Definition
A failure mode in composite materials in which the reinforcing fibers themselves break, rather than the surrounding resin (matrix) cracking or the layers separating. Fiber fracture typically occurs when a composite structure is overloaded in tension or bending beyond the strength of the fibers, and it represents a serious form of damage because the fibers carry most of the structural load.
Plain English
The strong threads inside a composite part have actually snapped. Since those threads are what give the part most of its strength, breaking them is real structural damage, not just surface or cosmetic damage.
Context Anchor
Seen in discussions of composite aircraft inspection, especially after an impact, dropped tool, hard landing, or other event that may have damaged a composite surface.
Derivation
‘Fiber’ comes from the Latin fibra, meaning thread or filament. ‘Fracture’ comes from the Latin fractura, meaning a break. Together the term simply names what has happened: the thread-like reinforcements inside the composite have broken.
Why Pilots Care
Undetected fiber fractures can allow a structural component to fail under normal flight loads.
Analogy
Think of a rope with some inner strands snapped. The outside may still look mostly intact, but the rope is no longer as strong as it was.
Intuition Check
Do not assume fiber fracture means only a visible crack on the surface. In composite structure, the important damage may be broken fibers inside the material.
Example Sentence 1
After a hard landing, the composite wing skin was inspected for fiber fracture as well as surface cracks.
Example Sentence 2
Fiber fracture near a fastener hole required repair before the wing could return to service.