encroachment vs strengthening

encroachment

noun
  • That which is gained by such unlawful intrusion. 

  • An intrusion upon another's possessions or rights; infringement. 

  • An unlawful diminution of the possessions of another. 

  • An entry into a place or area that was previously uncommon; an advance beyond former borders; intrusion; incursion. 

strengthening

noun
  • The process by which something is strengthened. 

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