Upon inspecting the mast for the first time in a year, I lifted the rig by the spreader only to find the bracket to start separate from the rig. It was clear that this was going to need to be repaired so I dug a little further.
These spreaders where attached to the rig in 2010 using a Plexus brand methacrylate adhesive. The stainless steel bracket was prepare with the correct etch primer supplied by Plexus. The carbon section was abraded and cleaned with acetone.
I think the done fall was that these spreaders had come from a 50-55mm OD mast. We bent the bracket in as best we could, but some large gaps still needed filling near the mast track. The adhesive used was supposed to support gap filling properties of up to 8mm. Large gaps do present problems though. The stiffness of the adhesive is different in areas of different thicknesses, which can mean that load might not be shared evenly by the adhesive.
Generally adhesives sensitive to the glue line thickness, where the strength peaks for a given adhesive thickness (I have seen 0.3mm thick thrown around a lot for lower viscosity adhesives).
Side note: An interesting technique for ensuring a 0.3mm bond thickness can be to mix in a small quantity of 0.3mm OD glass spheres/balls into the adhesive. This allows the parts to be clamped together without squeezing out too much glue. I have also seen the appropriate thickness fishing line used as well to space surfaces/parts.
Additionally, corrosion and corrosion cracking can be a common failure mode in adhesives particularly used in a marine environment.
In this case it seems the bond in the substrate or carbon laminate was the weakest link when the adhesive line was thin. Towards the aft edge of the fitting it looks like the thicker layer seperated from the steel and also had a lot of trapped air pockets, which would have held sea water promoting corrosion.
The way ahead.
My plan is to design and build a complete new set of spreaders in carbon fibre based on the original CST composites adjustable spreaders. My plan is the build the bracket off the mast using a 3D printed mould and vacuum bagging techniques. The spreaders will then be bonded to the mast with epoxy resin and glue powder/fibre over a larger area.
Unfortunately the hound fitting is bonded on the same way. It was wrapped in carbon as a regatta repair when the corner of the bracket was seen peeling off. So the hound fitting will be getting ground off and also re-designed / manufactured.
While i'm at it I will be assessing if my original spreader and hound position choice was a good one. The image below shows the original positon on the right and a more conventional position on the left.