Sharp Internal Corner Stress Failure Modes

Theory

The stress concentration factor Kt at a sharp internal corner follows Kt ≈ 1 + 2√(t/r), where t is the section thickness and r is the corner radius. As r → 0, Kt → ∞. In FDM, layer lines also terminate at sharp corners: adjacent perimeters meet at a point rather than a continuous curve, leaving a micro-void. This micro-void is the preferred site for crack initiation. SLA and SLS parts have a more isotropic structure but the geometric Kt remains the same regardless of process.

Application

Always treat sharp internal corners as failure-critical features. Use the formula r ≥ 0.5t as the minimum fillet for stress non-critical parts and r ≥ t for load-bearing features. Under repeated loading (fatigue) the stress at a sharp FDM corner may exceed the layer adhesion strength by 3–10×: even with correct wall thickness. In SLS nylon (PA12), the higher ductility allows some plastic redistribution, but corner cracks still initiate first.

Common mistakes

Finite element analysis (FEA) of 3D-printed parts must model the corner radius explicitly: simulating with r=0 gives meaningless infinite stress. Test samples should include the exact corner geometry of the production part. Failure in SLA parts at sharp corners is often sudden and brittle: add 0.5 mm minimum fillet even on non-structural inside corners. Paint or plating post-processes can hide a crack until it propagates fully.

Related terms: Fillets vs Sharp Internal Corners, Minimum Wall Thickness, Overhang / Self-Supporting Angle