Experimental Studies on Surface Layer Effects in Fatigue Strength of Aluminium Alloys

Abstract

An unnotched specimen having the emery- or carborundum-paper polished surface is generally used in ordinary fatigue tests in order to avoid the effects of a hardened surface layer and the roughness induced in the machining process on the fatigue strength, but the surface has thin hardened layers owing to the mechanical polishing. In the present study 11 kinds of alminium alloys were examined to determine the fatigue strength of the specimens from which the hardened surface layers were removed by the electrolytic polishing method, and the effect of the annealing treatments after the mechanical polishing.The fatigue test was conducted on an Ono's rotating-beam fatigue tester and the fatigue strength was determined at 107 cycles.The specimen polished with carborundum-paper had hardened layer on the surface about 0.03mm deep. When the layers were removed by the electrolytic polishing, the fatigue strength dropped lower than the specimen polished with carborundum-paper. The reduction of fatigue strength of O, T4 and T6 treated alloys are 4.2-19.2%, 10.6-21.1% and 1.6-6.8%, respectively.In case of notched condition, the specimen electrolytically polished showed lower fatigue strength than that shown in the machined specimen, but no difference was detected in the fatigue strentgth in either of the surface conditions of 6061 alloy.The fatigue strength of the specimen polished with carborundum-paper had the highest value, which was followed in the declining order by the specimen electrolytically polished, that annealed in vacuum, and that atmospherically annealed.