Influence of hydrogen and oxygen content on the mechanical behavior of zirconium between 275 and 325°C and titanium at 20°C

International audienceThe mechanical behaviors of α-Zr and α-Ti with various oxygen and hydrogen contents were investigated by loading/relaxation/unloading tensile tests and tests with strain rate jumps, at 300°C and 20°C, respectively. Solute oxygen and hydrogen atoms were found to have antagonist effects. While the former increases the yield stress and hardening exponent, reduces the activation volume and enhances static and dynamic strain aging phenomena, the latter significantly reduces these effects, as long as the O content is not too high

Experimental and numerical study of crack healing in a nuclear glass

International audiencea b s t r a c t An experimental study of thermally or water-induced crack healing in an inactive borosil-icate glass, chemically analogous to that used in France for the vitrification of nuclear waste was carried out. Partial welding of glass plates was observed after annealing in air at 425 °C (77 °C below T g) when at least 20 MPa compressive stress was applied, while annealing at 450 °C under 20 MPa led to a complete disappearance of the interface. Closure of indenta-tion-induced cracks was observed during annealing at 400 °C in an ESEM as a result of viscous relaxation of residual stresses but it did not constitute a sufficient proof of crack healing. DCDC specimens were thus pre-cracked in an ESEM and then either annealed at various temperatures (350–490 °C) in secondary vacuum or in air, or left in water at 70–90 °C, sometimes under a compressive stress normal to the crack face. The specimens were then reloaded in the ESEM and the crack opening displacements under a given load were compared to those measured during pre-cracking. The cracks were bridged by an alteration layer over a distance from the crack tip which decreased as loading increased. The restraining effect of these bridges on crack opening was assessed via finite element simulations, using interface elements. The tensile strength of the bridging layer was estimated as 27–39 MPa after vacuum annealing at 400 °C, 11–20 MPa after 15 days in water at 90 °C and 44–78 MPa after 11 days in water at 70 °C under 5 MPa normal compression. Partially healed cracks did not resume propagation from their former crack tip, but due to branch cracks re-initiated a few hundred microns behind it which grow avoiding the healed area. This behaviour was explained using finite element simulations

Etude expérimentale et numérique de la rupture ductile à faible triaxialité

On étudie la rupture ductile de deux métaux à faible taux de triaxialité en menant des essais multiaxiaux à rupture sur des éprouvettes tubulaires entaillées, correspondant à différentes valeurs de triaxialité des contraintes T et de paramètre de Lode µ. Les déplacements sont mesurés au moyen d'extensomètres et de stéréo-corrélation d'images numériques. Des calculs EF permettent de déterminer les valeurs de T, µ et de déformations plastiques équivalentes à la rupture. Enfin, l'observation au MEB de faciès et de coupes permettent la mise en évidence les mécanismes d'endommagement

A constitutive model for a rate and temperature-dependent, plastically anisotropic titanium alloy

Aircraft engine fan blades are notably designed to withstand impact loading involving large deformation, high strain rate, non-proportional loading paths and self-heating. Due to their high strength-to-weight ratio and good toughness, Ti–6Al–4V titanium alloys are promising candidates for the blades leading edge. An extensive experimental campaign on a Ti–6Al–4V titanium alloy provided in the form of cold rolled plates has been carried out. The thermo-mechanical characterization consisted in tension, compression and shear tests performed at various strain rates and temperatures, and under monotonic as well as alternate loading paths. A constitutive model has been accordingly developed accounting for the combined effect of plastic orthotropy and tension/compression asymmetry, nonlinear isotropic and kinematic strain hardening, strain rate hardening, and thermal softening. The constitutive model has been implemented as a user material subroutine into the commercial finite element computation code LS-DYNA. The performances of the model have been estimated by conducting numerical simulations considering a volume element under various loading paths as well as the specimens used for the experimental campaign

Ultrafine versus coarse grained Al 5083 alloys: From low-cycle to very-high-cycle fatigue

The fatigue performance of coarse and ultrafine-grained (UFG) 5083 Al alloy were compared, from low to very high cycle fatigue. The UFG alloy exhibited cyclic hardening and predominant kinematic hardening. At high plastic strain amplitude (and only in this regime), it showed easier crack initiation and a lower fatigue resistance. Its resistance to HCF was hardly better than that of its coarse grained counterpart until 2.10 6 cycles, but 43% higher in VHCF, until 5.10 8 cycles. Beyond that point, internal crack initiation occurred, and the fatigue resistance of the UFG material decreased, which was explained using Fracture Mechanics

Surface versus internal fatigue crack initiation in steel : Influence of mean stress

Stress-controlled fatigue tests were run at different R ratios (= sigma min / sigma max) up to at most 3 million cycles on a 2.5%Cr–1%Mo steel (ASTM A182 F22) used in riser tubes connectors for offshore oil drilling. The fatigue lives, as well as the slope of the S – N curves were found to decrease with increasing R and the endurance limit to follow Gerber’s parabola. Surface crack initiation without any defect involved, was most often observed for R=-1, 0.5 and 0, while an R ratio of 0.25 triggered crack initiation from either surface or internal pores or chemically inhomogeneous areas, leading, in the latter case, to fish-eye patterns for relatively low numbers of cycles. A further increase in R ratio to 0.5 promoted only defect-initiated surface cracks, while no fatigue fracture was observed within 10 million cycles above R=0.6. These transitions in crack initiation mode are discussed based on X-ray diffraction analyses of residual stresses, elastic–plastic F.E. computations on a unit cell model containing a pore and some fracture mechanics analyses, with a particular attention to environmental effects.The authors acknowledge the assistance of Dr. E. Heripre for FIB sectioning and observations with a dual beam microscope bought within the Equipex Matmeca funding program

Le titane et ses alliages dans les transports : atouts et problèmes

National audienc

Crack initiation mechanisms in torsional fatigue

The development of fatigue damage in Co45Ni specimens during push—pull and reversed torsion tests, performed inside a scanning electron microscope, was observed and the different stress states compared. It appeared that transgranular crack initiation and development is delayed and intergranular crack initiation promoted under torsional loading. This was explained in terms of reduced surface distortion at the emergence of persistent slip bands (PSBs) and smaller compatibility stresses at the PSB‐matrix interfaces. The influence of the mechanical strength of grain boundaries on the difference between tensile and torsional fatigue lives is discussed

A discrete dislocation model of Stage I fatigue crack growth and an analysis of Stage II to Stage I transition at low ΔK₁

Simulations of dislocations nucleation and glide ahead of a crystallographic mode II crack are performed for push-pull and reversed torsion. An influence of the normal stress on the friction of crack flanks, as well as on the condition for dislocation emission is introduced. The crack growth rates are deduced from the dislocation fluxes at the crack tip. A comparison between the loading modes is made. Taking into account the presence of grain boundaries, the repeated decelerations and sometimes the arrest that characterise Stage I crack growth are described by the model. An analysis of the transition from mode I to mode II crack growth observed at low ΔK1 in single crystals is proposed