Keep in mind, often the names of trees are regionally specific. That is to say, "skillet" and "pan" are the same depending on which part of the country you are from. I may be a bit rusty but, I believe that American Hornbeam is also called Blue Beech and/or Muscle Wood. Which I feel identifies the muscular smooth bark tree in your pic. It is also a bit blue in color. Hop hornbeam is a tree with a segmented bark very similar to shagbark hickory only the "shags" are much smaller. I've heard both American and Hop hornbeams called, Ironwood.