High-speed X-ray Phase Contrast Imaging Research Articles

Synchrotron X-ray imaging of the onset of ultrasonic horn cavitation.

High-power ultrasonic horns operating at low frequency are known to generate a cone-shaped cavitation bubble cloud beneath them. The exact physical processes resulting in the conical structure are still unclear mainly due to challenges associated with their visualization. Herein, we address the onset of the cavitation cloud by exploiting high-speed X-ray phase contrast imaging. It reveals that the cone formation is not immediate but results from a three-step phenomenology: (i) inception and oscillation of single bubbles, (ii) individual cloud formation under splitting or lens effects, and (iii) cloud merging leading to the formation of a bubble layer and, eventually, to the cone structure due to the radial pressure gradient on the horn tip.

High-speed x-ray phase-contrast imaging of single cavitation bubbles near a solid boundary

Laser-induced cavitation bubble dynamics at different distances from a rigid boundary is investigated using high-speed synchrotron x-ray phase-contrast imaging. This is achieved through the design of a tailored experimental chamber specifically designed to reduce the x-ray absorption along the path length in water while mitigating boundary effects. The highly resolved undistorted radiographs are able to visualize a sharp bubble interface even upon complex shapes, which can serve as high-quality benchmarks for numerical simulations. Here, the measured bubble shapes are compared to simulations using the incompressible boundary integral method. The direct optical access to the high-speed liquid jet provides accurate measurements of the evolution of the jet speed, which is contrasted to the simulated results. After the jet has impacted the opposite side of the cavitation bubble, the cavity assumes a toroidal shape, the volume of which can be accurately measured from the radiographs and its temporal evolution compared to the bubble-ring model. Thanks to the clear optical access to the cavity lobes throughout the collapse, non-axisymmetric splashing within the bubble resulting from the jet impact, also known as Blake's splashing, is observed and characterized for stand-off parameters of γ<1. Measurements extracted from the highly resolved visualizations provided herein have been validated against scaling laws for droplet impact on a thin liquid film, which contribute to confirm and elucidate the splashing phenomenon.

Multiscale dynamic experiments on fiber-reinforced composites with damage assessment using high-speed synchrotron X-ray phase-contrast imaging

Fiber-reinforced composites (FRCs) have been widely used as protection materials to replace heavier metals. However, predictive capabilities for the impact-induced failure of FRCs have not been well-developed. In this work, we introduced an integrated technique to visualize the dynamic failure of FRCs, with the modified Kolsky bar loading and the high-speed synchrotron X-ray phase-contrast imaging (PCI). We summarize our experimental studies across the FRCs' structural levels by using this integrated technique, including single-fiber tensile experiments, fiber/matrix debonding experiments, and single-edge notched bending experiments on composite laminates. It is revealed that the high-speed synchrotron X-ray PCI technique possesses sufficient spatial resolution to identify the failure behavior of a single fiber in the 10-μm scale level, as well as temporal resolution to identify the time sequence of internal dynamic damage evolution. The technique can penetrate through the opaque composite sample and provide real-time visualization of small-scale damaging mechanisms such as the embedded fiber's sliding in the matrix and the fiber's transverse debonding with the matrix. At an expanding scale, the technique can identify the crack initiation and track the cracking behavior inside a single ply or between different plies of a composite laminate. High-speed visualization inside the material by this integrated technique provides critical deformation and failure information to develop a physical understanding of the damage inside composites under impact.

End-of-injection fuel dribbling dynamics of multi-hole GDI injector

The fuel dribbling at the end-of-injection (EOI) transient is believed to be harmful to the emission performance. However, quantitative characterization and detailed explanation of the fuel dribbling process for GDI injectors are rarely reported, and the mechanism is not thoroughly understood. This study conducted a comprehensive investigation on the fuel dribbling process at the EOI transient for a multi-hole sac-type GDI injector using the high-speed X-ray phase-contrast imaging (XPCI) technique. Simultaneous visualization of needle bouncing, in-nozzle flow, and fuel dribbling directly illustrated that the fuel dribbling process was strongly correlated with needle bouncing. Further quantitative analysis of dribble volume and needle bouncing showed that the fuel dribbling process could be divided into the pre-needle-bouncing stage and the needle-bouncing stage. Distinct mechanisms of the two stages were explained through the dribbling source and the driving force. For the pre-needle-bouncing stage, the dribbling source was the residual fuel in the sac, and the driving force was the pressure difference between the sac and the ambient. For the needle-bouncing stage, the dribbling source was the fuel fed to the sac from upstream, and the driving force was the needle bouncing. Possible control strategies of fuel dribbling were also discussed according to the dribbling mechanisms of the two stages. A high injection pressure tended to increase the dribble amount but to reduce the dribble size and dribbling duration, which was believed to be overall favorable to restrain the adverse effect of fuel dribbling.

