Once a cycle helmet has been cracked, please don't use it any more, even if you still wear it but just a psychological comforting, do not have any basic usefulness. It should be okay in the terms of small knock and small touch, but once falling down with strong shock, the helmets could not be able to protect our brain anymore. Don’t look down upon a small crack, since it has changed stress and component force effect on the helmet’s physical structure.
No matter the helmet has been fallen down or impacted, we should replace our helmets after 3 years. Since sun, rain, sweat and vibrations affect the protective qualities of the material. It will weaken the safety factor and damage the protective value.