A quasi-one-dimensional theory for anisotropic propagation of excitation in cardiac muscle

It has been shown that propagation of excitation in cardiac muscle is anisotropic. Compared to propagation at right angles to the long axes of the fibers, propagation along the long axis is faster, the extracellular action potential (AP) is larger in amplitude, and the intracellular AP has a lower maximum rate of depolarization, a larger time constant of the foot, and a lower peak amplitude. These observations are contrary to the predictions of classical one-dimensional (1-D) cable theory and, thus far, no satisfactory theory for them has been reported. As an alternative description of propagation in cardiac muscle, this study provides a quasi-1-D theory that includes a simplified description of the effects of action currents in extracellular space as well as resistive coupling between surface and deeper fibers in cardiac muscle. In terms of classical 1-D theory, this quasi-1-D theory reveals that the anisotropies in the wave form of the AP arise from modifications in the effective membrane ionic current and capacitance. The theory also shows that it is propagation in the longitudinal, not in the transverse direction that deviates from classical 1-D cable theory

Trucking Livestock to South St. Paul

This archival publication may not reflect current scientific knowledge or recommendations. Current information available from University of Minnesota Agricultural Experiment Station: http://www.maes.umn.edu

Experimental characterisation of the mechanical properties and microstructure of Acrocomia mexicana fruit from the Yucatan Peninsula in Mexico

A study of the mechanical properties and microstructure at different drying conditions of the Cocoyol fruit endocarp of Acrocomia Mexicana palm found in the Yucatan Peninsula in Mexico was performed. Quasi-static uniaxial compression was carried out on endocarp samples. The experimental results showed that the fruit exhibited an average peak force and displacement at failure of 4.23 kN and 2.43 mm, respectively. The average energy absorbed by the fruits before failure was 6.06 J. Optical and scanning electron microscopy of cross-sections of the equatorial region revealed that the endocarp has complex hierarchical structure. The micrographs showed that the structure is made of bundles of randomly oriented tubes and bubble-like cells, showing entangled network of hollow micro channels, which are in the order of tens of microns. The results and the microstructure presented herein encourage further research for bioinspired man-made materials.Australian Research Council (ARC) Centre of Excellence for Design in Light Metals (CE0561574). Consejo Nacional de Ciencia y Tecnologia, CONACyT (CB-2008-01, reg. 101608)

Effects of heat treatment and strain rate on the microstructure and mechanical properties of 6061 Al alloy

In the present work, the effects of heat treatment and strain rate on mechanical behaviour and microstructure evolution of aluminium alloy (AA) 6061 have been investigated. The micro-crack initiation and crystallographic texture evolution are obtained from scanning electron microscope (SEM) and electron back-scatter diffraction (EBSD) experiments. Quasi-static and high strain rate compression tests are conducted on AA6061 specimens that underwent two different heat treatments: the as-received material with the original T6 heat treatment and the heat treated and artificially aged (HT) specimens. For the high strain rate compression (~2000 s-1 and ~4000 s-1) tests, the split Hopkinson pressure bar apparatus is used. It is observed that the additional heat treatment has significantly reduced the yield strength of the material. Furthermore, EBSD results show that the higher the applied strain rate is, the less significant change will happen to the texture. SEM images show that, for both T6 and HT specimens, the number and size of micro-cracks in the dynamic compressed specimens are smaller than in the quasi-static deformed specimen. Therefore, the strain rate is considered to be the dominant factor in forming micro-cracks.Australian Research Council through Centre of Excellence for Design in Light Metals (CE0561574). Discovery Projects (DP130101291 and DP140100945). LIEF Project (LE100100045). The National Natural Science Foundation of China through Grant No. 11232003

Effects of heat treatment and strain rate on the microstructure and mechanical properties of 6061 Al alloy

