Laurent Babout

Characterization by X-ray computed tomography of decohesion, porosity growth and coalescence in model metal matrix composites

Abstract Damage mechanisms of model materials have been studied using in situ tensile tests coupled with high resolution X-ray tomography. This non destructive technique revealed that 50% of the particles were pre-damaged by the extrusion. The initiation and growth phases of the damage process were quantified using the three dimensional images. The growth phase, measured both locally (on isolated particles) and globally (in the entire block) was compared with the Rice and Tracey prediction which was shown to overestimate the global prediction and to give a reasonable agreement of the local growth rate. Discrepancies between prediction and experiments could be partly quantified by introducing the effect of the growth threshold in the Rice and Tracey analysis. The scatter in the measured thresholds and growth rates were attributed to local crystallography and to local spatial arrangement effects.

X-ray microtomographic observation of intergranular stress corrosion cracking in sensitised austenitic stainless steel

Abstract Intergranular stress corrosion cracking in a sensitised type 302 stainless steel wire has been observed in situ using high resolution X-ray microtomography. Tomography enables the development and failure of crack bridging ligaments to be studied in detail in three dimensions. Direct comparison of these features has been made with scanning electron microscopy fractography. The crack bridges failed in a ductile manner, with a morphology that is consistent with non-sensitised low energy grain boundaries.

Recent results on 3D characterisation of microstructure and damage of metal matrix composites and a metallic foam using X-ray tomography

High resolution X-ray tomography experiments were carried out at the European Synchrotron Radiation Facility in Grenoble as well as with a lower resolution tomograph at institut national des sciences appliquees in Lyon. 3D numerical images were collected which helped to study the microstructure and also the damage of different light materials. Recent results obtained with this technique are presented in this paper, concerning model Al/ZrO2 composites, Ti based continuous fibres reinforced composites and an Al foam.

Hole filling in 3D volumetric objects

The construction of hole filling (or hole segmentation) method for 3D volumetric images is a new challenging issue in computer science. It needs a geometrical approach since from a topological point of view 3D holes (tunnels) are not well-delimited subsets of three dimensional space. In this paper, the authors propose an original, efficient, flexible algorithm of hole filling for volumetric objects. The algorithm has been tested on artificial objects and very complicated crack propagation tomography images. The qualitative results, quantitative results and features of proposed approach are presented in the paper. According to our knowledge it is the first algorithm of hole filling for volumetric objects.

High-resolution, in-situ, tomographic Observation of Stress Corrosion Cracking

This paper presents high resolution X-ray tomographic observations of intergranular stress corrosion crack nucleation and growth. In-situ experiments have been performed on beam line ID19 at the European Synchrotron Radiation Facility (ESRF). High-resolution tomography provides non-destructive, three-dimensional information of the shape and depth of damage. To the authors? knowledge, these are the first such observations of stress corrosion, and demonstrate the potential for high resolution, synchrotron, X-ray tomography as a tool for observing pitting, intergranular corrosion and intergranular cracking.The application of high-resolution synchrotron X-ray tomography to two in-situ experiments is described. Localised corrosion and intergranular cracking in sensitised 5083 aluminium alloy has been studied. These results show the progressive development, transition, and coalescence of two forms of damage within the bulk of the sample. In-situ observations of intergranular cracks in sensitised 302 stainless steel have also been obtained. These provide evidence for crack bridging ligaments, caused by the high resistance of special grain boundaries. Further applications of high resolution X-ray tomography are described, such as in-situ studies of pitting and the transition from pitting to cracking in aluminium alloys and stainless steels, including the effects of near-surface residual stress.

