Investigation on some factors affecting crack formation in high resistance aluminum alloys

Aluminum alloys having good mechanical properties are Al-Zn-Mg alloys (7xxx) and Al-Cu-Li alloys (Weldalite®). These alloys may be subjected to stress corrosion cracking. In order to overcome this problem the Al 7050 alloy has been developed and it is widely used for aerospace applications. Despite that, some components made of this alloy cracked during the manufacturing process including machining and chemical anodization. In a previous work cracked Al 7050 components have been analyzed in order to identify possible causes of crack formation. In this work the susceptibility of this alloy to intergranular corrosion has been analysed and compared with that of other high resistance aluminum alloys

The effect of disorder on the fracture nucleation process

The statistical properties of failure are studied in a fiber bundle model with thermal noise. We show that the macroscopic failure is produced by a thermal activation of microcracks. Most importantly the effective temperature of the system is amplified by the spatial disorder (heterogeneity) of the fiber bundle. The case of a time dependent force and the validity of the Kaiser effects are also discussed. These results can give more insight to the recent experimental observations on thermally activated crack and can be useful to study the failure of electrical networks.Comment: 22 pages, 11 fgure

Failure time and microcrack nucleation

The failure time of samples of heterogeneous materials (wood, fiberglass) is studied as a function of the applied stress. It is shown that in these materials the failure time is predicted with a good accuracy by a model of microcrack nucleation proposed by Pomeau. It is also shown that the crack growth process presents critical features when the failure time is approached.Comment: 13 pages, 4 figures, submitted to Europhysics Letter

Failure time and critical behaviour of fracture precursors in heterogeneous materials

The acoustic emission of fracture precursors, and the failure time of samples of heterogeneous materials (wood, fiberglass) are studied as a function of the load features and geometry. It is shown that in these materials the failure time is predicted with a good accuracy by a model of microcrack nucleation proposed by Pomeau. We find that the time interval $$ between events (precursors) and the energy $\varepsilon$ are power law distributed and that the exponents of these power laws depend on the load history and on the material. In contrast, the cumulated acoustic energy $E$ presents a critical divergency near the breaking time $\tau$ which is % E\sim \left( \frac{\tau -t}\tau \right) ^{-\gamma }. The positive exponent $$ is independent, within error bars, on all the experimental parameters.Comment: to be published on European Physical Journa

Induced seismicity in a salt mine environment evaluated by a coupled continuum-discrete modelling

International audienceWith the objective to better understand the induced microseismicity in a salt mine environment due to an underground solution mining, an in situ experiment is undertaken by GISOS in the Lorraine salt basin. The overburden overlying the salt cavity is characterized by the presence of two competent layers where most microseismic events are expected. This paper presents a coupled continuum-discrete modelling to simulate the mechanics of fracture initiation and propagation in the rock mass overlying the cavity: discrete approach for the competent layers and continuum approach for marls, salt and other rocks and soils. For the competent layers, numerous calibrations of the model microparameters based on the laboratory results are firstly performed. The first coupled modelling results suggest that the mechanism of fracturing in the competent layers is predominantly tensile as it could be expected. The results also show that the microseismic events associated to the progressive damage in the competent layers through microcraks development can be modelled. This opens interesting perspectives to assess the feasibility of seismic monitoring of underground cavities by comparing, in the future, the numerical modelling results with the recorded seismicity of the study area.Dans l'objectif de mieux comprendre la microsismicité induite dans des mines de sel, une expérimentation in situ a été entreprise par le GISOS sur une cavité saline de la région Lorraine. Le recouvrement est caractérisé par la présence de deux bancs raides dans lesquels l'essentiel de l'activité microsismique est attendue lors de la reprise de l'exploitation. Ce papier présente une modélisation basée sur une approche couplée : continue (pour les marnes et le sel) et discrète (pour les bancs raides) pour évaluer les mécanismes de développement des fractures dans le recouvrement. De nombreuses calibrations des microparamètres du modèle discret à partir des paramètres macroscopiques ont été nécessaires. Les résultats de la modélisation montrent que le mécanisme de fracturation dans les bancs compétents résulte essentiellement d'efforts en traction. Les premiers résultats montrent aussi que les évènements microsismiques peuvent très bien être mesurés à travers la formation des microfissures. Ceci ouvre des perspectives intéressantes pour des confrontations avec les mesures qui seront enregistrées lors de l'effondrement provoqué

Microwave transport approach to the coherence of interchain hopping in (TMTSF)2PF6

We report a microwave study of the longitudinal and transverse transport properties of the quasi-one-dimensional organic conductor (TMTSF)$_2$PF$_6$ in its normal phase. The contactless technique have provided a direct measurement of the temperature profile of the resistivity along the {\bf b'} direction and in magnetic fields up to 14 T. A characteristic energy scale ($T_x \sim 40 K$) has been observed which delimits a transient regime from an insulating to a metallic behavior. This anomalous profile is discussed in terms of the onset of coherent transport properties along the {\bf b'} direction below 40 K. This is also supported by the observation of a finite longitudinal and transverse magnetoresitances only below 40 K, indicative of a two-dimensional regime. Below $T_x$, however, strong deviations with respect to a Fermi liquid behavior are evidenced.Comment: 4 pages, 5 figures, submitted to Euro.Phys.J.

From RE-211 to RE-123. How to control the final microstructure of superconducting single-domains

This paper reviews the usual techniques for producing YBCO-type single-domains and the microstructure of the as-obtained samples. The problems of seed dissolution and parasite nucleations are discussed in details. Formation of microstructural defects, such as pores and cracks, are examined. An important part of this review is devoted to the study of the influence of RE-211 particles [RE2BaCuO5 where RE denotes Y, Yb, Nd, Sm, Dy, Gd, Eu or a mixture of them. Generally Nd4Ba2Cu2O10 is preferred to Nd2BaCuO5] on the microstructure and properties of RE-Ba-Cu-O single-domains. Trapping/Pushing theory is described in order to explain the spatial distribution of RE-211 particles in the RE-123 [(RE)Ba2Cu3O7-d] monoliths. Formation of RE-211-free regions is discussed. Different ways to limit the RE-211 coarsening are reviewed. Microstructural defects in the RE-123 matrix caused by the RE-211 particles are presented. It is also shown that RE-211 particles play a significant role on the mechanical properties of single-domain samples. We finish this review by discussing the Infiltration and Growth process as a good technique to control the microstructure.Comment: review paper to be published in Supercond. Sci. Technol.; 19 figures; 137 references; 37 page

Characterization of the fracture surfaces obtained in direct tensile tests

The study presented herein deals with an investigation of the dependence of the mode I fracture energy on the fracture path. The relief of the fracture surfaces resulting from direct tensile tests conducted on different types of granites was characterized by means of classical parameters, mean roughness and root-mean-square roughness. These parameters were obtained from the data acquired by a 3D laser topographical inspection system used to capture the texture of fracture surface profiles at various locations of the surface. A scattered linear correlation between fracture energy and the mean (micro)roughness was attained. Additionally, it was found that microstructural aspects, like planear anisotropy and grain size, as well as the weathering state, influence the fracture surface relief