incur vs ought to

incur

verb
  • to render somebody liable or subject to 

  • to bring upon oneself or expose oneself to, especially something inconvenient, harmful, or onerous; to become liable or subject to 

ought to

verb
  • Indicating duty or obligation. 

  • Indicating advisability or prudence. 

  • Indicating desirability. 

  • Indicating likelihood or probability. 

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