Elsevier

Mechanics of Materials

Volume 122, July 2018, Pages 58-68
Mechanics of Materials

Research paper
Influence of axial and torsional cyclic loading on the fatigue behavior of 304LN stainless steel using solid and hollow specimens

https://doi.org/10.1016/j.mechmat.2018.03.012Get rights and content

Highlights

  • Specimen geometry influences the fatigue life under axial loading.

  • Cyclic deformation behavior observed for shear and axial loading in tubular specimens was considerably different.

  • Work hardening was higher for tubular specimens subjected to shear loading.

  • Substructural evolution under shear loading resulted in different cellular arrangements along with martensite.

Abstract

Effect of loading path and geometry on fatigue behavior was evaluated for 304LN stainless steel using solid and thin walled hollow specimens. Hollow specimens were subjected to both axial and shear loading under strain control. Solid specimens were used only for strain controlled axial loading. Fatigue life was highest for shear loading despite the higher stress response and the least for the axial loading for hollow specimens. The cyclic deformation behavior marked by Masing behavior, cyclic stress strain curves and probability density function analysis was found to be remarkably different for shear loading. All disparities in the cyclic deformation behavior due to difference in loading path has been accounted by the dislocation dynamics and martensitic transformation investigated through TEM.

Introduction

Factors influencing the cyclic plastic deformation behavior of 304LN stainless steel has been studied expansively for decades (Sivaprasad et al., 2010, Paul et al., 2011c, Ray et al., 2010, Paul et al., 2011b). These studies are predominantly based on tension-compression uniaxial loading experiments designed specifically for solid specimens and are absolutely relevant to gain an elementary understanding of cyclic deformation behavior of the material. Nevertheless, it is difficult if not imprudent to directly extend this data for elucidation of the behavior observed in reality, in-service conditions; wherein more complex loading schemes and design of the components are liable to alter the cyclic response of the material. Therefore, it becomes mandatory to study the cyclic deformation behavior under varied loading schemes along with different specimen profiles. Additionally, it is not reasonable to investigate all possible combinations of loading schemes and specimen geometries, consequently the factors to be probed have to be selected carefully to extract useful information. In this regard, multiaxial loading using thin walled hollow specimens have attracted considerable attention owing to the complex pattern of loading coupled with the specimen geometry that has relatively closer resemblance with the components used in-service conditions. In fact, the focus would be on developing an unified approach that facilitates the extension of understanding developed on uniaxial fatigue, to multiaxial condition. However to do so, Manson 1966 has cautioned that specimen geometry influences the fatigue properties largely, therefore would require newer procedure for predicting lives of specimens of different profiles using data generated from a specific specimen profile. Similar arguments have also been put forward by Fash et al. 1988. Therefore, studies depicting the cyclic deformation behavior of hollow and solid specimens are useful for developing such unified methods.

Investigations in this regard by Miller and Chandler 1970 on hot rolled mild steel have revealed identical stress range versus strain relationship for solid and thin walled cylinders. However, it has been reported that the fatigue behavior of thin tubes is dependent on the thickness of the tube wall. This behavior has been accounted by the fact that the fatigue properties are dependent on the stress field surrounding the crack initiation zone which in turn is dependent on the geometry of the specimen. Studies on 316 stainless steel have revealed similar results wherein it was reported that the fatigue lives of solid specimens were substantially higher than the corresponding hollow specimens for identical experimental conditions (Van Den Avyle, 1983). It was further reported that the crack growth rates increased with decreasing wall thickness along with relatively less cycles spent in the propagation stage. Recent investigation by Sivaprasad et al. 2014 on SA 333 steel has also shown that the hollow specimens had shorter lives than the corresponding companion solid specimens. Also this variation in life of different specimen geometries was found to be dependent on the strain amplitude wherein the differences in lives were least at the highest strain amplitudes. In this case, the investigators have also evoked the possibility of microstructural conditions contributing to the disparity in lives in addition to the geometrically controlled state of stress. Further, it was shown both through experimentation and simulation, that the ratio of internal plastic strain of hollow and solid specimens was consistently higher than unity implying the greater internal plastic strain energy density in hollow specimens which explains the shorter lives for the hollow geometry (Ho and Bok, 2011).

All these investigations further strengthen the need to dig deeper into the aspect of influence of geometry on the fatigue performance of a material. Although these studies strongly corroborate the disparity in lives due to geometry, but the detailed analysis of the effect of geometry on cyclic deformation behavior as such and the corresponding microstructural evolution has not yet been explored at large. Therefore, this paper intends to address these issues for stainless steel grade 304LN by using solid and hollow specimens. Furthermore, failure solely due to torsional loading which is seldom reported, has also been examined and the corresponding effect on cyclic deformation behavior compared with that of axial loading. The present contribution thereby focuses on detailed analysis of the effect of different loading schemes and geometry on the cyclic plastic behavior which is understood from the stress strain hysteresis loops, Masing behavior and variation in the proportional limit. In addition, probability density function based approach is employed to understand the alterations in cyclic yielding behavior. Microstructural studies have also been carried out and correlated with the results obtained.

Section snippets

Experimental

The material chosen for this study was 304LN stainless steel used in the pressurized heavy water reactors (PHWRs) in the Indian nuclear power plants which was available in the form of pipe. The pipe had an outer diameter of 320 mm and 25 mm wall thickness. Its chemical composition (in wt. %) was: C, 0.03; Si, 0.65; Ni, 8.17; Cr, 18.73; Mo, 0.26; Cu, 0.29; N, 0.08; S, 0.02; P, 0.034 and remaining Fe. Corresponding microstructure which is primarily austenitic is depicted in Fig. 1. Uniaxial

Comparison of fatigue life and resultant cyclic hardening softening behavior

In order to analyze the effect of geometry, if any, on the cyclic deformation process it is prudent to begin by evaluating the fatigue life of hollow/ solid specimen both of them being axially loaded. Besides, the different loading conditions such as shear and axial can also possibly alter the cyclic response of the material for the same specimen geometry which also needs further insights. The low cycle fatigue life versus the total strain amplitude for the solid and hollow specimens coupled

Conclusion

In this paper investigation on the cyclic deformation behavior of 304LN was performed under axial and shear loading for solid and hollow specimens. The experimental observations and analysis have led to the following conclusions:

  • (a)

    Fatigue life was marginally higher for shear loaded hollow specimens than the axial counterpart. The life under shear straining for hollow geometry was comparable with the solid specimen life, loaded axially. The stress response was highest for shear loading and was

Acknowledgment

The authors are thankful to Mr. Bhupesh Mahato for his immense support provided in imaging during TEM investigations.

References (41)

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