Flexural strength is a measurement that indicates a material's resistance to deforming when it is placed under a load. The values needed to calculate flexural strength are measured by experimentation, with rectangular samples of the material placed under load in a 3- or 4-point testing setup. (A 3-point setup is shown in the accompanying photo.)
Gather the following data from the load experiment:
Maximum applied load, which we will call "P";
Material span length between points in the test setup, which we will call "L";
Width of the material specimen, which we will call "b"; and
Average depth of the specimen, which we will call "d."
Convert all measurements to the following units:
Maximum applied load in pounds;
Material span length in inches;
Width of the material specimen in inches; and
Average depth of the specimen in inches.
Substitute the numerical values for this data in the following equation for calculating flexural strength:
R = P_L/b_d^2
where R = Flexural strength, in units of lbs. per square inch, and d^2 indicates the quantity d squared.
To further clarify, multiply P by L and then divide that quantity by the quantity of b times d squared. The result is the calculated flexural strength.
During experimentation, the flexural yield strength is usually reported instead of flexural strength for materials that do not crack under the loads placed upon them in the flexure test.