Godfrey, Douglas & Bailey, John M. Coefficient of friction and damage to contact area during the early stages of fretting 1: Glass, copper, or steel against copper, report, September 1953; (https://digital.library.unt.edu/ark:/67531/metadc56786/m1/1/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .