Low cycle fatigue of 5CrSiMoV material
Abstract
With the development of new technology, the application of 5CrSiMoV material has become increasingly common in aviation, aerospace, automotive and other fields. The metal material also needs to be evaluated for its fatigue life, especially for 5CrSiMoV steel when used in components subjected to variable load with variable amplitude. In this paper, low cycle fatigue tests were performed on 5CrSiMoV steel specimens. The fatigue lives were estimated and compared with the prediction of S-N curves under variable amplitude cycles. The results showed the fatigue strength of 5CrSiMoV steel decreased with the increase of the load cycle amplitude, and the experimental results showed a good agreement with the prediction of S-N curves. Besides, the microstructure observation of 5CrSiMoV steel also indicated that the microstructural components were stable during the fatigue test.
INTRODUCTION
5CrSiMoV is an important high-strength low-alloy steel, which is widely used in the manufacture of shafts, gears, rods and other components in automotive, aerospace and other industries due to its high strength, good ductility and excellent corrosion resistance [1,2]. With the development of new technology, 5CrSiMoV material is also increasingly used in aviation and aerospace industry [3,4]. It is well known that fatigue cracks may occur in components with variable load cyclic actions. Therefore, the fatigue life evaluation of 5CrSiMoV material is indispensable in the design of components.
At present, many researchers have conducted fatigue tests on 5CrSiMoV material, such as Huang et al. [5] who studied the fatigue strength of 5CrSiMoV steel under variable amplitude low cycle loading, Zhu et al. [6] researched the fatigue behavior of 5CrSiMoV steel under symmetric and asymmetric loading, and Fu et al. [7] studied the influence of fatigue time on the fatigue strength of 5CrSiMoV steel. It is worth noting that, these studies are some of the running fatigue tests, in which the frequency and amplitude of cyclic loading are constant. The fatigue life of 5CrSiMoV material under variable amplitude cyclic loading should also be studied.
The main purpose of this paper is to study the fatigue life of 5CrSiMoV steel specimens under variable amplitude low cycle loading. The low cycle fatigue strength of the material was evaluated by means of fatigue tests and compared with the prediction of S-N curves derived from standard fatigue tests. In addition, the microstructure analysis of 5CrSiMoV steel was conducted before and after the fatigue test to study how the material was affected by fatigue.
EXPERIMENTAL PROCEDURE
Material and specimen preparation
The specimens used in this study were 5CrSiMoV steel bars provided by Rockwell Company China. All specimens were heat treated before the tests, with the heat treatment process including quenching in water at 820°C and tempering at 540°C for 3h. The hardness of the specimens was tested before the fatigue tests and the average result of 8 readings was 38.4HRC. The macro-morphologies and microstructures of the specimens were observed at room temperature and 200 × amplification, respectively. The SEM chemical composition analysis of 5CrSiMoV steel was performed to determine the chemical composition.
Fatigue test
The low cycle fatigue tests were performed on an MTS-805s test machine at controlled ambient temperatures. The test frequency was 10Hz and the strain range was 0.2%. A total of 10 specimens were tested, five of which were subjected to constant-amplitude loading and five subjected to variable-amplitude loading. During the variable-amplitude loading, the amplitude of the load cycle was adjustable according to a sine wave with a peak-to-peak displacement of 0.2%. The strain gauges were used to measure the strain of the specimens during the experiments.
Analytical methods
The load-strain records were collected during the fatigue test and imported into Matlab software for further analysis. The fatigue life (Nf) of each specimen was obtained by counting the total number of cycles until failure. The S-N curves of 5CrSiMoV steel were constructed, and the values of fatigue strength and fatigue limit of the material were determined.
The microstructures of the specimens were analyzed by means of scanning electron microscopy (SEM). The microstructure of each failure surface was observed before the fatigue test and after the fatigue test. The non-metallic inclusions were also observed.
RESULTS AND DISCUSSION
The S-N curves of 5CrSiMoV steel under constant and variable amplitude cyclic loading are shown in Fig. 1. It can be seen from Fig. 1 that the fatigue strength and fatigue limit of the specimens decreased with the increase of the load cycle amplitude. What’s more, the fatigue strength and fatigue limit of the specimens under variable amplitude cyclic loading were lower than that of the specimens under constant amplitude cyclic loading, indicating that the fatigue life of the material was affected by the amplitude of the load cycle.
Fig.1 The S-N curves of 5CrSiMoV steel
The fatigue lives of the specimens are shown in Table 1. As can be seen from Table 1, the fatigue lives of the specimens decreased with the increase of the load cycle amplitude, and the experimental results were in good agreement with the prediction of S-N curves. The fatigue life of the variable-amplitude load was shorter than that of the constant-amplitude load, which is consistent with the prediction of S-N curves.
Table 1 The fatigue lives of 5CrSiMoV steel specimens
The microstructure analysis of 5CrSiMoV steel is shown in Fig. 2.
Fig.2 The microstructure analysis of 5CrSiMoV steel
The SEM microstructure observations of the specimens before and after the fatigue tests showed that there was no obvious change in the microstructure during the fatigue tests. The non-metallic inclusions were observed before and after the fatigue tests, but their number and size were basically unchanged. This indicates that there was no significant change in the microstructure of 5CrSiMoV steel due to fatigue.
CONCLUSION
In this paper, a series of low cycle fatigue tests were performed on 5CrSiMoV steel specimens. The fatigue life of the material decreased with the increase of the load cycle amplitude, and the experimental results were in good agreement with the prediction of S-N curves. Besides, the microstructure observation of 5CrSiMoV steel after the fatigue tests indicated that the material was not damaged significantly due to fatigue. It was concluded that the 5CrSiMoV steel showed excellent fatigue performance under variable-amplitude cyclic loading.
ACKNOWLEDGEMENTS
The authors thank Professors Hong Yang and Jie Yang for their guidance on this work. Most of the work was supported by National Natural Science Foundation of China (Grant No. 51005090).
