The bending strength or flexural strength of a material is defined as its ability to resist deformation under load. During a bending test described in ASTM D790 the maximum achieved flexural stress value is noted as flexural strength. For materials that deform significantly but do not break, the load at yield, typically measured at 5% deformation/strain of the outer surface, is reported as the flexural strength or flexural yield strength. The test beam is under compressive stress at the concave surface and tensile stress at the convex surface. In the figure (source: ASTM.org) three examples are shown.
Curve a: Specimen that breaks before yielding.
Curve b: Specimen that yields and then breaks before the 5% strain limit.
Curve c: Specimen that neither yields nor breaks before the 5% strain limit.
The analogous test to measure flexural strength in the ISO system is ISO 178. The values reported in the ASTM D790 and ISO 178 tests seldom differ significantly. These tests also give the procedure to measure a material's flexural modulus (the ratio of stress to strain in flexural deformation).