Effect of pre-corrosion and corrosion/fatigue alternation frequency on the fatigue life of 7B04-T6 aluminum alloy
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Influence of corrosion/fatigue factors on the fatigue life of 7B04-T6 aluminum alloys were studied in this paper. Different kinds of alternating corrosion-fatigue tests were carried out. Three factors influencing fatigue life were investigated: pre-corrosion time, corrosion/fatigue alternation frequency, and specimen’s size. The results show that: with the extension of pre-corrosion time, fatigue lives decreased; although the total corrosion time was the same (192 h), as the corrosion/fatigue alternation frequency increased, the total fatigue lives increased. The total fatigue lives of specimens subjected to two, four, and six corrosion/fatigue cycles (n 5 2, 4, 6), in which specimens underwent pre-corrosion for 96 h per cycle (n 5 2), 48 h per cycle (n 5 4), and 32 h per cycle (n 5 6), respectively, was measured. It was found that the fatigue lives increased by 56.4, 84.4, 136%, as the number of cycles increased, compared to tests in which no alternation took place (192 h of continuous corrosion). When exposed to the same corrosion times, different sized specimens demonstrated different fatigue lives. The fatigue lives of small specimens were shorter than the larger specimens. These results are all due to the specific properties of corrosion/fatigue factors.
I. INTRODUCTION
During its service period, an aircraft is subject to different loads and environments.1 The alternating load may, in the long run, cause fatigue fracture of the structure; at the same time, the environment may lead to corrosive damage.2,3 One of the characteristics of military aircraft service is that more than 95% of its service time is parked at the airport and less than 5% is on duty or training in the air.4 When parked, the main damage to the aircraft structure is corrosion due to environmental.5 When flying in the air, the main damage is fatigue due to the alternating load.6 At present, the common method to forecast the service life of aircrafts in the USA is to carry out tests on the specimens under different load/environment spectra.7 The load spectrum simulates flying condition in the air, whereas the environmental spectrum simulates parking condition in the airport. The total time in which the aircraft is undergoing corrosion-fatigue cycles represents its service life. There are some environmental spectra now in China that apply to coastal areas,8–10 and they are always refereeing to the part of Subtropical Coastal Areas and the Structure Covered by Coating (CASS) designed by the U.S. Air Force to adapt to subtropical coastal areas.11 In the CASS, one corrosion test cycle is equivalent to one year of corrosion on the Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.441
ground. For example, 192 h dry–wet cyclic immersion corrosion in the lab corresponds to one year corrosion in coastal areas. But in reality, the aircraft is on duty frequently, and it not parking in the airport one year and flying in the air one year. So, using one-year corrosion spectra and one
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