Microbubble dynamics and heat transfer in boiling droplets

Dissipating large heat fluxes from a surface is critically important in numerous industrial and natural applications. Boiling based spray cooling and surface texturing are two of the most promising methods being investigated to address this problem. Although our understanding on these topics has significantly improved over past decades, critical gaps remain in the knowledgebase stymieing the realization of their full potential. As an example, while bubble growth in pool boiling have been investigated in detail, comparatively little is known about how the bubbles evolve inside boiling drops. In the present work, we have investigated for the first time, the microbubble dynamics inside water droplets boiling on superhydrophilic textured substrates using high-speed X-ray phase contrast imaging (XRPCI). Our observations show that the transient bubble density variation follows similar characteristics irrespective of the texture spacing at a given surface temperature. For an example microstructure, we found that the number of discrete bubbles on the surface decreases as temperature is increased although their growth rate increases. We observe that bubble growth is highly non-uniform during the lifetime of a drop on the surface. Initially, bubbles grow under diffusion-limited regime, but at later times they grow as ~t1.45 due to combined effects of coalescence and evaporation. In some conditions, we found that bubbles shrink dramatically after the initial growth spurt presumably due to severe quenching of the surface, and migration of bubbles on the surface. Using the bubble sizes, for the first time we analyzed the heat flux removed by a single bubble and also by all the bubbles at a given time. We find that the highest dissipation through latent-heat component (~600 W/cm2) occurs just in the beginning and thereafter it decreases. We expect that our findings and the analysis would guide further work on the topic and will aid in the overarching goal of engineering surfaces that are more efficient in boiling heat transfer.

High-speed synchrotron X-ray phase-contrast imaging for evaluating microscale damage mechanisms and tracking cracking behaviors inside cross-ply GFRCs

We integrated the high-speed synchrotron X-ray phase-contrast imaging (PCI) with a modified Kolsky compression bar loading platform to visualize the dynamic failure processes of cross-ply glass fiber reinforced composites (GFRCs). Four S-2 glass/SC-15 composite specimens were prepared, having similar thicknesses but different stacking sequences, namely as [012/9012], [9012/012], [08/908/08], and [908/08/908]. Three-dimensional synchrotron X-ray computed tomography and scanning electron microscopy (SEM) were employed to examine the microstructures and quantify the fiber volume fractions. Each specimen was notched and subjected to a dynamic three-point flexural loading. The onset of cracking close to the notch tip, crack propagation in 0° or 90° plies and their interface, crack opening, and ultimately failure of the specimen were captured by high-speed synchrotron X-ray PCI. Additional dynamic experiments were performed to determine the average time when the stress wave propagated through the specimen and correlate the X-ray images with the specimen's force-deflection response. Finally, the surface morphology of each specimen after the dynamic loading was imaged by SEM. Comparison between real-time X-ray images and post-fracture SEM images demonstrated the capability of the X-ray method to record damage evolution inside composites. Furthermore, the high spatial and temporal resolutions of the X-ray setup and edge enhancement by PCI enabled the identification of microscale damage features within 1 μs. Two damaging processes were identified, crack growth was quantified, and fracture toughness of the composites was evaluated. The method is deemed useful to reveal microscale damage mechanisms and track cracking behaviors inside cross-ply GFRCs under dynamic loading in real time.

Rate-dependent strength and deformation heterogeneity of B4C-reinforced Al composite: Time-resolved imaging with synchrotron X-rays

Dynamic (up to 5500 s−1) and quasi-static compression tests are conducted on a 15 wt% B4C particle-reinforced Al (B4C/Al) composite. In situ, high-speed synchrotron X-ray phase contrast imaging and digital image correlation are employed to map mesoscale deformation fields at μm and μs scales. The bulk stress–strain curves show significant strain and strain-rate hardening under dynamic compression. The strain-rate sensitivity exponent is an order of magnitude higher at high strain rates (>103 s−1) than that at low strain rates (<10−2 s−1). Strain field mapping demonstrates distinct compressive strain localizations for both quasi-static and dynamic loading. Nevertheless, compressive strain localizations appear denser in spacing under dynamic loading, owing to spontaneous dislocation nucleation in both weak and strong zones. This results in a higher density of geometrically necessary dislocations, which contributes to the higher strain and strain-rate hardening of B4C/Al under dynamic loading. The ratio of the maximum local strain to the bulk average is ~1.5, and the local strain-rate enhancement cannot explain the increased rate sensitivity of B4C/Al under dynamic loading. Therefore, the rate-dependent deformation heterogeneity dominates the strain-rate hardening of B4C/Al. Postmortem analyses help correlate deformation features to particle- and grain-scale microstructures, yielding consistent results with mesoscale strain fields.

