hairpin turn vs incurvation

hairpin turn

noun
  • Synonym of hairpin bend 

incurvation

noun
  • The state of being curved or bent; any curved shape or formation; curvature; a curve; a bend. 

  • The act of acquiring or being given a curved form; an instance of curving or bending. 

  • A curving inwards; the condition of being curved inwards. 

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