TMM4175 Polymer Composites

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Questions and problems¶

Composites – basic concepts¶

1) Metallic alloys fall generally into the simple definition of composites. They are however, not commonly classified as composite materials. Explain why.

2) Are semi-crystalline polymers composite materials? Explain why and/or why not.

3) On semantics: Could there be any real and significant difference between the terms polymer composites and polymer matrix composites?

4) Discuss/critisize the following statement found on the the site compositeslab.com: A composite is a material made from two or more different materials that, when combined, are stronger than those individual materials by themselves.

Fibers¶

5) Natural fibers are fibers that are produced by geological processes (minerals), or from the bodies of plants (cellulose) or animals (proteins). Describe how these fibers differ with respect to: a) physical properties, b) mechanical properties c) moisture dependence d) flammability and fire e) chemical resistence

6) Explain the differences between: strand, yarn, tow, roving and filament.

7) If continuous fiber composites are more efficient (better performance on almost all metrics), why using short fibers when most of the short fibers are chopped continuous fibers?

8) On the macro scale, short fiber composites are usually considered to be isotropic materials. Why?

9) Sometimes, short fiber composites are not isotropic but anisotropic. What can be the origin of that?

10) Carbon and Kevlar fibers have negative longitudinal coefficient of thermal expansion. Give a simple, physical explanation of the mechanism behind this behavior.

11) What is the difference between carbon fibers and graphite fibers?

12) What is the difference between aramid fibers and Kevlar fibers?

13) If your brain is wired such that knowledge is primarily a bunch of relative relations to a few familiar references, the following problem is specifically made for you: In the first step, estimate (that is guess as an engineer) the typical properties of human hair. Then, read the experimental study Properties of human hair and compare the experimental results with your guess work: a) Diameter b) Elastic modulus c) Yield strength (elastic limit) d) Tensile strength e) Elongation to fracture

14) The fiber volume fraction of a composite can be determined by the Burn-off method. One weakness with the method is the decomposition of the sizing of the fibers. What is a fiber sizing and what is its functions?

Polymers – basic concepts¶

NOTE:The following questions/learning goals refer to lectures, lecture notes and recordings found on Blackboard.

15) Describe the structure of simple polymers such as PE, PP, PVC and PS.

16) What is the principal difference between Polyethylene and Paraffin?

17) Give examples of natural polymers

18) Be able to identify the chemical structures: a: Alcohol b: Carboxylic acid c: Ester d: Amide e: Carbonate

19) Describe how esters and amides are important groups relevant for polymer composites

20) Describe the concepts of geometrical isomers and stereoisomers

21) Polyethylene terephthalate (PET) is also called polyester, or thermoplastic polyester. What is the explanation of the two different names for the same substance?

22) Describe briefly radical addition polymerization

23) Describe briefly catalytic addition polymerization

24) Describe briefly condensation polymerization

25) What is the difference between a cross-linked polymer and a network polymer?

26) Describe how unsaturated polyester dissolved in styrene becomes cross-linked during curing

27) What is a copolymer?

28) What is the difference between thermoplastics and thermosets?

29) Give examples of typical amorphous polymers

30) Give examples of typical semi-crystalline polymers

31) Describe how the temperature influences the properties of common polymers relevant for polymer composites, including: PE, PET, PA, Epoxies and Unsaturated polyesters. Glass transition temperature and melt temperature are relevant parameters for the answer.

Computational problems and procedures¶

32) Derive the expression (1) found in Geometrical aspects and volume fractions: \begin{equation} V_f = \frac{A_f}{A} = \frac{ \frac{1}{4}\pi d^2 }{ a^2 } =\frac{\pi}{4}\bigg(\frac{d}{a}\bigg)^2 \tag{1} \end{equation}

33) Derive the expression (2) found in Geometrical aspects and volume fractions: \begin{equation} V_f = \frac{A_f}{A} = \frac{\pi d^2 }{ 4 a^2\text{sin}(60)} = \frac{\pi }{ 2 \sqrt{3}}\bigg(\frac{d}{a}\bigg)^2 \tag{2} \end{equation}

34) An E-glass/Epoxy composite contains 1.4 kg fibers and 0.8 kg matrix. The composite contains 5% voids (volumetric). When the densities of fibers and matrix are 2540 kg/m³ and 1250 kg/m³ respectively, what is the mass of 1.0 cm³ of a representative sample of the composite?

35) Compute the mass (g/mol) of the repeat unit of PET

36) Compute the number of Nitrogen atoms in 1 kg of Kevlar fiber

37) Consider the five UD materials in Table-1. Assume that the tensile strengths ($X_T$) are governed by the tensile strengths of the fibers. a) Compute the failure strains of the five different fibers when assuming that materials behave linearly elastic. b) Rank the materials with respect to the maximum strain energy density (strain energy per volume) that can be stored before fracture when loaded in the fiber direction. c) Based on the previous answer; what material is most suitable (apparently) for a spring?

Numerical answers¶

34) Mass of 1 cubic centimeter: 1.75456 g 35) Mass of PET monomer: 192 g/mol 36) Number of Nitrogen atoms is approximatly 5E24 37c) S-glass/Epoxy with a strain energy density of 17.6 MPa = 17.6 MJ/m³ before fracture

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