Investigation of Uniaxial Asymmetric High-cycle Fatigue Failure Behavior of 7075-T651 Aluminum Alloy Sheet

Abstract

In this investigation, the high cycle fatigue life of 7075-T651 aluminum sheets under different stress levels were tested, and S-N curves were obtained by fitting the test data. For the problem that the traditional S-N curve fitting methods without considering the effect of fatigue limit on the S-N curve fitting accuracy, the traditional fitting methods were improved, the results show that the improved method can reduce the fitting error. The highcycle fatigue fracture morphology of 260MPa was observed by scanning electron microscopy (SEM). The observations reveal that the high cycle fatigue fracture morphology shows ductile fracture features. The main reason of the crack nucleation is that the second phase particles on the sub-surface, but the small dispersed precipitates can improve the fatigue performances. The morphology of the fatigue crack propagation is “stair” distribution. The fatigue crack transient beak region is characterized by the mixed-rupture characteristics of quasi-cleavage and dimples. Introduction With the advantages of small density, high strength, good machinability and heat treatment performances, the aluminum alloy of 7075 are widely used in the fields of aerospace industry, transportation, electronic communication [1-2]. These components suffered from the external alternating loads inevitably in service, eventually, the materials of components appear the irreversible fatigue failure, and either affect the normal operation of the equipment, or causes a serious disaster. Therefore, it is very significance to further investigate the fatigue performances and the fatigue mechanism of the 7075 aluminum alloy so that it can provide a guide for the structural design to reduce the fatigue accidents. In the past decade, many domestic and foreign scholars have done a lot of work on the fatigue performance investigation of aluminum alloy and have obtained a series of achievements. E. Cirik [3] investigated the effect of anodic oxidation on the fatigue performance of 7075-T6 alloy and fatigue tests with pre-corroded specimens showed that fatigue life of coated specimens was significantly affected by pre-corrosion, except for the specimen with the thickest coating layer. T. W. Zhao [4] tested the fatigue crack propagation rate of 7075-T651 aluminum alloy with the standard and non-standard compact specimens under the conditions of different stress ratio (R), overload, underload, high-low order, by improving the model of Wheeler [5], a good prediction results that the loading condition influence on the fatigue crack growth were obtained. R. S. Sachin [6] revealed the fretting fatigue behavior in 7075-T6 aluminum alloy use constant amplitude cyclic loading, using a Zeiss confocal microscope and a Hitachi Scanning Electron Microscope (SEM) to characterize the fretting fatigue damage, the results showed that fretting degradation depends on the microstructure of the material. J. Han [7] discovered the fatigue mechanism and fracture characteristics of the 7075-T651 aluminum alloy use the standard cylinder specimen, meanwhile, he obtained the fatigue life and S-N curve under different stress amplitudes. Despite comprehensive work, many questions remain to be answered, the fatigue mechanism of 7075-T651 aluminum alloy based on the low cycle fatigue (LCF) testing and use the cylinder International Conference on Material Science and Application (ICMSA 2015) © 2015. The authors Published by Atlantis Press 252 specimens usually, and there are less investigation of the fatigue performance of 7075-T651, especially, the high-cycle fatigue (HCF) performance should be studied more in-depth. In the current investigation, the fatigue performance often based on the medium thick sheet, but the fatigue performance of the aluminum alloy sheet was less studied. In this investigation, high-cycle fatigue experiments will be conducted using 7075-T651 aluminum alloy sheet under the conditions of different stress amplitude load, so that it can provide a fatigue performance guide for the application of 7075-T651 aluminum alloy.