J.-Y. Buffiere

Characterization of internal damage in a MMCp using X-ray synchrotron phase contrast microtomography

The initiation and development of damage inside a 6061 Al alloy reinforced with SiC particles has been studied during in situ mechanical tests using high resolution synchrotron X-ray tomography. The high coherence of the X-ray beam used improves the detection of reinforcements in the matrix as well as the detection of cracks. Qualitatively, the same damage mechanisms are observed at the surface and in the bulk of the sample, the rupture of the SiC particles being the dominant mechanism for the early stages of plastic deformation. Quantitatively, however, it is found that the geometrical characteristics of surface SiC particles differ from those of bulk particles and that the damage growth rate is larger inside the sample. This result can be understood in terms of elastic energy and normal stress levels in the SiC particles as calculated by finite element method (FEM) analysis.

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.

Study of the interaction of a short fatigue crack with grain boundaries in a cast Al alloy using X-ray microtomography

Abstract Synchrotron Radiation X-ray microtomography is used to visualize and analyze simultaneously the three-dimensional shape of crystallographic grains containing a short fatigue crack in a cast Al alloy. The visualization of the grains is based on the decoration of Al grain boundaries by liquid Ga which serves as a selective contrast agent. The intricate three-dimensional shape of the fatigue crack, as well as the crack stops observed on the sample surface, are correlated to the grain structure of the material. Complementary measurements of the grain orientation on the sample surface by electron backscattering diffraction (EBSD) allow us to discuss and interpret the observations in terms of possible crack propagation mechanisms.

Nanoscale zoom tomography with hard x rays using Kirkpatrick-Baez optics

To overcome the limitations in terms of spatial resolution and field of view of existing tomography techniques, a hard x-ray projection microscope is realized based on the sub-100-nm focus produced by Kirkpatrick-Baez optics. The sample is set at a small distance downstream of the focus and Fresnel diffraction patterns with variable magnification are recorded on a medium-resolution detector. While the approach requires a specific phase retrieval procedure and correction for mirror imperfections, it allows zooming nondestructively into bulky samples. Quantitative three-dimensional nanoscale microscopy is demonstrated on an aluminum alloy in local tomography mode.

X-ray microtomography studies of localised corrosion and transitions to stress corrosion cracking

Abstract Two forms of high resolution X-ray tomographic experiments (i.e. synchrotron based X-ray microtomography and desktop microfocus computed X-ray tomography) are demonstrated in the present paper to illustrate the wide application of these techniques for qualitative and quantitative studies of localised corrosion and environmentally assisted cracking. Specifically, synchrotron based X-ray tomography was used to investigate the localised corrosion morphology within aluminium specimens when exposed in situ to a chloride environment while microfocus computed X-ray tomography was used to investigate the morphology and quantify the transition from localised corrosion to stress corrosion cracking in steel specimens exposed ex situ to a simulated corrosive condensate environment.

Three-dimensional visualisation of fatigue cracks in metals using high resolution synchrotron X-ray micro-tomography

Abstract The present paper reviews recent developments in high resolution synchrotron X-ray micro-tomography for the study of fatigue cracks in metals. The possibilities and limitations of the tomography technique are described. A fatigue machine used for in situ cycling of samples is presented and three-dimensional images of the growth of fatigue cracks initiated in various Al alloys on natural or artificial defects are shown. Examples of quantitative use of such 3D images are given.

A synchrotron X-ray study of a Ti/SiCf composite during in situ straining

X-ray synchrotron radiography and high spatial resolution strain measurements have been combined to build-up a picture of the micromechanics of the damage that occurs, and the internal stress that causes it, within a Ti/SiC monofilament composite test-piece that was progressively strained in situ on the beam-line. The sample was designed to include a row of fibre ends within the test-piece. The radiographs show that fibre breakages occur during straining, even in the vicinity of the fibre ends. Axial and transverse strain measurements were made along each fibre in the composite as the load was progressively increased using a small X-ray spot size. In the present case interfacial sliding is believed to occur by slipping in or near the 4 μm carbon coating of the SCS-6 fibres. In order to interpret the strain measurements the predictions of a simple axi-symmetric finite element model of the composite are compared with the measured strains.

Sic single fibre full-fragmentation during straining in a Ti-6Al-4V matrix studied by synchrotron X-rays

High spatial resolution synchrotron X-ray strain measurements and radiography/tomography have been combined to study the progressive fragmentation process during a single fibre full-fragmentation test. In this manner it has been possible to observe the fragmentation sequence and to determine the interfacial shear stress as a function of position along the fibre. The SCS-6 fibre failed after it was elastically strained locally to 1.5%. The interfacial frictional shear strength inferred from the longitudinal strain profile of the broken fibre was calculated to be around 200 MPa, whereas a conventional post mortem full fragmentation analysis based on the mean fragment length would have incorrectly given a value of ∼700 MPa. This is due to the degradation of the fibre strength after the initial fibre fracture event. After failure, interface sliding was observed along considerable lengths of the fibre matrix interface. Upon unloading some reverse frictional sliding was found to take place. The morphology of the fibre cracks as well as the occurrence of matrix cracks as the sample approaches saturation fragmentation were also determined from radiographic and tomographic images.

Damage assessment in an Al/SiC composite during monotonic tensile tests using synchrotron X-ray microtomography

Abstract High resolution X-ray tomography is used to study the evolution of damage in an Al/SiC composite during monotonie tensile tests at room temperature. Two main damage mechanisms are observed when the plastic regime is reached: (i) The cracking of the matrix on brittle oxides resulting from the processing of the material; (ii) the cracking of SiC particles. The aspect ratio of the broken particles along the tensile direction is found to be high and the damage accumulation rate is different at the surface and in the bulk of the material. Those results are discussed with respect to the resolution of the imaging technique and to the strain level reached during the tests.

Crack initiation processes in acrylic bone cement

A major constraint in improving the understanding of the micromechanics of the fatigue failure process and, hence, in optimizing bone cement performance is found in the uncertainties associated with monitoring the evolution of the internal defects that are believed to dominate in vivo failure. The present study aimed to synthesize high resolution imaging with complementary damage monitoring/detection techniques. As a result, evidence of the chronology of failure has been obtained. The earliest stages of crack initiation have been captured and it is proposed that, in the presence of a pore, crack initiation may occur away from the pore due to the combined influence of pore morphology and the presence of defects within regions of stress concentration. Furthermore, experimental evidence shows that large agglomerations of BaSO(4) are subject to microcracking during fatigue, although in the majority of cases, these are not the primary cause of failure. It is proposed that cracks may then remain contained within the agglomerations because of the clamping effect of the matrix during volumetric shrinkage upon curing.