Very high cycle bending fatigue behaviors of FV520B steel under fretting wear
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Very high cycle bending fatigue behaviors of FV520B steel under fretting wear were studied by the ultrasonic fatigue technique. The specimen system for ultrasonic bending testing was designed and the stress distribution of fatigue specimen was obtained by finite element method. The microstructure of FV520B steel was characterized by means of optical microscope, transmission electron microscope, and energy-dispersive spectroscope. The P–S–N curve was drawn based on fatigue data. The micromorphology characteristics of fretting wear surface and fracture surface for fatigue specimen were observed. The results indicate that the microstructure of FV520B steel is mainly composed of lath martensite, ferrite, and precipitation particles, with some randomly distributed internal inclusions. The P–S–N curve shows that there exists no “conventional fatigue limit” and the fatigue life decreases continuously with the increase of applied stress Smax. Most of fatigue cracks are observed on fractography and initiate from the overlap region of fretting wear zone and stress concentration zone. The fracture failure for tested specimen is ascribed to fretting wear and bending vibration fatigue.
I. INTRODUCTION
Fatigue failure is mainly the predominant failure mode for engineering structures and components and may account for 50–90% of all the mechanical failure modes. The relevant research on fatigue behaviors of materials can be traced back to the middle of the 19th century. In general, low-cycle fatigue is associated with high applied stress and relatively few fatigue cycles (,105), whereas high-cycle fatigue is associated with low applied stress and a fatigue regime of 105 to 107 cycles. The conventional fatigue limit normally is equal to the constant fatigue strength at 107 cycles. Below this fatigue limit, it is assumed that a material has an infinite life. However, according to the test results of carburized steel in 1984,1 the specimens continued to fail even over 108 stress cycles, which was in contrast to the classical concept of fatigue limit. In the following decades, based on the observations that an endurance limit can not be obtained for most engineering materials,2–4 the fatigue property of materials in the very high cycle (VHC) fatigue regime (.107 cycles) has become a considerable research field. With the aging of a great deal of industrial equipment, the security and reliability of many components in continuing service in VHC fatigue regime need urgently to be verified because these crucial components were designed based on the theory of fatigue limit when Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.169
manufactured. Furthermore, modern engineering, such as aerospace industry, high-speed railway industry, heavy-load equipment, and so on, requires that most of the engine parts should be designed to have a minimum fatigue life of giga-cycles. On the other hand, the application of an ultrasonic fatigue testing system with a cyclic freq
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