What is flexural strength and how is it measured?

What is flexural strength? 

Flexural strength, also known as bending strength or transverse rupture strength, is a material property defined as the maximum stress before a material yields in a bending test.  

When a specimen (usually a beam or rod) bends, it experiences various stresses throughout its depth. On the inside of the bend, the stress will be at its maximum compressive stress, and on the other side, the stress will be at its maximum tensile stress value.

These inner and outer edges of the sample are called extreme fibers. Most materials fail due to tensile stress before failing under compressive stress. This is caused by small defects of various sizes on the surface that grow under tensile stress.

Therefore, the maximum tensile stress value under bending before failure of a beam or rod is considered as its bending strength.

How is flexural strength measured? 

Typically, material specimens are tested in a three-point bending setup where the load is applied to the specimen centered between two supports.

This setup will produce the greatest bending moment at the center of the specimen, which is not a good representation of the general behavior of the material, since larger imperfections near the support point will not affect the measured flexural strength.

To better represent the actual defect density, a four-point bend test was used, which distributes the maximum bending moment over a larger area of the sample.

The ring-to-ring setup is a variation of the plate or disc four-point bend test.

Typical values for common flexural strengths