Real-time damage characterization for GFRCs using high-speed synchrotron X-ray phase contrast imaging

We report the application of high-speed synchrotron X-ray phase contrast imaging (PCI) in real-time damage characterization for glass fiber reinforced composites (GFRCs) subjected to dynamic loading. Dynamic single-edge notched bending (DSENB) experiments on pre-notched S-2 GFRCs were performed on a modified Kolsky compression bar. During loading, the synchrotron X-ray beam penetrated through the composite specimen from the side to detect damage evolution inside the material. Entire dynamic events were recorded by a high-speed camera as image sequences.0° and 90° unidirectional and cross-ply composites were investigated. An optical imaging technique was also employed to capture similar dynamic events in comparison with the radiographic imaging. It is demonstrated that high-speed X-ray PCI had sufficient phase contrast to characterize a crack initiation at a 20-μm spatial resolution within 920 ns and track the crack geometry during propagation, thereby providing reliable data to quantify the dynamic damage resistance of GFRCs. Furthermore, being capable of recognizing microscopic damage-related features at a sub-10-μm resolution, high-speed X-ray PCI provided fundamental material failure mechanisms to reveal the essential of macroscale structural failure of composites. It can also track the damage evolution inside and between individual plies of laminated composites. However, current high-speed X-ray PCI technique only supports in-situ observation and the high timing and spatial resolutions are limited within a field of view of ~2.5 mm in square, preventing its application in the three-dimensional and larger-area damage detection for GFRC structures.

High-speed x-ray phase contrast imaging and digital image correlation analysis of microscale shock response of an additively manufactured energetic material simulant

The performance of energetic materials subjected to dynamic loading significantly depends on their micro- and meso-scale structural morphology. The geometric versatility offered by additive manufacturing opens new pathways to tailor the performance of these materials. Additively manufactured energetic materials (AMEMs) have a wide range of structural characteristics with a hierarchy of length scales and process-inherent heterogeneities, which are hitherto difficult to precisely control. It is important to understand how these features affect AMEMs’ response under dynamic/shock loading. Therefore, temporally and spatially resolved measurements of both macroscopic behavior and micro- and meso-level processes influencing macroscopic behavior are required. In this paper, we analyze the shock compression response of an AMEM simulant loaded under several impact conditions and orientations. X-ray phase contrast imaging (PCI) is used to track features across the observed shock front and determine the linear shock velocity vs particle velocity equation of state, as well as to quantify the interior deformation fields via digital image correlation (DIC) analyses. Photon Doppler velocimetry is simultaneously used to measure the particle velocities of the specimens, which are consistent with those obtained from x-ray PCI. The DIC analyses provide an assessment of the average strain fields inside the material, showing that the average axial strain depends on the loading intensity and reaches as high as 0.23 for impact velocities up to 1.5 km/s. The overall results demonstrate the utility of x-ray PCI for probing “in-material” equation of state and interior strains associated with dynamic shock compression behavior of the AMEM simulant.

The effect of loading direction on the fracture behaviors of cortical bone at a dynamic loading rate

In this study, the dynamic cracking processes in porcine cortical bone were visualized in real-time using the high-speed synchrotron X-ray phase-contrast imaging (PCI) technique in three osteon orientations: in-plane transverse, out-of-plane transverse and in-plane longitudinal. The dynamic flexural loading applied on the pre-notched bone specimens was introduced by a modified Kolsky compression bar. High-speed X-ray images of the entire loading events were documented with a high-speed camera. Three-dimensional X-ray micro-computed tomography was conducted to examine the intact microstructures and obtain the basic material properties of the bone material used for mechanical characterizations. The onset location, where crack initiated, and the subsequent direction, along which the incipient crack propagated, were measured quantitatively using the high-speed X-ray images and the latter was found dependent on the osteon direction significantly. The crack propagation velocities were dependent on crack extension over the entire crack path significantly for all the three directions while the initial velocity for in-plane longitudinal direction was lower than the other two directions. Straight-through crack paths were observed for in-plane longitudinal specimens while the cracks were deflected and twisted in the in-plane transverse direction. For out-of-plane transverse direction, the cracks follow paths with tortuosity fall in between the other two directions, showing a mixed mode of fractures of the former two extreme cases. The toughening mechanisms, visualized by the high-speed X-ray images, and the corresponding fracture toughness, evaluated in terms of fracture initiation toughness and crack growth resistance curve (R-curve), were also found significantly different among the three osteon directions, suggesting an overall transition from brittle to ductile-like fracture behaviors at the dynamic displacement rate (5.4 m/s) as the osteon orientation varies from in-plane longitudinal to out-of-plane transverse, and to in-plane transverse eventually.

