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The main aim of the study was to analyse the strain rate sensitivity of the compressive deformation response in bulk 3D-printed samples from 316L stainless steel according to the printing orientation. The laser powder bed fusion (LPBF) method of metal additive manufacturing was utilised for the production of the samples with three different printing orientations: 0∘, 45∘, and 90∘. The specimens were experimentally investigated during uni-axial quasi-static and dynamic loading. A split Hopkinson pressure bar (SHPB) apparatus was used for the dynamic experiments. The experiments were observed using a high-resolution (quasi-static loading) or a high-speed visible-light camera and a high-speed thermographic camera (dynamic loading) to allow for the quantitative and qualitative analysis of the deformation processes. Digital image correlation (DIC) software was used for the evaluation of displacement fields. To assess the deformation behaviour of the 3D-printed bulk samples and strain rate related properties, an analysis of the true stress–true strain diagrams from quasi-static and dynamic experiments as well as the thermograms captured during the dynamic loading was performed. The results revealed a strong strain rate effect on the mechanical response of the investigated material. Furthermore, a dependency of the strain-rate sensitivity on the printing orientation was identified.

Presented paper deals with experimental study on compressive properties of auxetics with controlled stiffness of strut joints. The variable strut joints properties were simulated by adding extra amount of material in the struts’ intersection regions. Four groups of inverted honeycomb structures were prepared by multi-jet 3D printing and tested in quasi-static compression. The structure collapsed gradually, however after the first collapse, failure in entire cross-section occurred due to the brittle nature of the base material. The behavior up to the first collapse was consistent among the specimens within each group, while differed slightly subsequently. With higher reinforcement in the joints, results showed increasing stress at the first collapse (ultimate compressive stress) while the strain at the first collapse remained unchanged. The auxetic behaviour became less significant with increasing joints’ reinforcement.

Light-weight cellular solids, such as aluminium foams, are promising materials for use in ballistic impact mitigation applications for their high specific deformation energy absorption capabilities. In this study, three different types of aluminium alloy based in-house fabricated cellular materials were subjected to dynamic penetration testing using an in-house experimental setup to evaluate their deformation and microstructural response. A two-sided direct impact Hopkinson bar apparatus instrumented with two high-speed cameras observing the impact area and the penetrated surface of the specimens was used. An advanced wave separation technique was employed to process the strain-gauge signals recorded during the penetration. The images captured by one of the cameras were processed using an in-house Digital Image Correlation method with sub-pixel precision, that enabled the validation of the wave separation results of the strain-gauge signals. The second camera was used to observe the penetration into the tested specimens for the correct interpretation of the measured signals with respect to the derived mechanical and the microstructural properties at the different impact velocities. A differential X-ray computed tomography of the selected specimens was performed, which allowed for an advanced pre- and post-impact volumetric analysis. The results of the performed experiments and elaborate analysis of the measured experimental data are shown in this study.

Direct impact testing with a Hopkinson bar is, nowadays, a very popular experimental technique for investigating the behavior of cellular materials, e.g., lattice metamaterials, at high strain-rates as it overcomes several limitations of the conventional Split Hopkinson Pressure Bar (SHPB). However, standard direct impact Hopkinson bars (DIHB) have only single-sided instrumentation complicating the analysis. In this paper, a DIHB apparatus instrumented with conventional strain-gauges on both bars (a so called Open Hopkinson Pressure Bar - OHPB) is used for dynamic impact experiments of cellular materials. Digital image correlation (DIC) is used as a tool for investigating the displacements and velocities at the faces of the bars. A straight-forward wave separation technique combining the data from the strain-gauges with the DIC is adopted to increase the experiment time window multiple times. The experimental method is successfully tested at impact velocities in a range of 5 − 30 m · s − 1 with both linear elastic and visco-elastic bars of a medium diameter. It is shown that, under certain circumstances, a simple linear elastic model is sufficient for the evaluation of the measurements with the visco-elastic bars, while no additional attenuation and phase-shift corrections are necessary. The applicability of the experimental method is demonstrated on various experiments with conventional metal foams, hybrid foams, and additively manufactured auxetic lattices subjected to dynamic compression.

