Which type of composite material is used when stiffness is the prime requirement?

• A. Kevlar
• B. Fiberglass
• C. Graphite (carbon)
• D. Basalt fiber

Answer: (C)

Stiffness in composites is governed mainly by the modulus of the reinforcing fiber. Among common fiber options, graphite (carbon) fibers have the highest elastic modulus, so a carbon fiber–reinforced polymer delivers the greatest resistance to deflection for a given weight. Kevlar is very tough and impact-resistant but has a much lower stiffness. Fiberglass provides moderate stiffness, while basalt fiber is stronger than fiberglass but still doesn’t reach the stiffness level of carbon fiber. So when stiffness is the prime requirement, carbon fiber reinforced composites are the best choice. (Also note that the stiffness is highest along the fiber direction, so plies are oriented to align with the load to maximize stiffness.)