fading vs weakening

fading

noun
  • The process by which something fades; gradual diminishment. 

weakening

noun
  • An instance or process of loss of strength. 

  • A structural principle of mathematical logic that states that the hypotheses of any derived fact may be freely extended with additional assumptions. 

How often have the words fading and weakening occurred in a corpus of books? (source: Google Ngram Viewer )