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Chapter 15: Problem 5

In your own words, briefly describe the phenomenon of viscoelasticity.

Short Answer

Expert verified
Provide examples of viscoelastic materials and their applications. Answer: Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when subjected to stress or strain. These materials combine the fluid-like properties of viscosity (resistance to flow and deformation) and the solid-like properties of elasticity (the ability to return to their original shape after being deformed). Viscoelastic materials resist deformation and return to their original shape but with a time delay, making them ideal for applications requiring damping or energy absorption. Examples of viscoelastic materials include polymers, elastomers, certain biological tissues, and specific foams like memory foam. They are widely used in applications such as automotive suspension systems, memory foam pillows and mattresses, protective equipment, and noise and vibration damping systems in buildings and machinery.

Step by step solution

01

Introduction to Viscoelasticity

Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when subjected to stress or strain. These materials have a unique behavior, combining the fluid-like properties of viscosity and the solid-like properties of elasticity.
02

Understanding Viscosity

Viscosity refers to a fluid's internal resistance to flow and get deformed due to an external force. Viscous materials, such as honey or syrup, flow slowly and exhibit high resistance to deformation.
03

Understanding Elasticity

Elasticity refers to a solid's ability to return to its original shape after being deformed due to an applied force. Elastic materials, such as rubber bands or springs, possess the capacity to store energy in the form of stress and release it as the material returns to its original shape.
04

Combination of Viscosity and Elasticity

Viscoelastic materials exhibit a unique combination of viscosity and elasticity that allows them to resist deformation and return to their original shape, but with a time delay. The response of viscoelastic materials to external forces does not exhibit an instant recovery to its original shape. Instead, these materials exhibit a gradual recovery process, making them ideal for applications that require damping or energy absorption.
05

Examples of Viscoelastic Materials

Common examples of viscoelastic materials include polymers (plastics, resins, and gels), elastomers (rubber and silicone), certain biological tissues (skin, cartilage, and tendons), and certain foams (memory foam).
06

Applications of Viscoelastic Materials

Viscoelastic materials are widely used in various applications, such as automotive suspension systems to provide comfort and reduce vibrations, memory foam pillows and mattresses for sleep comfort, and protective equipment like helmets and padding. These materials are also used in noise and vibration damping systems in buildings and machinery.

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Most popular questions from this chapter

(a) Compare the fatigue limits for polystyrene (Figure 15.11) and the cast iron for which fatigue data are given in Problem 8.20. (b) Compare the fatigue strengths at \(10^{6} \mathrm{cy}\) cles for poly(ethylene terephthalate) (PET, Figure 15.11) and red brass (Figure 8.34).

For each of the following pairs of polymers, do the following: (1) state whether it is possible to decide whether one polymer has a higher tensile modulus than the other; (2) if his is possible, note which has the higher ensile modulus and then cite the reason(s) or your choice; and (3) if it is not possible o decide, then state why. a) Random acrylonitrile-butadiene copolyner with \(10 \%\) of possible sites crosslinked; ilternating acrylonitrile-butadiene copolymer with \(5 \%\) of possible sites crosslinked (b) Branched and syndiotactic polypropylene with a degree of polymerization of 5000 ; linear and isotactic polypropylene with a degree of polymerization of 3000 (c) Branched polyethylene with a numberaverage molecular weight of \(250,000 \mathrm{~g} / \mathrm{mol}\); linear and isotactic poly(vinyl chloride) with a number-average molecular weight of \(200,000 \mathrm{~g} / \mathrm{mol}\)

For some viscoelastic polymers that are subjected to stress relaxation tests, the stress decays with time according to $$ \sigma(t)=\sigma(0) \exp \left(-\frac{t}{\tau}\right) $$ where \(\sigma(t)\) and \(\sigma(0)\) represent the timedependent and initial (i.e., time \(=0\) ) stresses, respectively, and \(t\) and \(\tau\) denote elapsed time and the relaxation time; \(\tau\) is a timeindependent constant characteristic of the material. A specimen of a viscoelastic polymer whose stress relaxation obeys Equation \(15.10\) was suddenly pulled in tension to a measured strain of \(0.6\); the stress necessary to maintain this constant strain was measured as a function of time. Determine \(E_{r}(10)\) for this material if the initial stress level was \(2.76\) MPa (400 psi), which dropped to \(1.72 \mathrm{MPa}\) (250 psi) after \(60 \mathrm{~s}\).

Why must fiber materials that are melt-spun and then drawn be thermoplastic? Cite two reasons.

On the basis of the curves in Figure \(15.5\), sketch schematic strain-time plots for the following polystyrene materials at the specified temperatures: (a) Crystalline at \(70^{\circ} \mathrm{C}\) (b) Amorphous at \(180^{\circ} \mathrm{C}\) (c) Crosslinked at \(180^{\circ} \mathrm{C}\) (d) Amorphous at \(100^{\circ} \mathrm{C}\).
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