Breaking when the clamps were released might indicate there is a problem besides joint design.
First guess is the joint gap was too big. Ideal gap is between .003 and .005" thick and the strongest joint strength is at .0015 thickness.
Couple things to check was both sides of the joint brought up to temperature at the same time. Was care taken to not angle flame so it goes into gap. Inspection of the failed joint showing solder mostly on one side would indicate this.
You could of had the right gap but things moved when hot. Clamping the blades with pliers just after solder melting as gwilson mentioned or pushing down on top with something like a stainless steel pointed rod after melting and holding until freeze is completed can make a big differ in getting a good close fitting joint.
What did your flux look like. If all the flux turned dark brown or black it is an indication the flux has absorbed all the oxides it can hold and can no longer do its job of keeping joint clean. Spending too much time heating the joint or over heating the metal can cause this. Once metal is overheated and oxides form there is no chance of the solder sticking.