One of the cardinal rules in working with composite sandwich panels for structural applications is “No Point Loading”.
OK, so we use pads of “appropriate size” to distribute the loads. The conversation is incomplete (so far as I have been able to
follow) as I cannot find any recommendations on what the target PSI loads should be for the pads. Static loads.
Anyone have a reference I can consult for this?
Thanks
what standard are you using? I’ve been looking at ASTM C 393 for 3-4pt bend tests. I don’t see anything about how to sort out the pad size for load. It only says that you CAN use pads if local core crushing happens at the load points.
It depends what you are loading it with… our loads were based on test data… how much load caused crushing in the panel, were loads static or cyclic etc… but some tips.
Bolt clamp up loads can be ball parked with 5-7 times the torque divided by the diameter. Keep the units in pounds and inches. 1/4 bolt at 50 inch pounds would be 250-350 inch pounds divided by .25" or 1000 to 1400 pounds of clamping force at that bolt. That is why you need something to support the core.
Size of the support depends on the contact patch of what you are attaching or loading it up with. The smaller the contact patch, the tougher whatever you put there to support the skins needs to be. You could calculate the patch size and determine bearing loads to be sure your support was well over the applied load but you need to know the bearing allowables (Fbru) for your material and they vary a lot depending on the core material and construction. Phenolic Honeycomb core vs foam core or solid wood or metal core.
In aviation, for light loads, we used to drill the hole in precured panels, chuck the long end of an allen wrench in a drill, stick the short end in the hole and spin it to bust out the core. (it was honeycomb material), inject the space with potting compound (resin with micro balloons or milled fibers would work) and drill the hole out after it cured.
For higher loads, we would pre cut out the core at load locations prior to layup, fit in wood or aluminum plugs with foam adhesive to fill in any gaps and then layup and cure the panel with the skins. After cure, you drill the holes at the right locations and you’re good to go.
I’ve seen them use plywood for this on fiberglass boats. We used solid wood but selected type to match usage. Balsa is great for light stuff all the way up to maple where you need more support. If you use aluminum plugs with carbon fiber, you must pre-prime the aluminum with a couple coats of primer to isolate it from the carbon or it will corrode, you could also use a layer of fiberglass between the carbon and aluminum to isolate it as well.
If you think the panel will be flexed in use, you need to be careful at the transition from the plug to the core. If it flexes at that transition, the difference in material stiffness can cause the skin to break along that line. If you are worried about this, there are ways to soften the transition. You can also taper the core and build in a pocket. I’ll try and find a photo to show what I mean.