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00303 CORROSION FATIGUE CRACK GROWTH BEHAVIOR AND AIRCRAFT STRUCTURE LIFE TIME PREDICTION

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Product Number: 51300-00303-SG
ISBN: 00303 2000 CP
Author: Vinod S. Agarwala, A. Basir Shafiq
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Considerable amount of research has been performed in the area of lifetime assessment of aircraft (structural) materials; however, a methodology or a model is yet to surface that would accurately predict lifetime of an aircraft structure serving in aggressive environment. Literature survey reports some trends but many conflicting opinions (results) as well, which reduces confidence level in lifetime prediction. Some experimental results reported in this paper suggest that corrosion, even though a time dependent phenomenon plays a vital role in affecting fatigue crack growth rates when there arre active corrosion cells present. These rates are much higher than those obtained under pure fatigue induced crack growth rates. Up to an order of magnitude increase in the crack growth rates for specimens tested in aggressive environment as compared to those tested in air were observed. It was also found out that pre-corrosion is only significant during the crack initiation stage. Crack growth beyond initiation is independent of the nature and extent of crack tip pre-corrosion. A methodology for a mathematical model based on the superposition of corrosion (diffusion) mechanisms and fatigue crack growth rate is suggested and is expected to yield reasonable corrosion fatigue crack growth rate estimates. Any technique that can modify crack-tip processes would influence the life of structure. A chemical inhibitor system based on dichromate, nitrite, borate and molybdate (DNBM) quaternary complexes has been shown to be quite effective in retarding the crack growth rates resulting in a significant increase in life. Application of these inhibitors has shown to increase the lifetime by a factor of five. Keywords: corrosion fatigue, life prediction, crack growth rate, pre-fatigue corrosion, active corrosion cells, crack-tip corrosion mechanism, inhibitors, corrosion superposition model.
Considerable amount of research has been performed in the area of lifetime assessment of aircraft (structural) materials; however, a methodology or a model is yet to surface that would accurately predict lifetime of an aircraft structure serving in aggressive environment. Literature survey reports some trends but many conflicting opinions (results) as well, which reduces confidence level in lifetime prediction. Some experimental results reported in this paper suggest that corrosion, even though a time dependent phenomenon plays a vital role in affecting fatigue crack growth rates when there arre active corrosion cells present. These rates are much higher than those obtained under pure fatigue induced crack growth rates. Up to an order of magnitude increase in the crack growth rates for specimens tested in aggressive environment as compared to those tested in air were observed. It was also found out that pre-corrosion is only significant during the crack initiation stage. Crack growth beyond initiation is independent of the nature and extent of crack tip pre-corrosion. A methodology for a mathematical model based on the superposition of corrosion (diffusion) mechanisms and fatigue crack growth rate is suggested and is expected to yield reasonable corrosion fatigue crack growth rate estimates. Any technique that can modify crack-tip processes would influence the life of structure. A chemical inhibitor system based on dichromate, nitrite, borate and molybdate (DNBM) quaternary complexes has been shown to be quite effective in retarding the crack growth rates resulting in a significant increase in life. Application of these inhibitors has shown to increase the lifetime by a factor of five. Keywords: corrosion fatigue, life prediction, crack growth rate, pre-fatigue corrosion, active corrosion cells, crack-tip corrosion mechanism, inhibitors, corrosion superposition model.
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