Discussion on “Effect of out-of-phase loading on fretting fatigue response of Al7075-T6 under cyclic normal loading using a new testing apparatus” by F. Abbasi and G.H. Majzoobi

Fretting fatigue has been studied mainly with constant normal load. Abbasi and Majzoobi (2017) suggest an new testing method where contact pressure can be independently varied during the test. The authors compare the case of constant normal load, with that of in phase or 90° and 180° degrees out-of-phase loads, but at the same frequency. However, the results are not obvious to interpret, and it is hoped that a reply from the authors and a discussion could lead to some progress. In particular, contact mechanics (and even the authors' own numerical simulations) seem in contrast with some experiments. Also, the case of constant normal load is found to be the least damaging despite less frictional force is developed which seems to imply an oxidation phenomenon which depends on greater exposure to air, but the time of tests seems similar. If these surprising effects are confirmed, this varying normal load effect calls for some new models in fretting. Or there is something wrong in the experiments?