We have been responding to customers expressing concerns over surface (thermal) cracks on the rotor surface (see picture).

When we make racing (two piece) rotors they are formulated and heat treated in a way to be highly resistant to aggressive pads and to withstand the extreme heat cycles without warping or cracking (but not thermal cracking).

This kind of thermal cracking is not abnormal which can happen in 1-2 sessions, 20 sessions, or never depending on the braking (temperature) zones and cooling rates after heat up.

There are other important factors including use the right pads; for example you should never attempt to use cheaper street pads and run on these rotors at track, as you would (have to) put more pressure (stress) to the rotors and generate excessive heat (more rubbing than braking) and cause the rotor surface to pre-mature surface cracks. Nor you should use some race pads that can generate deposit on rotor surface which can also make the rotors more prone to cracks due to uneven temperature over the rotor surface.

The rotors with surface cracks are usually continuously serviceable as long as they are within minimum thickness, and the cracks are not expanding to catch your fingernail.

Our two-piece rotors are designed and made to deliver strong brake torque under high heat without warping and not the surface appearance. We believe we have offered the best value and well performed two-piece rotor in the market But before purchase of our two piece rotors please make sure you have good understanding on the pad selection and the superficial surface cracking which is not part of our product performance guarantee.  

Here are some discussion about surface cracks:
http://forums.corvetteforum.com/showthread.php?t=1791417