Flexural strength refers to how much stress a material will take before it tears, ruptures, breaks, or permanently bends, i.e. yields. The units of measure are force, as in newtons or pounds-force; or area, as in meters squared or square inches.
Knowing how much force a material can take before breaking can be useful during the design process. Other names for this material characteristic include bending strength and modulus of rupture. One interesting note is that it can be a good indicator of a material’s isotropic characteristics, that is, how homogeneous the material is. The more homogeneous a material, generally the stronger it is, but this may be directional. Designers can take advantage of strength and directional strength features for their designs.
Two methods exist to test this material property on a rectangular sample—a three-point bending test and a four-point bending test.
In the three-point method, a flexural test machine applies increasing force on the sample until the sample breaks or permanently bends. For a three-point test, the flexural strength (given the symbol σ) can be calculated using:
σ = 3FL / 2wd2
F = the maximum force applied
L = the length of the sample
w = the width of the sample
d = the depth of the sample.
In the four-point method, force is applied at two points simultaneously towards the center of the sample. For ease of calculation, one force is usually applied one-third of the way between the supports and a second force is applied at two-thirds of the way.
The four-point test equation looks like this:
σ = FL / wd2