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Introduction to composites - fibersCME/MSE 404G. Polymeric MaterialsFall 2012
Figures taken from:P.K. Mallick. Fiber-Reinforced Composites, Materials, manufacturing, and design. 3rd Ed., CRC Press. 2008
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9+Properties of commercial fibersaverage values from manufacturers
fiber D, mm
g/cmt Et, GPa
Yt, GPa
% strain
COTE
Poisson’s ratio
E-glass 10 2.54 72.4 3.45 4.9 5 0.2
S-glass 10 2.49 86.9 4.3 5 2.9 0.22
PAN, T300
7 1.76 231 3.65 1.4 -0.6 0.2
Pitch, P55
10 2.0 380 1.90 .5 -1.3 NA
Kevlar 49
11.9 1.45 131 3.62 2.8 -2 NA
Spectra 900
38 0.97 117 2.59 3.5 NA NA
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Fibers: 2012references; new research on fibers for composites
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In-class exercise
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Each team is to find composites applications for their fibers
assignments
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Team responses
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14+Fiber bundles
Typical fibers have very small diameters, so that fiber bundles are used for ease of handling.
Untwisted = strand, end (glass & Kevlar fibers); =tow (carbon fibers)
Twisted = yarn
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15+Single fiber test
ASTM D3379 - ASTM D3379-75(1989)e1 Standard Test Method for Tensile Strength and Young's Modulus for High-Modulus Single-Filament Materials (Withdrawn 1998)
A single filament is mounted along the centerline of a slotted tab using adhesive at each end
The tab ends are gripped in the tensile machine and the midsection is cut
Constant loading rate until failure
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16+Single fiber mounting for tensile test
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17+Tensile property determinations
Definitions Fu – force at failure
Af = average filament cross-sectional area (planimeter measurement via photos of filament ends
Lf = gage length
C = true compliance (via loading rate)
Tensile strength Tensile modulus
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18+Typical tensile strengths of fibers
Typical fibers have high strength, high orientation
Stress-strain curves are nearly linear up to failure
Most fail brittlely
Most fibers are prone to damage with handling and with contact to other surfaces
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Model for fiber tensile strengths
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Model application
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21+Typical applications: Weibull distribution
Time to failure: failure rate is proportional to time raised to the nth power, k=n+1
Cases 0 < k < 1: failure rate decreases with time. Example =
infant mortality or early failure of electrical circuits k = 1: failure rate is constant over time. Example = random
external events k > 1: failure rate increases with time. Example = aging
process
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Weibull: probability density
In-class question:Interpret each curve with respect to a time-to-failuredata set.
Hint: the integral of each curve = 1.
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Weibull: cumulative distribution
In-class question:Interpret each curve with respect to a time-to-failuredata set.
Hint: the upper limit of each curve = 1.
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Failure rates
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Quantile plots
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Figure 2.4
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27+Example data: failure strength at a given fiber length
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28+Weibull distribution
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30+Quantile plots
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31+Problem 2.5. Mallick
MSE 599 P2_5.xlsx
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+Analysis of flaws in high-strength carbon fibres from mesophase pitchJanice Breedon Jones, John Barr, Robert Smith, J. Materials Sci., 14, (1980), 2455-2465
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Data taken at two guage lengths, 20 mm and 3.2 mm
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34+Effect of gauge length on strengthwhy should there be an effect?
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Single mode of failure should show similar Weibull plot slopes
Similar slope suggests the same failure modes for each gauge length
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Extrapolate to 0.3 mm lengthexpected load transfer length for multifilament fibres of this diameter (3.8 Gpa)
If the failure mechanisms are similar, we can extrapolate the tensile strength to shorter gauge lengths, estimating the tensile strength for lengths that are difficult to measure experimentally.
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+Flaw strength distributions and statistical parameters for ceramic fibers: the normal distributionM. R’Mili, N. Godin, J. Lamon, Phys. Rev. E, 85, 051106 (2012)
Large sets of ceramic fibre failure strengths from tows of 500 – 1000 filamentsFlaws generated by ultrasonic Flaw strengths are distributed normally
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SiC-based Nicalon filaments
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Quasi-linear regressionfailure of fiber tows
For probabilities less than 4%, there is an under-estimate of the number of first failures. This is likely due to the detection of low energy events near the filtering threshold.This is probably not a bimodal distribution of flaws.
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Comparison of model and fiber failure data
Very good indeed.
