FYI the cracks arn't what you really need to be worried about, it's the inner core. The core is often designed to collapse on impact, and these are the parts of the helmet that REALLY protect your head, not the outer shell. Again though, it really depends on what kind of helmet you have, and how they are manufactured. In short, your helmet may have no crack in it at all, but at the same time be totally worthless for any substantive protection.
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