This study is focused on detection and characterisation of influenced zones in micro-scale specimens of aluminium foam after thermal and mechanical loading induced by preparation process for three-point bending test. Two cell-wall specimens were prepared from a slab of aluminium foam and influences of preparation process (machining) and thermal load on local mechanical properties were investigated using nanoindentation. Although the nanoindentation is powerful method for investigation of material properties of small zones, it can be reliably used only to obtain information about elastic properties. Due to limitation of the nanoindentation for reliable measurement of inelastic properties, plastic properties were determined using a set of indirect finite element simulations of nanoindentation tests. The procedure is based on fitting numerical results to experimentally measured force-depth curves.

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.

This paper is focused on determination of micro-mechanical material properties of aluminium metal foam at level of single cell wall. For the purpose of homogenization, complex and heterogenous inner structure of metal foams requires deep understanding of its deformation mechanisms and local mechanical properties. Therefore, a series of micro-scale bending tests of specimens prepared from foam's cells wall were performed. Initially, a technique of specimen preparation was developed. Dimensions and geometry of the specimens were inspected using scanning electron microscopy (SEM). Custom-designed three-point bending device suitable for testing at micro-scale was used for loading. Thin pure aluminium metal sheets were used for calibration of the described method. Six single cell wall specimens were tested. It can be concluded that the developed specimen preparation technique and loading method are suitable to determine local mechanical properties of thin walled metal foams.

A micro-radiographic system composed of microfocus X-ray tube and a large flat panel detector has been adapted for imaging complicated internal microstructure of trabecular bone under applied deformation. To capture the deforming microstructure a load was applied in small increments while the sample was tomographically scanned. Reconstruction of the internal structure is provided using backprojection algorithm for equiangular cone-beam projection data. From the reconstructed cross-sections a finite element (FE) model of the microstructure was developed and loaded according to the experiment. Deformation behavior of the FE model was compared to the experimentally determined response of the sample.

New developments in the fields of X-ray detectors, X-ray micro/nano sources and data processing bring new possibilities for engineering practice in many applications. Nowadays it is possible to observe an object, which was difficult to image before due to limitations such as low contrast, low dynamic range, low attenuation and/or small structural dimensions. A number of compelling problems in different applications are summarized. Digital radiograms in this work are acquired using micro- and nano-focus X-ray tubes together with semiconductor digital pixelated detectors. A beam hardening correction was done for relevant radiograms and an iterative OSEM (ordered subsets) method was applied for CT reconstructions.

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.

In this paper, a split Hopkinson pressure bar (SHPB) was used for impact loading of an auxetic lattice (structure with negative Poisson’s ratio) at a given strain-rate. High strength aluminum and polymethyl methacrylate bars instrumented with foil strain-gauges were used for compression of an additively manufactured missing-rib auxetic lattice. All experiments were observed using a high-speed camera with frame-rate set to approx. 135.000 fps. High-speed images were synchronized with the strain-gauge records. Dynamic equilibrium in the specimen was analyzed and optimized pulse-shaping was introduced in the selected experiments. Longitudinal and lateral in-plane displacements and strains were evaluated using digital image correlation (DIC) technique. DIC results were compared with results obtained from strain-gauges and were found to be in good agreement. Using DIC, it was possible to analyze in-plane strain distribution in the specimens and to evaluate strain dependent Poisson’s ratio of the auxetic structure.

In this paper, impact testing of auxetic structures filled with strain rate sensitive material is presented. Two dimensional missing rib, 2D re-entrant honeycomb, and 3D re-entrant honeycomb lattices are investigated. Structures are divided into three groups according to type of filling: no filling, low expansion polyurethane foam, and ordnance gelatine. Samples from each group are tested under quasi-static loading and dynamic compression using Split Hopkinson Pressure Bar. Digital image correlation is used for assessment of in-plane displacement and strain fields. Ratios between quasi-static and dynamic results for plateau stresses and specific energy absorption in the plateau are calculated. It is found out that not only the manufactured structures, but also the wrought material exhibit strain rate dependent properties. Evaluation of influence of filling on mechanical properties shows that polyurethane increases specific absorbed energy by a factor of 1.05–1.4, whereas the effect of gelatine leads to increase of only 5–10%. Analysis of the Poisson's function reveals influence of filling on achievable (negative) values of Poisson's ratio, when compared to unfilled specimens. The results for the Poisson's function yielded apparently different values as the assessed minima of quasi-static Poisson's ratio in small deformations are constrained by a factor of 15.

