There was a guitar. It had some years of service, and one day during the attempt to tune it a mistake was made - it was tuned an octave higher than it should be. The E string cracked, causing sudden change of the tension force on the neck. The already high load got compounded with a shock dynamic load. And the material failed. The wood cracked in the headstock area, between the nut and the pegheads, longitudally along the wood grain. The crack did not go through completely, but compromised the rigidity of the guitar neck and it did not keep tune anymore, and tended to open under string tension and possibly could progress. The guitar then became a gift for a mad scientist, to repair or do whatever he pleases.
After analysis of the structure of the material it was found the wood failed in tension, in the place where the force was held by cohesion of the wood grain. It was decided to glue the material together and reinforce it with metal plates, holding the affected region in compression.
Two bars were cut from a 25mm wide and 2mm thick aluminium strip. Three holes were drilled into them, for two M6 bolts and one M4 bolt, in a way to make the larger holes placed at near the ends of the crack. Matching holes were drilled into the headstock. Two grooves were filed into the bottom metal bar, to make it fit between the tuning keys.
The crack was filled with metal-filled epoxy resin, using a piece of cardboard to spread the resin inside. The inside layers of the metal bars were also smeared with epoxy, just for sure. The bars then were attached to the guitar, screws were inserted through the holes (with a washer under the middle one to spread the force; the screw near the nut was left without a washer to avoid interference between its head and the strings) and tightened with nuts on the other side as firmly as possible.
After several hours the guitar was tuned and tested, and after several more hours it was tested again in order to assess the tuning stability (and therefore the rigidity of the repair). The tuning drift proved to be below detection.
Crack in the guitar headstock
Drilled holes, bottom
Drilled holes, top
Finished repair, top side
Finished repair, bottom side