Petr Koudelka

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.

Dual-energy X-ray micro-CT imaging of hybrid Ni/Al open-cell foam

In this paper, we employ dual-energy X-ray microfocus tomography (DECT) measurement to develop high-resolution finite element (FE) models that can be used for the numerical assessment of the deformation behaviour of hybrid Ni/Al foam subjected to both quasi-static and dynamic compressive loading. Cubic samples of hybrid Ni/Al open-cell foam with an edge length of [15]mm were investigated by the DECT measurement. The material was prepared using AlSi7Mg0.3 aluminium foam with a mean pore size of [0.85]mm, coated with nanocrystalline nickel (crystallite size of approx. [50]nm) to form a surface layer with a theoretical thickness of [0.075]mm. CT imaging was carried out using state-of-the-art DSCT/DECT X-ray scanner developed at Centre of Excellence Telc. The device consists of a modular orthogonal assembly of two tube-detector imaging pairs, with an independent geometry setting and shared rotational stage mounted on a complex 16-axis CNC positioning system to enable unprecedented measurement variability for highly-detailed tomographical measurements. A sample of the metal foam was simultaneously irradiated using an XWT-240-SE reflection type X-ray tube and an XWT-160-TCHR transmission type X-ray tube. An enhanced dual-source sampling strategy was used for data acquisition. X-ray images were taken using XRD1622 large area GOS scintillator flat panel detectors with an active area of [410 × 410]mm and resolution [2048 × 2048]pixels. Tomographic scanning was performed in 1,200 projections with a 0.3 degree angular step to improve the accuracy of the generated models due to the very complex microstructure and high attenuation of the investigated material. Reconstructed data was processed using a dual-energy algorithm, and was used for the development of a 3D model and voxel model of the foam. The selected parameters of the models were compared with nominal parameters of the actual foam and showed good correlation.

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.

DEFORMATION RESPONSE OF GELLAN GUM BASED BONE SCAFFOLD SUBJECTED TO UNIAXIAL QUASI-STATIC LOADING

This study is focuses on an investigation of the reinforcement effect of the bioactive glass nano-particles in the gellan gum (GG) scaffolds used in bone tissue engineering. The investigated material was synthesized as the porous spongy-like structure improved by the bioactive glass (BAG) nano-particles. Cylindrical samples were subjected to a uniaxial quasi-static loading in tension and compression. Very soft nature of the material, which makes the sample susceptible to damage, required employment of a custom designed experimental device for the mechanical testing. Moreover, as the mechanical properties are significantly influenced by testing conditions the experiment was performed using dry samples and also using samples immersed in the simulated body fluid. Material properties of the pure GG scaffold and the GG-BAG reinforced scaffold were derived from a set of tensile and compression tests under dry and simulated physiological conditions. The results are represented in the form of stress-strain curves calculated from the acquired force and displacement data.

Micromechanical Properties of Biocompatible Materials for Bone Tissue Engineering Produced by Direct 3D Printing

Bone implants in form of artificial scaffolds manufactured from poly-lactic acid (PLA) represent an attractive alternative to traditional surgical treatments of defective bones (i.e. autografts and allografts). In this work factors influencing biocompatibility and primary stability of implants manufactured from PLA using direct 3D printing were assessed using nanoindentation. For this reason bulk sample of the PLA material and a printed object were subjected to nanomechanical measurement. Quasi-static nanoindentation was employed to identify elastic modulus and hardness distribution on surface and within volume of the samples. Moreover mechanical properties along scanning direction and interlayer characteristics were also assessed. Gradients in mechanical properties have been identified within volume of the material, within the printing layers and at contact between individual layers.

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.

Identification of Stress-Strain Relation of Aluminium Foam Cell Wall by Spherical Nanoindentation

The aim of this paper is to identify, in addition to elastic properties, inelastic properties of tiny aluminium foam cell walls that can be directly deduced from the loaddepth curves of spherical indentation tests using formulations of the representative strain and stress. Constitutive parameters related to plastic material with linear isotropic hardening, the yield point (122 ± 17 MPa) and tangent modulus (950 ± 377 MPa), were obtained in this work. Spherical indentation and uniaxial tension experiments have also been performed on a standard aluminium alloy EN AW 6060 to explore the accuracy of the analytical models used to predict the uniaxial stressstrain in wide strain ranges. Some deviations received from different tests arose and, therefore, their effect on the evaluation of inelastic properties was discussed.

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.

Experiment E7/0,3 – Time Behaviour of Active Pressure of Non-Cohesive Sand after Wall Translative Motion

A new experiment denominated as E7/0,3 with active pressure of non-cohesive quartz sand on a rigid moved wall was performed at the Institute of Theoretical and Applied Mechanics in the last year. The wall was translative moved towards active direction (out of the mass) at a position of supposed acting of active pressure value, then the wall motion was stopped and time pressure stability was monitored. After more than three months the wall was moved at the last position of 100 mm from original position before the experiment. The experiment ran four and a half months.