My 1980's M-1 pattern Helmet, the same style since WWII. I have blogged about Helmets before. It is a very identifying way to show who a soldier belonged to.
The helmet became particularly important as huge artillery pieces were used to soften positions in rolling barrages before soldiers stormed the trenches.
Artillery could use various types of shells like anti-personnel and fragmentation shells that could rain metal on the heads and shoulders of soldiers in the trenches.