Microstructural features of a mortar as seen by computed microtomography

While low resolution limits the use of computed microtomography for microstructural investigation, the method has a unique advantage in that no specimen preparation is required; the images produced are thus entirely free of specimen preparation artifacts. In the present communication we report examination of the microstructure of a w:c 0.6 mortar, with the tomographic information resolved on a set of more than 500 successive planar images each 1.2 μm apart. Selected images are compared with backscatter SEM images taken from similar materials at comparable magnifications. The same features are observed, including sand grains, air voids, residual cement grains, hydrated cement paste components, and hollow-shell pores. The “patchy” microstructure of different areas of hardened cement paste, previously reported for mortars and concretes in backscatter SEM, is clearly also present in images derived from computed microtomography, and are not artifacts of SEM specimen preparation.     

Synchrotron X-ray computed microtomography investigation of a mortar affected by alkali–silica reaction: a quantitative characterization of its microstructural features

Alkali–silica reaction (ASR) is one of the most important weathering processes in cement-based materials. The damages caused by ASR have been qualitatively investigated with a number of different techniques. In this study, we present a procedure to obtain quantitative morphological parameters of the ASR reaction effects using synchrotron X-ray microtomography data. We found three different kinds of voids due to the effect of three different mechanisms: (i) cracks from ASR expansion, (ii) irregular-shaped voids due to the aggregate particles dissolution, and (iii) bubbles due to the cement paste preparation. We were able to separate them using morphological parameters (such as surface/volume ratio and aspect-ratio) calculated for each object, thus obtaining, e.g., volume fractions for each kind of voids. From the orientation data, we also studied if any shape preferred orientation was present in the sample, concerning the fractures network, and we found no appreciable preferred orientation. The new analysis procedure we applied in this study proved to be an effective approach for the quantitative characterization of the effects (cracks and porosity development by aggregate weathering) of the ASR reaction in mortars.     

3D in situ observations of glass fibre/matrix interfacial debonding

X-ray microtomography was used for 3D in situ observations of the evolution of fibre/matrix interfacial debonding. A specimen with a single fibre oriented perpendicular to the tensile direction was tested at a synchrotron facility using a special loading rig which allowed for applying a load transverse to the fibre. Three distinguishable damage stages were observed: (i) interfacial debond initiation at the free surface, (ii) debond propagation from the surface into the specimen and (iii) unstable debonding along the full length of the scanned volume. The high resolution microtomography provides both qualitative and quantitative 3D data of the debonding initiation and propagation. Thus, microtomography is demonstrated as a promising technique which can assist micromechanical model development.     

Mechanical and leakage behaviour of the dentin – adhesive interface

Dentine bonding systems (DBS) have been developed in order to bond restorative materials (i.e. composite) to the inner walls of the tissues when function and integrity as to be restored. Adhesion to dentine results from the penetration of DBS into the demineralised substrate constituted by a swollen collagen network. The short-term stability of a restored tooth is mainly affected by the presence of defects which act as stress raiser, while the long-term stability of a restored tooth is mainly affected by the seal of the restorative material on the dental structures. In order to determine the properties of the material interface, bonding to dentine is analysed using micro-tensile static and dynamic tests, assisted by the finite element modelling (FEM) and by the X-ray computed microtomography (micro-CT). The effect of voids and porosity in the composite layer of the DBS on the stress distribution has been investigated. Tensile adhesive strength for a particular DBS was measured on cylindrical specimens. The dual energy absorption technique, with the synchrotron beam light, has been developed to investigate, in a non-destructive manner, the leakage at the dentine-DBS interface of a silver nitrate staining solution as a function of mechanical cycling. The results indicate that leakage occurs radially through the dentine-adhesive interface and is influenced by the porosity in the adhesive and composite layers.     

