A break like that looks like a moisture (or lack there of) problem. The heat treating might have killed it. Go out and bend a nice green sapling and see what it looks like when it breaks. Now get a dry dead branch an do the same . Note the difference. All of the points about design are Excellent and quite valid, but if your bow were made out of green wood ,it would still be in one piece (would be a piss poor bow.......... but would have still bent) It's all about the ballance of design,wood selection, moisture content etc.etc.. IMHO. ....to dry from the heat was the proverbial straw.