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Abstract: Seismic velocity at the near surface drops during ground motions due to remote earthquakes and can recover afterwards over decades. In the laboratory, seismic velocity of rock samples decreases after dynamic deformations (e.g., shaking) and gradually recovers towards the original level. These observations at different scales are referred to as slow dynamics in granular materials (e.g., rocks and concrete), but the underlying mechanisms remain debated. We explore the physics behind seismic velocity transients during and after dynamic deformations using Stór Mjölnir — a triaxial pressure loading apparatus featuring two piezoelectric transducers mounted in the top and bottom pistons and an X-ray transparent aluminium pressure vessel that houses a cylindrical core sample of Clashach sandstone (10 mm in diameter and 25 mm in length). We present the mechanical, acoustic, and X-ray microtomography results of two triaxial loading experiments, conducted at room temperature with a confining pressure of 20 MPa and a pore fluid pressure of 5 MPa. Both experiments involve first increasing the ram pressure at a constant strain rate of 1x10-5 s-1 until the onset of sample yielding, indicated by a deviation from the linear stress–strain curve. In the first experiment, we further hold the ram pressure constant and then abruptly reduce the pressure by 30 MPa before rapidly returning the pressure to the previous hold level; this perturbation is repeated for 32 cycles until catastrophic failure of the rock sample. In the second experiment, we apply the same cyclic loading protocol after sample yielding, except for the abrupt pressure drop of 150 MPa; the sample survives only two loading cycles before catastrophic failure. These cyclic loading protocols are designed to induce transient seismic velocity responses, which are monitored by active acoustic surveys acquired every 8 s and in-situ 3D X-ray tomography synthesised every 6 min at the beamline I12-JEEP, Diamond Light Source (Oxfordshire, UK). We observe nearly linear relationships between the small stress perturbations (30 MPa) and corresponding seismic velocity changes, indicating minimal slow dynamics in the rock sample. In contrast, large stress perturbations (150 MPa) cause nonlinear velocity changes, although the recovery time scale is limited by the small size of the experimental sample. The time-resolved 3D X-ray volumes from both experiments show no resolvable transient structural changes in the rock samples, despite ongoing microfracture accumulation and pore enlargement driven by background creep until catastrophic failure. These results demonstrate that active seismic waves can detect nonlinear velocity transients in triaxial loading experiments, which likely originate from microstructures (e.g., grain contacts) below the X-ray imaging resolution (voxel edge length ~ 7.9 µm). These experiments also motivate further study on seismic velocity transients using our next-generation experimental apparatus that accommodates larger samples (18 mm in diameter and 45 mm in length) and six acoustic transducers. Ultimately, we aim to assess seismic velocity transients as a proxy for rocks’ susceptibility to small stress perturbations, which could provide a method to map the proximity to catastrophic failure and hence help mitigate induced seismicity associated with hydraulic fracturing.

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Abstract: In situ tomography enables non-destructive, time-lapse imaging of biological tissues under load, offering insights into structural and mechanical changes. However, repeated scans can expose samples to high radiation doses, potentially altering tissue properties. This study evaluated the feasibility of low-dose synchrotron computed tomography (sCT) for high-resolution, in situ imaging of intact bovine intervertebral discs (IVDs), and assessed the effects of repeated x-ray exposure on mechanical, microstructural, and molecular integrity. Intact oxtail IVD segments were imaged using propagation-based phase contrast sCT at 54 keV. Scan parameters were optimised to achieve high image quality within 66 seconds per scan, resulting in a total absorbed dose of ∼30 kGy over six scans. Mechanical properties were assessed under cyclic loading, microstructural changes via digital volume correlation (DVC), and molecular alterations using Raman spectroscopy. High-resolution imaging of soft and calcified tissues was achieved. Changes in sample stiffness, hysteresis, or stress recovery between irradiated and control were not identified. DVC revealed no microstructural damage or strain accumulation in the calcified endplate. Raman spectroscopy indicated minimal changes in soft tissues, with bone showing slightly increased collagen crosslinking and reduced mineralisation. Overall, this study demonstrates that high-energy, low-dose sCT enables repeated imaging of musculoskeletal tissues without compromising integrity, supporting its application in dynamic, time-lapse imaging studies. Importantly, larger, intact samples, such as whole bovine IVDs, were imaged overcoming limitations of previous studies that relied on small animal models. This approach supports more physiologically relevant investigations of tissue mechanics and degeneration in complex systems.

