The novel possibilities of phase feature detection in radiography at a third generation synchrotron radiation source are used to image, both in projection and in computed tomography, a cracked silicon single crystal and metal matrix composites strained in tension. Through an instrumentally very simple technique, based on Fresnel diffraction, phase jumps related to the interface between the matrix and the reinforcing phases of the composites are detected even when these phases show very similar x-ray attenuation. Strain-induced cracks with openings below the micrometer range are also visible through the phase modulation they introduce, illustrating the potential of the technique for assessing damage in materials with improved resolution and sensitivity.

This paper aims at illustrating the potential of X-ray tomography for studying the mechanical behaviour of materials through in situ experiments. Typical experimental tomography set ups which use laboratory and synchrotron X ray sources are described; advantages and limitations of both types of sources are presented. Dedicated experimental devices which allow deformation and/or temperature changes to be applied to various types of materials are described. Examples of results of in situ mechanical experiments are presented and discussed; they include monotonic tensile testing of steel fiber entanglements, This is the 3rd in a series of featured review articles to celebrate the 50th anniversary of Experimental Mechanics. These articles serve to touch on both areas of mechanics where the journal has contributed extensively in the past and emergent areas for the future.

No abstract

No abstract

X-ray diffraction contrast tomography ͑DCT͒ 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°rotation of the sample and are analyzed in terms of the Friedel pairs ͑͑hkl͒ and ͑hkl͒ reflections, observed 180°apart in rotation͒. 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 arise͒ 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.

A full three dimensional study of a fatigue crack in cast iron is presented. This analysis involves combining various tools, namely, Synchrotron X-ray microtomography Preprint submitted to Elsevier 4 October 2009of an in situ experiment, image acquisition and treatment, 3D volume correlation to measure 3D displacement fields, extraction of the crack geometry, extended digital image correlation to account for the crack displacement discontinuity, crack modeling in an elastic material exploiting the actual crack geometry, and finally estimation of stress intensity factors. All these different tasks are based on specific multiscale approaches.

The principle of the tomography technique and the different possible set‐ups, which can be used to obtain medium‐(10 μm) and high‐(1 μm) resolution, three‐dimensional, non‐destructive images, are shown in this paper. Illustrations are made of the applications of the technique in the field of materials science. Examples are given for medium‐resolution images of metallic foams and model metal matrix composites that are reinforced with spherical particles. High‐resolution examples are shown for aluminium alloys. For low‐absorbent materials we show that the phase contrast obtained using synchrotron radiation can provide a valuable solution. The quantitative use of these images, coupled with in‐situ tensile tests or used for the simple analysis of the initial microstructure of several structural materials, is also described.

Synchrotron X-ray microtomography has been utilized for the in-situ observation of steady state plane strain fatigue crack growth. A high resolution experimental configuration and phase contrast imaging technique have enabled the reconstruction of crack images with an isotropic voxel with a 0.7 µm edge. The details of a crack are readily observed, along with evidence of the incidence and mechanical influence of closure. After preliminary investigations of the achievable accuracy and reproducibility, a variety of measurement methods are used to quantify crack opening displacement (COD) and closure from the tomography data. Utilization of the physical displacements of microstructural features is proposed to obtain detailed COD data, and its feasibility is confirmed. Loss of fracture surface contact occurs gradually up to the maximum load. This is significantly different from tendencies reported where a single definable opening level is essentially assumed to exist. The closure behaviour is found to be attributable mainly to remarkable generation of mode III displacement which may be caused by local crack topology. Many small points of closure still remain near the crack-tip, suggesting that the near-tip contact induces crack growth resistance. The effects of overloading are also discussed.