X-Ray Phase Contrast Imaging of Liquid Film and Spray Development Inside an Aircraft Engine Swirler

Abstract Modern aircraft engines combine liquid fuel and air using an intricate flow device with many fuel and air flow passages. To date, the process by which the fuel atomizes within this swirler set has not been examined directly due to optical access limitations. In this work, high-speed X-ray phase-contrast imaging of a liquid spray inside a gas turbine engine swirler geometry is presented. Measurements were carried out at the 7-BM beamline of the Advanced Photon Source at Argonne National Laboratory using the high-energy broadband X-ray beam. The synchrotron X-ray source provides the necessary photon energy and flux to capture time-resolved fluid phenomena within the confines of the relevant geometry while liquid and air are flowing. Spray nozzle hardware and geometries were provided by the National Jet Fuels Combustion Program (NJFCP), allowing for characterization of the spray using a commercially relevant configuration. Modified swirlers were three-dimensional printed with acrylic to improve imaging access while maintaining influential internal features. Water was used as a surrogate fluid for these studies to demonstrate the visualization capabilities. The experiments were conducted at atmospheric exit pressure conditions with a pressure drop of 6% across the swirler. High-speed imaging of the pilot spray cone revealed sheet breakup several millimeters downstream of the orifice exit, upon interaction with the radial assist air flow. These droplets and ligaments were observed to impinge on the inner filming surface of the swirler and flow toward the exit while developing a tangential flow. Under these conditions, the liquid film grows up to several hundred microns in thickness on the filming surface, and subsequently forms ligaments up to several millimeters in length before breaking up. This work demonstrates the capability of X-ray diagnostics in visualizing liquid flows within solid geometries of technical relevance. Furthermore, the spatial quantification of filming flows and liquid interaction with the swirler air provides validation data for modeling of the multiphase flows and surface interactions within the swirler.

In Situ Observation of Adiabatic Shear Band Formation in Aluminum Alloys

We used high-speed X-ray phase contrast imaging and infrared thermal imaging techniques to study the formation processes of adiabatic shear bands in aluminum 7075-T6 and 6061-T6 alloys. A modified compression Kolsky bar setup was used to apply the dynamic loading. A flat hat-shaped specimen design was adopted for generating the shear bands at the designated locations. Experimental results show that 7075-T6 exhibits less ductility and a narrower shear band than 6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within the shear band zones for 7075-T6 and 6061-T6 respectively. This local high temperature zone and the resulting thermal instability were found to relate to the shear band formation in these aluminum alloys.

Real-time visualization of dynamic fractures in porcine bones and the loading-rate effect on their fracture toughness

We visualized, in real time, the dynamic fracture behaviors in porcine cortical bone from humerus and porcine trabecular bone from nasal bone at a high loading rate using high-speed synchrotron X-ray phase-contrast imaging (PCI). Dynamic three-point bending loading was applied on notched bone specimens by a modified Kolsky compression bar and images of the entire fracture events were recorded with an ultra-high-speed camera. Experiments at a quasi-static loading rate on material testing system (MTS) were also performed to identify the loading-rate effects on the fracture toughness of the two types of bone. Three-dimensional synchrotron X-ray computed tomography was conducted to examine the initial microstructures in the bone specimens before mechanical loading. At the dynamic loading rate, the onset locations of crack initiation were found to be independent from the bone types. The deleterious effect of dynamic loading rate on bone's fracture toughness was verified in this study and the crack was found to propagate at higher speeds in cortical bone than in trabecular bone. In a comparison of the observed more torturous crack paths at the quasi-static loading rate, cracks in dynamically loaded bone specimens generally followed the paths with less in-plane deflections and out-of-plane twists. However, our experimental results also indicated that, although the extent was diminished at dynamic loading rate, the crack deflections at osteon cement lines still played a role as a major toughening mechanism to dynamic fractures in transversely orientated cortical bone.

