Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel
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INTRODUCTION
THE crack growth behavior of 4340 steel under cyclic loading conditions is important in predicting the remaining lives of helicopter rotor components. This steel is commonly used in quenched and tempered state because it provides optimum strength and toughness properties. As a result, most researchers have focused their studies on the fatigue behavior of 4340 steel in the quenched and tempered condition.[1,2,3] However, microstructures that are optimum for strength and toughness are not necessarily best for fatigue applications. Further, microstructures resulting in high long-life fatigue resistance will generally yield lower thresholds for fatigue crack growth.[4] Thus, to thoroughly understand the fatigue behavior of 4340 steel, it is necessary to study the influence of microstructure on the fatigue crack growth resistance. Pearlite-ferrite is typically present in the microstructure of high strength low alloy steels in the annealed condition, whereas tempered martensite is found under the quenched and tempered condition. Interactions between microstructure and the growing crack are important in determining the fatigue crack growth resistance. The objective of this research is to better understand the interactions among growing fatigue cracks, the microstructure encountered by the cracks, and the cyclic plasticity that develops in a small zone ahead of fatigue crack tips in 4340 steels. While previous studies have attempted to relate microstructure to fatigue crack growth resistance, no previous FAN YANG, formerly a Postdoctoral Research Associate, School of Materials Science and Engineering, Georgia Institute of Technology, is a Consultant, Fairfax, VA 20171. ASHOK SAXENA, Professor and Chair, is with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. LAURA RIESTER, Scientist, is with the High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6062. Manuscript submitted September 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
work has investigated the role of crack tip plasticity in determining the microstructure/crack interactions. In this study, fatigue crack growth tests were performed in the pearliteferrite and tempered martensite microstructures. The microstructure/crack interaction was characterized using optical microscopy and scanning electron microcopy (SEM). Subsequently, nanoindentation hardness tests were performed to characterize the crack tip plasticity in various specimens. The results are used to shed more light on ways to improve the fatigue resistance of the helicopter rotor parts. Under cyclic plastic deformation, a metal may harden, soften, remain stable, or may have a mixed character, depending on the microstructure and testing conditions.[5] Since the hardness of a metal is related to its yield strength,[6] the cyclic deformation zone may be delineated with precise hardness measurement around the crack tip. Nanoindentation hardness is a unique technique, which allows one to deter
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