Erika Tudisco

Quantifying Bulk Electrode Strain and Material Displacement within Lithium Batteries via High-Speed Operando Tomography and Digital Volume Correlation

Tracking the dynamic morphology of active materials during operation of lithium batteries is essential for identifying causes of performance loss. Digital volume correlation (DVC) is applied to high‐speed operando synchrotron X‐ray computed tomography of a commercial Li/MnO2 primary battery during discharge. Real‐time electrode material displacement is captured in 3D allowing degradation mechanisms such as delamination of the electrode from the current collector and electrode crack formation to be identified. Continuum DVC of consecutive images during discharge is used to quantify local displacements and strains in 3D throughout discharge, facilitating tracking of the progression of swelling due to lithiation within the electrode material in a commercial, spiral‐wound battery during normal operation. Displacement of the rigid current collector and cell materials contribute to severe electrode detachment and crack formation during discharge, which is monitored by a separate DVC approach. Use of time‐lapse X‐ray computed tomography coupled with DVC is thus demonstrated as an effective diagnostic technique to identify causes of performance loss within commercial lithium batteries; this novel approach is expected to guide the development of more effective commercial cell designs.

Neutron tomographic imaging of bone-implant interface : Comparison with X-ray tomography

Metal implants, in e.g. joint replacements, are generally considered to be a success. As mechanical stability is important for the longevity of a prosthesis, the biological reaction of the bone to the mechanical loading conditions after implantation and during remodelling determines its fate. The bone reaction at the implant interface can be studied using high-resolution imaging. However, commonly used X-ray imaging suffers from image artefacts in the close proximity of metal implants, which limit the possibility to closely examine the bone at the bone-implant interface. An alternative ex vivo 3D imaging method is offered by neutron tomography. Neutrons interact with matter differently than X-rays; therefore, this study explores if neutron tomography may be used to enrich studies on bone-implant interfaces. A stainless steel screw was implanted in a rat tibia and left to integrate for 6weeks. After extracting the tibia, the bone-screw construct was imaged using X-ray and neutron tomography at different resolutions. Artefacts were visible in all X-ray images in the close proximity of the implant, which limited the ability to accurately quantify the bone around the implant. In contrast, neutron images were free of metal artefacts, enabling full analysis of the bone-implant interface. Trabecular structural bone parameters were quantified in the metaphyseal bone away from the implant using all imaging modalities. The structural bone parameters were similar for all images except for the lowest resolution neutron images. This study presents the first proof-of-concept that neutron tomographic imaging can be used for ex-vivo evaluation of bone microstructure and that it constitutes a viable, new tool to study the bone-implant interface tissue remodelling.

Characterization of the bone-metal implant interface by Digital Volume Correlation of in-situ loading using neutron tomography

Metallic implants are commonly used as surgical treatments for many orthopedic conditions. The long-term stability of implants relies on an adequate integration with the surrounding bone. Unsuccessful integration could lead to implant loosening. By combining mechanical loading with high-resolution 3D imaging methods, followed by image analysis such as Digital Volume Correlation (DVC), we aim at evaluating ex vivo the mechanical resistance of newly formed bone at the interface. X-rays tomography is commonly used to image bone but induces artefacts close to metallic components. Utilizing a different interaction with matter, neutron tomography is a promising alternative but has not yet been used in studies of bone mechanics. This work demonstrates that neutron tomography during in situ loading is a feasible tool to characterize the mechanical response of bone-implant interfaces, especially when combined with DVC. Experiments were performed where metal screws were implanted in rat tibiae during 4 weeks. The screws were pulled-out while the samples were sequentially imaged in situ with neutron tomography. The images were analyzed to quantify bone ingrowth around the implants. DVC was used to track the internal displacements and calculate the strain fields in the bone during loading. The neutron images were free of metal-related artefacts, which enabled accurate quantification of bone ingrowth on the screw (ranging from 60% to 71%). DVC allowed successful identification of the deformation and cracks that occurred during mechanical loading and led to final failure of the bone-implant interface.

Investigating the onset of strain localization within anisotropic shale using digital volume correlation of time‐resolved X‐ray microtomography images

Digital volume correlation analysis of time-resolved X-ray microtomography scans acquired during in situ triaxial compression of Green River shale cores provided time series of 3-D incremental strain fields that elucidated evolving deformation processes by quantifying microscopic strain localization. With these data, we investigated the impact of mechanical anisotropy on microscopic strain localization culminating in macroscopic shear failure. We conducted triaxial compression experiments with the maximum compressive stress, σ1, aligned perpendicular and parallel to lamination planes in order to investigate end-member stress states that arise within sedimentary basins. When the preexisting laminations were perpendicular to σ1, a lamination-parallel region with high axial compaction developed within the macroscopically linear deformation phase of the experiment and then thickened with increasing applied differential stress. Scanning electron microscopy images indicate that this axial compaction occurred within a lower density lamination and that more axial compaction occurred within the center of the core than near its sides. Boundary element method simulations suggest that this compacting volume promoted shear fracture development within the upper portion of the shale. When the laminations were parallel to σ1, lamination-parallel dilation bands formed, thickened, and intensified in dilation. Population densities of the distributions of incremental shear strain, radial dilation, and axial contraction calculated by digital volume correlation analysis enabled quantification of the evolving overall impact of, and interplay between, these various deformation modes. (Less)