Reliable Airway Tree Segmentation Based on Hole Closing in Bronchial Walls

Reliable segmentation of a human airway tree from volumetric computer tomography (CT) data sets is the most important step for further analysis in many clinical applications such as diagnosis of bronchial tree pathologies. In this paper the original airway segmentation algorithm based on discrete topology and geometry is presented. The proposed method is fully automated, reliable and takes advantage of well defined mathematical notions. Holes occur in bronchial walls due to many reasons, for example they are results of noise, image reconstruction artifacts, movement artifacts (heart beat) or partial volume effect (PVE). Holes are common problem in previously proposed methods because in some areas they can cause the segmentation algorithms to leak into surrounding parenchyma parts of a lung. The novelty of the approach consists in the application of a dedicated hole closing algorithm which closes all disturbing holes in a bronchial tree. Having all holes closed the fast region growing algorithm can be applied to make the final segmentation. The proposed method was applied to ten cases of 3D chest CT images. The experimental results showed that the method is reliable, works well in all cases and generate good quality and accurate results.

3D inspection of fabrication and degradation processes from X-ray (micro) tomography images using a hole closing algorithm

This paper presents relatively new examples of X-ray tomography applications to engineering materials. These examples illustrate the usefulness of the technique to inspect fabrication processes, but also to study degradation processes that may occur during service, for example from corrosion. It also shows how advanced image processing algorithms such as the hole closing method, can be used to extract features, or correct processed images.

Grain Boundary Engineering for Crack Bridging: A New Model for Intergranular Stress Corrosion Crack (IGSCC) Propagation

The first in-situ high-resolution X-ray tomography of stress corrosion cracking in a fully sensitised austenitic stainless steel indicates the build up of un-fractured ligaments within the cracking path. This is attributed to corrosion resistant grain boundaries and their associated triple junction (TJ) fractions. A simple grain bridging model has been introduced to quantify the effect of static stress and crack bridging on crack length development. Grain boundary engineering, by thermomechanical treatments, has been used to modify microstructural features, such as grain size, grain boundary character and TJ density distributions, studied using electron backscatter diffraction. A progressive shift to higher ?special? TJ compositions, in form of participating low energy grain boundaries, is achieved. The new model successfully predicts the effect of grain boundary engineering on the maximum crack length.

Geometric Approach to Hole Segmentation and Hole Closing in 3D Volumetric Objects

Hole segmentation (or hole filling) and hole closing in 3D volumetric objects, visualised in tomographic images, has many potential applications in material science and medicine. On the other hand there is no algorithm for hole segmentation in 3D volumetric objects as from the topological point of view a hole is not a 3D set. Therefore in the paper the authors present a new, geometrical approach to hole closing and hole filling in volumetric objects. Moreover an original and efficient, flexible algorithm of hole filling for volumetric objects is presented. The algorithm has been extensively tested on various types of 3D images. Some results of the algorithm application in material science for crack propagation analysis are also presented. The paper also includes discussion of the obtained results and the algorithm properties.

Grain boundary control for improved intergranular stress corrosion cracking resistance in austenitic stainless steels: new approach

Abstract The present paper provides an overview of a new approach which has focused on the behaviour of special grain boundaries in sensitised austenitic stainless steel. The aim of the work was to develop a general model for stress corrosion cracking, which would ultimately be capable of predicting the effects of the degree of sensitisation, the connectivity of special boundaries and the influence of stress gradients, such as those developed from surface preparation (machining or peening) or due to the stress concentration effect of pit formation. Experimental work using electron backscatter diffraction analysis and in situ high resolution computed X-ray tomography has correlated cracking with the microstructure in a type 304 austenitic stainless steel. In situ three-dimensional observations demonstrated that annealing twins cause local crack arrest and diversion, leaving non-fractured ligaments in the wake of the cracking path. The mechanical effects of the deformation and failure of these bridges have been modelled, demonstrating that special grain boundaries cause crack tip shielding. Increasing the fraction of special boundaries and decreasing grain size are both predicted to increase stress corrosion cracking resistance. Experimental observations using room temperature intergranular stress corrosion tests and high temperature autoclave tests confirm these predictions for thermomechanically processed microstructures. The effects of applied stress and stress gradients are also predicted by the model, which may be extended to include the kinetics of crack growth, clustering of grain boundary types and variation of the degree of sensitisation.