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Abstract: Directed energy deposition (DED) additive manufacturing (AM) can fabricate, repair, and join near-net-shaped components for high-performance engineering applications, including biomedical, energy, and transport sectors. The broader adoption of DED remains constrained by the limited number of alloys available that can be reliably manufactured without imperfections, hence limiting mechanical properties. Here, we designed an Al-Ni-Ce-Mn-Fe AM alloy that can achieve an ultra-fine microstructure (<5 μm), uniform distribution of intermetallics, low residual stress (<32 MPa), and superior mechanical properties in as-built DED components. Compared to DED AlSi10Mg in the as-build state using the same conditions, the yield increased by 70%, and the ultimate tensile strength by 50%. DED-AM involves rapid cooling and complex thermal conditions, which largely influence the property of the final components. Post-characterization cannot capture the time resolved thermal behavior, hence offer limited mechanism-based guide for alloy design. In this study, we develop a novel multimodal characterization methodology for correlative in situ X-ray imaging, X-ray diffraction, and infrared imaging, enabling quantification of the in situ thermal-related behavior, including phase evolution, temperature distribution and stress accumulation during DED. We elucidated key mechanisms driving the structure refinement and stress development in this alloy. The insights gained into the interplay between alloy composition, thermal-related behavior, and performance under specific AM conditions informs next-generation material design tailored for AM technologies.

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Abstract: Microdefects, including microcracks and resorption trenches, may be important contributors to bone fragility. 3D microdefect morphology was imaged using synchrotron micro-CT to develop a classification system for investigating the relationship with bone mechanics and hip-fractures. Femoral heads from ageing hip-fracture patients (n = 5, 74–82 years) were compared to ageing non-fracture controls (n = 5, 72–84 years). Two trabecular cores were prepared from the chiasma; one was imaged using synchrotron micro-CT to measure microdefects and one was mechanically tested to measure tensile strength. Morphological and mechanical data were compared and correlated using Mann Whitney U test and Pearson’s rank correlation. All the procedures performed were in accordance with the ethical standards of the Imperial College Tissue Bank (R13004) and the 1984 Declaration of Helsinki. Microdefects varied and were classified into four categories based on shape and measurable parameters. Hip-fracture donors exhibited significantly higher density of all microdefects (p < 0.05). Microdefect volume was strongly negatively correlated with ultimate tensile strength (p < 0.05) and stiffness (p < 0.05). Microdefects might contribute to loss of bone strength and fragility fracture via runaway resorption. Microcracks could promote focussed osteoclastic resorption and the formation of resorption pits which create stress risers leading to the re-formation of microcracks under continued load. CT-based classification methods should be used to explore the complex interaction between microdefects, metabolism, and bone fracture mechanics.

Open Access

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Abstract: Investigating the textural properties and 3D geometry of the connected pore network in volcanic products provides insight into magma ascent processes, due to their influence on magma permeability, outgassing efficiency and explosivity. Here, we used X-ray computed microtomography to investigate vesicle textures in tephra from the 2021 Tajogaite eruption (La Palma, Spain) and the relationship between these pore network parameters and eruptive style. We report a 3D dataset of pore network parameters for lapilli clasts collected throughout the eruption, associated with different eruptive styles (ash-rich jets, lava fountains, Strombolian activity). In clasts from Strombolian activity, the lower vesicle number density (VND) and tortuosity factor (m) suggests that there are fewer vesicles and that the channels which connect them are less tortuous than in clasts from fountain and ash-rich jet activity, favouring a lower degree of gas–melt coupling and thus, more efficient outgassing. Instead, for clasts of lava fountain and ash-rich jet activity, the higher VND and m suggest a higher number of vesicles connected by more tortuous channels, promoting some degree of gas–melt coupling and thus, less efficient outgassing. However, in clasts from ash-rich jets, the presence of narrower channels, as suggested by the lower throat-pore size ratio, favours a greater degree of gas–melt coupling with respect to fountain activity, leading to magma fragmentation. This work highlights the importance of textural and pore network analyses in understanding eruption dynamics, and provides a case study for investigating the interplay between pore network parameters, magma permeability and ascent dynamics for low-viscosity magmas.