In the present work, the effects of heat treatment and strain rate on mechanical behaviour and microstructure evolution of aluminium alloy (AA) 6061 have been investigated. The micro-crack initiation and crystallographic texture evolution are obtained from scanning electron microscope (SEM) and electron back-scatter diffraction (EBSD) experiments. Quasi-static and high strain rate compression tests are conducted on AA6061 specimens that underwent two different heat treatments: the as-received material with the original T6 heat treatment and the heat treated and artificially aged (HT) specimens. For the high strain rate compression (~2000 s-1 and ~4000 s-1) tests, the split Hopkinson pressure bar apparatus is used. It is observed that the additional heat treatment has significantly reduced the yield strength of the material. Furthermore, EBSD results show that the higher the applied strain rate is, the less significant change will happen to the texture. SEM images show that, for both T6 and HT specimens, the number and size of micro-cracks in the dynamic compressed specimens are smaller than in the quasi-static deformed specimen. Therefore, the strain rate is considered to be the dominant factor in forming micro-cracks.Australian Research Council through Centre of Excellence for Design in Light Metals (CE0561574). Discovery Projects (DP130101291 and DP140100945). LIEF Project (LE100100045). The National Natural Science Foundation of China through Grant No. 11232003

Normal faulting in the Simav graben of western Turkey reassessed with calibrated earthquake relocations

Western Turkey has a long history of large earthquakes, but the responsible faults are poorly characterized. Here we reassess the past half century of instrumental earthquakes in the Simav-Gediz region, starting with the 19 May 2011 Simav earthquake (Mw 5.9), which we image using interferometric synthetic aperture radar and regional and teleseismic waveforms. This event ruptured a steep, planar normal fault centered at 7–9 km depth but failed to break the surface. However, relocated main shock and aftershock hypocenters occurred beneath the main slip plane at 10–22 km depth, implying rupture initiation in areas of low coseismic slip. These calibrated modern earthquakes provide the impetus to relocate and reassess older instrumental events in the region. Aftershocks of the 1970 Gediz earthquake (Mw 7.1) form a narrow band, inconsistent with source models that invoke low-angle detachment faulting, and may include events triggered dynamically by the unilateral main shock rupture. Epicenters of the 1969 Demirci earthquakes (Mw 5.9, 6.0) are more consistent with slip on the south dipping Akdağ fault than the larger, north dipping Simav fault. A counterintuitive aspect of recent seismicity across our study area is that the largest event (Mw 7.1) occurred in an area of slower extension and indistinct surface faulting, yet ruptured the surface, while recent earthquakes in the well-defined and more rapidly extending Simav graben are smaller (Mw <6.0) and failed to produce surface breaks. Though our study area bounds a major metamorphic core complex, there is no evidence for involvement of low-angle normal faulting in any of the recent large earthquakes

Numerical investigation of the impact behaviour of bioinspired nacre-like aluminium composite plates

Inspired by the hierarchical structure of nacre, an aluminium alloy (AA) 7075 based composite featuring layer waviness and cohesive interface is studied as a low weight impact resistant material. To investigate the mechanical response and the ballistic performance of this laminated structure, a numerical study of the proposed nacre-like composite plates made of 1.1-mm thick AA 7075 tablets bonded with toughened epoxy resin was performed using Abaqus/Explicit. Target thicknesses of 5.4-mm, 7.5-mm and 9.6-mm impacted by a rigid hemi-spherical projectile were simulated. The epoxy material was modelled using a user-defined interface cohesive element with compressive strength enhancement. A significant performance improvement was recorded for the 5.4-mm nacre-like plate (compared to the same thickness bulk plate), which was explained by the hierarchical structure facilitating both localised energy absorption (by deformation of the tablet) and more globalized energy absorption (by inter-layered delamination and friction). For a given projectile, however, the performance improvement of using the proposed composite decreased with increasing laminate thickness, which was attributed to the increased likelihood of ductile failure occurring prior to perforation in thicker bulk plates. For 5.4-mm thick plates impacted at high velocity, the nacre-like plate had a better ballistic performance than that of the plates made of continuous (flat and wavy) layers, which was attributed to the larger area of plastic deformation (observed in the nacre-like plate after impact) due to the tablets arrangement.The Australian Research Council Centre of Excellence for Design in Light Metals (CE0561574); National Natural Science Foundation of China (No. 11232003); The Australian Research Council via project DP1093485