Microstructure and properties of hot isostatically pressed powder and extruded Ti25V15Cr2Al0·2C

Abstract

The influence of process route on the microstructure and tensile behaviour of specimens prepared from hot isostatically pressed powders and extruded ingot of the burn resistant alloy, Ti–25V–15Cr–2Al–0·2C (wt-%), has been investigated. Samples based on gas atomised (GA) and plasma rotating electrode process (PREP) powders have been studied. Microstructural examination shows that many PREP powder particles are single crystals, whereas GA particles are polycrystalline. The mechanical properties of hot isostatically pressed specimens have been assessed using tensile testing monitored by acoustic emission, while microstructures have been characterised by synchrotron X-ray microtomography and optical and analytical scanning electron microscopy. Tomographic examination revealed a small fraction (<0·002 vol.-%) of pores in samples made from hot isostatically pressed GA powders, but no porosity was detected in samples made from hot isostatically pressed PREP powder. In view of their similar tensile behaviour, it is concluded therefore that the porosity does not contribute to the scatter and poor ductility in these hot isostatically pressed samples. These pores increased in size and volume fraction after heat treatment above the hot isostatic press temperature. The large scatter in tensile properties of both hot isostatically pressed GA and PREP samples was correlated with the presence of large (100–400 μm) circular crack initiation sites on the fracture surfaces, but the origin of these initiation sites has not been identified.     

High-resolution tomographic imaging of a human cerebellum: comparison of absorption and grating-based phase contrast

Human brain tissue belongs to the most impressive and delicate three-dimensional structures in nature. Its outstanding functional importance in the organism implies a strong need for brain imaging modalities. Although magnetic resonance imaging provides deep insights, its spatial resolution is insufficient to study the structure on the level of individual cells. Therefore, our knowledge of brain microstructure currently relies on two-dimensional techniques, optical and electron microscopy, which generally require severe preparation procedures including sectioning and staining. X-ray absorption microtomography yields the necessary spatial resolution, but since the composition of the different types of brain tissue is similar, the images show only marginal contrast. An alternative to absorption could be X-ray phase contrast, which is known for much better discrimination of soft tissues but requires more intricate machinery. In the present communication, we report an evaluation of the recently developed X-ray grating interferometry technique, applied to obtain phase-contrast as well as absorption-contrast synchrotron radiation-based microtomography of human cerebellum. The results are quantitatively compared with synchrotron radiation-based microtomography in optimized absorption-contrast mode. It is demonstrated that grating interferometry allows identifying besides the blood vessels, the stratum moleculare, the stratum granulosum and the white matter. Along the periphery of the stratum granulosum, we have detected microstructures about 40 µm in diameter, which we associate with the Purkinje cells because of their location, size, shape and density. The detection of individual Purkinje cells without the application of any stain or contrast agent is unique in the field of computed tomography and sets new standards in non-destructive three-dimensional imaging.     

Effect of curing cycle on void distribution and interlaminar shear strength in polymer-matrix composites

The effect of temperature cycle on the void volume fraction, shape and spatial distribution was determined by means of X-ray microtomography in [0]₁₀ AS4/8552 composite laminates manufactured by compression molding. Cure temperatures were designed to obtain different processing windows while the overall degree of cure was equivalent, leading to laminates with average porosities in the range 0.4% and 2.9%. Regardless of the final porosity, voids were elongated, oriented parallel to the fibers and concentrated in channels along the width of the laminate as a result of the inhomogeneous process of consolidation and resin flow along the fibers. The interlaminar shear strength was found to be controlled by the void volume fraction in panels with porosity above 1%.     

Internal microstructure evolution of aluminum foams under compression

In this paper, the internal microstructure deformation of open-cell and closed-cell aluminum foams under compression was investigated by using synchrotron radiation X-ray computed tomography (SR-CT) technique and digital image analysis method. The reconstructed images were obtained by using filtered back projection algorithm based on the original images taken from SR-CT experiments. Several important parameters including cross-section porosity, total porosity and cross-section deformation were computed from the reconstructed images. The variation of these parameters provided useful evolution information of internal microstructure of aluminum foams under compression.     

