Daniel Kytýř

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.

High resolution X-ray imaging of bone-implant interface by large area flat-panel detector

The aim of the research was to investigate the cemented bone-implant interface be- havior (cement layer degradation and bone-cement interface debonding) with emphasis on imaging techniques suitable to detect the early defects in the cement layer. To simulate in vivo conditions a human pelvic bone was implanted with polyurethane acetabular cup using commercial acrylic bone cement. The implanted cup was then loaded in a custom hip simulator to initiate fatigue crack prop- agation in the bone cement. The pelvic bone was then repetitively scanned in a micro-tomography device. Reconstructed tomography images showed failure processes that occurred in the cement layer during the first 250 , 000 cycles. A failure in cemented acetabular implant — debonding, crumbling and smeared cracks — has been found to be at the bone-cement interface. Use of micro-focus source and high resolution flat panel detector of large physical dimensions allowed to r econstruct the micro-structural models suitable for investigation of migration, micro-motions and consecutive loosening of the implant. The large area flat panel detector with physical dimensions 1 20 × 120 mm with 50μm pixel size provided a superior image quality compared to clinical CT systems with 300 − 150 μm pixel size.

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.

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.

Numerical modelling of the reinforcing effect of geosynthetic material used in ballasted railway tracks

This article deals with finite-element (FE) modelling of the reinforcing effect of the geosynthetic material used in the construction of a ballasted track. Various different designs of geosynthetic material are studied and their reinforcing effects are evaluated in terms of the total settlement reduction. Three-dimensional FE models of the reinforced railway superstructure are compared to a reference FE model with no reinforcement. Each geosynthetic material is modelled respecting its material properties, and the interaction with the ballast material is simulated according to its primary function. A clear distinction in the modelling of the interaction with the ballast material is made between geogrids and the remaining geosynthetics. A new approach to model the reinforcing effect of a geogrid is proposed and evaluated by FE analysis. The results of numerical modelling are compared to those of experiments conducted using an experimental box instrumented with one half of an instrumented concrete sleeper. Different types of geosynthetics were used to reinforce the ballast material, and the settlement reduction was measured using linear variable differential transformer (LVDT) sensors. The proposed FE models enable quick evaluation of the reinforcing effect of a given geosynthetic and comparison to other possible solutions in terms of the total settlement reduction. Other design possibilities (e.g. the use of reinforcing geosynthetics in several layers) are briefly discussed, too.

Inspection of Post Impact Fatigue Damage in Carbon Fibre Composite Using Modulus Mapping Technique

This study is focused on inspection of damage extent induced into C/PPS composite material by fatigue and impact loading. Initial damage to specimens was induced by drop-weight out-of-plane impact damage. Several levels of damage states (intact specimen, fatigued and impacted specimen, ruptured specimen) were inspected using modulus mapping (MM) technique. Quantification of the damage level was based on comparison of results from MM obtained in distinct locations on the specimens. Regions of interest were selected in order to determine magnitude of damage after impact and to assess remaining loading capabilities of the material. For this purpose, material maps provided information about location where matrix had been inflicted by the damage. Results show that impact loading has no measurable influence on mechanical properties of the matrix. However, gradient in mechanical properties was detected in the vicinity of crack. Results were validated using quasi-static nanoindentation and constant strain rate continuous measurement that showed depth profile of mechanical properties.

Instrumentation for Micromechanics Research in Trabecular Bone

This article deals with description of instrumentation required for micromechanical testing of isolated trabeculae, the basic structural elements of cancellous bone. Process of tensile and bending tests is described in terms of development of the testing devices, challenges connected with the micro-scale testing and precision of the measurements. The paper covers the whole testing procedure from tissue harvesting, sample preparation, experimental procedure and data evaluation. Inverse finite element identification of elasto-plastic material model with damage based on threepoint bending for simulation of deformation behaviour of trabecular bone is shortly discussed as well.

Novel Device for 4-Point Flexural Testing of Quasi-Brittle Materials During 4D Computed Tomography

This paper deals with development and validation of a novel device for investigation of the crack behavior in quasi-brittle materials using radiographically observed flexural testing. Instead of standard horizontal arrangement of three-point bending devices, a novel approach consisting in a vertically oriented four-point bending setup is proposed. In the paper, technical description of the proposed device together with its advantages over existing methods and results of validation experiments with natural rocks are presented. The validation experiments were concentrated on the ability of the device to capture the fracture behavior of the samples using X-ray transmission radiography and 4D X-ray micro-tomography. Particular attention was paid to evaluate the ability to perform tomographic scans during post-peak softening, i.e. on intermittent loading of samples after formation of the crack. The acquired results showed very good performance in terms of both the mechanical characteristics of the device (stiffness and loading precision) and the X-ray imaging properties.

UTILIZATION OF IMAGE AND SIGNAL PROCESSING TECHNIQUES FOR ASSESSMENT OF BUILT HERITAGE CONDITION

Historical buildings represent invaluable heritage from the past and therefore their protection is a very important task. Assessment of their condition must not cause damage accumulation, thus the least possible volume removed from the structure is essential. As many historical buildings in the Czech Republic are built using sandstone that can be considered as a typical heterogeneous system, statistical signal processing is a promising approach for determination of the representative volume element (RVE) dimensions. Such calculations can be carried out on the domain of logical arrays representing binary images of the materials microstructure. This paper deals with processing of image data obtained using SEM-BSE and high resolution flatbed scanner for determination of RVE dimensions. Advanced image processing techniques are employed and results from calculation using grayscale data are presented and compared with results calculated on the basis of color input images.

SEMI–AUTOMATED ASSESSMENT OF MICROMECHANICAL PROPERTIES OF THE METAL FOAMS ON THE CELL-WALL LEVEL

Metal foams are innovative porous material used for wide range of application such as deformation energy or sound absorption, filter material, or microbiological incubation carrier. To predict mechanical properties of the metal foam is necessary to precisely describe elasto–plastic properties of the foam on cell–wall level. Indentation with low load is suitable tool for this purpose. In this paper custom designed instrumented microindentation device was used for measurement of cell-wall characteristics of two different aluminium foams (ALPORAS and ALCORAS). To demonstrate the possibility of automated statistical estimation of measured characteristics the device had been enhanced by semi-automatic indent positioning and evaluation procedures based on user-defined grid. Vickers hardness was measured on two samples made from ALPORAS aluminium foam and one sample from ALCORAS aluminium foam. Average Vickers hardness of ALPORAS foam was 24.465HV1.019 and average Vickers hardness of ALCORAS was 36.585HV1.019.