I claim the ornamental design for a sheet material, as shown and described.
The sole FIGURE is a top plan view of a sheet material showing my new design.
The surfaces or portions of the sheet shown in between the football shapes form no part of the claimed design.
The uniform broken lines illustrating the lacing lines, the sectional lines, and the perimeter of the drawing depict portions of the sheet material that form no part of the claimed design.
The ornamental design may be applied to a single-ply or multi-ply material such as a textile material, a t-shirt material, a laminate, paper, bath tissue, facial tissue, paper towels, wipes and the like.