FLEXURAL STRENGTH AND MODULUS
Flexural strength is the measure of the stiffness of a material, that is to say how well it resists bending.
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.
Flexural Test Machine
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).
Flexural Test Method
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 |
The higher the number the stiffer the material.
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 |
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