I Jandejsek

Evaluation of strain field in microstructures using micro-CT and digital volume correlation

X-ray digital micro-tomography was employed for precise strain measurement which is essential for evaluation of the experiments with small samples of trabecular bone. X-ray Digital Volumetric Correlation (DVC) method was used to identify the three-dimensional strain field in loaded complex microstructure. DVC relies on tracking selected sample points within the three-dimensional image data throughout the sequence of captured projections. In this study an improved DVC method is applied for evaluation of the strain field in trabecular bone sample subjected to compressive loading. The deformed sample was tomographically scanned using micro-focus X-ray tube and the single-photon counting silicon pixel detector Medipix2.

X-ray and finite element analysis of deformation response of closed-cell metal foam subjected to compressive loading

Time-lapse X-ray computed microtomography was employed to quantify the deformation behaviour of closed-cell aluminium foam. The specimen was incrementally loaded and tomographically scanned using a custom X-ray tomographic device to capture the deforming microstructure. Because of the very small thickness of the cell walls and the high ratio between pore size and cell wall thickness cone-beam reconstruction procedure was applied. A finite element (FE) model was developed based on the reconstructed three-dimensional data. The FE model was used for two purposes: i) the nodal points were used for tracking the displacements of the deforming structure, ii) verification of the material model for description of the foams deformational behaviour. Digital volumetric correlation (DVC) algorithm was used on data obtained from the time-lapse tomography to provide a detailed description of the evolution of deformation in the complex structure of aluminium foam. The results from DVC demonstrate the possibility to use the complex microstructure of the aluminium foam as a random pattern for the correlation algorithm. The underlying FE model enables easy comparison between experimental results and results obtained from numerical simulations used for evaluation of proposed constitutive models.

X-ray inspection of composite materials for aircraft structures using detectors of Medipix type

This work presents an overview of promising X-ray imaging techniques employed for non-destructive defectoscopy inspections of composite materials intended for the Aircraft industry. The major emphasis is placed on non-tomographic imaging techniques which do not require demanding spatial and time measurement conditions. Imaging methods for defects visualisation, delamination detection and porosity measurement of various composite materials such as carbon fibre reinforced polymers and honeycomb sendwiches are proposed. We make use of the new large area WidePix X-ray imaging camera assembled from up to 100 edgeless Medipix type detectors which is highly suitable for this type of measurements.

Utilization of dual-source X-ray tomography for reduction of scanning time of wooden samples

We present a novel dual-source/dual energy (DSCT/DECT) micro-tomography system including results of high-resolution DSCT reconstruction. The DSCT micro-tomography setup was designed as a multi-purpose X-ray imaging device equipped with two pairs of X-ray tubes and detectors in orthogonal arrangement with independent control of beam parameters. Both pairs (tube-detector) are mounted on a computer numerical control positioning system and can be independently set up to different geometries (e.g. with different magnification of each pair). In this work the simultaneous scanning of the object by two tube-detector pairs was used for approximately half reduction of tomography scanning time. The developed imaging procedure was applied for scanning of a wooden sample locally damaged during a semi-destructive test for assessment of wood quality. Prior to the tomography measurements the setup geometry was precisely adjusted in terms of magnification, horizontal and vertical tube-specimen-detector alignment of both pairs. DSCT measurements were carried out in sequence (2 × 90° for each tube) with identical 100μm image resolution. It was proven that the presented experimental setup combined with appropriate control technique significantly reduces tomography scanning time of materials with complex micro-structure.

Real-time X-ray microradiographic imaging and image correlation for local strain mapping in single trabecula under mechanical load

X-ray microradiography was employed to quantify the strains in loaded human trabecula. Samples of isolated trabeculae from human proximal femur were extracted and glued in a loading machine specially designed and manufactured for testing small specimens. The samples were then tested in tension and three-point bending until complete fracture of the specimen occured. To assess the deformation in the very small samples (thickness 100?m, length 1?2mm) a real-time microradiography in conjunction with digital image correlation (DIC) has been employed. Loaded samples were irradiated continuously by X-rays (Hamamatsu L8601-01 with 5?m spot) during the test. Radiographs were acquired using 0.25s exposure time with hybrid single-photon counting silicon pixel detector Medipix2. The distance between the source and detector was kept small to ensure radiographs of good quality for such a short exposure time. Design of the experimental loading device enables for precise control of the applied displacement which is important for the post-yield behavior assessment of trabeculae. Large dynamic range, high sensitivity and high contrast of the Medipix2 enables measuring even very small strains with DIC. Tested experimental setup enables to combine micromechanical testing of the basic building block of trabecular bone with time-lapse X-ray radiography to measure the strains and to assess the mechanical properties of single human trabecula as well as to capture the softening curve with sufficient precision.

