Since there is no way to hear the sound , I can't say what it was. It could have been your hands/wrist popping, or something else.
I'm guessing it is a hidden tang knife, most likely with a pin or rivet.
One possibility that can occur is the blade breaking in the handle. This happens most often just the other side of the guard. The stress of the bend is highest there.When this type of break happens, you can usually either wiggle the blade, pull it out, or it falls off in your hand. The second place it occurs is at the first pin/rivet. This is more likely if the pin hole takes up too much of the tang, leaving a weak area. This type of break often does not cause the knife to come apart, as the epoxy holds the shortened tang firmly in place. In such a case, the knife may perform fine....or the blade may come loose after a period of use.
When creating and finishing blades, always consider where the stresses will go under load, as well as where stress risers can form in HT. Some places where problems can lurk are:
Thin areas within 1" of the guard/ricasso are big problems ( as with a very deep finger choil or deeply cut back blades).
Sharp angles, as would be made by cutting the tang on a band saw, at the ricasso are blade killers.
Tangs that are too thin or narrow, and have holes drilled to close to the edge.
Holes with the edges not chamfered.
Deep filework, saw teeth,cute cuts and notches in the blade, etc. Any acute angle is a starting place for a crack.
Most of the above can be made with no problem if all sharp angles are rounded.
One final comment about blade design. The point of highest stress is going to be the ricasso/guard junction. This is the fulcrum of all leverage ( intended and unintended) applied to the blade, and should theoretically be the thickest and widest place on the blade. In practice it is often not the thickest and widest place, so pay extra attention to making sure there is enough metal there, and that you have added no other problems for the steel to deal with.
There is more than just dumb luck and bad luck involved in all this. Metallurgy is involved.
Since we have made the blade from a hardened ,and thus less ductile, steel, we have greatly lowered the plasticity of the steel. We need to design in the most strength for the shape we have made.
It is a property of metals that they be much less plastic if there is a notch. A 1" cylinder of steel with a 1/4" deep notch ground around it will break cleanly at the notch when it is stretched or bent. It will,however, stretch or bend far more if the same amount of reduction in diameter is made by a gradual reduction to 1/2" thick. This is called the notch effect. Sharp corners, notches, or sudden reductions in thickness should be avoided as much as possible.
Stacy
