Small Jumbo build commences (J-185 style)

I've just spent a little while gluing together five layers of veneer to make up one side of my laminated sides in a not dissimilar arrangement - but yours is more elegant. I just wish, as I peel the Cascamite off my hands and sleeves, that I had read the words "small paint roller" a couple of hours earlier!

Sides and linings in the body mould. Over size (width) at present so next job on these is to start forming the front and back profiles.
Not started on the front and back bracing yet so closing the box is some way off yet.

Test of carbon fibre 3k strand tow resin bonded to spruce spars

Carbon fibre 3k tow from: www.easycomposites.co.uk/#!/fabric-and-reinforcement/carbon-fibre-reinforcement/carbon-fibre-tow/carbon-fibre-tow-3k.html
Resin bonding from: www.easycomposites.co.uk/#!/resin-gel-silicone-adhesive/epoxy-resin/EL2-epoxy-laminating-resin.html
(fast resin hardener used 100:30 by weight)

Spruce spars: 236 x 10.56 x 6.55 (mm) vertical grain direction
1 plain spruce spar as a control
1 spruce spar with 1 x 3k strand of CF bonded to one edge**
1 spruce spar with 1 x 3k strand of CF bonded to two edges
** this spar was tested with the CF on top and bottom of the spar by reversing the spar orientation.
Weight applied 675 gms piece of brass bar suspended on a string and allowed to settle before measurements taken. Three sets of measurements were taken and averaged.

Spar Deflection (mm)
Plain spar 3.57
Single 3k strand on top 2.81
Single 3k strand on bottom 3.52
Single 3k strand to top and bottom edges 2.52

The hardening time for the resin is shown as approx. 6 hrs. 24 hrs was left after coating before testing.
Stiffness has increased when using the CF 3k tow. Most when applied to both top and bottom edges but significantly when just applied to the edge under tension. Little advantage when applied to the compression surface only.
Took some care with this testing by building an 18mm marine ply gantry to support the spars and the dial indicator. Spars were pushed into a slot in the gantry so ends were registered in the same position each time they were tested. Test were cycled through and repeated in three cycles.

Spars were slotted 36mm into the gantry, measurement point at 185mm from gantry and weight suspended at 190mm from gantry.

Francis
Test of carbon fibre 3k strand tow resin bo....doc (29.5 KB)        Word doc as the table hasn't displayed properly in the thread

R the F  Just about. But before I decide to go the full Falcate route I wanted a little reassurance....

Looking at the results I think they're due to the different nature of the CF and resin. The CF (embedded in the resin) is great in a tension mode, whereas in compression CF is poor and its the resin alone providing the additional compressive resistance.
The additional stiffness given by the CF would mean that braces could be lighter and still give the same soundboard support. Adding CF (or the resin) to the underside of braces does give additional stiffness but I'm dubious about sandwiching CF between the brace and the soundboard. I'd prefer to have brace and soundboard in direct contact and rely on the CF on the tension side of the brace to provide the extra stiffness.

Another thought! with the string loading on the bridge there is a change in the compression/extension operation of a brace travelling under the bridge. Infront of the bridge the soundboard/brace interface is compressive whereas the other edge is in tension, visa versa behind the bridge where the belly is being raised causing tension along the soundboard/brace intersection. Would that mean applying CF to different parts of a brace depending on its loading? Probably easier to apply to the full length on both brace edges. Need to channel the underside of the braces to retain the wood/wood contact?

I still intend to use laminated braces for the soundboard to make them easier to bend to the tight arcs on the secondary falcate braces. Bending the primaries would probably be OK with solid spruce but I will laminate anyway.

Comments on this thinking welcome...

Just had a response from a member of the ANZLF confirming that braces need to be shifted to keep them under the bridge with the vertices under the saddle line.

Hope your on the mend Rob.

I need to keep things symmetric about the centre line, vertical shifting is the nearest thing to symmetry I can get.
The laminated curved braces are all glued up now, I need to profile the bases to match the 20 foot soundboard radius then profile the tops of each brace before applying the carbon fibre tow/resin.

Interesting project this is turning out to be...