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Fatigue crack initiation Fatigue crack growth
a b s t r a c t
Along the hydrogen supply chain, metallic components, such as pressure vessels, com- pressors and valves, are facing high pressure hydrogen gas. The object of this paper is to address microstructural as well as mechanical aspects of fatigue crack initiation and growth at room temperature in a quenched and tempered (Q&T) low alloy steel under hydrogen pressurein the range 0.5e35 MPa. For such steel, the need to perform tests in- situ under hydrogen pressure is required. The influence of hydrogen gas on the total life in terms of crack initiation and crack propagation is analyzed. The experimental tech- niques developed to detect crack initiation in a pressure vessel under hydrogen pressure are presented. Thanks to these technical developments the influence of hydrogen gas on the total life duration including crack initiation and crack propagation is analyzed. It is shown that the effect of hydrogen pressure on crack initiation is important. At constant load ratio, the hydrogen pressure effect on fatigue crack growth (FCG) is dependent on the loading amplitude (in terms of DK). These results related to cracking behavior are enriched with information on fracture surfaces appearance. The results presented have been ach- ieved within the European project MATHRYCE  dedicated to Material Testing and Rec- ommendations for Hydrogen Components under fatigue. They are part of a process necessary to give a scientific background to the development of a design methodology where hydrogen enhanced fatigue damage is taken into account.
macroscopic tangential force, Q, and a classical shear stress over the surface which exhibits its maximum value, q max , at the stick zone limit (cf. Figure 2(b)). P, Q and are recorded during the test.
p max , q max and the loading frequency, f L , were chosen to characterize our tests. Tests performed are
characterized by a loading frequency equal to 1 or 10Hz and a maximal Hertzian pressure varying from 420 to 1000MPa. In this study, Variable Displacement Method  under partial slip conditions was used. This method will be developed later on in this work. For each maximal pressure, coefficients of friction at the transition between partial slip and gross slip regime were previously estimated. A study  showed that this coefficients of friction may be used to provide representative value of the friction within the sliding zone under partial slip condition.
potential equations), and in section 3 (Serre-Green-Naghdi system). Results of computations for traveling solitary waves and transient wave groups dis- cussed respectively in sections 5.1 and 5.2. In these sections, the respective roles of various terms involved in linear transfer functions to correlate sur- face elevation with bottom pressure are emphasized. It is generally admitted that linear theoryin time domain should not be used to reconstruct eleva- tion from bottom pressure distribution (Bishop and Donelan, 1987; Oliveras et al., 2012) when wave fields are nonlinear, and this result finds a new con- firmation here. In the meantime, conclusions about linear theoryin space domain are not straightforward. These new results are discussed in section 6.
Figure 3: Typical AFM height (a) and phase (b) images of the crack tip for a fracture propagating from top to bottom of the image (400x400 nm 2 ). The scale range is respectively 5 nm and 5 ◦ .
a condensed liquid phase. According to Eq. (??), the phase contrast on the condensate region corresponds to an increase in the energy dissipated by the tip-sample interaction [?]. Under humidity, the presence of thin water ﬁlms on both the external sample surface and the AFM tip entails energy dissipation due to the combination of two phenomena: (1) the formation and rupture of a capillary bridge at each oscillation of the AFM tip [?] and (2) the visco-elastic displacement in the thin ﬁlms [?]. The dissipated energy is very weak when scanning the glass surface, but it is greatly enhanced when the AFM tip crosses a portion of the crack tip ﬁlled with liquid water [?]. The resolution of the AFM technique does not allow to measure the opening of the crack accurately (a few nanometers here), but it allows to measure the length
This careful and exhaustive work will challenge oth- ers attempting to improve on its results, which agree well, within the reasonable error estimates, with the re- cent theoretical predictions of Tailoi et al. and Liu et al., both of whom employed approaches that use ab initio molecular dynamics simulations. These experi- ments are likely not the last word on the melting tem- perature of Ta. Other researchers will comment on them, and have questions that lead to further studies. For example, one may need more information on exactly how the temperatures were derived from the measure- ments. A theory paper by Burakovsky et al., also in Physical Review Letters, already raises questions regard- ing the results of Dewaele et al. The theorists predict, us- ing what they characterize as a “quantum-based” model that combines density-functional calculations with ab initio molecular dynamics simulations, that there may indeed be another stable phase in tantalum below its melting point. This too invites study, since it is not nec- essary that a phase transition should occur at a high pressure just because another structure has a lower free energy. (There are many examples of higher energy metastable structures that exist in simple materials.) In the theory paper, the melting temperature calculated for the bcc form of tantalum agrees with the predictions of Taioli et al. for the bcc structure. It is also interesting to note that the recent shock-wave study of Dai et al. did not report phase changes in Ta. In other words, the jury is still out in this aspect of high-pressure research. The current state of results for the melting curve of tantalum is summarized in Fig. 2.
