Thermo-mechanical FE model with memory effect for 304L austenitic stainless steel presenting microstructure gradient

The main purpose of this study is to determine, via a three dimensions Finite Element analysis (FE), the stress and strain fields at the inner surface of a tubular specimen submitted to thermo-mechanical fatigue. To investigate the surface finish effect on fatigue behaviour at this inner surface, mechanical tests were carried out on real size tubular specimens under various thermal loadings. X ray measurements, Transmission Electron Microscopy observations and micro-hardness tests performed at and under the inner surface of the specimen before testing, revealed residual internal stresses and a large dislocation microstructure gradient in correlation with hardening gradients due to machining. A memory effect, bound to the pre-hardening gradient, was introduced into an elasto-visco-plastic model in order to determine the stress and strain fields at the inner surface. The temperature evolution on the inner surface of the tubular specimen was first computed via a thermo-elastic model and then used for our thermo-mechanical simulations. Identification of the thermo-mechanical model parameters was based on the experimental stabilized cyclic tension-compression tests performed at 20^{\circ}C and 300^{\circ}C. A good agreement was obtained between numerical stabilized traction-compression cycle curves (with and without pre-straining) and experimental ones. This 3 dimensional simulation gave access to the evolution of the axial and tangential internal stresses and local strains during the tests. Numerical results showed: a decreasing of the tangential stress and stabilization after 40 cycles, whereas the axial stress showed weaker decreasing with the number of cycles. The results also pointed out a ratcheting and a slightly non proportional loading at the inner surface. The computed mean stress and strain values of the stabilized cycle being far from the initial ones, they could be used to get the safety margins of standard design related to fatigue, as well as to get accurate loading conditions needed for the use of more advanced fatigue analysis and criteria

Experimental and numerical analysis of hydrogen interaction with plastic strain in a high strength steel

8 pagesInternational audienceCyclic loading tests were performed on micro-notched samples of high-strength steel S690QT in air and under cathodic polarisation in a saline solution. These specimens were modelled and their behaviour simulated by finite elements calculations with a combined nonlinear isotropic-kinematic hardening constitutive law. This model can simulate cyclic softening and ratcheting effect of the high-strength steel. Stress and strain fields in the vicinity of the notch-tip were calculated. Results show that a strong dependence of the crack initiation with plastic strain accumulation. Hydrogen assisted cracking mechanism is discussed based on arrangements of dislocations structures

Microscopic Damage Evolution During Very High Cycle Fatigue (VHCF) of Tempered Martensitic Steel

AbstractDimensioning of high-strength steels relies on the knowledge of Wöhler-type S/N data and the assumption of a fatigue limit for applications where the number of load cycles exceeds 107. Very high cycle fatigue (VHCF) experiments applied to a 0.5C-1.25Cr-Mo tempered steel (German designation: 50CrMo4) revealed surface crack initiation at prior austenite grain boundaries in medium strength condition (37HRC) and internal crack initiation at non-metallic inclusions at high strength condition (48HRC). Despite the formation of small cracks during cycling up to 109 cycles, it seems that the medium strength condition exhibits a real fatigue limit. Application of automated electron back-scattered diffraction (EBSD) within the shallow-notched area of electro-polished fatigue specimens had shown that prior austenite grain boundaries act as effective obstacles to crack propagation. High resolution thermography during cycling of the specimens allowed the identification of local plasticity, which led to crack initiation at a later stage of the fatigue life. It was found that Cr segregation rows play a decisive role in the crack initiation process. By means of high-resolution electron microscopy in combination with focused ion beam milling (FIB), evolution of cyclic plasticity and crack initiation was correlated with the material's microstructure. The results are discussed in terms of the completely different crack initiation mechanisms of medium and high strength variants of the same steel. EBSD and crack propagation data are used to adapt numerical modeling tools to predict crack initiation and short crack propagation

A multiscale overview of modelling rolling cyclic fatigue in bearing elements

During service, bearing components experience rolling cyclic fatigue (RCF), resulting in subsurface plasticity and decay of the parent microstructure. The accumulation of micro strains spans billions of rolling cycles, resulting in the continuous evolution of the bearing steel microstructure. The bearing steel composition, non-metallic inclusions, continuously evolving residual stresses, and substantial work hardening, followed by subsurface softening, create further complications in modelling bearing steel at different length scales. The current study presents a multiscale overview of modelling RCF in terms of plastic deformation and the corresponding microstructural alterations. This article investigates previous models to predict microstructural alterations and material hardening approaches widely adopted to mimic the cyclic hardening response of the evolved bearing steel microstructure. This review presents state-of-the-art, relevant reviews in terms of this subject and provides a robust academic critique to enhance the understanding of the elastoplastic response of bearing steel under non-proportional loadings, damage evolution, and the formation mechanics of microstructural alterations, leading to the increased fatigue life of bearing components. It is suggested that a multidisciplinary approach at various length scales is required to fully understand the micromechanical and metallurgical response of bearing steels widely used in industry. This review will make significant contributions to novel design methodologies and improved product design specifications to deliver the durability and reliability of bearing elements

