In school education, people give more concern on visible injury on campus than the hidden damage that is invisible, intangible and implicit.

It is found that the MMM signal has apparent relationship with the damage, and that by the zero value of the MMM signal, the hidden damage in the ferromagnetic component can be detected.

Hereby the article states the definitions of responsibilities of "Hidden Damage" by analyzing typical cases with the concerned clauses in hope of benefiting the handling of "Hidden Damage" of FCL.