X-ray color imaging with 3D sensitive voxel detector

X-ray imaging is today widely used in a broad range of applications. Nevertheless some limitations are represented by the inability to distinguish between a thick layer of low Z material and a thin layer of high Z material, and by the beam hardening, where the incident X-ray spectrum is modified as the beam traverses the sample. Such effects cause problems in many applications (e.g. CT reconstruction) generating artifacts and worsening the spatial resolution. This work presents a new technique allowing spectral sensitivity using a new 3D voxel detector based on the Timepix pixel detector. The device is designed as a layered stack of several Timepix sensors. The readout chip is thinned down to reduce the amount of insensitive absorbing material. Every single layers in the stack act as a filter, i.e. each stack layer visualizes a different part of the spectrum attenuated by the object giving further information about the object composition. The comparison of attenuation levels observed in different detector layers can be used to estimate the extent of the beam hardening effect in the imaged object and thus point out differences in the material composition.

High resolution micro-CT of low attenuating organic materials using large area photon-counting detector

To overcome certain limitations of contemporary materials used for bone tissue engineering, such as inflammatory response after implantation, a whole new class of materials based on polysaccharide compounds is being developed. Here, nanoparticulate bioactive glass reinforced gelan-gum (GG-BAG) has recently been proposed for the production of bone scaffolds. This material offers promising biocompatibility properties, including bioactivity and biodegradability, with the possibility of producing scaffolds with directly controlled microgeometry. However, to utilize such a scaffold with application-optimized properties, large sets of complex numerical simulations using the real microgeometry of the material have to be carried out during the development process. Because the GG-BAG is a material with intrinsically very low attenuation to X-rays, its radiographical imaging, including tomographical scanning and reconstructions, with resolution required by numerical simulations might be a very challenging task. In this paper, we present a study on X-ray imaging of GG-BAG samples. High-resolution volumetric images of investigated specimens were generated on the basis of micro-CT measurements using a large area flat-panel detector and a large area photon-counting detector. The photon-counting detector was composed of a 010× 1 matrix of Timepix edgeless silicon pixelated detectors with tiling based on overlaying rows (i.e. assembled so that no gap is present between individual rows of detectors). We compare the results from both detectors with the scanning electron microscopy on selected slices in transversal plane. It has been shown that the photon counting detector can provide approx. 3× better resolution of the details in low-attenuating materials than the integrating flat panel detectors. We demonstrate that employment of a large area photon counting detector is a good choice for imaging of low attenuating materials with the resolution sufficient for numerical simulations.

Visualization of delamination in composite materials utilizing advanced X-ray imaging techniques

This work is focused on the development of instrumental radiographic methods for detection of delaminations in layered carbon fibre reinforced plastic composites used in the aerospace industry. The main limitation of current visualisation techniques is a very limited possibility to image so-called closed delaminations in which delaminated layers are in contact practically with no physical gap. In this contribution we report the development of innovative methods for closed delamination detection using an X-ray phase contrast technique for which the distance between delamination surfaces is not relevant. The approach is based on the energetic sensitivity of phase-enhanced radiography. Based on the applied methodology, we can distinguish both closed and open delamination. Further we have demonstrated the possibility to visualise open delaminations characterised by a physical gap between delaminated layers. This delamination type was successfully identified and visualized utilizing a high resolution and computed tomography table-top technique based on proper beam-hardening effect correction.

Verification of Numerical Model for Trabecular Tissue Using Compression Test and Time-Lapse X-Ray Radiography Based on Material Model Determined from Three-Point Bending Test of Single Trabecula

The aim of this study is to determine constitutive constants for elasto-plastic material model with damage for single trabecula based on the indirect simulation of micromechanical testing and its verification at macro level using compression test of the cylindrical sample of the trabecular tissue. Three-point bending test of isolated trabeculae was performed in a shielding box and deflection of the sample was acquired using X-ray microradiography. Measured values (displacements of markers) were used for indirect identification of the material model for single trabecula using finite element (FE) method. The bending test was simulated and results were fitted to experimentally obtained values and the appropriate set of material constants was determined. To verify the applicability of the identified material model the compression test of the complex sample was carried out. Cylindrical sample was incrementally loaded and each loading state was captured using the micro-computed tomography. Material model identified from three-point bending test was applied to the model of complex sample and the simulation of the compression test was performed.

High resolution radiography of ambers with pixel detectors

Radiography serves as a powerful non-destructive technique for studying inner structure of biological samples and materials. In the last years X-ray imaging has taken advantage of the developments in instrumentation such as table-top micro-focus X-ray tubes and quantum counting pixel detectors. The imaging setups used for our measurements allow for the observation of tiny samples including fossils in amber. The main goal of the study was to apply microradiography as representative of non-destructive and non-invasive methods for imaging fossils in amber. Those fossils are generally not easy to visualize, especially their internal structures. We investigated a combination of sources and detectors: (a) an X-ray unit for mammography with tungsten anode, emissive spot of 100 μm and an amorphous selenium imager; (b) a micro-focus X-ray tube with tungsten anode, emissive spot of 5 μm and a flat panel imager; (c) a nano-focus X-ray tube with tungsten anode, with gauge of emissive spot of 1 μm and as imager the pixel semiconductor detector Medipix2. The study of fossils in amber can be for example not well visible because of the presence of organic detritus from various sources. The amber preserves various ancient biological objects which are fully or partly saturated with amber resin. These samples attenuate X-rays similarly, but the use of pixel detectors enables capturing these differences, without permanent destruction of the samples (cracking, slicing, etc.). Microradiographic studies are completed by the observation of amber fossils in scanning and transmission electron microscopes.