Sandwich Panel Flexure and Core Shear
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Sandwich Panel Flexure and Core Shear are the two most common types of safety margins calculated in the aircraft cabin interiors industry for honeycomb sandwich panels. The flexure and core shear allowable stresses are developed using coupon tests. A test coupon is simply a specimen that is cut from the panel raw material. The direction along which the long axis of this coupon lies will influence the final allowable stresses. You can read more sandwich panel design at this link: Hexcel. Also check out this video for more on the Honeycomb Sandwich Panel Manufacture Process.
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We can see in the figure 1 above, this coupon is cut from the panel raw material in such a way that its long axis lies along the core 'ribbon' or the core 'L direction. The middle white area in the image with the core visible is simply a pictorial representation of the skin peeled off locally to expose the core for demonstration only, obviously in real life you cannot see the core this way.
We can also see in the inset picture at the bottom left in figure 1, the sheets of nomex aramid fiber. The bond lines or node lines are in red, along the length of the sheets, and the dashed core hexagon cell will take shape after expansion of the core. At the bottom right of the figure, we see what the core looks like after expansion.
The core ribbon direction 'L' is along the width of this core raw material in the bottom right inset image. The red arrows in the top figure show the continuous flow of the nomex aramid fiber material. The foil thickness alternates between 't' (no bond) and '2t' (bonded red strips) where t is the thickness of each sheet. Finally this expanded core is doused in the resin material (typically phenolic or epoxy resins) and cured at high temperatures.
In Figure 2 above, we now see that this coupon is cut from the panel raw material in the other perpendicular direction, thus the long axis of this coupon now lies along the weaker core 'W' or core expansion direction. The reason this direction is weaker is that there gaps in the flow of material, namely the nomex aramid fiber is absent within the hexagonal cells and only present at the bond lines. If the coupon is subject to bending along the ribbon 'L' axis, then only the weaker bond lines are able to resist the bending loads.
It is common practice to test coupons along both of these directions and document the A-Basis and B-Basis flexure and core shear allowable stresses. It is also typical that the flexure and core shear allowable stress values are higher for the 'L' direction coupons than the 'W' direction coupons. In the follow up post to this one, we will dig a little deeper into the typical flexure and core shear test set ups.
Related to Sandwich Panel Flexure and Core Shear, here is a cool video that goes into the manufacturing process of composite sandwich panels: