Cyclic Deformation and Low-Cycle Fatigue for 316LN Stainless Steel under Non-proportional Loading

Yajing Li,Xu Chen,Bin Ren,T Palin-Luc,A Carpinteri,F Morel

doi:10.1051/matecconf/201930008002

Abstract

The effects of loading path and strain amplitude ratio on the cyclic behavior and fatigue life were investigated on a 316LN nuclear grade stainless steel employing a series of symmetrically strain-controlled fatigue tests at room temperature. The loading paths of Uniaxial, Torsional, Proportional, Rhombic, Rectangular, and Circular were employed with the constant equivalent strain amplitude of 0.5%. The strain amplitude ratio of 2.35, 1.73 and 1.27, defined by the ratio of shear strain amplitude to the axial strain amplitude, was realized by changing the shear strain amplitude under Proportional, Rhombic, Rectangular and Elliptical loading paths. As expected, the significant non-proportional additional hardening was observed. It’s interesting to note that the axial cyclic stress response varied with the strain amplitude ratio, and the law was different under different loading paths. The fatigue life of all the tests were evaluated by three critical plane criteria proposed by Smith-Watson-Topper (SWT), Fatemi-Socie (FS) and Chen-Xu-Huang (CXH). Results show that the SWT criterion significantly overestimated the fatigue life of non-proportional loading because the effect of shear damage was not considered. The CXH criterion for tensile-type failure yielded good prediction results except for two torsional data points. The FS criterion provided better predictions than other models.

Highlights

316LN austenitic stainless steel (ASS) has become the main pipeline material of primary circuit of China’s third-generation AP1000 pressurized water reactor nuclear power plant
One can remark that the material exhibited two-staged cyclic stress response regardless of the loading path, initial hardening for the first dozens of cycles, followed by cyclic softening until final failure
It is shown that the SWT criterion performs well for fatigue life prediction under uniaxial, torsional and proportional loadings

Summary

Introduction

As structural components, they are often subjected to complex multiaxial fatigue loading owing to the repeated start and stop-operation during service. There are generally three categories of criteria: equivalent strain criteria, critical plane criteria and energy criteria. Among these criteria, the equivalent strain criteria have been reported to perform non-conservatively for multiaxial non-proportional owing to neglecting the non-proportional additional hardening. Three classic critical plane criteria were applied to predict the fatigue life under different loadings

Material and Experiments

Cyclic deformation behavior

Fatigue life prediction

Conclusions