Compacting an assembly of soft balls far beyond the jammed state: Insights from three-dimensional imaging.

Very soft grain assemblies have unique shape-changing capabilities that allow them to be compressed far beyond the rigid jammed state by filling void spaces more effectively. However, accurately following the formation of these systems by monitoring the creation of new contacts, monitoring the changes in grain shape, and measuring grain-scale stresses is challenging. We developed an experimental method that overcomes these challenges and connects their microscale behavior to their macroscopic response. By tracking the local strain energy during compression, we reveal a transition from granular-like to continuous-like material. Mean contact geometry is shown to vary linearly with the packing fraction, which is supported by a mean field approximation. We also validate a theoretical framework which describes the compaction from a local view. Our experimental framework provides insights into the granular micromechanisms and opens perspectives for rheological analysis of highly deformable grain assemblies in various fields ranging from biology to engineering.

Seismic events miss important kinematically governed grain scale mechanisms during shear failure of porous rock.

Catastrophic failure in brittle, porous materials initiates when smaller-scale fractures localise along an emergent fault zone in a transition from stable crack growth to dynamic rupture. Due to the rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to image directly. Here, we observe these micro-mechanisms directly by controlling the microcracking rate to slow down the transition in a unique rock deformation experiment that combines acoustic monitoring (sound) with contemporaneous in-situ x-ray imaging (vision) of the microstructure. We find seismic amplitude is not always correlated with local imaged strain; large local strain often occurs with small acoustic emissions, and vice versa. Local strain is predominantly aseismic, explained in part by grain/crack rotation along an emergent shear zone, and the shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation.

The effect of high relative humidity on a network of water-sensitive particles (couscous) as revealed by in situ X-ray tomography.

Water significantly influences the mechanical behaviour of all granular materials but none as much as hygroscopic amorphous particles. With sufficiently high water content, particles can swell, agglomerate and their mechanical properties can be reduced, having direct effects on the macroscopic response of the material. In the food and pharmaceutical industry this can cause loss of product functionality. Despite their relevance, very little is known about the microscopic processes that induce these phenomena. Previous studies focused on single particle behaviour, the strength of agglomerated particles and the material flowability, leaving unexplored the link connecting the particle behaviour and the bulk response. This experimental study aims to investigate this aspect with quantitative measurements at both particle and macroscopic scales. A sample of fine couscous is exposed to a high relative humidity (RH) air flow, while being subjected to oedometric conditions, in order to reproduce the storage-silo conditions. In the meantime, X-ray tomographies are acquired continuously and the resulting images are analysed. The designed spatial resolution allows each particle of the sample to be identified and tracked, allowing volumetric evolution to be compared to the properties of the whole sample. The analysis reveals a dilation-compaction macroscopic behaviour, a result of the competition between the particle swelling and the higher deformability as the water content increases. The number, orientations and inter-particle contacts are computed. Their area is related to the applied boundary conditions, and is found to be consistent with the particle swelling and dependent on the applied stress direction.

Imaging local soil kinematics during the first days of maize root growth in sand.

Maize seedlings are grown in Hostun sand with two different gradings and two different densities. The root-soil system is imaged daily for the first 8 days of plant growth with X-ray computed tomography. Segmentation, skeletonisation and digital image correlation techniques are used to analyse the evolution of the root system architecture, the displacement fields and the local strain fields due to plant growth in the soil. It is found that root thickness and root length density do not depend on the initial soil configuration. However, the depth of the root tip is strongly influenced by the initial soil density, and the number of laterals is impacted by grain size, which controls pore size, capillary rise and thus root access to water. Consequently, shorter root axes are observed in denser sand and fewer second order roots are observed in coarser sands. In all soil configurations tested, root growth induces shear strain in the soil around the root system, and locally, in the vicinity of the first order roots axis. Root-induced shear is accompanied by dilative volumetric strain close to the root body. Further away, the soil experiences dilation in denser sand and compaction in looser sand. These results suggest that the increase of porosity close to the roots can be caused by a mix of shear strain and steric exclusion.

Unearthing real-time 3D ant tunneling mechanics.