Vertebral body defects represent one of the most common orthopedic challenges. In order to advance the transfer of stem cell therapies into orthopedic clinical practice, we performed this study to evaluate the safety and efficacy of a composite bioartificial graft based on a hydroxyapatite bone scaffold (CEM-OSTETIC(®)) combined with human mesenchymal stem cells (MSCs) in a rat model of vertebral body defects.Under general isoflurane anesthesia, a defect in the body of the L2 vertebra was prepared and left to heal spontaneously (group 1), implanted with scaffold material alone (group 2), or implanted with a scaffold together with 0.5 million MSCs (group 3) or 5 million MSCs (group 4). The rats were killed 8 weeks after surgery. Histological and histomorphometrical evaluation of the implant as well as micro-CT imaging of the vertebrae were performed.We observed a significant effect on the formation of new bone tissue in the defect in group 4 when compared to the other groups and a reduced inflammatory reaction in both groups receiving a scaffold and MSCs. We did not detect any substantial pathological changes or tumor formation after graft implantation.MSCs in combination with a hydroxyapatite scaffold improved the repair of a model bone defect and might represent a safe and effective alternative in the treatment of vertebral bone defects.

There are generally two types of bone tissue: trabecular and cortical. Cortical bone is the more dense tissue found on the surface of bones and trabecular bone is a highly porous structure that fills the proximal and distal ends of all long bones (e.g. femur or tibia) and is also present as a filler in other bones (e.g. in vertebral bodies). While at themolecular level both the cortical and trabecular bone are made of the same constituents, their overall mechanical properties are quite different.

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.

The paper describes hybrid approaches to testing of mechanical properties of historic materials which are mostly available in very small amounts and bulks unsuitable for standard testing procedures and techniques. Three case studies are presented. In all three, simple mechanical testing procedures were combined with special optical measurements and tailored software tools.

Development and validation of detailed 3-D finite element model of human skull used for explicit dynamic simulation of impact conditions is presented in the paper. The FE model is based on the series of computer tomography scans of resolution 512×512 pixels taken in 1mm slices. Fully automated direct generation of the volumetric tetrahedral mesh based on the Marching Cubes Algorithm, Laplacian smoothing and Delaunay tetrahedralisation is used to develop the geometry of both the human skull and the brain.

This paper is focused on investigation of mechanical properties of rat vertebrae during compressive loading in the longitudinal direction of rat's spine. High-resolution time-lapse micro-tomography was used as a tool to create models of the inner structure and deformed shape in pre-defined deformation steps. First, peripheral areas of vertebra specimen were embedded in polymethyl methacrylate to obtain proper boundary conditions of contact between specimen and loading plattens. Experimental loading device designed for application in X-ray setups was utilized to compress the vertebrae in several deformation steps. High-resolution micro-tomography scanning was carried out at each deformation step. Specimen was irradiated in tomography device equipped with microfocus X-ray tube with 5μm focal spot size and large area flat panel detector. Spatial resolution of reconstructed three-dimensional images was approximately 10μm. Digital volume correlation algorithm was utilized in order to assess displacements in the microstructure in every loading increment. Finite element model of vertebra was created from volumetric data reconstructed from tomography of the undeformed specimen. Simulated compressive test of the developed finite element model was performed in order to compare stiffness and displacements obtained by digital volume correlation and finite element simulation.