A method to quantify the 3D microstructure of fibrous materials containing mineral fillers using X-ray microtomography: application to paper materials

This article proposed and validated an original and automatic method based on synchrotron X-ray microtomography to characterise non-destructively, in 3D, the mineral fillers that may be present in fibrous composite materials. The approach consists of (i) obtaining the 3D internal structure of the sample in a non invasive way, (ii) identifying the fillers in the 3D microstructure using appropriate image processing tools, (iii) calculating the filler content on the numerical data, and (iv) validating the representativity of the data sets by evaluating the representative elementary volume. This method was successfully applied in the case of paper samples. The numerical filler content were in good agreement with standards. This method opens new perspectives in terms of characterisation of filler spatial repartition.     

Mechanical properties of NBR/clay nanocomposites by using a novel testing system

NBR/clay nanocomposites are prepared by two different filler types: clay microparticles and clay nanoparticles. The morphology properties of all specimens are explored by XRD and SEM. The mechanical properties are characterized by means of a novel video-controlled method under uniaxial tension. Apart a limited increase in tensile stress at small strains, the ultimate stress at rupture of nanocomposites is much higher than microcomposites. The most dramatic phenomenon is the development of volume strain while the materials are stretched. The nucleation of voids is much more active in composites containing the filler with higher specific surface when the cavitation occurs at the poor interface between the clay platelets and the rubber matrix. In turn, the existence of very diffuse voids hinders the propagation of cracks and retards the rupture process. DMA results reveal that the interfacial action of NBR molecules with layered silicates increases with the degree of intercalation.     

3D mapping of water in oolithic limestone at atmospheric and vacuum saturation using X-ray micro-CT differential imaging

Determining the distribution of fluids in porous sedimentary rocks is of great importance in many geological fields. However, this is not straightforward, especially in the case of complex sedimentary rocks like limestone, where a multidisciplinary approach is often needed to capture its broad, multimodal pore size distribution and complex pore geometries. This paper focuses on the porosity and fluid distribution in two varieties of Massangis limestone, a widely used natural building stone from the southeast part of the Paris basin (France). The Massangis limestone shows locally varying post-depositional alterations, resulting in different types of pore networks and very different water distributions within the limestone. Traditional techniques for characterizing the porosity and pore size distribution are compared with state-of-the-art neutron radiography and X-ray computed microtomography to visualize the distribution of water inside the limestone at different imbibition conditions. X-ray computed microtomography images have the great advantage to non-destructively visualize and analyze the pore space inside of a rock, but are often limited to the larger macropores in the rock due to resolution limitations. In this paper, differential imaging is successfully applied to the X-ray computed microtomography images to obtain sub-resolution information about fluid occupancy and to map the fluid distribution in three dimensions inside the scanned limestone samples. The detailed study of the pore space with differential imaging allows understanding the difference in the water uptake behavior of the limestone, a primary factor that affects the weathering of the rock.     

An investigation of mortars affected by alkali-silica reaction by X-ray synchrotron microtomography: a preliminary study

A first attempt to investigate samples affected by alkali-silica reaction (ASR) by synchrotron X-ray microtomography has been made. The setup available at the SYRMEP beamline, at the third generation synchrotron Elettra (Trieste, Italy), allowed collecting phase-contrast enhanced images, with a detectability approaching that of optical microscopy (a few microns). In this study, mortar cylinders were prepared and immersed in a 1-M NaOH solution at 80 °C for 14 days to enhance the ASR. The weathered samples were studied using the traditional 2D techniques such as optical microscopy and scanning electron microscopy as well as using the 3D micro-CT. Over the aged samples, the 3D imaging allows the ASR weathering to be studied, showing the reactive aggregate progressive dissolution with subsequent deposition of gel and microcracks development. This technique has proven to be a valuable, non-destructive, method which allows the rendering of the microstructural features in specimen affected by ASR.     

Strain Profiling of a Ferritic‐Martensitic Stainless Steel Sheet — Comparing Synchrotron with Conventional X‐Ray Diffraction

To improve the fatigue resistance of stainless steel sheet, it is a common practice to induce compressive residual stress in the surface through shot‐peening or tumbling. Stress depth profiles obtained by tumbling of thin stainless steel tensile rods were analysed using laboratory and synchrotron X‐Ray Diffraction (XRD). Both the non‐destructive synchrotron and the laboratory XRD etch‐depth profile gave similar results: a residual stress profile decaying over a depth not exceeding 50 µm into the material.     

