Which statement best describes brittle materials in aviation contexts?

• A. Seldom used on aircraft structures; fail on a 45-degree plane to the load.
• B. They are highly ductile and absorb energy before failing.
• C. They always yield before breaking.
• D. They are immune to impact damage.

Answer: (A)

Brittle materials fail without significant plastic deformation, so they don’t yield to absorb energy the way ductile materials do. In aviation contexts this means they’re used sparingly for primary structural loads because a crack can propagate rapidly and without warning. When a brittle material does fail under tensile or bending loads, the fracture surface tends to form along planes of maximum shear, which in a typical uniaxial load shows up at about 45 degrees to the applied load. That combination—limited plastic deformation, sudden fracture, and a 45-degree fracture plane—is what makes the statement about being seldom used for aircraft structures and failing on a 45-degree plane to the load the best description.

As a reminder, brittle materials don’t absorb much energy before breaking, they don’t necessarily yield before breaking, and they’re not immune to impact damage; these characteristics distinguish them from ductile materials and explain why their use in primary structures is limited.