Material selection

Advanced composite materials have unique mechanical properties in comparison with steel or aluminum alloys: fatigue strength, specific strength (ratio of strength to weight), specific rigidity (ratio of modulus of elasticity to weight), strength redundancy and high resistance of damaged structures to external loads. Contrary to metals, crack resistance of modern composite materials increase as strength increases. Crack resistance of composite materials depends on fiber tensile strength, its scatter, matrix tensile strength, and bond shear strength. Splitting of the bond between matrix and fiber helps material to stop macrocracks.

Unique properties of a composite can be obtained by proper choice of filament, fiber content, layer orientation, etc.

The use of composite materials decreases the weight of a passenger airplane by 20 - 40%.

Use of composite materials decreases cost of many engineering structures. Fiber content increase does not excessively raise the cost of the composite.

One of the most important stages of composite structure manufacturing is nondestructive testing:

A. Ultrasonic testing can reveal porosity, voids, incorrect volume fraction, foreign inclusions, translaminar cracks, delaminations, disbonds in adhesive, poor fillet in honeycomb construction.
B. Eddy currents technique is limited to materials with a conducting phase.
C. Thermography can be used as rapid technique for inspecting large-scale composite structures.
D. Holography is very expensive technique with restricted list of discovered defects, such as disbonding and delamination.
E. Radiography is used in testing of boron or glass reinforced composites, not carbon fiber reinforced composites.

Ultrasonic testing is the most widely used technique for many composite materials.

Group fiber breakage is the most dangerous defect in a composite. It can be revealed by one of the nondestructive methods from the above list.