hammer away vs persist

hammer away

verb
  • To continue discussing (a subject) or trying to solve (a problem). 

persist

verb
  • To repeat an utterance. 

  • To go on stubbornly or resolutely. 

  • To cause to persist; make permanent. 

  • To continue to exist. 

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