G. Peix

Observation of microstructure and damage in materials by phase sensitive radiography and tomography

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.

Reduction of porosity content generated during Nd:YAG laser welding of A356 and AA5083 aluminium alloys

Abstract Porosity formation is greatly influenced in aluminium alloys by the low vaporisation point element (Mg, Zn) content, or by process instability such as key-hole closures that tend to entrap occluded gases during welding. Another important contribution comes from the hydrogen content, because of a very high solubility in molten aluminium that favours microporosity generation. In this paper, cw YAG laser welds on two aluminium alloys were carried out: a AA5083-O wrought alloy with a high Mg content (4.5%) and a A356 cast alloy with 7% Si and a cast oxide layer. The porosity content in laser beads was extensively studied, with the use of different experimental method (X-ray radiography+image analysis, tomography), in order to check the influence of mechanical surface preparation as well as process parameters (single or dual spot, different welding speeds). It was concluded that surface preparation as well as dual beam welding are adequate methods for reducing porosity formation tendency in laser assemblies.

EFFECTIVE ATOMIC NUMBER IN THE RAYLEIGH TO COMPTON SCATTERING RATIO

Abstract Detection and counting X-ray photons scattered by the Rayleigh and Compton processes enable matter to be characterized locally. A theoretical relation was first established which simulates the result of a Rayleigh to Compton ratio measurement. It can thus be shown that a correct choice of scattering angle and photon energy enables a result to be obtained which is almost independent of X-ray attenuation inside the sample. With this condition, the Rayleigh to Compton scattering ratio depends only on the mixture under study and provides a local measurement of certain complicated functions of the atomic number Z and of the weight percentage of the different elements which constitute the compound. This function is usually called the “effective atomic number”, Zeff. Different methods of calculation of Zeff are found in the literature, four of them, those used most frequently, were tested. There is no unique relation between the computed Zeff and 80 experimental results performed on aqueous solutions with different concentrations of eight elements, having Z values ranging from 13 to 64. This observation led us to the conclusion that any effective atomic number is valid only for given experimental conditions. Finally, a new method of calculating Zeff was developed for the Rayleigh to Compton scattering ratio, which is applicable for any material, scattering angle or photon energy.

High-resolution computed tomography for architectural characterization of human lumbar cancellous bone: relationships with histomorphometry and biomechanics.

Abstract: The aim of the present study on human vertebral cancellous bone was to validate structural parameters measured with high-resolution (150 μm) computed tomography (HRCT) by referring to histomorphometry and to try to predict mechanical properties of bone using HRCT. Two adjacent vertical cores were removed from the central part of human L2 vertebral body taken after necropsy in 22 subjects aged 47–95 years (10 women, 12 men; mean age 79 ± 14 years). The right core was used for structural analysis performed by both HRCT and histomorphometry. Two cancellous bone specimens were extracted from the left core: a cube for HRCT and a compression test, and a cylinder for a shear test. Significant correlations were found between HRCT and histomorphometric measurements (BV/TV, trabecular thickness, separation and number, and node-strut analysis), but with higher values for most of the tomographic parameters (BV/TV and trabecular thickness determined by HRCT were overestimated by a factor 3.5 and 2.5 respectively, as compared with histomorphometry). The maximum compressive strength and Young’s modulus were highly correlated (ρ= 0.99, p<0.0005). Significant correlation was obtained between bone mineral density (determined using dual-energy X-ray absorptiometry) and the maximum compressive strength (ρ= 0.64, p= 0.002). In addition the maximum compressive strength and architectural parameters determined by HRCT or histomorphometry showed significant correlations (e.g., for HRCT, BV/TV: ρ = 0.88, p<0.0005, N.Nd/TV: ρ= 0.73, p<0.001). The shear strength was significantly correlated with BV/TV (ρ= 0.62, p= 0.002), Tb.Sp (ρ=−0.58, p= 0.004) and TSL (ρ= 0.55, p= 0.006) measured by HRCT. In conclusion, an HRCT system with 150 μm resolution is not sufficient to predict the true values of the structural parameters measured by histomorphometry, although high correlations were found between the two methods. However, we showed that a resolution of 150 μm allowed us to predict the mechanical properties of human cancellous bone. In vivo peripheral systems with such a resolution should be of interest and would deliver an acceptable radiation dose to the patient.

