• Wood was used on most early airplanes and is now mainly used on homebuilt airplanes. Wood is lightweight and strong, but it also splinters and requires a lot of maintenance.
• Aluminum (blended with small quantities of other metals) is used on most types of aircraft because it is lightweight and strong. Aluminum alloys don’t corrode as readily as steel. But because they lose their strength at high temperatures, they cannot be used for skin surfaces that become very hot on airplanes that fly faster than twice the speed of sound.
• Steel can be up to four times stronger and three times stiffer than aluminum, but it is also three times heavier. It is used for certain components like landing gear, where strength and hardness are especially important. It has also been used for the skin of some high-speed airplanes, because it holds its strength at higher temperatures better than aluminum.
• Graphite-epoxy is one of several types of composite materials that are becoming widely used for many aircraft structures and components. These materials typically consist of strong fibers embedded in a resin (in this case, graphite fibers embedded in epoxy). Thin sheets of the material can be stacked in various ways to meet specific strength or stiffness needs. Graphite-epoxy is about as strong as aluminum and weighs about half as much.
• Titanium is about as strong as steel and weighs less, though it is not as light as aluminum. It holds its strength at high temperatures and resists corrosion better than steel or aluminum. Though titanium is expensive, these characteristics have led to its greater use in modern aircraft.

Structures Can Buckle under Stress

Sufficient stress will buckle—and in some cases can destroy—a structure. Aircraft are designed to resist such buckling (though some parts and small areas are allowed to buckle to some extent). Buckling depends not only on the physical properties of the structural material but also on thickness and shape.