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Correct me if I'm wrong,
Melted piston can occur when egt is too high = lean condition.
Broken rod = detonation.
I think it's a bit of a broad scenario - broken rods can be caused by excessive RPM as well, and detonation can also cause excessive heating of the combustion chamber which then causes pre-ignition, which can also cause melted pistons... as well as detonation causing sandblasted/cavitated-looking/melted pistons as well...
Maybe someone with more experience will chime in and correct me, but I think that there are multiple causes for the symptoms you are describing.
Like Matt said its a broad scenario, I am not sure if a rod could be broken with severe detonation but I think that
before that happens the engine is going to have blown head gaskets, broken pistons rings and melted pistons .
So its not a good idea to wait for a broken rod to worry about detonation.
Here is a good article on engine detonation
As well a useful webinar for that subject
I believe in almost ever case of a melted piston it is due to detonation. A melted piston is just a severe case of detonation. I also think a piston is likely to let go before a rod goes and then the rod goes through the side of the block. The rod then gets the blame. I have seen a few cases of broken rod bolts cause catastrophic failure however. I have also seen one case of what I expect was pre-ignition cause a piston pin and rod to bend while the piston crown itself looked fine.
If you had a lean mixture under high load and relatively retarded timing and no detonation (due to the timing or good fuel) would the temps get hot enough to melt the piston... Or does the boundary layer still protect the piston and it needs detonation to break through that layer and expose the alloy to combustion temps?
As I learn, melted piston is due to high EGT resulting from lean condition.
Broken piston is due to detonation, broken rod can result from too much toque for the rob or detonation.
Nice looking piston that just melted like on the picture is from high EGT, lean condition - no detonation is visible here.