Which statement best describes brittle materials?

• A. They are seldom used on aircraft structures; fail on a 45-degree plane to the load.
• B. They are the primary choice for structural components due to high ductility.
• C. They deform significantly before failure.
• D. They bend without warning and never fail in tension.

Answer: (A)

Brittle materials fail with little to no plastic deformation and tend to fracture suddenly. Under loading, the fracture often occurs along a plane approximately 45 degrees to the applied load, which reflects the dominant shear stress direction at failure. Because of this sudden, catastrophic failure and the limited energy absorption, brittle materials are not favored for primary aircraft structures.

That’s why the statement about them being seldom used on aircraft structures and failing along a plane at about 45 degrees to the load best describes their behavior. The other descriptions describe ductile behavior—deforming significantly before failure, or bending without warning and never failing in tension—which does not fit brittle materials.