What is fail-safe design and how is it implemented in aircraft structures?

Fail-safe design ensures the structure can sustain required loads after failure of a single structural element or presence of discrete damage. Implementation methods include: Multiple load paths (redundant structure so load redistributes if one element fails), Crack arrest features (tear straps, frames that stop crack propagation), Backup structure (secondary structure carries load if primary fails), and Designed crack paths (direct crack growth to inspection zones). Examples: wing with redundant spars, fuselage with tear straps and crack-stopping frames, fail-safe bolted splice designs. Analysis demonstrates residual strength at continued safe flight loads with element failed. This philosophy complements damage tolerance.