A hollow stave Ian Wilmut – August 2011

LBL stave At the March UG week LBL showed a prototype asymmetric stave. This prompted consideration of the strip stave to see if some of the same ideas could be applied. This is being looked at in the context of the 45gsm UD K13D2U If the fibre needs to bend around a 5mm radius the stiff fibre must approach the bend at 15° to the axis of the radius (assumes a 25mm minimum bend radius)

The basic idea Build a stave from skins, but leave out the core and instead install simple tubes that form both the skin to skin coupling and the close outs in one. The tubes should also contribute to the thermal path from the periphery of the stave to the Pocofoam. Because the contact areas between elements are large, flat and thin they are good candidates for co-curing. Thus it might be possible to eliminate most of the cold cure adhesives. This would serve to save the material overhead of the glue.

Proposed Structure Four tubes are made around mandrels as per the US Pixel structure, there are two layups that will result in CW and CCW twist on one member of each type Two skins are also made as asymmetric layups. Again one will twist CW and one CCW – this is not central to the idea, but a ±15° skin will curl less than a 0-90° one. The stave could then be assembled using a 0° layer (in red) as a glue layer. It would need the mandrels back in the tubes and outer platterns The gluing of the Poco Foam needs a lot more consideration It would be desirable to glue poco to tubes at the sides with a “heat proof” adhesive to constrain thermal load of the pipe. K13D2U +15° K13D2U -15° K13D2U 0° K13D2U -15° K13D2U +15° K13D2U –15° K13D2U +15° K13D2U -15° K13D2U +15° K13D2U -15° K13D2U –15° K13D2U +15°

Benefits Loss of cold cure glue, saving on “non-productive” material. The close outs become an integrated part of the design. The fibres in the tubes should serve to conduct heat from the hybrids into the poco-foam. – Note, it could be expected that the ends of the stave might be warmer as not as many fibres conduct to the foam (~75mm). There might be a saving of material from this design – depending on how thin the skins can be made. The skins should be able to be made very flat and smooth to minimise the amount of glue needed below the modules. As the fibres in the skins are no longer at 0° and 90° the widthways and length ways performance can be tuned to have appropriate length and widthways performance. ±15° fibres provide 75:25% lengthways: widthways split in performance. So 4 layers at ±15° perform as 0,0,0,90 but can be made symmetric to alleviate curling during cure

Drawbacks When the modules are mounted a “filler” will probably be needed to support the tube. This design has may cases of asymmetric fibre layups so many parts will be twisted or bowed before they are integrated, and it might be that if the final assembly stage is a co-cure then the final object contains distortions. There are less width wise fibres so the thermal performance might be lower – it is expected there will be the equivalent of one widthways thin layer of fibres.

Additional Locking points would clearly not seem to be immediately compatible – however if a cut out was added to the outer edge the present design could be installed in much the present way The end close outs would have to be carefully considered. It is obvious that they can only be present at one end (probably z=0). The forms to build the tubes have been shown by LBL to be successfully made from Aluminium. Here the high CTE of aluminium allows there removal once the structure has cooled The module mounting forms need to be able to expand to fill the appropriate tube as the fit will need to be excellent to prevent excess glue use during module mounting.

Conclusion Simple FEA and study of material follows in additional presentation This looks like it might be feasible, but unless offers concrete improvements to the present design is probably not worth worrying about. To understand if it is worth pursuing it will be necessary to: – Understand thermal performance – including ends – Understand mechanical performance – Prototype small section