Hi again Nodd. Boy that glider sure did turn out nice. Beautiful sleek lines!! Too bad you had a wreck, but that happens from time to timedoesn't it
Anyhow, I don't know it this will be of any help to you, but when I built my enlarged Sagitta wing (111" span, pictured below) here is how I did it:
The spar caps, top and bottom are 1/8 X 3/8 spruce, the shear webs are progressively smaller. They start the full width of the spar capsnear the root and go down from there. All this is combined with a full D-tube leading edge structure. The only place where I wrapped the spar with anything was at the root, the section that has the joiner tube inside it. This is simply because as you put a G load on the wing, the tube is actually trying to "pry" against the spar caps and spread them apart. Outside of that point there is significantly reduce force trying to spread the spars apart, and the bond between the spar caps and webs is enough to do the job. This wing will go up on a high start that is big enough for 3-4m gliders and the wing barely even flexes. Even crash tested it a couple weekends ago.....no damage to the wing after two gust wind cartwheel landings!!
Just looking at the photos, I am with Bruce, I would bet that the shear web on the original wing was the major player in the failure.
EDIT: Forgot to say, the following is not directly aimed at Mr. Nodd, as he is pretty knowledgeable in designing, but more intended for newer fellow's to designing and building.
Something to watch out for, are "built in sheer points". In other words, a place where there is a change in the structure that could cause a failure. A good example of this is the end of a joiner tube, or the end of a double, or maybe even the edge of the top sheeting. Any of these points will be a spot where there could be a failure. A perfect example of this is how in a crash, most aircraft will break the fuselage right at the trailing edge of the wing/wing mount structure.
What you want to do to alleviate those shear points is spread them out. If you have the joiner tube, the root spar doubler, and the sheeting all and at the same rib, immediately followed by the small sheer webs, I guarantee that is where the wing will break.Some things I did on my wing was the leading edge sheeting of course extends to the tip, so that helps spread that out a bit.The ply doublers on the front and back of the spar caps go out to the end of the joiner tube, but are then cut at an angle from the bottom up so that the top of the ply is onebay longer than the bottom, allowing that shear point to be spread a bit. Also, right up against the joineris wherethe shear webs are full spar width, therebyhelping to spreadthe heavy loading out along an even greater length.
Remember, this is pretty basic stuff, but many guys forget it......the closer in to the fuselage you get, the stronger thebending loads are on the wing. One needs to build accordingly strong at the root, and get lesser towards the tip to keep the tips light where there is less load.
Hereare the promised pics of the glider, and a simple "schematic" drawing of the spar structure.