NO, it is not okay to use a cracked or damaged abrasive wheel.
Explanation
While an abrasive wheel might contain fine cracks impossible to see with naked eyes, damaged wheels can explode when in contact with ceramics at high speed. In addition to that, bits of shattering wheel could easily fly to cause an accident when the machine is at high speed. Please reject the wheels that sound hollow and dead when tapped.
No. If it is cracked or damaged THROW IT AWAY. I can tell you from personal experience, not much worse than a wheel breaking apart while spinning and embedding chunks of the wheel in your chest and face.
