TABLE 3-2 Typical Properties of Unidirectional Composites

 

E-Glass/Resin

KEVLAR Resin

Graphite/Resin

S-Glass/Resin

Fiber Direction

Modulus, GPa

44.8

75.8

145

56

Strength tension, MPa

1124

1241

1517

1980

Strength compression, MPa

896

276

2068

626

Coefficient of thermal expansion, microstrain/K

8.8

-4.0

-0.45

5.5

Transverse Direction

Modulus, GPa

11.0

5.52

10.3

11.4

Strength tension, MPa

31

14

48

31

Strength compression, MPa

138

55

138

138

Coefficient of thermal expansion, microstrain/K

22.1

57.6

25.2

23.3

Shear (Inplane)

Modulus, GPa

4.14

2.41

5.52

4.48

Strength, MPa

71.7

34.5

82.7

71.7

Poisson Ratio

Axial-Transverse

0.27

0.34

0.30

0.27

Thickness Direction

Modulus, GPa

11.0

5.5

10.3

11.4

Poisson ratio

0.44

0.38

0.55

0.44

To further illustrate the cost differences based on fibers, consider a hybrid composite with a carbon/glass volume ratio vc/g. The ratio of the cost of the hybrid composite to an all-glass composite to provide the same structural stiffness is given by

The parameters in Equation (1) are defined as follows: $, price per unit mass; s, specific gravity; E, Young's modulus; g, glass; and c, carbon. On the other hand, the weight ratio, defined as the weight of the hybrid composite divided by that of the all-glass composite, is given by

E-glass fiber costs around $2/lb; a high-strength carbon fiber costs about $30/lb. Thus, an all-carbon fiber composite costs 3.5 times more than an all-glass fiber composite to provide the same structural stiffness at a weight savings of 76 percent. When a 50/50 glass/carbon hybrid composite is used, however, the calculated cost ratio is reduced to 2.9 with a weight savings of 58 percent.



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