Scarf joint strength / failure... Add splines? is it needed?

[Rangerdave1]

So I am pretty new at building. My first build came out great, sounds wonderful, has been stable and plays well. I did a scarf joint under the fingerboard for the neck with no issue. Now on to a few new builds I am presently working on, I did the same type of joint however I was curious how strong it really was. It was plenty strong when just a blank 3" all the way across before I started carving as I stood on it and bounced up and down.

Once I carved the neck and was 95% done I thought I'd flex it and see how strong it was. Well to my surprise it broke with some force just using my hands and arms. Now there was no fingerboard on it which I believe adds a lot of strength to the joint but still.. The surfaces were very flat before I glued them, Things were clamped well. As I inspect the failed joint, the wood is what appears to have failed as you can see in the photos. Now I am worried that there is not enough strength in this joint.

Should I consider adding a spline or some sort of reinforcement in the joint?? Was I expecting just too much out of this joint given the amount of wood there once carved? Will it be plenty strong enough once the fingerboard is glued on with a trust rod inserted in to the slot?

I drew out where I think a spline or carbon rod could go on each side of the truss rod slot but is it necessary?

What are your thoughts and experiences?

[John Parchem]

Interesting, It looks like your glue join held up just fine as I see wood on both sides. The neck should go back together without issue. I personally think that the fretboard acting as a strap will do more than the spine and may be all that is required. If I was to add a spline I would put it on both sides of the truss rod. If it is needed then having it on both sides would keep it from starting to split on the weak side.

I believe there was a discussion about where to place a scarf joint in the forum a month or so back. I place the joint so that one side terminates on the back side right under the nut and the other side terminates under the head stock veneer. That placement has a lot more glue surface area,

[Rangerdave1]

Yes If I were to add a spline it would be on both sides. However if most are not doing it and having good results, then I don't want to add extra steps if not necessary. I will look up the thread you are referring to.. I don't think I will reuse this neck but I may glue it back together with a fret board and test it's strength just out of curiosity and for my own peace of mind.