Comparsion of Stiffness

[mwemag]

I need to find a material that is as stiff as possible (bending stiffness). For the calculation in my application I want to find out how much stiffer a material is compared to steel.

Tungsten carbide, silicon carbide or Al2O3 for example have a higer young's modulus. 3.6 x, 2.5 x and 1.8 x higher than the steel. Tungsten carbide is very expensive, whilst the stiffness of SiC and Al2O3 might not be enough according to young's modulus.

My question is: Are there other properties affecting the stiffness than the young's modulus, as someone mentioned in thread 367-114094? Which property defines the bending stiffness?
Thanks for any help.

 

[MintJulep]

Osmium

E = 80 million psi

 

[BobM3]

There are geometric properties that affect stiffness. But for material properties, Young's Modulus is about it. Short beams can also deflect via shear so if you have a short beam you may need to consider shear modulus also.

 

[EdStainless]

The physical dimension in the direction of the load is the biggest factor, it is to the fourth power. After that it is all modulus.
Don't forget Mo, it is 47msi, and it can be worked with. The drawback is its density.
If you need light, how about Be. Not freindly, but light and stiff.
Of course you can build composites that are in the 40-50 million range, in a specified direction. Graphite fiber/epoxy can get you some great results.

SiC is, for most purposes, the stiffest engineering material available.

Look at the periodic table. Re, Os, Ir and Ru are the stiffest by a large margin. It is all down from there for pure materials and metals.

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[Compositepro]

Carbon fiber has about the highest specific modulus (modulus divide by density) but in the case of composite beams the shear modulus is very different from Young's modulus. Beam stiffness will depend on fiber orientation (and all zero degree is not necessarily optimal).

 

[mwemag]

Thank you for all the tips.
I'm still wondering, why there are certain materials with a high "flexural rigidity" referred to stiffness, despite a low "young's modulus". Look at the following example:

Zirconia: flexural rigidity 980 MPa, young's modulus 210 GPa
SiC: flexural rigidity 500 MPa, young's modulus 410 GPa
(

http://www.asuzac.com.vn/ceramics_products.html)

If rigidity is directly related to the YM how come that Zirconia has a much higher flexural rigidity than SiC, although SiC has a higher YM? The provider confirmes that the Zirconia has a higher stiffness than the SiC.

I would be lucky if that is true... What do you think about this "flexural rigidity" vs. "young's modulus"-relation?

 

[EdStainless]

What you are referring to as 'flexural rigidity' is not a material property, it is a function of geometry and modulus. It is something htat you can figure out after you design a part.
You might also note that ceramics have widely varying poisson ratios.

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[CoryPad]

I think they are using the word rigidity instead of strength. The flexural strength (as measure by three point or four point bending specimens) would be independent of Young's modulus.

Regards,

Cory

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[mwemag]

Yes, flexural rigidity as a function of geometry would be a plausible explanation. In a equation I found the flexural rigidity is E x I, young's modulus times the second moment of area of cross-section. However I wonder why they are using this value to define the stiffness of the material, since they are selling and comparing just the materials, regardless the shape.

Relating to the material, regardless the geometry, SiC having the higher YM - would it be stiffer than zirconia then?

CoryPad, as you mentioned the "flexural strength" would be independent of YM - is this a material or geometrical property? If it is the former can it be used to derive the stiffness rather than with YM?

 

[CoryPad]

Flexural strength is a material property (specifically, the stress required to fracture a beam loaded in three- or four-point bending), and it is unrelated to stiffness and modulus.

http://www.efunda.com/formulae/solid_mechanics/beams/casestudy_display.cfm?case=simple_centerload

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.