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 with the core inside is simply a pictorial representation of the skin peeled off there locally to expose the core for demosntration, obviously in real life you cannot see the core like this.
We can also see in the inset picture on the bottom left of the figure, the sheets of nomex aramid fiber. The bond or node lines are in red along the length of the sheets, and the dashed core hexagon cell will form after expansion of the core. On the bottom right of the figure, we can see what the core looks like after expansion. The core ribbon direction 'L' is along the width of this core raw material. The red arrows in the top figure show the continuous flow of the nomex aramid fiber. The foil thickness alternates between 't' (no bond adhesive) 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, we can see now 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.
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: