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Chap. 9 Design of Composite Materials9-1. Advantages of Composite Materials in Structural Design

The main advantages of using composites in structural design are as follows.

Flexibility Ply lay-up allows for variations in the local detail design. Ply orientation can be varied to carry combinations of axial and shear loads.

Simplicity Large one-piece structures can be made with attendant reductions in the numb

er of components. Selective reinforcement can be used.

Efficiency High specific properties, i.e., properties on a per-unit-weight basis. Savings in materials and energy.

Longevity Generally, properly designed composites show better fatigue and creep behavi

or than their monolithic counterparts.

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9-2. Fundamental Characteristics of Composite Materials

Composite materials come with some fundamental characteristics that are quite different from conventional materials. Heterogeneity : Composite materials, by definition, are heterogeneous. There i

s large area of interface and the in situ properties of the components are different from those determined in isolation.

Anisotropy : Composites in general, and fiber reinforced composites in particular, are anisotropic. The modulus and strength are very sensitive functions of fiber orientation.

Coupling Phenomena : Coupling between different loading modes, such as tension-shear, is not observed in conventional isotropic materials. These coupling phenomena make designing with composites more complex.

Fracture Behavior : Monolithic, conventional isotropic materials show what is called a self similar crack propagation. This means that the damage mode involves the propagation of a single dominant crack; one can then measure the damage in terms of the crack length. In composites, one has a multiplicity of fracture modes. A fiber reinforced composite, especially in the laminated form, can sustain a variety of subcritical damage (cracking of matrix, fiber/matrix decohesion, fiber fracture, ply cracking, delamination).

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9-3. Designing with Composite Materials

Characteristics of Composite Materials

1) Physical properties of composites depends on

- Physical properties of components

- Volume fraction, shape & geometric arrangement

of components

- Interface characteristics

2) Anisotropic

Modulus and Strength of Fiber Reinforced Composite

(65 vol.%) Carbon Fiber/Epoxy Laminated Composite - 3 layer

stacking sequence - 3 layers

Young's modulus, tensile strength vs volume fraction of plies

"Performance Charts" or "Carpet Plots"

can tailor theproperties of composites

ooo 90/45/0 ooo 90 ,45 ,0

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ex) Specification : Longitudinal tensile strength higher than steel.

Transverse modulus similar to Al.

Point A :

If we change the volume fraction of 0o ply and 90o ply,

Point B :

If we decrease the volume fraction of 0o ply and increase the volume fraction

of 90o ply,

Point C :

Reversing the x and y coordinates,

500MPa)( x 70GPa)(Ey

60% 0 20% 45 20% 90 with with with o o o, ,

90GPaE MPa725 xx

20% 0 20% 45 90 with with 60% with o o o, ,

40GPaE 340MPa= yy

40% 0 20% 45 90 with with 40% with o o o, ,70GPaE MPa580 xx

satisfy specification

70GPaE 580MPa yy

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Thermal Expansion Coefficient of Fiber Reinforced Composite B/Al, B/Epoxy, C/Epoxy Carbon, Kevlar Fiber - negative CTE parallel to fiber direction.

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Control of thermal expansion coefficient

→ important for aerospace application (-70°C ~ 200 °C)

electronic packaging application (R.T. ~ 150 °C)

ex) Leadless chip carrier (LCC)

Si + Alumina Chip Carrier + Carbon/Epoxy Composite

By controlling carbon fiber orientation,→ Thermal expansion mismatch = 0

→ No thermal fatigue problem

Si + SiCp/Al

K/1030~5 :C/Epoxy K/108.5 GaAs

K/106 : O Al K/101.4 : Si66

632

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/K106 6C/epoxy

K/106 :/AlSiC vol% 80 K/1023 : Al

K/107 :/AlSiC vol% 70 K/105.4 : SiC

6p

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6p

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Hybrid Composite

Composite using more than 1 type of fiber

Selection of fibers highest strength in highly

Control of fiber alignment stressed location and direction

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Carbon+Glass/Epoxy Composite

specific modulus

fatigue property

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ARALL (Aramid Aluminum Laminates)

Alcoa, 3M Company (1985)

Arrange aramid fibers between Al sheets.

High strength, excellent fatigue & fracture resistance.

Good formability & machinability - similar to metal.

Applications :

Aircraft - fuselage, lower wing, tail skin.

Increase the fatigue and fracture resistance.

Weight saving ~ 15-30%.