Flexural Strength

Theory

Flexural strength (σ_f) is determined from a three-point bend test: σ_f = 3FL/2bh², where F is peak load, L is span, b is width, h is thickness. The outer fibres experience the highest stress (tension on convex side, compression on concave). For brittle materials, σ_f > σ_UTS because large volumes are not at peak stress. Measured per ASTM D790 (plastics) or ISO 178.

Application

Flexural strength is directly relevant to shelves, brackets, beams, and housings that carry bending loads. In FDM, the orientation of the part relative to the bend axis critically affects σ_f. Layers perpendicular to the load direction (Z-printed beam) fail in tension at the bottom layer at much lower loads than XY-printed equivalents. Increasing number of perimeters raises flexural strength more than infill.

Common mistakes

Flexural strength is not the same as tensile strength: for plastics flexural strength is often 1.2-1.5× tensile strength due to the stress gradient. Three-point vs. four-point bend tests give different results. FDM parts have high variability in flexural strength depending on layer orientation; always test in the service orientation before finalising design.

Related terms: Tensile Strength, Compressive Strength, Bending Moment, Stiffness