Numerical modeling of the tensile deformation process of sintered 316L based on microtomography of porous mesostructures

This paper concerns the numerical modeling of the tensile deformation process of porous sinters prepared from 316L powder. In the research specimens with 41%, 33% and 26% porosity were used. To take into account the porosity of materials (at a mesoscale) in the numerical modeling, X-ray microtomography was used. Based on the micro-CT images three-dimensional models were generated which mapped porous structures of the materials. Then, the surfaces of the models were subjected to triangulation and saved as finite element meshes. Numerical calculations were performed using the finite element method (FEM). To define the material nonlinearity the true stress–strain curve of solid 316L was used. Two approaches were employed for modeling the deformation process. The essence of the applied methods was the reduction of the impact of the non-mapped geometries (of a size less than the micro-CT accuracy) on mechanical properties of the materials. As a result of the calculations carried out by two methods, stress and strain fields were obtained and nominal stress–strain curves of the porous materials were determined. Based on the results of numerical calculations the influence of material discontinuities at the mesoscopic scale on macromechanical properties was investigated.     

Analysis of the internal structure and lattice (mis)orientation in individual grains of deformed CP nickel polycrystals by synchrotron X-ray micro-diffraction and microscopy

Monotonic and cyclic loading of polycrystals causes a complex evolution of the dislocation structure and internal stresses. These phenomena were studied in sheet samples of commercially pure (CP) Ni in heat-treated (large-grained) states. Various microscopy tools were used, namely, scanning electron microscopy (SEM) with electron back-scattered diffraction (EBSD) to image the surface grain structure; Focused Ion Beam (FIB) with channelling contrast to visualise the through-thickness grain arrangement; and synchrotron scanning transmission X-ray microscopy (STXM) to obtain absorption-contrast images. In order to investigate the internal defects and lattice distortion caused by them, synchrotron X-ray diffraction was used in a variety of modes. Reciprocal space mapping (RSM) was used to quantify the amount of lattice re-orientation (rotation) due to plastic deformation. Micro-beam Laue diffraction was used to obtain 2D images containing multiple reflections that undergo “streaking” due to plastic deformation. The combination of reciprocal space mapping and Laue micro-diffraction provided improved insight into the deformation processes within individual grains during plastic deformation. The results are interpreted and discussed in conjunction with dislocation dynamics and finite element modelling of plastic deformation by crystal slip.     

X-ray microtomography (μ-CT) to evaluate microstructure of mortars containing low density additions

The present paper examines the effect of expanded perlite, expanded glass and cenospheres on cement mortars porosity. The mortars were scanned using X-ray microtomography (μ-CT) and the reconstructed images were treated with the 3D analysis software Morpho+. The mortar microstructure was studied in terms of porosity (open, closed and partial porosity) and the equivalent diameter of the pores. Porosity was seen to vary greatly, depending on the low density addition and its internal structure. The lowest open porosity was obtained in mortars made with cenospheres, whereas expanded perlite provided the highest open porosity. Capillary water absorption tests carried out to validate the porosity data confirmed that the expanded perlite mortars had the highest absorption. When additional software for voxel data visualization and light microscopy were used to visualize the mortar microstructure, the digital images and thin sections obtained showed good adhesion between the studied additions and the cement paste.     

Modelling of nucleation and void growth in dynamic pressure loading, application to spall test on tantalum

