You have been asked to manufacture a wiring junction box for ...

[nelaaney]

You have been asked to manufacture a wiring junction box for connecting electrical wires in a fighter aircraft. The box sits under the pilot's seat and is part of the seat ejection system. Sufficient forces are exerted on the box that it must be made of a composite material using intermediate to long fibers. What fiber, matrix, and manufacturing method would you use for this part, and why?
 
  What will be an ideal response?

Question 2

Why does crosslinking increase the environmental stress-crack resistance of polymers?
 
  What will be an ideal response?

[tjayeee]

Answer to Question 1

Several materials could be chosen but, because of its good electrical and heat insulative capability and its non-flammable nature, phenolic seems to be the appropriate resin. The fibers need to be strong, stiff, and tough enough to withstand the ejection impact. Therefore, aramid is a good choice for the fibers. The manufacturing process could be either compression molding or RTM, probably depending upon whether the fibers need to be longer than those that could conveniently be used in compression molding.

Answer to Question 2

Environmental stress-cracking is seen in polymers that have high internal orientation, such as extruded polyethylene tubing which has had a high drawdown. The polymer molecules are aligned in such cases and are stressed. When the environmental effects act on these polymers, cracking occurs. By crosslinking the polymers, the chains are held together and cracking is prevented.