Fatigue fracture surface roughness and the K-opening level

An investigation utilizing the aluminum alloy 8009 has been carried out in the near-threshold range in an effort to determine the factors involved in the rise in K-op with Delta K in this alloy. It is noted that whereas a number of structural aluminum alloys exhibit a rise in K-op with Delta K, not all do, nor do the steels which we have tested in the near-threshold range. For the 8009 alloy it is concluded crack closure is roughness-induced, and that the rise is in K-op with Delta K is wear-related, with more wear occurring the lower the Delta K value because more closure cycles are spent at a given location of the crack front the lower the crack growth rate. (C) 1997 Elsevier Science Ltd.