Effect of mean stress and ratcheting strain on the low cycle fatigue behavior of a wrought 316LN stainless steel
Introduction
Engineering components and structures are often subjected to a cyclic stressing with a mean stress, which may significantly affect their fatigue lives. It is mostly recognized for a variety of materials that a tensile mean stress leads to a reduced fatigue life [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. One reasonable explanation for the detrimental effect of tensile mean stress on fatigue life is that tensile mean stress facilitates the crack opening [2]. Meanwhile, the mean stress induced progressive accumulation of inelastic deformation, known as ratcheting strain [12], may also play an important role by causing additional fatigue damage for various materials such as SAE 1045 steel [1], AZ31B magnesium alloy [3], CP-Ti [6], Zircaloy-2 [7], 42CrMo steel [10] and 45 carbon steel [11]. For example, Yang [11] found that the failure mode for carbon steel 45 under cyclic stressing depended on the loading level: large ratcheting strain was the main cause of failure as characterized by apparent necking if the loading level was relatively higher, while low-cycle fatigue was the main cause of failure as featured by brittle fracture if the loading level was relatively lower. However, some recent research results showed beneficial effect of tensile mean stress on fatigue life of some stainless steels [13], [14], [15], [16], [17]. Such beneficial effect was found mainly associated with the reduced strain amplitude due to the mean stress induced hardening. Thus it seems that the effect of mean stress on fatigue life depends on the materials, which requires further investigation.
A great deal of efforts has been made to correct the effect of mean stress on stress-life diagram used for the engineering design [18], [19], [20]. The well-known Goodman equation, Morrow equation, Smith equation [21], and Walker equation [22], all describe the detrimental effect of tensile mean stress on fatigue life by introducing an effective stress amplitude higher than the actual stress amplitude. However, for the cases where tensile mean stress extends fatigue life as mentioned previously, these equations show their deficiencies. Liu et al. [23] developed a stress-based fatigue (SBF) model by addressing the maximum stress and stress ratio to consider the effect of mean stress and ratcheting strain, which yielded satisfactory predictions for 304 stainless steel; however, the SBF model requires substantial fitting to determine its stress ratio dependent parameter c. A relatively concise and feasible solution is to combine the mean stress, stress amplitude and strain amplitude as in the well-known Smith-Watson-Topper (SWT) model [21] or energy-based fatigue life models [2], [19]. Especially, more life-prediction models have been developed based on the SWT model [18], [24], [25], which yield higher accuracy of life prediction for specific materials.
In this study, fatigue tests were conducted on a wrought 316LN stainless steel under different control modes, i.e. symmetrical strain-control mode, symmetrical stress-control mode and asymmetrical stress-control mode, to explore the effect of mean stress and ratcheting strain on the low cycle fatigue behavior. The SWT model was evaluated for the prediction of fatigue life under different control modes. A modified fatigue life model based on the Coffin-Manson equation was proposed and validated.
Section snippets
Material and experiments
The material tested in this study is a nuclear grade wrought 316LN austenitic stainless steel which is used for primary circuit piping in the AP1000 PWR nuclear power plants designed by Westinghouse Inc. Its chemical composition (in wt%) is given in Table 1. The average grain size of the steel is about 100 µm, as shown in Fig. 1. Dumbbell-like solid specimens of 27 mm gauge length and 10 mm gauge diameter were machined following the suggested configuration in ASTM Standard E606. The gauge section
Monotonic tensile behavior
The monotonic tensile stress-strain curve of the steel is shown in Fig. 2. The steel displays the feature of significant and continuous nonlinear strain hardening from the yielding strength of 249.6 MPa to the ultimate tensile strength of 560.1 MPa. The fracture strain of 73.2% and the percent reduction in area of 82% indicate the excellent ductility of the steel.
Cyclic stress/strain response
The fatigue testing results including the cyclic stress/strain response and fatigue life under different control modes are presented in
Fatigue life modeling
For fully-reversed (strain and stress control) tests, Basquin-Coffin-Manson Eq. (1) can be employed to predict the fatigue life. Fig. 12 shows the relationship of the elastic, plastic and total strain amplitude with reversals to failure. The equation parameters for fully-reversed tests are presented in Table 3.where is the total strain amplitude, is the elastic strain amplitude, and is plastic strain amplitude, E is the elastic modulus, is the
Conclusion
Low cycle fatigue experiments under different control modes, i.e. symmetrical strain cycling, symmetrical stress cycling and asymmetrical stress cycling, were performed on a wrought 316LN stainless steel at room temperature. The effect of mean stress and ratcheting strain on the low cycle fatigue behavior of the steel were explored. The main observations and conclusions are as follows:
- (1)
Under symmetrical strain and stress cycling, the steel exhibits consistent cyclic hardening/softening
Acknowledgements
The authors gratefully acknowledge financial support for this work from the National Natural Science Foundation of China (Nos. 51435012 and 51505325) and Ph.D. Programs Foundation of Ministry of Education of China (No. 20130032110018).
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