Fig. 1: Typical indentation load (P) vs. displacement (h) curve for the (100) surface of BCC Fe single crystal.

The green line indicates the indentation load-displacement curve. The blue arrow and the red arrows indicate the first pop-in and subsequent pop-in events, respectively, in the indentation. The curved broken line is drawn by Hertz's contact theory fitted with the indentation load-displacement data before the first pop-in.