading rate or history and are immediately reversible, (4) the material is isotropic, and (5) the material behaves in the same way in tension and compression. 

In this approach the reviews concerned the rheology involving the linear viscoelastic behavior of plastics and how such behavior is affected by temperature. Next is to extend this knowledge to the complex behavior of crystalline plastics, and finally illustrate how experimental data were applied to a practical example of the long-time mechanical stability. 

When the magnitude of deformation is not too great, viscoelastic behavior of plastics is often observed to be linear, i.e., the elastic part of the response is Hookean and the viscous part is Newtonian. Hookean response relates to the modulus of elasticity where the ratio of normal stress to corresponding strain occurs below the proportional limit of the material where it follows Hooke s law. Newtonian response is where the stress-strain curve is a straight line. 

At the same time, the methods for determination of flex strength and flex modnlns of materials in general and composite materials in particnlar are rather tricky and may lead to gross deviations from correct valnes. Thongh, correct values also depend on the basic dehnitions, experimental setnps, and interpretations of experimental data. Viscoelastic behavior of plastics and plastic-based composite materials as well as a certain nonnniformity of the matrix of composite materials also add more complications to measurements and interpretations of experimental values of flex strength and modnlns. 

An exemplary phenomenmi of practical relevance in which the viscoelastic behavior of plastic melts plays a rheological role is expansion of the extruded mass (die swell) when melt flows freely out of an extrusion die (profile molding tool). 

Because of the viscoelastic behavior of plastics in general, and plastic melts in particular, this relaxation of the molecular chains is dependent on the factors time and temperature and is entropy elastic. The longer the die, the more time the molecules have to relax within a fixed die cross-section, so that the swell factor after the material exits from the die will be reduced. 

Plastics are viscoelastic. Their behavior is partly elastic and partly that of a very viscous fluid. Properties of strength and rigidity vary with amount of stress, the rate of loading, and the temperature at which the stress is applied. Viscoelastic behavior requires performance tests to measure time dependence. The viscoelasticity of plastics also severely limits the usefulness of many short-time tests such as impact, tensile, and flexural strengths and modulus. Unfortunately, such test data are very widespread because they are easier and cheaper to obtain than time- and temperature-dependent information. These data can cause much confusion and disappointment when used for plastics. Short-time data are useful for quality control and specification purposes, and if properly interpreted, can shed some light on plastic performance. However, they cannot be used in design and are more often than not misleading because they do not account for the viscoelastic behavior of plastics. 

To relate the viscoelastic behavior of plastics with an S-S curve the popular Maxwell model is used, this mechanical model is shown in Fig. 3.8. This model is useful for the representation of stress relaxation and creep with Newtonian flow analysis that can be related to plastic s non-Newtonian flow behavior. It consists of a spring [simulating modulus of elasticity (E)] in series with a dashpot of coefficient of viscosity (ij)- It is an isostress model (with stress the strain (e) being the sum of the individual strains in the spring and dashpot. 

Bruller O.S., Phenomenological characterization of the nonlinear viscoelastic behavior of plastics. Private Communication, Technical University of Munich, 1989. 

Papers Viscoelastic Behavior of Plasticized Polyvinyl Chloride at Large Deformation. III. The Effect of Filler, J. Potym. Scu., Part A2, 1909-192U (196I+) co-author R. Sabia. 

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