Suppose I wear a bicycle helmet, and I fall hard and my helmet cracks. Do I buy a new helmet or can I try repair the old one with some glue?
Did you crash it? Replace immediately.
Did you drop it hard enough to crack the foam? Replace.
Is it from the 1970's? Replace.
Is the outside just foam or cloth instead of plastic? Replace.
Does it lack a CPSC, ASTM or Snell sticker inside? Replace.
Can you not adjust it to fit correctly? Replace!!
(source: Bicycle Helmet Safety Institute)
Helmet manufacturers recommend replacing helmets after the crash, even if there are no visible cracks, just to be sure. If you see a crack, the helmet cannot do its purpose anymore. You might want to replace the helmet if it is getting old (a few years or so) even if there has been no crash.
Remember, most of us are making money with our brain, so we need to protect it.
You should NEVER repair any bike helmet under any circumstances! The helmet manufacturers even recommend replacing a helmet after several years of use. ( Imagine that! ) I don't replace my helmet that often, but do inspect it for structural damage, which occurrs mostly with the styrofoam liner that begins deterorating after some days in the sun and sweat/oils from the skin. I guess we all have to ask ourselves...how much is my head worth if there's an accident? I doubt anyone thinks their head isn't at least worth the $50 to $100 price tag of a helmet every 5 or so years.