# Sandwich material testing! XPS vs Divinycell

Discussion in 'Materials' started by Jussi_2, Jan 5, 2014.

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### bjnSenior Member

What I find hard to believe is the formula for core shear. Shouldn't the modulus of elasticity of the skins, and skin thickness, be in the formula?

To me it "feels like" a sandwich with thin elastic skins should have greater shear stresses in the core, compared to one with thick stiff skins, with the same load.

While writing this post, I found this text, which has the math behind the simplification of the shear stress formula. And it was a bit of an eureka moment for me =)
http://www3.eng.cam.ac.uk/DesignOff...2002/1b/paper8/mmm/notesC/1B2001P8MMML08F.pdf

The simplified formula assumes Eskin >> Ecore
But I must admit that the feeling of a more elastic skin will cause more shear, still remains...

Example
One core is skinned with 300g/m2 glass + epoxy, and another skinned with 600g/m2 + epoxy. Both beams are on a 1m wide support, with me standing in the middle. The lighter beam will flex 40mm, and the other 20mm. According to the formula "shear stress = shear force divided by width and thickness", both cores will have the same shear stress. But to me it feels like the one which bends 40mm will have twice the shear stress.

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### PericlesSenior Member

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### waikikinSenior Member

If you are really in the market for some foam I have some H100(maybe up to 40m2 x 12-15mm-cant really remember till I put the tape on it) longti kerfed foam at low cost(Kledgecel). PM.. pick up is from from Sydney or Wollongong.
That boat pictured deserves quality, gotta love a Crowther(Lock).

Regards from Jeff

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### Jolly MonJunior Member

Hi Groper, hey I tried this link but it returns an error. Maybe they’ve changed things. I realize this is an old thread. I did look up the Hexcel website and found a pdf of mechanical testing of sandwich panels, but I didnt see how to change the core or types or thickness of outside laminates. Any suggestions?

Rob

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### Eric ruttanSenior Member

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