|MadSci Network: Engineering|
Actually, glass is quite strong, and that's why it breaks when it is struck hard enough, rather than bending. One measure of the strength of glass is that it is difficult to scratch. Contrast with how easy it is to scratch many plastics. Reinforcing plastics with glass (or other) fibers is a way to get the benifits of plastics like formability and impact resistance while increasing the stiffness of the material. Parts made from such compounds are often lighter than the same part made from metal, and can have the same stiffness, and so are used in a variety of applications, like bicycle frames and airplane bodies where lighter weight is crucial.
You might find some books on polymers to learn more about these compound (or hybrid or graphite-epoxy) materials. You could also check out some of the web pages listed here.
Try the links in the MadSci Library for more information on Engineering.