Stress Concentration

Theory

Kt = σ_max / σ_nominal. For a circular hole in infinite plate: Kt = 3. Kt decreases with larger fillet radius. Sharp corners (r → 0) → Kt → ∞ (theoretical). Stress concentrations are primary crack initiation sites under cyclic loading (fatigue). FDM layer boundaries are inherent stress risers.

Application

Add generous fillets (≥ 1 mm radius, ideally ≥ 0.5× wall thickness) at all inside corners of printed parts. Avoid sharp re-entrant corners. For holes in thin walls, add boss reinforcements. Orient stress concentrations away from inter-layer planes in FDM.

Common mistakes

FDM layer lines are stress risers even in nominally smooth geometry. A perfectly designed fillet in CAD may have a stress concentration if the layer orientation is perpendicular to the fillet tangent. Model and simulate with the actual print orientation.

Related terms: Factor of Safety, Design Margin, Snap Fit, Build Orientation