The Fracture Behavior of 7085-T74 Al Alloy Ultra-Thick Plate During High Cycle Fatigue
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INTRODUCTION
THE lightweight 7000 series Al-Zn-Mg-Cu alloys are commonly used in the aeronautical industry because of its excellent properties of fracture toughness and strength.[1–6] Among alloys of this series, 7085 Al alloy has attracted increasing interest because of its unique combination of low quenching sensitivity and good thermal stability after T74 treatment.[7–10] In the last few years, 7085-T74 Al alloy ultra-thick plate has been mainly used in large structural components (such as the wings and tails) in advanced aircrafts, which are inevitably subjected to cyclic loading during the service.[7,9] Therefore, higher requirements are demanded for the fatigue resistance of 7085Al alloy plate. When used in aircraft as structural airframe parts, damage tolerance of the 7085-T74 Al alloy is an important parameter besides strength for the lifespans of aircraft wings and tails. Many investigations have been carried out on the fatigue properties of Al-Zn-Mg-Cu alloys.[1,11–14] For example, De et al. evaluated the grain refinement on the fatigue life of 7075 Al alloy. Their results show that fine-grained 7075 Al alloy presents longer fatigue life than the parent 7075-T6 material.[11] Wei et al. researched the fatigue behavior of
PAN DAI, XIAN LUO, YANQING YANG, ZONGDE KOU, and BIN HUANG are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P.R. China. Contact e-mails: [email protected]; [email protected] JINXIN ZANG and JIGANG RU are with the Beijing Institute of Aeronautical Materials, Beijing 100095, P.R. China. Manuscript submitted December 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
7050 Al alloy ultra-thick plate. They found that compared with 7050 Al alloy plate with less thickness, the ultra-thick plate shows an improved fatigue property due to the less densely populated particles.[1] Nevertheless, in contrast to previous studies on 7075 and 7050 Al alloys, there is no open literature concerning 7085-T74 Al alloy ultra-thick plates. Furthermore, the 7085 Al alloy is often processed into parts with steps, corners, keyways, oil holes, etc., which act as a notch effect and could change the stress state of the alloy and then cause stress concentration. Therefore, it is imperative to study the fatigue behavior and notch sensitivity of the commonly used 7085-T74 Al alloy ultra-thick plate under alternant load. Hence, in the present work, high-cycle fatigue (HCF) tests were carried out on 7085-T7452 Al alloy ultra-thick plate. A crack initiation mode of the alloy was further proposed to describe the damage mechanism during fatigue testing.
II.
EXPERIMENT PROCEDURE
The material used in this investigation was a 220-mm ultra-thick 7085-T7452 Al alloy plate with chemical composition (wt pct) of 7.56 Zn, 1.50 Mg, 1.45 Cu, 0.12 Zr, 0.06 Fe, 0.02 Si and balance Al. The fatigue tests were carried out on a MTS810 fatigue machine at ambient temperature according to the standard ASTM: E466-07. The tests were conducted at a frequency of 5 Hz under strain
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