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45+General effect of aspect ratio on tensile strength
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46+Dry glass bundle. 3000 filamentsSingle filament shows a linear stress-strain curve.
Bundle shows a nonlinear stress-strain curve prior to maximum stress, and progressive failure after maximum stress.
Both effects are due to statistical distribution of the filament strengths. Some fail as the load increase. After the maximum stress, highly loaded fibers continue to fail, but not all at once
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Fiber productionGlass fibers
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49+Types of glass fiberstensile strength = 3.45 GPa; surface flaws reduce this to 1.72 GPa
Continuous strand roving [strand = parallel filaments, n > 204]
Woven roving [roving = group of untwisted strans/ends wound on a cylindrical forming package]
Chopped strands – continuous strands cut to specific lengths; 3.2 – 12.7 for injection molding
Chopped strand mats - 50.8 mm for chopped strand mats
Woven roving mat
Milled glass fibers, 0.79 to 3.2 mm; plastic fillers
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Glass fiber compositions
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Glass fiber properties
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Sizing chemistries
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+Boron fibers
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http://www.angelfire.com
Boron Fibers
Boron is an inherently-brittle material. It is commercially made by chemical vapor deposition of boron on a substrate, that is, boron fiber as produced is itself a composite fiber. In view of the fact that rather high temperatures are required for this deposition process, the choice of substrate material that goes to form the core of the finished boron fiber is limited. Generally, a fine tungsten wire is used for this purpose. A carbon substrate can also been used. The first boron fibers were obtained by Weintraub by means of reduction of a boron halide with hydrogen on a hot wire substrate.
The real impulse in boron fiber fabrication, however, came only in 1959 when Talley used the process of halide reduction to obtain amorphous boron fibers of high strength. Since then, the interest in the use of strong but light boron fibers as a possible structural component in aerospace and other structures has been continuous, although it must be admitted that this interest has periodically waxed and waned in the face of rather stiff competition from other so-called advanced fibers, in particular, carbon fibers.
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synthesis
Reduction of boron Halide : Hydrogen gas is used to reduce boron trihalide:
2BX3 + 3 H2 = 2 B + 6 HX where X denotes a halogen: Cl, Br, or 1.
In this process of halide reduction, the temperatures involved are very high, and, thus, one needs a refractory material, for example, a high melting point metal such as tungsten, as a substrate. It turns out that such metals are also very heavy. This process, however, has won over the thermal reduction process despite the disadvantage of a rather high-density substrate (the density of tungsten is 19.3 g cm -3) mainly because this process gives boron fibers of a very high and uniform quality. There are many firms producing boron fibers commercially using this process.
In the process of BCI3, reduction, a very fine tungsten wire (10-12 micron diameter) is pulled into a reaction chamber at one end through a mercury seal and out at the other end through another mercury seal. The mercury seats act as electrical contacts for resistance heating of the substrate wire when gases (BCl3, + H2,) pass through the reaction chamber where they react on the incandescent wire substrate. The reactor can be a one- or multistage, vertical or horizontal, reactor. BCl3 , is an expensive chemical and only about 10% of it is converted into boron in this reaction. Thus, an efficient recovery of the unused BCl3, can result in a considerable lowering of the boron filament cost.
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Kevlar
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83+Carbon fibers
Graphitic orientationa. Circumferentially
orthotropicb. Radially orthotropicc. Transversely
isotropicd. Circumferential +
radiale. Circumferential +
random
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84+Carbon fibers
Graphitic orientationa. Circumferentially
orthotropicb. Radially orthotropicc. Transversely
isotropicd. Circumferential +
radiale. Circumferential +
random
In-class question: the most common orientation for pitch fibers
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+Filament failure under compression
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Compression failure cannot be determined directly by simple compression tests on filaments
Indirect methods are used, such as the loop test, in which a filament is bent into a loop until it fails.
The compressive strength is determined from the compressive strain at the fiber surface.
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88+Fiber compressive strength
fiber Tensile strength, GPa
Compressive strength, GPa
E-glass 3.4 4.2
PAN T-300 3.2 2.7-3.2
AS4 carbon 3.6 2.7
GY-70 carbon 1.86 1.06
P100 carbon 2.2 0.5
Kevlar 49 3.5 0.35-0.45
Boron 3.5 5
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Effect of fiber diameter on strength
Explain this phenomena
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Effect of fiber diameter on strength
Fiber that are formed by spinning processes usually have increased strength at smaller diameters due to the high orientation that occurs during processing.