Mechanical Properties of Ewe Vertebral Cancellous Bone Compared With Histomorphometry and High-Resolution Computed Tomography Parameters

The goal of the present study was to determine if a high-resolution computed tomography (HRCT) system with 150 microns resolution was sufficient to predict mechanical properties in ewe lumbar vertebrae. To answer this question, we used a triangular comparison between: HRCT; biomechanics (compression and shear tests); and histomorphometry, which was the reference method for the measurements of morphometric parameters. Two dissected lumbar vertebrae (L-4 and L-5) from 32 ewes were used. Both compressive and shear properties correlated significantly with amount of bone and structural parameters evaluated by histomorphometry (bone volume/tissue volume, trabecular thickness, trabecular separation), but no significant correlation was found with the trabecular number. With our shear test involving the trabecular architecture itself more significant correlations were found with the node-strut analysis parameters than from the compressive test. Significant correlations were also found between HRCT and histological parameters (bone volume/tissue volume, bone surface/bone volume, trabecular separation, trabecular number, total strut length, number of nodes, and number of termini). Correlations between HRCT structural parameters and mechanical properties on L-4 were of the same magnitude as the correlations between the histomorphometric structural parameters and mechanical results on L-5 but with the remarkable advantage the HRCT is a noninvasive method. In spite of the resolution (150 microns) of our HRCT system, which entailed mainly an enlargement of the thinnest trabeculae or their loss during the segmentation process, we obtained coherent relationships between mechanical and tomographic parameters. The thinnest trabeculae probably had little effect on the mechanical strength. Also, this type of resolution allows us to consider the possibility of perfecting an in vivo HRCT system. However, physical density and bone mineral density correlated much better with strength than either classical histomorphometric or tomographic parameters. The current conclusion is fairly negative with respect to the ability of HRCT to assess mechanical properties nondestructively as compared with dual-energy X-ray absorptiometry. But, the noninvasive nature of the imaging modality and the capacity for three-dimensional imaging at arbitrary orientation make HRCT a promising tool in the quantitative assessment of cancellous architecture.

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.

X-ray Compton backscattering techniques for process tomography: imaging and characterization of materials

Non-destructive evaluation by Compton scattering using an industrial x-ray tube allows three-dimensional (3D) imaging of materials. The x-ray tube and the detector are set on the same side of the object. Thus, non-destructive evaluation of the wall of a tank, even when it is full, is possible without the requirement for the x-ray beam to cross the whole object. Several applications were tried in our laboratory. Besides 3D imaging, a method allowing thickness measurement of a wall was developed, which was especially suitable for multilayer compounds. The accuracy is mm. Compton scattering techniques also allow point-by-point density measurements in the near-surface zone of any component (even dense and bulky ones). An accuracy of 1% was achieved for light composite materials and also for dense components () provided by powder metallurgy. A new application allows us to perform 3D imaging using a linear accelerator (6 MeV) as the photon source. Thus, testing can be performed inside a tank, even through a thick and dense wall (8 mm of steel).

In-line density measurement system using X-ray Compton scattering

Abstract A density measurement system based on X-ray Compton scattering has been constructed. Intended to be installed on an industrial production line, this computer-controlled system measures automatically the local density (within a few mm 3 ) of the middle products of powder metallurgy (green state). The measurement is nondestructive and is made from a single side of the sample. An industrial X-ray tube and a multihole collimator allow a measurement time of 40 s with an accuracy of better than 1%, for products obtained after compression of iron powder and whose density ranges from 6 to 7.3 g cm −3 .

Quantitative microtomography: measurement of density distribution in glass wool and local evolution during a one-dimensional compressive load

This paper proposes two applications of quantitative tomography. Assuming that the attenuation ratio of the x-ray beam throughout the investigated sample can be evaluated with high accuracy, this paper deals with quantitative characterization of glass wool structure. In a first part, we measure the 3D spatial distribution of density in a sample that has been extracted from a glass wool plate. The calibration is performed and an experimental validation of the technique is realized. The heterogeneity of the material is characterized and explained in relation to manufacturing methods. In a second step, we follow the evolution of this distribution during a mechanical test. For this purpose, an original device has been developed in the laboratory in order to perform a tomographic process during a mechanical load. The proposed method compares the density distribution between two different compression steps and determines the evolution of density inside the sample. The local strain is evaluated along the stress direction. These first results allow assumptions about the relationships between structure and properties to be expressed.

A Comparison of the Ball, Wire, Edge, and Bar/Space Pattern Techniques for Modulation Transfer Function Measurements of Linear X-Ray Detectors

Actual assessment of the modulation transfer function (MTF) of a physical system requires objective experimental data to be obtained. This paper presents four practical methods for measuring the MTF of a linear x-ray detector. These methods are based on using the ball, wire, edge, and bar/space pattern as stimuli. The measurement procedure and experimental conditions are described in detail. The MTF results obtained with these different techniques are analyzed and compared.