The internal moment at a cross-section of a beam resulting from transverse loads, generating tension on one face and compression on the other.
Bending moment M (N·mm) at a section equals the algebraic sum of moments of all forces to one side. The flexure formula gives the resulting stress: σ = M·y/I, where y is distance from the neutral axis and I is the second moment of area. Maximum stress occurs at the outermost fibres (y = h/2). Shear force V and bending moment M are related by V = dM/dx.
Bending moment diagrams guide placement of material in 3D-printed structural parts. Ribs and flanges increase I to reduce peak stress for a given M. Cantilever brackets, overhangs, and snap-fit arms all carry bending moments. In FDM, printing a beam with layers parallel to the bending axis (XY plane) is much stronger than printing with layers perpendicular: the maximum tension fibre then lies along the interlayer bond plane.
Bending moment diagrams assume linear elastic, isotropic, homogeneous material: none strictly hold for FDM. Stress concentrations at notches and holes create local M amplification that exceeds σ_f. For large deflections, geometry change must be accounted for (nonlinear analysis). FDM parts with infill below 40% have dramatically reduced effective I, underperforming bending calculations based on solid cross-section assumptions.
Related terms: Flexural Strength, Stiffness, Compressive Strength, Stress
| Field | mechanics, engineering |
|---|---|
| Also called | Flexural Moment |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert