Hi , FWIW , ..I don't think that polyurethane foam is suitable in this application ; it mainly lacking in the degree of compressive strength available for the job in hand , which is more of a requirement to resist a local kink which preceeds buckeling of the tube in question . ..The normal application is to stabalise the stressed surfaces which contain it , ie double skinned GRP(sic) monocoque stressed skin shells .
For the "cockpit strengthening" application , IF , one wished to use this idea , a better alternative , which would have a proven abilaty to withstand the loads associated with preventing a tube from buckeling would be , (USA),one of the tube bending filler products on offer from Cerro Metal Products Corp.,....The metal ones may be rather heavy and expensive, but the resin type would be a consideration on economical grounds alone .
There are of course all manner of practical difficulties of application and problems should repairs ever be necessary. This would need a LOT of consideration before its use .
A better alternative would be to insert ,as has been mentioned , a well oiled (sic) ,reasonably tight fitting length of ash or hardwood dowel , ie wood with a good resistance to compressive loading across the grain direction .(It could have even been previously soaked in a propriatory hardening resin ,).
However , out of all of this , it would be far simpler , in terms of any posible future repairs etc. .to use a short length insert of steel tubing ( realy only practical if useing metric size tube ) or to USE A THICKER GAUGE TUBE for this member in the first place . (Alt,is an OVERSIZE on dia,...Your choice! )
Ref Fokker cockpit tubes and the "corner" tube segments ; These could be replaced with a continuous length, (or at least for the pieces that lay along and adjacent to the top longerons ),making this section effectively of double tubeing , and , in order that the longeron is not affected unduely by welding heat ; these reinforcing tubes could be attached to the longerons via some other way , ie clips/plates and s/solder joints.Etc etc.or the cockpit reinforcing tube could be welded to a number of short external sleeves (fitting over the longeron ) , and these , after clean up ,sweat/soldered to the longeron...Again , thought has to be given to the various posible difficulties/ requirements concerning repairs that may be encountered later etc..However , the structure must still be of reasonably ballenced strength , otherwise there may arrise from this alteration , a problem elsewhere. .Regards
John M