Provide crushed oyster shell, free-choice in a dish. It's not expensive; all feed stores carry it. Oyster shell will help make the eggshells hard enough so they don't crack easily. Also, put a wooden egg or a golf ball into each nestbox -- this also helps a hen learn to leave the eggs alone.
Hens often use their beaks to test a newly laid egg - to see if it has a good shell for hatching a chick. If the shell is weak, the egg would probably crack while the embryo is developing, killing the new chick and endangering the rest of the hatch. So, taping the egg with the beak helps ensure her reproductive success.
OTOH, some hens have developed a habit of breaking open even a hard-shell egg. So, gather your eggs often, make sure the hens have adequate levels of good-quality protein, calcium supplement (oyster shell), and plenty of fresh clean water always available.