Flexural strength, also known as modulus of rupture, or flexural strength, is a material property that manifests itself as stresses occurring just before yielding in a bending test.[1]​ Most frequently, the transverse flexural test is used, in which a circular or rectangular section specimen is arched until it fractures or yields when subjected to a three-point test. It is the highest stress occurred within the material at its moment of failure. It is represented by the sigma symbol: .

When an object made of a single material, such as a wooden beam or a steel rod, is flexed (fig. 1), a range of stresses occur at depth (fig. 2). At the edge of the object, inside the arch (concave face), the value of the compressional stress is the maximum, and on the outside of the curve (convex face) the value of the stress is the maximum tensional stress. These inner and outer edges of the beam or rod are called "end fibers." Most materials fail under tensile stresses. The value of the maximum tensile stress that can be sustained before the beam or rod fails constitutes its flexural strength.

In homogeneous material, the flexural resistance is equivalent to the tensional resistance. In most materials there are defects of varying magnitude, which locally concentrate their strength and cause weakness. When a material is arched, only the end fibers exert the greatest resistance. If the fibers are free of defects, the strength of the intact fibers controls their flexural strength.

If the same material was subjected only to tensile forces, all its fibers are subject to the same resistance. When the weakest fiber reaches its limit tensile stress, it begins to fail. Therefore, for the same material, the flexural strength is commonly higher than the tensile strength. Conversely, in a homogeneous material with defects only on the surface (e.g., due to tears) its tensile strength could exceed its flexural strength.

If defects of any kind are not taken into account, the material obviously fails under a bowing force less than the corresponding tensile strength. Both forces induce the same failure stress, the value of which depends on the strength of the material.

In a rectangular specimen, the stress resulting from an axial force is determined by the following formula:.