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Chapter 15: Problem 9

(a) Contrast the manner in which stress relaxation and viscoelastic creep tests are conducted. (b) For each of these tests, cite the experimental parameter of interest and how it is determined.

Short Answer

Expert verified
Answer: Stress relaxation tests apply a constant strain to the material and measure the stress over time, while viscoelastic creep tests apply a constant stress and measure the strain over time. The key parameter in stress relaxation tests is the stress change over time, while in viscoelastic creep tests, it is the strain change over time. Both tests are essential for understanding a material's response to stress and strain under time-dependent loading conditions.

Step by step solution

01

Explain stress relaxation test

Stress relaxation test is conducted on a material by applying a constant strain and measuring the reduction in stress over time. As the material deforms, its response changes and will eventually reach an equilibrium state. The test aims to determine the material's ability to relieve stress under constant strain.
02

Explain viscoelastic creep test

Viscoelastic creep test, on the other hand, applies a constant stress to the material and measures the resulting strain over time. The material deforms over time because of the applied stress and the test aims to determine the material's viscoelastic response under a constant stress condition.
03

Discuss experimental parameters in stress relaxation test

In a stress relaxation test, the experimental parameter of interest is the stress value at various time intervals after the application of constant strain. The stress is measured using instruments such as a load cell or a strain gauge, and the relaxation response is captured in the form of stress-time plots. From these plots, one can determine the material's relaxation behavior and derive relevant material properties, such as relaxation modulus.
04

Discuss experimental parameters in viscoelastic creep test

In a viscoelastic creep test, the experimental parameter of interest is the strain value at various time intervals after the application of constant stress. The strain is measured using tools like linear variable displacement transducer (LVDT) or a strain gauge, and the creep response is captured in terms of strain-time plots. From these plots, one can determine the creep behavior of the material and derive relevant material properties, such as creep modulus and creep compliance.
05

Summarize the differences

To summarize, stress relaxation tests apply a constant strain to the material and measure the stress over time, while viscoelastic creep tests apply a constant stress and measure the strain over time. In stress relaxation tests, the key parameter is the stress change over time, while in viscoelastic creep tests, it is the strain change over time. Both tests are essential for understanding a material's response to stress and strain under time-dependent loading conditions.

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Most popular questions from this chapter

For each of the following pairs of polymers, (1) state whether it is possible to determine whether one polymer has a higher melting temperature than the other; (2) if it is possible, note which has the higher melting temperature nd then cite reason(s) for your choice; and 3) if it is not possible to decide, then state why. (a) Isotactic polystyrene that has a density of \(1.12 \mathrm{~g} / \mathrm{cm}^{3}\) and a weight-average molecular weight of \(150,000 \mathrm{~g} / \mathrm{mol}\); syndiotactic polystyrene that has a density of \(1.10 \mathrm{~g} / \mathrm{cm}^{3}\) and a weight-average molecular weight of \(125,000 \mathrm{~g} / \mathrm{mol}\) (b) Linear polyethylene that has a degree of polymerization of 5000 ; linear and isotactic polypropylene that has a degree of polymerization of 6500 (c) Branched and isotactic polystyrene that has a degree of polymerization of 4000 ; linear and isotactic polypropylene that has a degree of polymerization of 7500

List two important characteristics for polymers that are to be used in fiber applications.

What is the distinction between dye and pigment colorants?

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 \(60 \mathrm{~s}\).

Normal butane and isobutane have boiling temperatures of \(-0.5\) and \(-12.3^{\circ} \mathrm{C}\) (31.1 and \(\left.9.9^{\circ} \mathrm{F}\right)\), respectively. Briefly explain this behavior on the basis of their molecular structures, as presented in Section 14.2.
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