Factor of Safety

Theory

FoS = failure load / working load. For yield-critical parts, FoS = σ_yield / σ_applied. Typical values: structural plastic parts 2–4, safety-critical 4–10. FDM parts have lower effective strength due to layer adhesion, so use higher FoS than injection-molded equivalents.

Application

Use FoS ≥ 3 for FDM structural parts under cyclic load. Printed brackets, snap-fits, and hinges are especially sensitive to interlayer delamination. Run a stress simulation (FEA) and apply anisotropy knock-down factors before finalizing wall thickness.

Common mistakes

FDM FoS is direction-dependent. Z-axis tensile strength can be 30–50% lower than XY. A FoS calculated on isotropic material properties will be dangerously optimistic for printed parts unless orientation is accounted for.

Related terms: Stress Concentration, Design Margin, Topology Optimization