Three-dimensional grain mapping by x-ray diffraction contrast tomography and the use of Friedel pairs in diffraction data analysis

X-ray diffraction contrast tomography (DCT) is a technique for mapping grain shape and orientation in plastically undeformed polycrystals. In this paper, we describe a modified DCT data acquisition strategy which permits the incorporation of an innovative Friedel pair method for analyzing diffraction data. Diffraction spots are acquired during a 360 degree rotation of the sample and are analyzed in terms of the Friedel pairs ((hkl) and (hkl -) reflections, observed 180 degrees apart in rotation). The resulting increase in the accuracy with which the diffraction vectors are determined allows the use of improved algorithms for grain indexing (assigning diffraction spots to the grains from which they arise) and reconstruction. The accuracy of the resulting grain maps is quantified with reference to synchrotron microtomography data for a specimen made from a beta titanium system in which a second phase can be precipitated at grain boundaries, thereby revealing the grain shapes. The simple changes introduced to the DCT methodology are equally applicable to other variants of grain mapping. Copyright 2009 American Institute of Physics

Experimental study of the three-dimensional character of short fatigue cracks in a cast aluminium alloy

Images obtained by high-resolution X-ray tomography are used to show experimentally the three-dimensional character of short fatigue cracks in a cast aluminium alloy. The pores present in this alloy are preferred initiation sites for fatigue cracks, but no correlation can be established with the three-dimensional size of the pores. A strong interaction between fatigue cracks and grain boundaries is clearly shown, both at the surface and in the bulk, using post mortem 3D observations as well as in situ observations

Study of Interaction of Short Fatigue Cracks in 2024T351 Aluminium Alloy

ABSTRACT. In the present pape

Three‐dimensional study of fatigue crack propagation using synchrotron X‐ray microtomography

International audienc

Status and perspectives of combined 3D imaging and diffraction experiments at the ESRF

International audienceno abstrac

A 3D measurement procedure for internal local crack driving forces via synchrotron X-ray microtomography

Synchrotron X-ray microtomography has been utilized for the in situ observation of fatigue crack opening/closure during load cycles. High-resolution phase contrast imaging technique has enabled the reconstruction of clear crack images together with the details of microstructural features. Physical displacements of micro-pores in a crack-tip stress field are used to obtain local mixed-mode crack driving forces along a crack front, and its feasibility is confirmed. Complicated crack closure behaviour are observed due to the combined effects of local modes II and III displacements. The technique used provides a highly effective way of assessing local crack driving forces together with supplementary ways of verifying and interpreting it by visualising and quantifying various forms of crack-tip shielding behaviour. The proposed technique is clearly advantageous compared to the limited procedures available in the current literature, where detailed internal information can only be achieved for limited types of material

¿Cómo deciden les jóvenes la transición a la educación postobligatoria? Diferencias entre centros públicos y privados-concertados

El objetivo del presente artículo es examinar el papel que juega la titularidad del centro en el que se cursa la educación secundaria obligatoria (ESO) en la decisión de transitar a la educación postobligatoria. En dicha encrucijada se contemplan también otras variables que se han constatado de gran peso en la toma de decisión como son las notas previas obtenidas, el nivel formativo familiar, el sexo, las expectativas y las aspiraciones, partiendo del modelo decisional de transiciones educativas[1] que contempla el modelo de Boudon sobre los efectos primarios y secundarios. Los resultados obtenidos son los derivados de un estudio tipo panel que se inicia con la entrevista a jóvenes de secundaria a los 15 años, de diferentes institutos de la ciudad de Barcelona, cuando están cursando el último año de la ESO, y se les vuelve a entrevistar en los dos años siguientes para saber cuál ha sido su trayectoria y experiencia educativa. Los análisis llevados a cabo permiten afirmar que el nivel formativo familiar es el factor que tiene una mayor incidencia en el rendimiento de los jóvenes, pero no en la elección de qué hacer al finalizar la ESO, donde pesa más la titularidad del centro. [1] Se puede consultar su explicación completa y extendida en la página web del grupo GRET (Troiano, 2015) http://grupsderecerca.uab.cat/gret/content/un-modelo-conceptual-para-el-an%C3%A1lisis-de-la-decisi%C3%B3n-de-ir-la-universidad-y-de-la-elecci%C3%B3n, consultado 26 de marzo de 2015

The three-dimensional microstructure of polycrystalline materials unravelled by synchrotron light

Synchrotron radiation X-ray imaging and diffraction techniques offer new possibilities for non-destructive bulk characterization of polycrystalline materials. Minute changes in electron density (different crystallographic phases, cracks, porosities) can be detected using 3D imaging modes exploiting Fresnel diffraction and the coherence properties of third generation synchrotron beams. X-ray diffraction contrast tomography, a technique based on Bragg diffraction imaging, provides access to the 3D shape, orientation and elastic strain state of the individual grains from polycrystalline sample volumes containing several hundred up to a few thousand grains. Combining both imaging modalities allows a comprehensive description of the microstructure of the material at the micrometer length scale. Repeated observations during (interrupted) mechanical tests provide unprecedented insight into crystallographic and grain microstructure related aspects of polycrystal deformation and degradation mechanisms in materials, fulfilling some conditions on grain size and deformation state