In-Situ Visualization of Tensile Failure in Additively Manufactured 316 L Stainless Steel

Additive manufacturing has become an enabling technology for the production of complicated engineered structures once thought impossible to produce. As these technologies develop, the mechanical behavior of these materials/structures must be characterized in a variety of harsh environments. To assess the loading-rate sensitivity of additively manufactured 316 L stainless steel, dynamic and quasi-static tension experiments were performed. High-speed X-ray phase contrast imaging was performed during dynamic experiments at Argonne National Laboratory’s Advanced Photon Source. These images reveal the evolution of porosity and intrinsic defects within the material, and their influence on the mechanisms of dynamic failure in real time. Stress-strain histories were recorded for experiments, on which the performance of the material is addressed. High degrees of localized yielding were observed as a precursor to ductile crack growth and propagation. No transition in the mechanism of fracture was observed. However, from the stress strain-histories the influence of defects was shown to decrease with increasing strain-rates. Comparisons between failure mechanisms of the additively manufactured specimens were made to those of wrought material subjected to identical loading.

High-speed X-ray visualization of dynamic crack initiation and propagation in bone

The initiation and propagation of physiological cracks in porcine cortical and cancellous bone under high rate loading were visualized using high-speed synchrotron X-ray phase-contrast imaging (PCI) to characterize their fracture behaviors under dynamic loading conditions. A modified Kolsky compression bar was used to apply dynamic three-point flexural loadings on notched specimens and images of the fracture processes were recorded using a synchronized high-speed synchrotron X-ray imaging set-up. Three-dimensional synchrotron X-ray tomography was conducted to examine the initial microstructure of the bone before high-rate experiments. The experimental results showed that the locations of fracture initiations were not significantly different between the two types of bone. However, the crack velocities in cortical bone were higher than in cancellous bone. Crack deflections at osteonal cement lines, a prime toughening mechanism in bone at low rates, were observed in the cortical bone under dynamic loading in this study. Fracture toughening mechanisms, such as uncracked ligament bridging and bridging in crack wake were also observed for the two types of bone. The results also revealed that the fracture toughness of cortical bone was higher than cancellous bone. The crack was deflected to some extent at osteon cement line in cortical bone instead of comparatively penetrating straight through the microstructures in cancellous bone. Statement of SignificanceFracture toughness is with great importance to study for crack risk prediction in bone. For those cracks in bone, most of them are associated with impact events, such as sport accidents. Consequently, we visualized, in real-time, the entire processes of dynamic fractures in notched cortical bone and cancellous bone specimens using synchrotron X-ray phase contrast imaging. The onset location of crack initiation was found independent on the bone type. We also found that, although the extent was diminished, crack deflections at osteon cement lines, a major toughening mechanism in transversely orientated cortical bone at quasi-static rate, were still played a role in resisting cracking in dynamically loaded specimen. These finding help researchers to understand the dynamic fracture behaviors in bone.

Abdominal pumping involvement in the liquid feeding of honeybee

Honeybee drinking is facilitated by a “mop-like” tongue, which helps honeybees suck in the sucrose solution from the environment. However, the liquid-transport mechanism from the pharynx to the crop, especially the natural link between abdominal pumping and dipping behavior on the sucrose solution intake, remains obscure. A significant increase in abdominal pumping frequency is observed when honeybees drink the sucrose solution. Abdominal pumping exhibits a function other than respiration. This second function assists in driving the sucrose solution from the pharynx to the crop. We combine the experimental measurements using high-speed video and X-ray phase contrast imaging with theoretical modeling to investigate the effect of abdominal pumping in liquid feeding of honeybee. Experimental results show that a honeybee performs abdominal pumping in the abdomen at a faster rhythm during sucrose solution feeding than during other physiological activities. In addition, the period of abdominal pumping is in concordance with that of dipping cycles. Theoretical analysis demonstrates that the abdomen, which is comparable with a micro pump, changes its volume rhythmically. Such expansion reduces pressure in the abdomen, which also reduces pressure in the crop and helps propel the sucrose solution from the pharynx to the crop. Abdominal pumping can help honeybees improve their feeding efficiency and save foraging time. This research work reveals a specific feeding mechanism of insects fed on sucrose solution and opens a new way for the design of microfluidic pump.