10 Se 90 ). Density functional theory (DFT) was used to describe the electronic struc-
ture that evolved self-consistently with time. We have adopted a generalized gradient approach (GGA) using the exchange energy obtained by Becke 38 and the correlation energy according to Lee, Yang and Parr (LYP) 39 . The
BLYP approach was used due to its account on valence electron localization effects. The valence electrons have been treated explicitly in conjunction with norm conserving pseudopotentials of the Trouiller-Martins type to account for core-valence interactions. The wave functions have been expanded at the G point of the supercell on a plane-wave basis set having an energy cut-off 20 Ry. A fictitious electron mass of 200 a.u. was used in the first-principles molecular dynamics (FPMD) approach. The time step for integrating the equations of motion was set to 0.1 fs. The temperature control was achieved for both ionic and electronic degrees of freedom using Nosé-Hoover thermostats. The initial coordinates of the atoms have been constructed using the atomic positions in a GeSe crystal. To achieve the correct compositions, randomly chosen Ge atoms were substituted by Se atoms depending on the target composition. Each composition was first melted at T = 2000 K for a period of 22 ps to lose the memory of initial configuration, then simulated along selected isotherms (1373 K, 1050 K, 800 K, and 600 K for 22–30 ps each) before thermalizing at 300 K for 40 ps. Three independent quenches with 1050 K being the starting temperature were performed in order to have a statistical average of the 300 K trajectories and the ener- getically best structure among these quenches were used to apply compression at 300 K by means of decreasing the cell size. The first 4 ps at each isotherm have been discarded.
Simulated annealing is a thermodynamic approach which mimicks this physical process. The simulation starts from an initial configuration; then the atoms are equilibrated at a high constant temperature through the Monte Carlo method, or molecular dynamics, where periodic local quenching is performed. After that, by gradually cooling the system, the structure moves towards lower energy phases. Finally, if thermodynamical equilibrium is preserved throughout the cooling process, the system will crystallize at the ground state. However, in practice, this method is only applicable for simple energy landscapes, since the system can easily get trapped in a local minimum, i.e., metastable structure, especially if this structure is cycled through a very high energy barrier. Therefore, the selection for the initial structures has significant influence to the whole simulation. The simulated annealing approach has been applied to predictive studies of inorganic solids. 10 Mellot-Draznieks et al. predicted the inorganic-organic hybrid structure,
the two spin states under various stimuli including temperature, pressure, light, magnetic and electrical fields [1, 2]. Among these stimuli, pressure is one of the most appealing thermodynamic parameters used to probe the spin transition properties of spin-crossover materials since it allows for tuning both the crystal field as well as the interactions between SCO centers. Since the pioneering works of Drickamer [3, 4], Kӧnig  and Gütlich,[6, 7], the pressure effect has continuously attracted the interest of the scientific community, being intensively studied until nowadays. The pressure effect on spin-crossover materials has been investigated using various techniques, such as Mössbauer spectroscopy , X-Ray and neutron diffraction [8-13], X-Ray spectroscopies [14, 15], Raman spectroscopy [16, 17], optical reflectivity [18-25], magnetometry [26-31] and dielectric spectroscopy . In general, the application of an external pressure induces a transformation of the SCO complex to the low-spin state, i.e. to the state of lower molecular volume, and has the effect of shifting the switching temperatures towards higher values. However, the interpretation of the pressure effect on molecular SCO phenomena has been, in some cases, compromised by the relatively poor or incomplete information resulted from experimental difficulties related to the need for hydrostatic conditions at low temperatures. The practical implementation consists of immersing the sample in a pressure transmitting fluid meant to support no shear. However, the melting point of fluids is usually increased underpressure and solidification inevitably occurs at some pressure. Beyond this point, the pressure across the experimental volume is generally inhomogeneous and differential as well as shear stresses appear . Depending on the type of measurement, this can lead to a more or less dramatic
In this paper, we seek to explain why a government might want to give favor to some foreign …rms and hurt other foreign …rms. Mod- els that explain government behavior toward foreign …rms fall into two major categories. The traditional view o¤ers the “benevolent government model,” according to which the home government max- imizes some social objective, by setting tax rates, or tari¤ rates. A more modern view sees the government as reacting to pressure groups. This view has given rise to the “political economy” approach of trade polices. To model government behavior under this approach, a con- venient way is to postulate that the government seeks to maximize a political support function, without explicitly modelling the behavior of pressure groups 6 . Alternatively, one can be more explicit about the optimization behavior of pressure groups, by, for example, adopting the common agency model proposed by Berheim and Whinston (1986). In the context of international trade, Grossman and Helpman (1994) posit pressure groups seeking protection as “principals” and the gov- erment as their “common agency.” The principals non-cooperatively o¤er the government a menu of payments conditional on actions that the government may take. Such menus are called “contribution sched- ules.” In the common agency model, favors come from competing contributions.
