Polycrystal modelling of fatigue: Pre-hardening and surface roughness effects on damage initiation for 304L stainless steel

Abstract

a b s t r a c tThe 304L stainless steel is a major component of residual heat removal circuits of pressurized water reactors (PWRs). The main purpose of this study is to understand the risk of thermal fatigue damage resulting from the machining of the 304L steel pipes inner surface (pre-hardening gradient, residual stresses and scratches), at the scale of the microstructure. This work is based on previous results obtained for pipe specimens thanks to a macroscopic elasto-visco-plastic model. Applied to the pipe specimens, this modelling showed that a thermal loading with temperature gradient, induced a cyclic non-linear biaxial loading at the inner surface of the pipe. In this paper, a polycrystal plasticity model, implemented in a Finite Element (FE) code, is adapted to cyclic loading. An elementary volume (3D aggregate), representing the inner surface and sub-surface of the 304L steel tube, is built from successive polishings and orientation mappings thanks to an Electron Back Scattering Diffraction method. At the grain scale, the polycrystal model is used as a ''numerical microscope'' to compute the local mechanical fields. Different fatigue criteria are tested to determine their sensitivity to surface properties (roughness, residual stress and pre-hardening) and to the microstructure of the material (crystallographic orientation and grain size). Pre-hardening leads to a lower and more homogeneous distribution of local strain amplitudes in the aggregate, but slightly higher stresses when compared to initial material without hardening. By contrast, surface roughness leads to large localized strain and stress fields in grains located at the bottom of scratches. To determine the surface micro-structural ''hot spots'' features and to test the sensitivity of different surface conditions, three different fatigue criteria (Manson-Coffin, Fatemi-Socie and Dissipated Energy criteria) have been computed. We point out that the pre-hardening may have a complex effect on fatigue resistance, since it reduces local plastic strain amplitudes, but increases local stresses. Moreover, the pre-hardening has a positive effect on fatigue since it delays damage initiation. By contrast, the surface roughness leads to a negative effect. However, we have shown that the three different fatigue criteria do not deliver similar quantitative predictions. Relevant criteria for high cycle fatigue, such as stress based criteria, are not considered in this paper, since the thermal loading used for computation is large enough to reduce cyclic plastic strain straining within all grains of 304L pipe inner surface for midlife of experiments.

Material characterisation and finite element modelling of cyclic plasticity behaviour for 304 stainless steel using a crystal plasticity model

“…The backstress , which accounts for the Bauschinger effect [18], satisfies the nonlinear evolution rule:…”

“…The dislocation density evolution law [18] is able to describe the dislocation multiplication and annihilation for the slip system , as follows:…”

“…Le Pécheur et al [18] used the dislocation density-based model by constructing a 3D aggregate to investigate the effect of pre-hardening, which leads to a more homogeneous local stress and strain distribution at the stabilised fatigue region. In addition, a variety of damage initiation criteria were applied to aggregates of different surface roughness to investigate the sensitivity of these criteria to the roughness profile and pre-hardening.…”

Influence of the microstructure and voids on the high‐cycle fatigue strength of 316L stainless steel under multiaxial loading

“…However, despite the drastic simplifications used to reduce the number of degrees of freedom, it is still significantly high, and given the number of realizations, loading conditions and defect sizes studied, the computations could not be achieved in a reasonable time if a crystal plasticity model is used. Nevertheless, several studies dealing with HCF account for the crystal plasticity in their modelling . In particular, in Refs , the effect of the constitutive model on the mechanical responses and on the predictions of the probabilistic fatigue criterion discussed in the present study has been evaluated.…”

Competition between microstructure and defect in multiaxial high cycle fatigue

“…A promising approach, consisting in computing, by FE method, the mechanical response of explicitly modelled polycrystalline aggregates, allows to take into account microstructural details generally neglected in the homogenisation schemes and to deepen the analysis of the mesoscopic mechanical responses of metals under cyclic multiaxial loading. In recent years, several works have involved this kind of numerical simulations to contribute to the study of the HCF behaviour [8][9][10][11][12]. The present study falls within this framework and a specific effort is made to quantify the multiaxial fatigue performance of metals in the presence of defects through statistical modeling of the microstructure.…”