Basic wall wart ps but awkward V for replacement, just died, not been left on or abused . Rated 115 deg C ,3A in the primary side, In use about 30mA of mains. I don't think I've sliced into one before and am not sure what I would expect if fused from overtemp or old-age etc. Probably as all the body of the fuse would be at the same over-temperature then all the Woods metal/ high bismuth/antimony solder or whatever, would have all melted and coalesced into one lump or a lump either side , would have been my guess. If failed from current surge/ old age / shock/vibration/ chemical breakdown or something then a tiny fracture or a change of texture to powder or something , instead of shiny metal. Looking under x30 mag This one between the terminal wires about 3mm run of the low temp metal with about 0.5mm gap on one side, the end of shiney "fuse-wire" run rounded as though locally melted , all in its original posistion with no obvious channel in the casing to mechanically retain it in place.
So not really any more forward on failure modes of these fuses.