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Structural modification of lipid A, the lipid anchor of LPS, is one of the strategies used by Gram-negative bacteria to evade host innate immunity. Francisella tularensis is a human pathogen that infects and replicates within phagocytic cells. It produces an atypical lipid A, whose structure precludes an efficient recognition by both innate immune players, TLR4 and cationic antimicrobial peptides. Interestingly, a recent report indicates that the lipid A of Francisella (LVS vaccinal strain) undergoes polar modifications when bacteria are grown in human macrophages as compared to in broth. To characterize the structural modifications of lipid A that may be induced intracellularly, Francisella novicida , a surrogate strain for the highly virulent F. tularensis , was submitted to different stress conditions mimicking the harsh environment encountered in the macrophages. To analyze lipid A directly from intact bacteria without any chemical treatment or purification steps, we used a rapid and sensitive MALDI-TOF mass spectrometry approach. Among the many conditions tested, only bacteria exposure to acidic pHs (from 6 to 5) induced a change in lipid A structure. These changes were characterized by an increase in the relative abundance of molecular species bearing an additional hexose unit on the diglucosamine backbone, similar to species present when bacteria are grown under reduced environmental temperature. This lipid A glyco-form, which is observed in trace amounts in normal in vitro growth conditions at 37 °C, may contribute to the intracellular parasitism of macrophages by Francisella . [ABSTRACT FROM AUTHOR]

FFT-based homogenization methods aim at calculating the effective behavior of heterogeneous materials with periodic microstructures. These methods operate on a regular grid of voxels, and hence require an appropriate spatial discretization of periodic microstructures. However, when different microstructural length scales are involved, it is not always possible to have sufficient spatial resolutions to explicitly consider the influence of fine microstructural features (e.g. voids, second-phase particles). To circumvent this difficulty, one solution consists of using composite voxel methods to define the effective properties and the effective internal variables of heterogeneous voxels. In this work, different composite voxel methods are proposed to deal with inelastic materials with multiple length scales. These methods use simple homogenization rules to calculate the effective behavior of heterogeneous voxels. The first part of this paper is dedicated to the description of the composite voxel methods, which are based either on the Voigt, laminate structure or Mori–Tanaka approximations. In the second part, these methods are used to model the elasto-plastic behavior of a pearlitic steel polycrystalline aggregate. According to the results, the Voigt approximation, which ignores morphological features, is not appropriate for treating heterogeneous voxels. When morphological information is accounted for, with either the laminate structure or Mori–Tanaka approximations, a better agreement with experimental observations is obtained. Though none of these methods is universal, they offer some possibilities to investigate the mechanical behavior of heterogeneous materials involving multiple length scales. [ABSTRACT FROM AUTHOR]

The macroscopic behavior of polycrystalline materials is largely influenced by the shape, the arrangement and the orientation of crystallites. Different methods have thus been developed to determine the effective behavior of such materials as a function of their microstructural features. In this work, which focuses on polycrystalline materials with an elastic–viscoplastic behavior, the self-consistent, finite element and spectral methods are compared. These common methods are used to determine the effective behavior of different 316L polycrystalline aggregates subjected to various loading conditions. Though no major difference is observed at the macroscopic scale, the hardening rate is found to be slightly overestimated with the finite element method. Indeed, spatial convergence cannot be guaranteed for finite element calculations, even when fine mesh resolutions, for which the computational cost is important, are used. Also, as the self-consistent method does not explicitly account for neighborhood effects, important discrepancies between the self-consistent method and the other methods exist regarding the mechanical response of a specific grain. The self-consistent method nevertheless provides a reasonable description of the average response obtained for a group of grains with identical features (e.g. shape, orientation). [ABSTRACT FROM AUTHOR]

• Microstructure-sensitive framework has been extended and applied to SLM 316L. • As-built surface roughness has been considered explicitly in the finite element model. • Textured microstructure of SLM 316L has been evaluated concerning fatigue strength. • Competitive effects of surface defect and microstructure have been distinguished. This paper aims to study the individual and competitive effects of surface defects and microstructural components in an AM 316L by numerical methods. Based on the microstructure sensitive modeling framework, numerical simulations of models in different surface states with virtual quasi-realistic explicit microstructures are performed. Problems encountered during the establishment of the numerical polycrystalline aggregate model are discussed, including the grain morphology, the strongly textured microstructure with preferential crystallographic directions, roughness and surface defect. A statistical method based on generalized extreme value distribution along with a non-local method is used to describe the stochasticity of microstructural attributes. Simulations of differently configured explicit microstructure models are analyzed and discussed. [ABSTRACT FROM AUTHOR]

This paper deals with the effect of defect size and shape under high cycle fatigue for metallic alloys. A large simulation campaign based on a multiaxial fatigue criterion and a non-local approach is presented. A relative defect size based on a ratio between the defect size and a characteristic length introduced by the non-local approach is defined. A normalized Kitagawa–Takahashi diagram is then obtained. A competition between the highly stressed volume size and the local maxima due to the defect is observed and seem dependent on the relative defect size. The effect of the loading mode (uniaxial and pure shear) and of the plasticity are discussed. Finally, a comparison of the simulation results with experimental data on a 316L L-PBF demonstrates the robustness of the proposed approach and explains the negligible effect of the defect morphology compared to its size. • The effect of defect size and shape under high cycle fatigue is investigated. • A multiaxial fatigue criteria with a non-local approach is applied. • A competition between the highly stressed volume and the local maxima is underlined. • A simulated normalized Kitagawa–Takahashi diagram is proposed. • The effect of the loading mode and the plasticity is discussed. [ABSTRACT FROM AUTHOR]

• Explicit virtual microstructure is introduced to the numerical models. • Different configurations of grain size and morphology are investigated. • Stress-based fatigue criteria combined with two non-local methods are applied. • Kitagawa-Takahashi effect is reproduced and explained via the proposed framework. This study aims at investigating the effect of defect on the high cycle fatigue behavior of polycrystalline aggregates. An explicit virtual microstructure finite element model is created to conduct fatigue simulations. Different stress-based criteria frequently applied in fatigue assessment are tested with a combination of the non-local methods. Two realizations of the non-local method are used and compared. Simulation results are compared to experimental results of 316L. From the parametric simulations, better qualitative understandings on the choice of non-local method as well as its parameters are obtained. The effect of Kitagawa-Takahashi can be reproduced and explained by this proposed framework of microstructure modeling-simulation. [ABSTRACT FROM AUTHOR]

• Experimental torsion and bending fatigue strengths of 90°, 45°, and 0° LPBF building direction were determined. • Kitagawa diagrams were provided in bending and torsion loading at different building directions. • Bending fatigue behavior is very sensitive to building direction compared to torsion. • Torsion fatigue behavior of LPBF 316L is not sensitive to lack-of-fusion defects. The present study aims to investigate the high cycle fatigue (HCF) performance of steel 316L fabricated by the laser powder bed fusion (LPBF) process. Bending and torsional fatigue test specimens built horizontally (0°), inclined (45°), and vertically (90°) have been prepared and tested. Stress-relieving heat treatment was carried out to reduce the residual stresses. The 90°, 45°, and 0° built near-net-shape and polished specimens have fatigue strengths between 140 and 250 MPa under bending loading and 150 and 170 MPa under torsion loading. This difference illustrates a more pronounced defect sensitivity in bending compared to torsion. Fractographical analyses revealed the fatigue failure mechanisms. As the building direction inclines from vertical to horizontal, the effective areas of inherent defects diminish contributing to improving fatigue strength under bending loading whilst the differences from building directions in torsional fatigue strengths are minor. Microstructural features are seen to compete with inherent defects to affect fatigue performance in the condition that the effective defect sizes are close to the critical fatigue crack size. [ABSTRACT FROM AUTHOR]

• Torsion fatigue limit is found higher than bending fatigue limit for LPBF 316L. • Effect of lack-of-fusion defect dominates over the effect of stress gradient. • Skipping contouring introduces clustering defects without increasing surface roughness. • Clustering surface defects act synergistically as one to initiate the fatigue crack. The mechanical performances of additive manufactured (AM) material are highly dependent on the fabrication process which inevitably results in surface imperfection as well as porosity. In the present study, the high cycle fatigue (HCF) behavior of an AM stainless steel 316L is experimentally investigated to characterize and evaluate the effect of the inherent surface defects. Profilometry and Computed Tomography are used. A series of fatigue experiments is carried out under different loading modes including tension, bending, and torsion fatigue tests. For each loading condition, different surface preparations are used to investigate the effect of surface state. Fatigue tests reveal that surface treatment can improve fatigue performances, the improvements observed being higher under tension/bending loading than under torsion loading. The fractographic analysis is performed for all the available tested specimens to reveal the mechanism of fatigue crack initiation. Lack-of-fusion (LoF) defects play the predominant role in the fatigue performance of SS 316L fabricated by laser powder bed fusion (LPBF). The presence of multiple LoF defects at the surface or subsurface is detrimental to the endurance under cyclic loading. By using Murakami approach modeling the relationship between fatigue strength and defect size, it is found that the multiple clustering defects act synergistically as one large virtual crack to initiate the fatigue crack. [ABSTRACT FROM AUTHOR]

Multiaxial high cycle fatigue modeling of materials is an issue that concerns many industrial domains (automotive, aerospace, nuclear, etc.) and in which many progress still remains to be achieved. Several approaches exist in the literature: invariants, energy, integral and critical plane approaches all of them having their advantages and drawbacks. These different formulations are usually based on mechanical quantities at the micro or mesoscales using localization schemes and strong assumptions to propose simple analytical forms. This study aims to revisit these formulations using a numerical approach based on crystal plasticity modeling coupled with explicit description of microstructure (morphology and texture) and proposes a statistical procedure for the analyses of numerical results in the HCF context. This work has three steps: First, 2.5D periodic digital microstructures based on a random grain sizes distribution are generated. Second, multiaxial cyclic loading conditions corresponding to the fatigue strength at 106 cycles are applied to these microstructures. Third, the mesoscopic Fatigue Indicator Parameters (FIPs), formulated from the different criteria existing in the literature, are identified using the finite element calculations of the mechanical fields. These mesoscopic FIP show the limits of the original criteria when it comes to applying them at the grain scale. A statistical method based on extreme value probability is used to redefine the thresholds of these criteria. These new thresholds contain the sensitivity of the HCF behavior to microstructure attributes. Finally, the biaxiality and phase shift effects are discussed at the grain scale and the loading paths of some critical grains are analyzed. [ABSTRACT FROM AUTHOR]