While to my knowledge there were no US M26 fragmentation grenades made of plastic, there were plenty of similar shaped training grenades trialed and eventually dropped for various reasons. Generally, however, the reason is not due to the durability of the plastic. Those problems were solved decades ago, as study of rifle ammunition shows, you can tailor make plastic for nearly any application.
While the US has not gone to a plastic fragmentation grenade, many other countries have. The Austrians paved the way with Arges in the 70s, PRB of Belgium had many for export, China has also manufactured different models, the Czechs - the list goes on.