There was an old movie about WWI, wherein the character played by James Cagney, shielded his comrades from a grenade in exactly the same way: by placing his helmet over it and lying atop it.
I think the movie was called the Fighting 69th.
There's a great song called "The Fighting 69th" by the band Dropkick Murphys, but it's about the original civil war unit.
I had actually wondered for a long time if a kevlar helmet would protect one from a grenade. I wish it was still an unanswered question.