Neutron imaging and digital volume correlation to analyse the coupled hydro-mechanics of geomaterials

A new approach to characterise the evolution and coupling of deformation and fluid flow in geomaterials is presented. The method exploits some key features of neutrons, namely penetration of dense materials used for triaxial pressure cells, sensitivity to hydrogen and the possibility to distinguish hydrogen from its isotope deuterium (in normal water, H2O, and heavy water, D2O, respectively). Illustration of the approach is provided with results from a combined fluid flow/triaxial compression test on a cemented sand specimen performed in-situ (i.e., acquiring images during loading) at a neutron imaging station. Quantitative analysis of neutron tomography images acquired at different stages of deformation is made by Digital Volume Correlation to provide full 3D strain fields that highlight the evolution of localised deformation features. Spatio-temporal tracking of the effect of the evolution of the permeability in the sample was possible by neutron radiographies acquired during pressure driven flow of H2O into the sample saturated with D2O. By exploiting the H2O/D2O neutron transmission contrast and similarities of their flow behaviour, the tracking of the H2O/D2O front can be considered as an indicator of the permeability of the sample that is correlated to the measured evolution of the deformation. (Less)

Experimental study on the cementation level in an artificial rock with crushable grains

The results of an experimental investigation of the effects of the degree of cementation on the mechanical behaviour of a porous artificial rock with crushable grains are presented. The studied material is an analogue of real cemented granular materials, such as pyroclastic weak rocks, carbonate sands, calcarenites and compacted decomposed granite. The present study aims to clarify the role of the degree of cementation in the relative importance or the sequential nature of mechanisms in natural material, i.e., granular rearrangement and breakage are expected to appear after de-bonding of particles. Cemented samples with different cement content were investigated. To understand the influence of the cementation, uncemented samples were reconstituted from a mixture of crushable grains and cement (not hydrated) in the same percentage by weight as the cemented (hydrated) samples, in order to obtain the same fraction of fines. Preliminary results show that, for the same confining stress, the cemented sample are more compressible than uncemented ones during isotropic compression; while, during axial loading, the cemented samples show a more rigid behaviour and a lower resistance.

Timelapse ultrasonic tomography for measuring damage localization in geomechanics laboratory tests.

Variation of mechanical properties in materials can be detected non-destructively using ultrasonic measurements. In particular, changes in elastic wave velocity can occur due to damage, i.e., micro-cracking and particles debonding. Here the challenge of characterizing damage in geomaterials, i.e., rocks and soils, is addressed. Geomaterials are naturally heterogeneous media in which the deformation can localize, so that few measurements of acoustic velocity across the sample are not sufficient to capture the heterogeneities. Therefore, an ultrasonic tomography procedure has been implemented to map the spatial and temporal variations in propagation velocity, which provides information on the damage process. Moreover, double beamforming has been successfully applied to identify and isolate multiple arrivals that are caused by strong heterogeneities (natural or induced by the deformation process). The applicability of the developed experimental technique to laboratory geomechanics testing is illustrated using data acquired on a sample of natural rock before and after being deformed under triaxial compression. The approach is then validated and extended to time-lapse monitoring using data acquired during plane strain compression of a sample including a well defined layer with different mechanical properties than the matrix.

Full-field Laboratory imaging of localised deformation in sandstone

Rocks are neither truly homogeneous nor are laboratory test boundary conditions perfect plus at some stage of loading deformation in a specimen generally localises into shear/compaction bands, tensile/shear cracks or fractures. In the presence of such localised strains and heterogeneity point-wise measurements at a boundary do not well characterise the mechanics of the system. Therefore “Full-field” measurements are needed to better study the behaviour. In this paper full-field measurements, including x-ray tomography, 3D-volumetric digital image correlation and ultrasonic tomography, are used to characterise localised deformation phenomena in a sandstone deformed under triaxial compression. X-ray tomography allows 3D characterisation of the internal structure of laboratory specimens and the consequences of deformation are visible where there has been sufficient change in material density. Complementing x-ray tomography by 3D-volumetric digital image correlation (DIC) techniques allows quantification of 3D displacement and strain fields between different stages of loading by comparison of the corresponding x-ray tomography images. In addition ultrasonic tomography provides full-field measurement of ultrasonic velocities, and thus elastic properties, in laboratory specimens. DIC and x-ray tomography can provide information on structural changes due to deformation and the associated kinematics and strains, whilst ultrasonic tomography gives insight into damage (degradation of elastic properties), which are to be expected, e.g., due to compaction and related porosity reduction or grain crushing.