35. Kimura, T., Lawes, G., Goto, T., Tokura, Y. & Ramirez, A. P. Magnetoelectric phase diagrams of orthorhombic RMnO 3 (R = Gd, Tb and Dy). Phys. Rev. B 71, 24425
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= V /N ,
a 0 is the Bohr radius, V the volume and N the number of
valence electrons) at temperatures up to 3000 K, and of the known crystalline phases: bcc, f cc, and cI16, between 0 and 90 GPa. Density functional theory with a plane wave basis set and the Born-Oppenheimer approximation were used for all simulations . They were performed in the N V T ensemble with cubic supercells and periodic boundary conditions. For the simulations of bcc and cI16 solids, and all liquids we used 128 atom supercells with Γ- point sampling of the Brillouin zone. Simulations of the f cc solids were performed with 108 atoms and 8 k-points (the Γ-point only was found to be insufficient). Most simulations ran for at least 10 ps, with some up to 20 ps, while for obtaining the melting temperatures we have computations as long as 200 ps. Extensive convergence tests were carried out for size effects, various simulation parameters, and the validity of the pseudopotential ap- proximation . Good agreement was also obtained with existing liquid experimental data .
It is surprising and counterintuitive that the loss of a RhoA GAP, ARHGAP18, would lead to decreased Myosin-II activation and potentially decreased tension (Porazinski et al., 2015). In many cases, a loss of a RhoA GAP increases active RhoA and actomyosin contraction (Miller and Bement, 2009), which is the opposite result observed in medaka YAP mutants. So how would a GAP that inactivates RhoA promote tissue tension? One possibility is that a delicate balance of activation and inactivation of RhoA, mediated by GEFs and GAPs, respectively, is required to generate tissue tension or cellular contraction (Figure 1). An excellent example of balanced GTPase activation via GEFs/GAPs is observed during cytokinesis. While Ect2 GEF activates RhoA, MgcRacGAP can restrict or inhibit RhoA activity to organize actomyosin organization throughout cytokinetic furrow formation (Loria et al., 2012; Miller and Bement, 2009). Yet, it was recently shown that MgcRacGAP also promotes RhoA activation and Myosin- II accumulation during in C. elegans cytokinesis (Loria et al., 2012).
The trial crystal structures were fully optimized and candidates with the lowest enthalpies were selected. To make sure that the selected crystalline structures indeed correspond to local energy minima, each structure was annealed to 300 K and equilibrated for 10 ps, using a molecular dynamics (MD) simulation in NVT ensemble, followed by a second structural optimization. In the MD annealing simulations, the VASP program and same potentials as described above were used. MD trajectories were sampled with a 1 fs time interval. The annealed structures were then compared with the original ones to examine possible structural changes. Sometimes new structures were produced in this annealing procedure. The structural searches have been performed at five pressures, 1 atm, 10 GPa, 20 GPa, 50 GPa, and 100 GPa, in order to obtain a complete spectrum of the most stable crystalline structures of B 2 H 6 in different pressure regions.
