Investigation into Corrosion Pit-to-Fatigue Crack Transition in 7075-T6 Aluminum Alloy
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JMEPEG (2017) 26:2535–2541 DOI: 10.1007/s11665-017-2697-4
Investigation into Corrosion Pit-to-Fatigue Crack Transition in 7075-T6 Aluminum Alloy V. Sabelkin, S. Mall, and H. Misak (Submitted December 1, 2016; in revised form March 16, 2017; published online May 1, 2017) Transition of corrosion pit to crack under fatigue condition was investigated in high-strength 7075-T6 aluminum alloy. The pit was formed at the edge of a hole in a specimen. Specimen was subjected to a constant stress during the pit formation. Two types of corrosion pit were considered: corner-pit and through-pit. Two sizes were tested for each pit type. Also, the baseline data of cycles to initiate a 250-lmlong crack were established when the corrosion pit was created without any applied stress on the specimen, i.e., Sappl = 0. The cycles to initiate a 250-lm-long crack initially decreased with increasing Sappl relative to the baseline value and then increased with increasing Sappl such that this increase was significant with higher value of Sappl. The transition between this increase and decrease occurred when the Sappl was greater or less than a value which caused the onset of plastic deformation at the root of the pit, respectively. Microstructural analysis showed that this decrease in cycles to initiate the crack was due to microcracks at the pit front which developed at the lower level of Sappl, and the increase was due to plastic deformation at the higher levels of Sappl. Keywords
aluminum alloy, corrosion pit, corrosion under load, crack initiation
1. Introduction Fatigue is the primary load in the aircraft structures. Corrosion is a concern in the aircraft structures especially in the aging aircraft fleet. Combination of corrosion and fatigue, which is commonly referred to as the corrosion fatigue, is therefore an important issue which has received a great deal of attention by the researchers and aircraft users (Ref 1). Damage progression under corrosion fatigue has been broadly divided into three phases: corrosion pit formation and growth, initiation (nucleation) of crack(s) from corrosion pit which is commonly referred as ‘‘pit-to-crack transition,’’ and finally fatigue crack growth (both small and large crack) (Ref 2-5). These phases may be subdivided further. The present study deals the pit-tocrack transition aspect which has received a good amount of attention as discussed next (Ref 6-11). The effects of corrosion formed by EXCO exposure in 7075-T6511 on the fatigue life were studied by Kim et al. (Ref 8). They developed a corrosion-modified equivalent initial flaw size using fracture mechanics principles. Burns et al. investigated the transition from corrosion pit to fatigue crack in 7075T6511 which showed that the transition depends upon the topographic and microscopic features of the corrosion pit (Ref The views expressed in this article are those of the authors and do not reflect the official policy or position of the United State Air Force, Department of Defence, or the US Government. V. Sabelkin, S. Mall, and H. Misak, Department of
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