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Abstract: Strain rate hardening caused by the changed deformation mode is a fascinating phenomenon in cellular metamaterials where the material’s stiffness and energy absorption capabilities increase as the strain rate increases. This unique behaviour is attributed to a combination of micro-inertia effects, base material’s strain rate hardening and inertia effects. At high strain rates, the metamaterial’s inertia influences its deformation response, which changes to shock mode. This work briefly presents the geometry and fabrication of different metallic metamaterials. Then, it evaluates their mechanical response at different strain rates, ranging from quasi-static to intermediate dynamic and shock, determined by experimental and computational investigation. The three deformation modes can be separated into two critical loading velocities, unique for each metamaterial, which are also presented and compared in this work for various metamaterials. The investigations show that the deformation mode change in metallic metamaterials depends on their porosity. The critical velocities separating the deformation modes decrease with increasing porosity, i.e., decreased density of the metamaterial results in reduced critical loading velocities. The shock deformation mode in cellular metamaterials is thus attainable at much lower loading velocities than in homogeneous (nonporous) materials.
Keywords: metamaterials, cellular structures, high strain rate, experimental testing, computational modelling, compression loading, mechanical properties
Published in DKUM: 22.05.2024; Views: 204; Downloads: 24
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Abstract: We examined activation patterns of the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), soleus (SO), and tibialis anterior (TA) muscles in eight older (58.4 ± 3.3 years) and seven young (23.1 ± 2.9 years) participants, before and after 14 days of horizontal bed rest. Visual feedback on the exerted muscle torque was provided to the participants. The discharge patterns of individual motor units (MUs) were studied in three repetitions of isometric plantar flexion at 30 and 60% of Maximum Voluntary Contraction (MVC), before, and 1 day after the 14-day bed rest, respectively. In the GL and GM muscles, the older participants demonstrated higher MU discharge rates than the young, regardless of the contraction level, both before and after the bed rest. In the TA and SO muscles, the differences between the older and young participants were less consistent. Detailed analysis revealed person-specific changes in the MU discharge rates after the bed rest. To quantify the coactivation patterns we calculated the correlation coefficients between the cumulative spike trains of identified MUs from each muscle, and measured the root mean square difference of the correlation coefficients between the trials of the same session (intra-session variability) and between different sessions (inter-session variability) in each participant (intra-person comparison) and across participants (inter-person comparison). In the intra-person comparison, the inter-session variability was higher than the intra-session variability, either before or after the bed rest. At 60% MVC torque, the young demonstrated higher inter-person variability of coactivation than the older participants, but this variability decreased significantly after the bed rest. In older participants, inter-person variability was consistently lower at 60% than at 30% MVC torque. In young participants, inter-person variability became lower at 60% than at 30% MVC torque only after the bed rest. Precaution is required when analyzing the MU discharge and coactivation patterns, as individual persons demonstrate individual adaptations to aging or bed rest.
Keywords: mišičnoskeletni sistem, mišice, bed rest, staranje, elektromiografija, high density electromyography, muscle disuse, motor units, discharge rate, aging
Published in DKUM: 07.07.2023; Views: 539; Downloads: 61
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Keywords: aluminium foam, powder gun, high strain rate
Published in DKUM: 10.07.2015; Views: 1739; Downloads: 50
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Abstract: In present work the extraction of feverfew flower heads was performed using supercritical carbon dioxide at pressures from 200 to 800 bar and at temperatures of 40, 60 and 80 °C. For comparison, the conventional extractionswith organic solvents have been performed. Dry feverfew flower heads were used as starting material. Supercritical fluid extraction (SFE) wasperformed using a semi continuous flow apparatus in a laboratory and pilot scale. The influence of process parameters on the total yield and amount of parthenolide isolated was investigated. Dynamic behaviour of the extraction runs followed by single-step separation was analysed by a mathematical model for initial constant rate extraction period and the subsequent time-dependant diffusion controlling mass transfer rate period. In order to concentrate the obtained extract in parthenolide, a two-step separation was employed.
Keywords: chemical processing, high pressure extraction, conventional extraction, supercritical CO2, parthenolide, feverfew, extraction rate, mass transfer
Published in DKUM: 01.06.2012; Views: 2114; Downloads: 109
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