3.2. Tensile asymmetric (R = 0.1) tests
The results of the tests on forged VT3-1 titanium alloy with superimposed static stress (R = 0.1) are illustrated in Fig. 6 (a). The character of fatigue strength decreasing (with the fatigue life) is similar to fully-reversed data and to the results of  . However, continuous decreasing of the fatigue strength versus the number of cycles is not always observed in the two-phase titanium alloy. For example in  it was reported about clear step of SN - curve for positive stress ratios. We also observed the same thing on the VT3-1 alloy. The results of tensile fatigue tests show a higher resis- tance of VT3-1 under R = 0.1 cyclic loads in terms of maximum stress. The fatigue strength of VT3-1 under fully-reversed loading is about 50 MPa lower compared to R = 0.1. However, representa- tion of tensile fatigue results in Haigh diagram shows that the resistance of the forged VT3-1 titanium alloy against cyclic loading with imposed static stress is extremely low, Fig. 6 (b). The results of fatigue tests under R = 0.1 are well below Gerber and Goodman predictions of fatigue strength. Such fatigue behavior of VT3-1 tita- nium alloy can be explained by using the idea of Oguma and Naka- mura  . They have performed VHCF tests on Ti–6Al–4V titanium alloy under positive stress ratio R = 0.1. Titanium alloy was pro- cessed by two different treatments producing dual-phase microstructure with alpha-grains sizes of 4 and 10 l m respec- tively. Oguma and Nakamura have found that the alloy with the bigger alpha-grains has the lower fatigue strength. The difference in fatigue strength at 10 8 cycles between Ti-alloys with different
showing a clear similarity between the shape of magnetic phases above 8GPa. In contrast to an abundant number of antiferromag-
netic (AF) Kondo lattice systems, for which the AF quan- tum critical point (QCP) has been intensively investi- gated, until recently there was no good equivalent exam- ple for a direct transition from a FM to a paramagnetic ground state. Another interesting compound that may help to find a reason for the scarcity of systems with FM QCPs is the intensively studied system YbRh 2 Si 2 . Its
The observed anisotropy of layered compounds appears to be a direct consequence of the bonding-type anisotropy in- side their structures. For example, it is now well established 34 that the layered covalent crystals exhibit two distinct kinds of bonding, a covalent one inside the layers with van der Waals type forces between the slabs. The situ- ation appears to be quite different for ionic-layered com- pounds such as BaFX (which may also be right for other ionic-layered compounds like PbI2 or CdX2), where only one type of force is present. A layer structure for ionic crystals with large halogen atoms (high polarizability due to the large size) appears to be the most favorable framework in terms of stability. Hence, the anisotropic position of the halogen at- oms modulate the magnitude of the Coulomb forces along
Figure 4 | Neutron diffraction measurements of Yb 2 Ti 2 O 7 under applied pressure. (a) Diffraction data sets from 400 to 100 mK after the 800 mK data
set has been subtracted. Error bars are not shown for clarity. (b) Ordered moment versus temperature determined by neutron diffraction for P ¼ 11(2) kbar (blue, left axis) and magnetic fraction determined by mSR for P ¼ 12.2 kbar (orange, right axis). Error bars of the ordered moment represent s.d. of the reﬁnement. Schematic spin structure of the ice-like splayed ferromagnet with y ¼ 14° for P ¼ 0 (c) and y ¼ 5° for P ¼ 11 kbar (d), where y is the splay angle between the  direction and the magnetic moment, tilted towards the local  directions of the tetrahedron.
claim of complete loyalty to the ruler, even if he is corrupt and unjust, as well as a will to transcend local and national contexts by delivering a universal message. Salafists, then, aim to preserve all Muslims from chaos (fitna) by not engaging in any kind of politics and not participating in elections, demonstrations, or revolutions. Instead, they believe they can play a role in orienting state policies through secret advice given to the ruler. They usually condemn violence and have long been critical of terrorist operations targeting civilians. In fact, the Salafi leader al-Wadi’i remained very critical of the jihadists’ strategy at the global level as well as inside Yemen from the early 1990s onward. At the time, he accused Usama bin Ladin--who, following Afghanistan, was trying to launch new wars-- of preferring to invest in weapons rather than in mosques. He even botched some of his plans for jihad against the socialist elites of South Yemen. 21
Located a few kilometers from Montpellier, Villeneuve-lès-Maguelonne in the gulf of Aigues-Mortes belongs to the C2 category. Like Le Rove, this commune provided itself with the relevant local planning documents whenever they were created by law, but it did not come under an inter-municipal master plan before 1983, undoubtedly due to a lack of interest by the State. Lying both on the seashore and in the lagoon complex of Palavas-les-Flots, Villeneuve-lès-Maguelonne is an old settlement that hosted the episcopal headquarters of its region in the Middle Ages. It is a unique place, with strong reminders of the past (the ruins of the medieval cathedral still remain on the island where the town stood at that time) and outstanding natural landscape and biodiversity. Conserving the lagoon and old salt marshes was the incentive behind the protection of large expanses of the area (Coastal conservancy, wetlands under the Ramsar Convention, Natura 2000), but overall Villeneuve-lès-Maguelonne has been less protective than Le Rove. Moreover, a smaller proportion of the land is protected, probably because a major part of the municipal area has long been dedicated to agriculture, which is now suffering the effects of urban expansion (Abrantes et al., 2010). Today, the northern part of the commune, which is inland and near the city of Montpellier (approximately 270,000 inhabitants), is the main urbanized area. It therefore falls under the very strong urban dynamics fostered by Montpellier, the capital of the province, explaining the strong demographic increase of recent years (+214% between 1982 and 2012) and jeopardizing the management of urban sprawl in the area.