Dynamic ductile fracture is a three stages process controlled by nucleation, growth and finally coalescence of voids. In the present work, a theoretical model, dedicated to nucleation and growth of voids during dynamic pressure loading, is developed. Initially, the material is free of voids but has potential sites for nucleation. A void nucleates from an existing site when the cavitation pressure p _c is reached. A Weibull probability law is used to describe the distribution of the cavitation pressure among potential nucleation sites. During the initial growth, the effect of material properties is essentially appearing through the magnitude of p _c. In the later stages, the matrix softening due to the increase of porosity has to be taken into account. In a first step, the response of a sphere made of dense matrix but containing a unique potential site, is investigated. When the applied loading is a pressure ramp, a closed form solution is derived for the evolution of the void that has nucleated from the existing site. The solution appears to be valid up to a porosity of 0.5. In a second part, the dynamic ductile fracture of a high-purity grade tantalum is simulated using the proposed model. Spall stresses for this tantalum are calculated and are in close agreement with experimental levels measured by Roy (2003, Ph.D. Thesis, Ecole Nationale Supérieure de Mécanique et d’Aéronautique, Université de Poitiers, France). Finally, a parametric study is performed to capture the influence of different parameters (mass density of the material, mean spacing between neighboring sites, distribution of nucleation sites...) on the evolution of damage.     

Transient liquid phase bonding of AA 6082 aluminium alloy

Transient liquid phase bonding (TLP) on AA 6082 samples were performed under ambient non‐vacuum conditions, which was possible by a suitable pre‐treatment. This treatment involves a zincate treatment followed by copper plating, which is a common industrial process and can be performed in large batches. This treatment allows to remove the natural aluminium oxide layer and to protect the aluminium surface from excessive oxidation. Different bonding conditions were investigated and showed the feasibility of the transient liquid phase bonding process for AA 6082. Energy dispersive X‐ray spectroscopy (EDX) investigations showed that the isothermal solidification is already terminated after 5 min. The microstructure of the bonding zone showed no metallurgical discontinuity such as eutectic microstructure or intermetallic Al–Cu phases. However the microstructure shows numerous voids with a size of approximately 30 µm in the bonding zone. It is assumed that these voids were formed during the bonding process due to solidification shrinkage and the presence of interfacial oxide layers. The transient liquid phase bonded samples that were mechanically tested under tensile load showed an average strength of approximately 270 MPa, the minimum yield strength required for the base material according to EN 754‐2 is 255 MPa. Due to the notch effect of the voids, the tensile sample failed under forced fracture and showed no plastic deformation.     

Void configuration under constrained deformation in ductile matrix materials

SiC particle reinforced Al 2025 aluminum alloy composite is used for tensile tests. The ductile fracture by nucleation, growth and coalescence of micro voids, particle cracking and the interfacial debonding under the different constraint conditions, which are obtained by changing the notch radius, is analyzed. The effect of the local constraint on the respective damage phenomenon is analyzed using the axi-symmetric unit cell FE model by changing the local stress triaxiality and side constraint. The results show that the fracture process of the notched tensile bar is simulated well and the damage phenomena agree qualitatively with the experimental ones. Finally the effect of constraint on the void configuration and coalescence is investigated experimentally using three-dimensional fracture surface observations using the SEM (scanning electron microscope) and three-dimensional imaging analysis method. The constraint effect is analyzed by changing the specimen’s shape. The experimental results show that the void aspect ratio is decreased with increases of SiC particle volume fraction in aluminum alloy. The final void volume fraction at fracture also changes by the specimen shape and the material’s structure.     

Effect of ettringite morphology on DEF-related expansion

In this study, time dependent ettringite formation in heat-cured mortars has been investigated. In order to clarify the effect of formation place and morphology of ettringite on expansion, secondary electron images of cracked surfaces of mortars at three ages were analysed by SEM–EDS. Also, the X-ray microtomography analysis has been performed to observe the crack formation. The expansive role of delayed formed ettringite was related with its time dependent morphology as a function of formation place. From these observations, mechanism of ettringite reformation after heat curing has been proposed. Alumina rich species were the primary sources of ettringite formation as the starting nuclei. At later ages, if S and Al sources are readily available, the mentioned alumina rich nuclei will grow up and build ball ettringite. At long term, ball type ettringites (non-expansive) converted to massive type (expansive). These conversions can only take places if the form of available space is narrow (preformed micro-cracks). Massive ettringites exert pressure in these narrow spaces and cause expansion of mortar. If the form of the available space is spherical (entrapped air voids) ball ettringites preserve their initial form and do not cause any expansion.