In order to ensure sustainability if historic buildings their technical state has to be inspected on regular basis. Damage assessment has to be preferably carried out using non-destructive methods otherwise damage accumulation may occur during life-cycle of the constructions. According to character of detected damage appropriate intervention measures (i.e. strengthening, consolidation, etc.) have to be then efficiently applied. Among other factors significantly influencing life span of constructions weathering agents (rain, erosion, dissolution, etc.) may cause rapid degradation of mechanical properties. In this paper X-ray radiograhical imaging was used to describe fluid penetration process in porous Maastricht limestone that is commonly used for restoration purposes. The imaging was performed in custom radiography device simulating practical in-situ measurements using microtube device. This device is a modified Karsten tube capable of determining absorbed volume and its speed even on inclined surfaces. However actual fluid penetration process in terms of saturation depth/volume ratio and shape of fluid wave propagating through microstructure is indeterminable using microtube. For this purpose real-time radiography imaging of fluid saturation process was performed to investigate behaviour of fluid in the material. Furthermore X-ray computed microtomography was performed to develop finite element model for simulation of fluid flow in the porous microstructure. Using the real-time imaging relations between penetration speed, penetration depth and penetrated volume were assessed. These results can be used to validate results from microtube measurements including nonlinear regions present when semi-spherical wave propagates through the material. Using a set of finite element simulations of the microtube experiment fluid velocity distribution in the material together with effective Darcy's flux were calculated and results were compared to those from real-time imaging.

This study presents an improved radiographic method for strain measurement in very small samples of a single trabeculae. X-ray micro-radiography was used to track the deformation behaviour of individual trabecula during mechanical loading. As the X-ray micro-radiography images of a single trabecula show no significant features applicable for digital image correlation (DIC) a random pattern of markers was created on the surfaces of the samples to improve the accuracy of tracking. Metal plated borosilicate glassmicro-spheres (mean diameter 10 μm) were used as the markers for trabecular displacement tracking. Two different X-ray imaging setups were used for this purpose. The specimens of isolated trabeculae were loaded by a micro-mechanical testing device developed with respect to radiographical observation. This compact device enables a high precision three-point bending measurement. The specimen was continuously irradiated during the loading procedure by a micro-focus X-ray source. The radiographs were acquired by a single-photon counting silicon pixel detector and s flat panel sensor with CsI flipped scintillator plate. Circular Hough transform was used to locate positions of the spherical markers in the sequence of acquired radiographs and to calculate the strain in the loaded sample. The gold-coated micro-spheres provide clearly visible features in the sequence of radiographs after beam hardening correction, which in conjunction with pattern recognition algorithm enables to substantially improve the accuracy of strain measurements.

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 foam's 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 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 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.

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.

Digital volumetric correlation (DVC) method can be employed for evaluation of full-volume displacement and strain fields in deformed material with clearly recognizable microstructure. This microstructure can be defined by complex inner geometrical topography, as trabecular bone and metal foam have for instance, or/and by significant variations of chemical composition, as aluminum grainy alloys have for instance. Volumetric image data were acquired using time lapse X-ray micro-CT during gradually specimen loading. Tetrahedral FE mesh was generated describing related volumetric data. The displacement fields were measured in set of control points by DVC utilizing vertices of this tetrahedral FE mesh. The advantage of the approach is that the resulting strain can be directly compared with FE simulation of the experiment. Problems of accuracy, especially in evaluation of the strain field from the noisy measured displacements and problems of computational efficiency of the method are discussed.

Stem cells biology is one of the most frequent topic of physiological research of today. Spinal fusion represents common bone biology challenge. It is the indicator of osteoinduction and new bone formation on ectopic model. The purpose of this study was to establish a simple model of spinal fusion based on a rat model including verification of the possible use of titanium microplates with hydroxyapatite scaffold combined with human bone marrow-derived mesenchymal stem cells (MSCs). Spinous processes of two adjacent vertebrae were fixed in 15 Wistar rats. The space between bony vertebral arches and spinous processes was either filled with augmentation material only and covered with a resorbable collagen membrane (Group 1), or filled with augmentation material loaded with 5 × 106 MSCs and covered with a resorbable collagen membrane (Group 2). The rats were sacrificed 8 weeks after the surgery. Histology, histomorphometry and micro-CT were performed. The new model of interspinous fusion was safe, easy, inexpensive, with zero mortality. We did not detect any substantial pathological changes or tumor formation after graft implantation. We observed a nonsignificant effect on the formation of new bone tissue between Group 1 and Group 2. In the group with MSCs (Group 2) we described minor inflamatory response which indicates the imunomodulational and antiinflamatory role of MSCs. In conclusion, this new model proved to be easy to use in small animals like rats.