Sometimes things happen. Fortunately, there were no injuries. I was using this lump of crud to clamp some wood pieces while the glue cured. Or rather, I was trying to do so. The tension I had on the clamp was modest; certainly didn’t have bear down on the screw. Oh well, cheap tools. This is a 4 inch C-clamp. Note that the pivot separated too.
There isn’t any sign of currosion or other pre-existing fracture. The crystaline structure in the metal is quite variable, which implies the casting temperature wa poorly managed.