Cyclic Stressing Frequency Effect on Fatigue Strength

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Cyclic Stressing Frequency Effect on Fatigue Strength

A Study of Frequency Effect with Particular Reference to the Fatigue Strength of Certain Aluminium Alloys

Harris, W.J.

This article concerns the experimental investigations of the effect of frequency of cyclic stressing on the reversed bend fatigue strength of the aluminium alloys 4·4 per cent Cu-Mg-Si-Mn (L.70, L.71, L.72 and L.73) and the 5 per cent Zn-Mg-Cu type (D.T.D.687) over the frequency range 10 to 1,000 cycles per second. The central theme of the theoretical analysis of the experimental work has been based on the working hypothesis that the so-called frequency effect on fatigue strength, as normally measured by fatigue tests in air, contains, in most cases, an appreciable corrosion fatigue factor together with the true frequency effect. The corrosion fatigue factor, being time and frequency dependent, becomes dominant after an appreciable time exposure whilst the true frequency effect recedes to insignificance. This state of affairs becomes reversed when considering fatigue strength on a much shorter time basis. A study of the kinetics of corrosion fatigue, and fatigue under normal ambient conditions where complete corrosion protection has been afforded, has led to a method whereby the true frequency effect can be separated from the net effect of corrosion and frequency as measured by ambient fatigue strength values. The extraction of the true frequency effect by this method has resulted in its formulation thus: