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The dynamic strain aging (DSA) phenomenon is present in C–Mn steels and associated welds commonly used in many industrial applications. This phenomenon, which appears in metals containing solute atoms interacting with dislocations, induces an increase in flow stress and ultimate tensile strength (UTS) as well as a decrease in ductility. The DSA phenomenon was studied by tensile, strain rate sensitivity and internal friction tests between 20°C and 350°C in two C–Mn steels with very different sensitivities to DSA (a fully killed A42 steel and a semikilled A48 steel). The internal friction technique, which allows one to evaluate the balance between carbon and nitrogen atoms free in the lattice and carbon and nitrogen atoms interacting with mobile dislocations, appears to give results well correlated with those obtained by tensile (and strain rate sensitivity) tests. The ductility loss or the UTS increase is shown to be proportional to Snoek peak height. It is concluded that, even in industrial materials, a single internal friction test can give a good evaluation of DSA sensitivity. [ABSTRACT FROM AUTHOR]

Cold drawing steel wires lead to an increase of their mechanical strength and to a drop of their ductility. The increase of their mechanical strength has long been related to the reduction of the various material scales by plastic deformation, but the mechanisms controlling their elongation to failure have received relatively little attention. It is usually found that heavily deformed materials show a tendency to plastic strain localization and necking. However, in this paper it is shown that, though the steel wires are plastically deformed up to strain levels as high as 3.5, a significant capability of plastic deformation is preserved in as-drawn wires. This apparent contradiction is resolved by the existence of residual stresses inside the wire. Finite element analyses have been conducted in order to show that residual stresses, inherited from the drawing process, are sufficient to produce a significant hardening effect during a post-drawing tensile test, without introducing any hardening in the local material behaviour. The main conclusion of this paper is that once the material has lost its hardening capabilities, residual stresses, inherited from the process, control the elongation of cold drawn wires. The finite element method allowed also the determination of the residual stress field that would lead to the best agreement between the simulated and the experimental stress strain curve of as-drawn wires. [ABSTRACT FROM AUTHOR]

Cold drawing of steel wires leads to an increase of their mechanical strength and to a drop in their ductility. The increase of their mechanical strength has long been related to the reduction of the various material scales by an intense plastic deformation. Besides, it was discussed in the companion paper that large plastic deformation leads to the loss of the material hardening capabilities and that, in such a case, residual stresses preserve the elongation to failure of wires. Experimental measurements of residual stresses inside the wire have therefore been undertaken. In this paper, lattice parameters as measured using synchrotron diffraction are compared with those calculated using the residual stress fields as determined by the finite-element method. There is a major disagreement between experimental and numerical results that is too large to be attributed to the errors of the finite-element analyses. Therefore, neutron diffraction experiments have also been performed. These measurements show that there is a significant variation of the lattice parameter with the drawing level, which is not inherited from residual stresses, and that variation is very sensitive to the cooling rate after processing. It is therefore proposed that cold drawing would induce a phase transformation of the steel, possibly a martensitic transformation. [ABSTRACT FROM AUTHOR]

Two reduced scale (1:3) railway wheel/axle assemblies have been tested before and after low cycle fatigue in rotary three point bending. The comparison between the residual stress fields before and after low cycle fatigue tests shows that the high stress zone is shifted towards the mid plane of the assembly, thus indicating that residual stresses resulting from wheel/axle fitting are progressively reduced by cyclic loading. Numerical modelling shows analogue results. A comparison between the two methods is proposed. [ABSTRACT FROM AUTHOR]

Fatigue crack initiation was studied in a bimodal TA6V titanium alloy. A ghost structure inherited from the forging process, the scale of which is roughly 100 times the apparent grain size, was found to govern the initiation process. In these macrograins, that we have labelled macrozones, most of the primary alpha grains (α[sub p]) are found to display the same crystallographic orientation. Fatigue cracks are initiated on the basal plane or, if basal slip is difficult, on the prismatic plane. Thus in macrozones, where basal or prismatic slip is easy, numerous neighbouring tiny cracks appear over the whole macrozone, which have the size of the primary α[sub p] grains. In these macrozones the contribution of crack coalescence to crack growth is consequently very significant. On the contrary, if basal and prismatic slips are both difficult in the macrozone, no crack can be found in the corresponding macrozone. The crack initiation process is thus highly heterogeneous at the scale of the macrozone. Furthermore, this microstructure is found to induce a large scatter in the fatigue life of notched samples. [ABSTRACT FROM AUTHOR]

Abstract: Ductile fracture of a C–Mn steel was characterized by tensile tests performed in a large temperature range (from 20 to 350°C) on round notched and CT specimens. The experimental results revealed a sharp decrease in fracture strain and fracture toughness around 200°C. These temperatures correspond to the domain of dynamic strain ageing (DSA). The Portevin-Le Chatelier (PLC) effect, which is the most classical manifestation of DSA, was simulated for round notched and CT specimens with a mechanical constitutive model which includes the strain ageing effect and the stiffness of the testing machine. It is shown that changes in stiffness can amplify the DSA effect. 3D-Modeling was used to correctly capture the complex space-time correlation of strain localization, particularly in side-grooved CT specimens. The results were compared to classical elastic–plastic simulations. The local approach to fracture was then applied to predict the ductile fracture of round notched specimens using the Rice and Tracey criterion. In the DSA domain, the approach used in this study predicts a decrease of the fracture strain which is less than observed experimentally. [Copyright &y& Elsevier]

• Complex foods with identical composition but different microstructures were designed. • Food structure strongly influences both the kinetics and final extent of lipid digestion. • The key moments of digestion are highlighted by multidimensional statistical analysis. • The food structure impacts on the micronutrient bioaccessibility. • This impact depends on the hydrophilicity vs hydrophobicity of the micronutrient. Digestion is a mechanical and chemical process that is only partly understood, and even less so for complex foods. In particular, the issue of the impact of food structure on the digestion process is still unresolved. In this study, the fate of four micronutrient-enriched foods with identical compositions but different microstructures (Custard, Pudding, Sponge cake, Biscuit) was investigated using the 3-phase in vitro model of human digestion developed by the INFOGEST network. Matrix disintegration and hydrolysis of macronutrients (proteins, lipids and carbohydrates) were monitored during the three phases of digestion using biochemical techniques, size-exclusion chromatography, thin-layer chromatography and gas chromatography. Micronutrient release (vitamin B9 and lutein) was monitored using reverse-phase chromatography. Food structure did not greatly influence macronutrient hydrolysis, except for lipolysis that was four-times higher for Biscuit compared to Custard. However, the bioaccessibility of both micronutrients depended on the food structure and on the micronutrient. Vitamin B9 release was faster for Biscuit and Sponge cake during the gastric phase, whereas lutein release was higher for Custard during the intestinal step. Extensive statistical analysis highlighted the impact of food structure on the digestion process, with different digestion pathways depending on the food matrix. It also made it possible to characterise the gastric step as a predominantly macronutrient solubilisation phase, and the intestinal step as a predominantly hydrolysis phase. [ABSTRACT FROM AUTHOR]