When does it make sense to test in 3 point compared to 4 point bending? The highest or maximum bend stress occurs under the loading anvil in three point flexural bend tests. In four point bend tests, the maximum flexural stress is spread over the section of the beam between loading points. Also, a three point test best applies where the material is homogeneous, such as plastic materials. A four point test tends to be the best choice if the material is not homogeneous, such as composites or wood.

The stress concentration of a three point test is small and concentrated under the center of the loading point, whereas the stress concentration of a four point test is over a larger region, avoiding premature failure.

A three point test is easier to perform than a four point test. The deflection measurement in three point tests is commonly measured using the machine’s crosshead position sensor (typically a digital encoder), whereas the four point bend test is commonly measured using a deflectometer.

Conversion kits are available that convert a three point bend fixture to or from a four point bend fixture (you can expand in both directions).