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Unformatted text preview: nion fairings and wing landing gear doors
(graphite/Kevlar)
• Brakes (structural carbon)  v v For any class of ﬁberreinforced composites, the ones with the highest speciﬁc strength
and modulus values generally have all their ﬁbers aligned in one direction (i.e., unidirectional ﬁberreinforced composites). If the loading direction is known (and is always the
same), then the composite can be designed and fabricated so that the strong and stiff
(ﬁber) direction coincides with the loading direction. In this case the “weakness” in the
other directions is not a problem. If, however, the loading direction is not known, or varies  eText Main Menu  Textbook Table of Contents pg581 [R] G1 727060 / IRWIN / Schaffer iq Chapter 14 12.01.98 plm QC2 rps MP Composite Materials 581 FIGURE 14.2–4
Glass/epoxy 4 A plot of speciﬁc strength
versus speciﬁc elastic modulus for several monolithic
structural materials and
polymerbased composite
materials. The data in the
ﬁgure demonstrate the superiority of composites as
structural materials.
(Source: Adapted from C.
Zweben.) Specific tensile strength = Tensile strength/density
(Arbitrary units) Kevlar/epoxy 3 Graphite/epoxy (T300) Boron/epoxy
2 Graphite/epoxy (HMS)
1
Titanium Steel Aluminum Beryllium Magnesium
Graphite/epoxy (GY70) 0 0 1 2
3
4
5
6
7
Specific tensile modulus = Elastic modulus/density
(Arbitrary units) 8 with time, then a nearly isotropic composite is required. The ﬁbers must be arranged so
that a portion of them is oriented in each of several directions within the material. This
type of ﬁber architecture yields properties that are between those of the “strong” and
“weak” directions in aligned ﬁber composites. That data at the lower left end of the ranges
in Figure 14.2–4 correspond to nearly isotropic composites. .......................................................................................................................................
EXAMPLE 14.2–1
A robotic arm with a circular cross section used in a space vehicle is subjected to a constant tensile
force along its axis. The length of the arm is ﬁxed by other design considerations. Derive an
expression for use in selecting a material that will minimize the weight of the arm. Assume that the
applied stress cannot exceed 50% of the ultimate strength. How would the material choice change
if the design requirement called for a ﬁxed stiffness value? [Hint: From mechanics, the bending
stiffness S of the arm is given by S
EI , where E is the elastic modulus and I is the bending
moment of inertia (I for a circular rod is d 4 64).]
Solution
Since this is unidirectional loading, we can use the properties of unidirectional composites when
comparing materials. If the applied force is F, the stress is F A
F d 2 4 where d is the arm
diameter. We can thus write,
4F
d2 0.5 u where u ultimate tensile strength. The mass M of an arm with length l and density is M
d 2 4 l. If we solve the stress equation for d 2 and substitute this expression into the mass
equation, we ﬁnd:
2 Fl...
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This note was uploaded on 02/25/2013 for the course PHYS 2202 taught by Professor Sowell during the Spring '10 term at Georgia Institute of Technology.
 Spring '10
 sowell
 Physics, The Crucible, ........., Tensile strength, IRWIN / Schaffer

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