As an option:
from the forums:
@Clamp-o-Tron

Have scrapyard track flight time instead of flights. This is simpler for the user but more code to write/change for you.

from here

Final Frontier tracks accumulated mission time, EVA time, and mission counts for kerbals, not sure how they are doing it code-wise, but its seems to work pretty good. Another option might be just to track how old it is, (i.e. when was it first used), then use its age and generation as a base on how likely it is to fail.