What causes this?

If my eyes are not deceiving me the neck joint at the headstock is a 'finger joint' rather like the one Bob Taylor uses. I cannot see how this joint could be strong but hey, I build ukuleles, I am not a wood engineer (like Bob ;) )If the box was undamaged it could only be that the instrument was faulty and this occured in transit inside the box... . Anyway - send it back.

Really, the only time a joint breaks (especially a finger joint - that's some serious joinery), at the joint, is if there isn't enough. Enough what? Here's a short list - glue, pressure, long grain, drying time, clamping time, etc.

There are many who agree that a glued joint is stronger than wood. I've experienced this, first hand. Glued joints don't break, if glued properly.

If it were my guess, yours is a combination of things, lack of glue being the first. The grain on the neck is not vertical, so there's probably some end grain in the glue joint somewhere - this would also be a contributing factor.

I'd send it back. -Aaron

If you look closely the joint is nearly intact in one part but it has failed 75% od its width. This seems to me a combination of faulty construction and impact damage.

I'm kinda surprised to see the individual "fingers" of the finger joint sticking out like they are. Kinda looks like it wasn't glued very well...

All agreed...I am a lifelong woodworker and was raised in the lumber industry. A correctly executed and glued finger joint actually adds considerable strength...as was mentioned Taylor does this (I am a Taylor fan, but don't let that throw you, it really is a good design). A finger joint (the way Taylor uses them) allows them to save wood and money when they produce an NT neck, while actually increasing the strength of that otherwise weak area. I have seen tests done on different types of wood joinery and when you do a joint like this correctly, well, the break would not be on the joint, but on the wood just to the north or south of it, typically. Joints like this are also used in laminated beams and in furniture. They are strong and spread the joint load over a greater surface, thus the strength. My guess is that this joint was almost totally missed by the dude with the glue gun…otherwise the break would not result in us seeing exposed fingers like that, they’d be embedded in the remainders and the break would have occurred just off the joint. It is probably what Pete said, a combination of a weak, poorly executed joint (with too little glue OR faulty glue, which would not be the fault of the luthier) and a good jolt from a careless shipper.
 
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