Flexural strength

 What is flexural strength? 

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

When a specimen, usually a beam or rod, is bent it experiences a variety of stresses across its depth. At the inside of the bend, the stress will be at its maximum compressive stress, while at the opposite side the stress will be at its maximum tensile stress value.

These inner and outer edges of the specimen are called the extreme fibers. Most materials fail due to tensile stress before they fail under compressive stress. This is caused by small defects of various sizes at the surface, which will grow under tensile stress.

Therefore the maximum tensile stress value under bending before the beam or rod fails is considered its flexural strength.

How is flexural strength measured? 

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

This setup will create the largest bending moment at the center of the specimen, which does not give the best representation of the materials general performance because larger defects near the support points will not affect the measured flexural strength.

To allow a better representation of the actual defect density a four-point bending test is used, which spreads the maximum bending moment over a larger area of the sample.

A ring-on-ring setup is a variation of the four-point bending test for plates or discs.

What are the typical values for flexural strength? 

Aluminium oxide (0.1% porosity) 400 MPa
Aluminium oxide (2% porosity ) 300 MPa
Aluminium nitride 200 MPa
Boron Carbide 450 MPa
Silicon Carbide 630 MPa
Silicon Nitride 930 MPa
Titanium Diboride 277 MPa
Titanium Oxide 137 MPa
ZTA 910 MPa

 

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Sources

Mechanical Behavior of Materials: Marc André Meyers, Krishan Kumar Chawla, Cambridge University Press, 2009