The ornamental design for a sledge hammer head, as shown and described.
FIG. 1 is a perspective view of our new design.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines having the dash-dash appearance illustrate portions of the sledge hammer head that are not claimed. The broken lines having the dot-dash-dot appearance defines the boundary of the claimed design.