| The skins provide the bulk of the load path for a composite panel in bending in-plane tension and in-plane compression; as you point out.

Thanks for that Rick.

| I neglect the contribution of foam core when determining composite bending strength or in-plane properties of the panel. However when the panel is bent the core experiences shear. If the core is brittle with little shear strength (or limited shear strain capability) then the shear stress created in the core will fail the core. That means there are now two intact skins no longer attached to an homogeneous core. Basically the composite is delaminating.

Given the part has a design load, at what percentage of that load would a core expect to see enough strain to damage it?

Are you saying an ideal core would have higher shear strain, and we really don't care about shear strength, because if the core is doing strain work, you are far into yeild/overload and all bets are off? Thus assumes you do not want delamination in an overload, which, you might.

| I have sometimes used low density XPS for bulkheads and I add glass on the surfaces if I want the bulkhead to take shear loads - as is the case in closing a tubular section that will carry torsion. However it always worries me that the shear strain of the low density XPS is limited and the glass will shear the XPS bond in high shear conditions.

Do you expect these parts to see design loads that would cause these shear issues? Or are you explaining the dynamics in an overload condition?

| A low density XPS foam is far more brittle than PVC foam. The shear breaking strain is around 3% for 30kg/Cu.m Styrofoam compared with 8% for Divinycell H80. Given that E-glass has a tensile breaking strain around 6.5% you quickly see that the low density XPS is prone to core failure before skin failure when sandwiched between glass skins.

Again, if the glass is at 3% strain, what % of design loads is that? Given you can choose where a fail happens, would you rather the glass fail in an overload?

| That said I have recently found Bunnings have a pinknsh colored XPS foam core made in Germany that is less brittle than the blue XPS Styrofoam I have used in the past. I have not compared the properties but the Knauf board feels almost rubbery. It will rebound a little if compressed.

Do you have a data sheet for it?
Rob has a sample of the 700kps XPS, which, I believe, is not available in AU. I would be interested in your thoughts if you get a chance to look at it.

| This points out the need for compatibility of certain mechanical properties for cores and skins. In fact XPS may be a better match for a carbon skin that is good for about 1.5% strain, which is much lower than the breaking strain of glass around 7% despite the carbon having higher tensile strength.

| Divinycell and glass work nicely together. I am always amazed at how a glass/divinycell/epoxy composite works structurally. Such easily worked materials can be readily turned into a robust and durable structure.

Perhaps what I have missed is these structures are not 'math engineered'/(full FEA)so much as 'experience engineered'(formula assisted). So a material that handles unexpected loads in a very predictable and understood way is better?

What are the failure modes of glass/pvc foam? Is fail detection and repair easy? How are they detected/repaired?

In my limited experience shear fails are not a big deal.
Because shear fail only happens in an overload event, which you want to detect, and you can easily detect the extent of the failure (tap test) and repair it by injecting epoxy as this failure mode has the part mostly still intact, just cracks in the foam.