A new relation for equalization of load spectrum to a constant amplitude load suitable for fatigue laboratory testing

Fatigue is one of the most important phenomena in the life determination of parts in various industries. The life determination of the part through the test procedures, due to the real loading (spectral loading) is very complicated. Thus, it is necessary to equalize the fatigue real cycle to test cycles applicable in the laboratory. In this paper, by using the available equations in equalization of fatigue cycles, some equations have been studied for the load spectrum. Then the deviation percentage of these equations has been investigated for two very applicable materials in aviation industries (Aluminum 7075-T6 and Steel 4130) by means of block loading spectrum. It has been observed that the errors is very large and not acceptable in some situations. After that, in order to decrease the errors, a new method has been proposed to determine the number of equivalent cycles in fatigue test, considering equalization of the real load and converting it to an applicable load in the laboratory. In this equalization process, constant amplitude loading was obtained for a sample loading block for each of the mentioned materials in such a way that the rate of fatigue damage to be equal to the real loading. Finally, some standard specimens have been tested by fatigue loading and has been observed that the new proposed procedure is capable to predict the fatigue life. The maximum error is equal to 5.5 per cent.