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tienne (Aeronautics) (OP)
12 Nov 12 18:25
Hello everybody,

I am currently studying the behavior of composite plates in 4 points bending and in-plane shear and especially the strain in the panel depending on the fiber orientation.

Let’s take a plate with a fiber orientation 0/90 degree (weave fabric) subjected to a 4 points bending loads and the same plate -45/45 fiber orientation.
In term of strain it is the configuration two that would give the maximum strain allowable because it is the matrix that “works” which much more ductile than the fiber.

If I take the same panel working in in-plane shear I would obtain an even greater results (strain before failure) knowing that shear modulus of the Epoxy is about 3 times lower than the E modulus. Correct ?

Is there any other explanation to these results? I have read that a resin like the Epoxy works better in shear than flexure but with no explanation.


Thanks,
RPstress (Aerospace)
13 Nov 12 11:44
The linear shear modulus G (for the so-called Hookean region of deformation) is probably very roughly E/3; a resin probably behaves more or less as an isotropic-ish solid over the early part of its stress/strain curve. With a Poisson's of, say, 0.38, the shear modulus is probably E/(2*(1+0.38)) ~= E*0.36. There's not that much data about for shear modulus vs. Young's modulus. The 3M AF 163 film adhesive spec quotes E = 161 ksi for E, 60 ksi for G and a Poisson's of 0.34 (ratio G/E = 0.373 or implied Poisson's of 161/(60*2) - 1 = 0.342—close). I can't find any other resin with both G and E quoted, not epoxy nor any thermoplastics. If anyone has any other examples do please tell us.

At failure, shear strains do tend to be greater than direct strains at tensile failure. However, this depends a lot on the strain after yield, which it is hard to generalise about.

Shear strain at failure often seems to be above 0.5 even for the brittle-ish epoxy matrix resins (I'm basing this on remembered in-plane shear test stress/strain data; I'll have to look the data out, if I can find it, to be sure), and often above 1.0 for the softened epoxy film adhesives. Epoxy resins do yield but rarely seem to deform more than six or seven percent in tension (often only two or three). Just finding a shear strain to failure and a tensile strain to failure for the same material is quite hard, even for metals.

I would guess that this dearth of information is partly because we rarely need it. What do you want it for? Just curious.

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