Today, I have some pictures that will help you recognize a cracked garage door panel, when the door is in the opened position, the cracked panel will bow downward in the middle. When you close the door you will hear a popping or snapping sound when the panel pops back into proper place. Check behind middle hinge for cracks. See photos below. Thanks
Bottom 2 panels bowing due to cracking behind hinge
Cracking behind hinge
2 bottom panels cracking (outside picture)