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When to use flexural modulus 1

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prdave00

prdave00

Mechanical
Jul 24, 2008
181
US
I've been recently looking at switching over from a metal to a plastic for one of my components. The plastic supplier has provided values for tensile (Young's) modulus and flexural modulus. I'm doing some hand calculations to predict deflection of a circular beam under a simply supported 3-point bend scenario and my default is to use Young's modulus.

My question is when is flexural modulus used in stress and strain calculations for elastic beams? Or is flexural modulus only used to compare different materials? I'm guessing the later given my understanding of the test methods such as ASTM D790.
 
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I'd use flexural modulus in cases where my part will be under flex.

I would use tensile modulus if my part will be stretched.

I'd use compression modulus if the part will be compressed.

I am not being flippant. Use the property that fits how the part will be used.

Deflection of a circular beam under load = flexural modulus

If you have only one and need the other tensile and flex usually work out close to one another. There are exceptions.

Chris DeArmitt PhD FRSC CChem
 
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Thanks Chris for setting me straight. I didn't take your reply to be flippant at all.
 
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I just gave a tailor-made webinar to a company that's moving from metals to plastics and wanted to understand plastics from a metallurgist point of view. I put together some slides comparing the two and prdave00 is right, the specific modulus of plastics (even filled) is far lower than metals but the specific strength is as good or, when filled, better than metals.

Chris DeArmitt PhD FRSC CChem
 
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Just in case anyone's curious the switch is from a metal to a PAN-based carbon-fiber reinforced plastic. Here are the "typical" values provided by the supplier for different modulus of elasticity of the material I'm looking at.

[blue]Flexural Modulus per ISO 178 =[/blue] 21.2 GPa

[blue]Tensile Modulus per ISO 527 =[/blue] 15.0 / 9.9 (parallel / perpendicular to fill direction)

[blue]Compression Modulus per ISO 604 =[/blue] 11.1 / 9.7 GPa (parallel / perpendicular to fill direction)

[blue]Shear Modulus per ISO 15310 =[/blue] 3.6 GPa

The flexural modulus can be about twice that of the tensile or compressive modulus!
 
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