Dynamic crack propagation from a circular defect in a unidirectional carbon fiber reinforced plastic composite

A single-ply unidirectional IM7/8552 carbon fiber reinforced plastic composite with artificially introduced circular defects is subjected to dynamic tensile loading using a modified Kolsky tension bar. A high-speed X-ray phase contrast imaging method is integrated with the Kolsky bar setup to record the crack initiation from the defects and subsequent propagation in the material in real time during the tensile loading. The tensile loading was applied either in longitudinal (0° to fibers) or transverse (90° to fibers) direction of the specimens. Shear failure of the matrix and axial splitting along the loading/fiber direction were observed in longitudinal specimens to initiate from the edge of the artificial circular defects. Debonding of fiber and matrix was observed in transverse specimens, which initiated from the top and bottom edge of the hole. The dynamic tensile loading history during the crack propagation was recorded using a piezoelectric load cell and synchronized with the observed damage and failure processes.

High-speed visualization of vortical cavitation using synchrotron radiation

High-speed X-ray phase-contrast imaging of the cavitating flow developing within an axisymmetric throttle orifice has been conducted using high-flux synchrotron radiation. A white X-ray beam with energy of 6 keV was utilized to visualize the highly turbulent flow at 67 890 frames per second with an exposure time of 347 ns. The working medium employed was commercial diesel fuel at flow conditions characterized by Reynolds and cavitation numbers in the range of 18 000–35 500 and 1.6–7.7, respectively. Appropriate post-processing of the obtained side-view radiographs enabled the detailed illustration of the interface topology of the arising vortical cavity. In addition, the visualization temporal and spatial resolution allowed the correlation of the prevailing flow conditions to the periodicity of cavitation onset and collapse, to the magnitude of the underlying vortical motion, as well as to the local turbulence intensity.

Visualization of dynamic fiber-matrix interfacial shear debonding

To visualize the debonding event in real time for the study of dynamic crack initiation and propagation at the fiber–matrix interface, a modified tension Kolsky bar was integrated with a high-speed synchrotron X-ray phase-contrast imaging setup. In the gage section, the pull-out configuration was utilized to understand the behavior of interfacial debonding between SC-15 epoxy matrix and S-2 glass fiber, tungsten wire, steel wire, and carbon fiber composite Z-pin at pull-out velocities of 2.5 and 5.0 m s−1. The load history and images of the debonding progression were simultaneously recorded. Both S-2 glass fiber and Z-pin experienced catastrophic interfacial debonding whereas tungsten and steel wire experienced both catastrophic debonding and stick–slip behavior. Even though S-2 glass fiber and Z-pin samples exhibited a slight increase and tungsten and steel wire samples exhibited a slight decrease in average peak force and average interfacial shear stress as the pull-out velocities were increased, no statistical difference was found for most properties when the velocity was increased. Furthermore, the debonding behavior for each fiber material is similar with increasing pull-out velocity. Thus, the debonding mechanism, peak force, and interfacial shear stress were rate insensitive as the pull-out velocity doubled from 2.5 to 5.0 m s−1. Scanning electron microscope imaging of recovered epoxy beads revealed a snap-back behavior around the meniscus region of the bead for S-2 glass, tungsten, and steel fiber materials at 5.0 m s−1 whereas those at 2.5 m s−1 exhibited no snap-back behavior.

In situ observation of fracture processes in high-strength concretes and limestone using high-speed X-ray phase-contrast imaging.

The mechanical properties and fracture mechanisms of geomaterials and construction materials such as concrete are reported to be dependent on the loading rates. However, the in situ cracking inside such specimens cannot be visualized using traditional optical imaging methods since the materials are opaque. In this study, the in situ sub-surface failure/damage mechanisms in Cor-Tuf (a reactive powder concrete), a high-strength concrete (HSC) and Indiana limestone under dynamic loading were investigated using high-speed synchrotron X-ray phase-contrast imaging. Dynamic compressive loading was applied using a modified Kolsky bar and fracture images were recorded using a synchronized high-speed synchrotron X-ray imaging set-up. Three-dimensional synchrotron X-ray tomography was also performed to record the microstructure of the specimens before dynamic loading. In the Cor-Tuf and HSC specimens, two different modes of cracking were observed: straight cracking or angular cracking with respect to the direction of loading. In limestone, cracks followed the grain boundaries and voids, ultimately fracturing the specimen. Cracks in HSC were more tortuous than the cracks in Cor-Tuf specimens. The effects of the microstructure on the observed cracking behaviour are discussed.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.