Flexural strength is the measure of the stiffness of a material, that is to say how well it resists bending.

Testing Strength and Modulus

A simple beam of material is supported either end and a load applied at the mid-point. The load is pushed onto the specimen at a constant rate of 2mm per minute.

Measurements are taken using at least five values of load and deflection. From these results a deflection curve is plotted from which the flexural modulus is calculated. As with tensile modulus this is a ratio of stress to strain. Values are reported in MPa (psi).

The only difference between ASTM D790 and ISO 178 is the size of the test bar – 125mm x 12.5mm x 3mm and 80mm x 10mm x 4mm respectively. Therefore, the values reported in ASTM and ISO seldom differ significantly.

Typical results for a selection of materials as follows:

Material Flex Strength Flex Modulus
ABS 70 2.5
Acetal 85 2.5
30% Glass Filled Acetal 150 7.5
Polypropylene 40 1.5

There are various measures which are defined as follows:

Term Definition
Flexural Modulus The ratio of stress to strain
Flexural Stress at Yield The stress corresponding to test specimen yield
Flexural Stress at Break The stress corresponding to test specimen failure
Flexural Strength The maximum stress sustained by a specimen during testing