Granular excavation is the removal of solid, discrete particles from a structure composed of these objects. Efficiently predicting the stability of an excavation during particle removal is an unsolved and highly nonlinear problem, as the movement of each grain is coupled to its neighbors. Despite this, insects such as ants have evolved to be astonishingly proficient excavators, successfully removing grains such that their tunnels are stable. Currently, it is unclear how ants use their limited information about the environment to construct lasting tunnels. We attempt to unearth the ants' tunneling algorithm by taking three-dimensional (3D) X-ray computed tomographic imaging (XRCT), in real time, of Pogonomyrmex ant tunnel construction. By capturing the location and shape of each grain in the domain, we characterize the relationship between particle properties and ant decision-making within an accurate, virtual recreation of the experiment. We discover that intergranular forces decrease significantly around ant tunnels due to arches forming within the soil. Due to this force relaxation, any grain the ants pick from the tunnel surface will likely be under low stress. Thus, ants avoid removing grains compressed under high forces without needing to be aware of the force network in the surrounding material. Even more, such arches shield tunnels from high forces, providing tunnel robustness. Finally, we observe that ants tend to dig piecewise linearly downward. These results are a step toward understanding granular tunnel stability in heterogeneous 3D systems. We expect that such findings may be leveraged for robotic excavation.

Dual modality neutron and x-ray tomography for enhanced image analysis of the bone-metal interface.

The bone tissue formed at the contact interface with metallic implants, particularly its 3D microstructure, plays a pivotal role for the structural integrity of implant fixation. X-ray tomography is the classical imaging technique used for accessing microstructural information from bone tissue. However, neutron tomography has shown promise for visualising the immediate bone-metal implant interface, something which is highly challenging with x-rays due to large differences in attenuation between metal and biological tissue causing image artefacts. To highlight and explore the complementary nature of neutron and x-ray tomography, proximal rat tibiae with titanium-based implants were imaged with both modalities. The two techniques were compared in terms of visualisation of different material phases and by comparing the properties of the individual images, such as the contrast-to-noise ratio. After superimposing the images using a dedicated image registration algorithm, the complementarity was further investigated via analysis of the dual modality histogram, joining the neutron and x-ray data. From these joint histograms, peaks with well-defined grey value intervals corresponding to the different material phases observed in the specimens were identified and compared. The results highlight differences in how neutrons and x-rays interact with biological tissues and metallic implants, as well as the benefits of combining both modalities. Future refinement of the joint histogram analysis could improve the segmentation of structures and tissues, and yield novel information about specimen-specific properties such as moisture content.

The shape of hanging elastic cylinders.

Deformations of heavy elastic cylinders with their axis in the direction of earth's gravity field are investigated. The specimens, made of polyacrylamide hydrogels, are attached from their top circular cross section to a rigid plate. An equilibrium configuration results from the interplay between gravity that tends to deform the cylinders downwards under their own weight, and elasticity that resists these distortions. The corresponding steady state exhibits fascinating shapes which are measured with lab-based micro-tomography. For any given initial radius to height ratio, the deformed cylinders are no longer axially symmetric beyond a critical value of a control parameter that depends on the volume force, the height and the elastic modulus: self-similar wrinkling hierarchies develop, and dimples appear at the bottom surface of the shallowest samples. We show that these patterns are the consequences of elastic instabilities.

A peek into the origin of creep in sand.

This paper presents the results of an experimental study of the particle scale mechanisms that underpin creep, on-going deformations under constant external load, in dry non-cemented sand under 1D oedometric compression loading at 2500 kPa. Traditional observations on the boundary of the sample are complemented with simultaneous measurements of the 3D kinematics of both the entire grain assembly and details of grain-scale mechanisms using synchrotron based X-ray tomography at two different spatial resolutions. Both the continuum response and the local grain scale response are captured using two spatial resolutions, i.e.  6.5 µ m and 0.65 µ m respectively. The results, for the first time, illustrate that small displacements measured at the boundary can be the result of rather pronounced fracturing at the individual grain scale.

Application of microtomography and image analysis to the quantification of fragmentation in ceramics after impact loading.

Silicon carbide ceramics are widely used in personal body armour and protective solutions. However, during impact, an intense fragmentation develops in the ceramic tile due to high-strain-rate tensile loadings. In this work, microtomography equipment was used to analyse the fragmentation patterns of two silicon carbide grades subjected to edge-on impact (EOI) tests. The EOI experiments were conducted in two configurations. The so-called open configuration relies on the use of an ultra-high-speed camera to visualize the fragmentation process with an interframe time set to 1 µs. The so-called sarcophagus configuration consists in confining the target in a metallic casing to avoid any dispersion of fragments. The target is infiltrated after impact so the final damage pattern is entirely scanned using X-ray tomography and a microfocus source. Thereafter, a three-dimensional (3D) segmentation algorithm was tested and applied in order to separate fragments in 3D allowing a particle size distribution to be obtained. Significant differences between the two specimens of different SiC grades were noted. To explain such experimental results, numerical simulations were conducted considering the Denoual-Forquin-Hild anisotropic damage model. According to the calculations, the difference of crack pattern in EOI tests is related to the population of defects within the two ceramics.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.