Crack propagation mechanism and life prediction for very-high-cycle fatigue of a structural steel in different environmental medias

The influence of environmental medias on crack propagation of a structural steel at high and very-high-cycle fatigue (VHCF) regimes is investigated based on the fatigue tests performed in air, water and 3.5% NaCl aqueous solution. Crack propagation mechanisms due to different crack driving forces are investigated in terms of fracture mechanics. A model is proposed to study the relationship between fatigue life, applied stress and material property in different environmental medias, which reflects the variation of fatigue life with the applied stress, grain size, inclusion size and material yield stress in high cycle and VHCF regimes. The model prediction is in good agreement with experimental observations

Comparison of KI calculation methods

This paper compares different numerical methods for mode I stress intensity factor (SIF) calculations. Both 2D and 3D models are used to calculate KI for the compact tension specimens. J-integral and interaction integral provide relatively accurate results. The analysis of a reactor pressure vessel subjected to pressurized thermal shock is performed using the finite element method (FEM) and extended finite element method (XFEM). XFEM method shows advantages in modeling cracks but oscillations in 3D problems due to extraction domains for J and interaction integrals. The best results are obtained with domain integrals using a FEM with a refined mesh.The authors are grateful for the financial support of the PISA Project provided by the Swiss Federal Nuclear Safety Inspectorate (ENSI) (DIS-Vertrag Nr. H-100668). Guian Qian is also grateful for the visiting invitation provided by Key Laboratory of Pressurized Systems and Safety (East China University of Science and Technology), Ministry of Education, China.Qian, G.; González Albuixech, VF.; Niffenegger, M.; Giner Maravilla, E. (2016). Comparison of KI calculation methods. Engineering Fracture Mechanics. 156:52-67. https://doi.org/10.1016/j.engfracmech.2016.02.014S526715

A statistical model of fatigue failure incorporating effects of specimen size and load amplitude on fatigue life

Among many contributing factors, the load range, number of load cycles and specimen geometry (including configuration and size) are three major variables for fatigue failure. Most existing statistical fatigue models deal with only one or two of these three variables. According to the statistical distribution of microcracks with respect to their size and spatial location, a weakest-link probabilistic model for fatigue failure is established to incorporate the combined effect of load range, number of load cycles and specimen size. The model reveals a compound parameter of load range and number of load cycles reminiscent of the empirical formulae of fatigue stress-life curve and its correlation with another compound parameter of cumulative failure probability and specimen size. Four sets of published fatigue test data are adopted to validate the model

Fatigue properties and S-N curve estimating of 316L stainless steel prepared by SLM

This paper investigates the fatigue properties of 316L stainless steel manufactured by selective laser melting technology (SLM) under hot-isostatic-pressing (HIP) heat-treatment. The fatigue tests of 316L specimens were carried out under pulse tensile cyclic loading to obtain the fatigue performance. To avoid falling into local optima and improve the convergence speed, the dynamic multiswarm particle swarm optimizer (DMS-PSO) algorithm was first introduced for parameter optimization of the three-parameter Weibull distribution model, and then the S-N curves based on the model were obtained. The S-N curves were highly consistent with the metallographic and fractographic phenomena. Besides, both the S-N curves and fractographic analysis show that the resistance of high-stress fatigue fracture is significantly improved by using HIP heat-treatment

Nanograin formation in dimple ridges due to local severe-plastic-deformation during ductile fracture

Nanograin materials have superior properties in mechanics, physics and chemistry. Here, we found a new phenomenon that nanograin formation spontaneously occurs in the process of ductile fracture for a titanium alloy, and the dominating mechanism is local severe-plastic-deformation (LSPD). The microstructure evolution during the entire process of monotonic tension was revealed to further understand the ductile fracture from plastic deformation to necking, and to final failure, especially in the post uniform deformation stage, in which the voids nucleate, grow and coalesce. The process of the LSPD can potentially provide a new concept and approach to design and produce high ductile materials, in which nanograin formation will consume massive strain energy to enable the large elongation after specimen necking in the post uniform deformation. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

A new local approach model for cleavage fracture in ferritic steels and its validation

The paper discusses and applies a statistical approach to correlate the fracture behavior of a notched and fracture mechanics specimen. The method can be used for fatigue analysis. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch toughness of ferritic steels according to a new local approach to cleavage fracture. The temperature independence of the two Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the two Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with two different microstructures

Influence of processing parameters of selective laser melting on high-cycle and very-high-cycle fatigue behaviour of Ti-6Al-4V

Orthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti-6Al-4V were fabricated. The results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in highcycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The S-N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Then, the tests on the validation group were performed to verify the optimal combination of SLM processing parameters. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack-of-fusion defects in the cases of surface and internal crack initiation

Microstructure features induced by fatigue crack initiation up to very-high-cycle regime for an additively manufactured aluminium alloy

Fatigue failure can still occur beyond 10(7) cycles, i.e. very-high-cycle fatigue (VHCF), in many metallic materials, such as aluminium alloys and high-strength steels. For VHCF of high-strength steels, a fine granular area (FGA) surrounding an inclusion is commonly identified as the characteristic region of crack initiation on the fracture surface. However, no such FGA feature and related crack initiation behaviour were observed in VHCF of conventionally cast or wrought aluminium alloys. Here, we first reported the distinct mechanisms of crack initiation and early growth, namely the microstructure feature and the role of FGA in VHCF performance for an additively manufactured (AM) AlSi10Mg alloy. The AM pores play a key role in fatigue crack initiation similar to that of the inclusions in high-strength steels, resulting in almost identical FGA behaviour for different materials under a range of mean stress with a stress ratio at R 0. The profile microstructure of FGA is identified as a nanograin layer with Si rearrangement and grain boundary transition. This process consumes a large amount of cyclic plastic energy making FGA undertake a vast majority of VHCF life. These results will deepen the understanding of VHCF nature and shed light on crack initiation mechanism of other aluminium and AM alloys. (c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology

Poèmes à voir : Albert-Birot et Tardieu

La quatrième séance du séminaire « Livre/Poésie : une histoire en pratique(s) » sera consacrée aux poèmes à voir de Pierre Albert-Birot et Jean Tardieu. Intervenants : Frédérique Martin-Scherrer (Association Jean Tardieu – ENS-LSH), Marianne Simon-Oikawa (Université de Tokyo) Modération : Hélène Campaignolle-Catel, Isabelle Chol Séance le jeudi 23 février, de 16h à 18h, à l’Université Paris 3 – Sorbonne Nouvelle, centre Censier, salle Las Vargnas (3e étage)

A new local approach model for cleavage fracture in ferritic steels and its validation

The paper discusses and applies a statistical approach to correlate the fracture behavior of a notched and fracture mechanics specimen. The method can be used for fatigue analysis. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch toughness of ferritic steels according to a new local approach to cleavage fracture. The temperature independence of the two Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the two Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with two different microstructures