A multi-scale crystal plasticity model for cyclic plasticity and low-cycle fatigue in a precipitate-strengthened steel at elevated temperature

peer-reviewedIn this paper, a multi-scale crystal plasticity model is presented for cyclic plasticity and low-cycle fatigue in a tempered martensite ferritic steel at elevated temperature. The model explicitly represents the geometry of grains, sub-grains and precipitates in the material, with strain gradient effects and kinematic hardening included in the crystal plasticity formulation. With the multiscale model, the cyclic behaviour at the sub-grain level is predicted with the effect of lath and precipitate sizes examined. A crystallographic, accumulated slip (strain) parameter, modulated by triaxiality, is implemented at the micro scale, to predict crack initiation in precipitate-strengthened laths. The predicted numbers of cycles to crack initiation agree well with experimental data. A strong dependence on the precipitate size is demonstrated, indicating a detrimental effect of coarsening of precipitates on fatigue at elevated temperature. (C) 2016 Elsevier Ltd. All rights reserved.ACCEPTEDpeer-reviewe

Crack propagation in TRIP assisted steels modeled by crystal plasticity and cohesive zone method

The influence of transformation induced plasticity (TRIP) on materials mechanical behaviours, as well as failure phenomena including crack propagation and phase boundary debonding in multiphase steels (e.g. dual phase steels, TRIP steels) are studied by using an advanced crystal plasticity finite element method. We have coupled the crystal plasticity model Ma and Hartmaier (2015), which explicitly considers elastic-plastic deformation of ferrite and austenite, austenite-martensite phase, with a cohesive zone model designed for crack propagation, to study the deformations of several representative microstructural volume elements (RVE). Results shows that, the transformation induced plasticity enhances materials strength and ductility, hinders crack propagation and influences interface debonding. Furthermore, the martensitic transformation kinetics in TRIP steels was found depending on the crystallographic orientation and the stress state of a retained austenite grain. The current simulation results helps to investigate and design multiphase steels with improved mechanical properties

Influence of hydrogen on the hydraulic fracture behavior of a 42CrMo4 steel welds: Effect of the prior austenite grain size

The influence of hydrogen on the mechanical behavior of a quenched and tempered 42CrMo4 steel has been evaluated by means of high internal pressure fracture tests carried out on hydrogen precharged notched cylindrical specimens. The notched cylindrical specimens were precharged in a 1 M H2SO4 + 0.25 g/l As2O3 solution for 3 h with 1.2 mA/cm2. Hydraulic fracture tests were performed at different loading rates. Hydrogen embrittlement resistance increased with grain size refinement although the fine grained specimen had a higher hydrogen content than the coarse grained one. Fractographic analysis showed hydrogen enhanced decohesion fracture was less pronounced with decreasing grain size. Hydrogen embrittlement susceptibility is discussed in terms of the prior austenite grain size (PAGS) and the operative fracture mechanisms.The authors would like to thank the Spanish Government for the financial support received to perform the research projects TED2021-130413B-I00 and PID2021-124768OB-C21. This work was supported by the Regional Government of Castilla y León (Junta de Castilla y León) and by the Ministry of Science and Innovation MICIN and the European Union Next Generation EU/PRTR (MR5W.P3) and PRTR (MR4W.P2). L.B. Peral is grateful for his Margarita Salas Postdoctoral contract (Ref.: MU-21-UP2021-030) funded by the University of Oviedo through the Next Generation European Union

Numerical analysis of stress distribution in polycrystalline microstructure

The paper presents a numerical analysis of stress distribution in micromechanical models in order to simulate microstructural mechanisms of microcrack initiation and propagation in polycrystalline metals. The analysis is based on plane-strain finite element crystal elasticity models. The microstructure is generated using Voronoi tessellation, encompassing random crystallographic orientation and position of grains that have different shapes and sizes. Since the correlation between the physical mechanisms of deformation and the microstructure is essential for sound understanding of crack initiation and propagation, the paper considers development of microstructural models of behaviour of anisotropic linear-elastic and elastic-plastic polycrystalline metals. The results indicate that the key factor for good agreement with the data obtained from polycrystalline microstructure, is the correct and proper interpretation of material heterogeneity between grains. The attention should be placed on proper material characterization, crystallographic slip mechanism representation and orientation