Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the ...regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.
Two of the key parameters that are useful for understanding the deformation kinetics in metals with ultrafine grain (UFG) and/or nanocrystalline (nc) microstructure are the strain rate sensitivity ...(SRS) of the flow stress and the flow stress activation volume. In this paper, we present experimental results from our in-house tests on UFG Cu, Fe and Ta, along with those available in literature, regarding the behavior of SRS and activation volume of UFG/nc metals with face-centered cubic (fcc) and body-centered cubic (bcc) structures. The UFG metals were produced via severe plastic deformation (SPD), and confirmed using transmission electron microscopy. Our own results, as well as those in the literature, indicate that the SRS of the UFG/nc fcc metals is elevated, whereas that of UFG/nc bcc metals is much reduced compared to their coarse-grained counterparts. These trends appear to hold independent of the processing routes used to refine the grain size and of the testing method employed. The activation volumes in UFG fcc and bcc metals also exhibit different behavior. We present discussions to explain the observed trends. The implications of these findings for plastic instability are also discussed for the two lattice structures.
We have investigated the plastic deformation behavior of ultrafine-grained (UFG) bulk Fe. The UFG microstructure (grain size ∼300 and ∼150 nm) was obtained through combined equal channel angular ...extrusion and low-temperature rolling, and confirmed using transmission electron microscopy. Mechanical properties were measured under both quasi-static (strain rate in the range of 10
−4–10
0 s
−1) and dynamic (strain rate up to ∼10
3 s
−1 using the Kolsky bar technique) compressive loading. Dynamic stress–strain curves of the UFG Fe exhibit significant flow softening, in addition to a significant reduction in the strain rate sensitivity of the flow stress. Consequently, under dynamic loading the plastic flow mode changes from the uniform deformation of conventional coarse-grained Fe to substantial localized deformation in UFG Fe. Both in situ high-speed camera movies and post-loading optical and scanning electron microscopy show the development of adiabatic shear bands, observed for the first time in pure Fe under compressive loading conditions. The morphology of the shear bands and the increased propensity for flow localization due to the ultrafine grain structure are explained using established models for adiabatic shear banding.
The conservation of chemoselectivity becomes invalid for multiple electrophilic warheads during protein bioconjugation. Consequently, it leads to unpredictable heterogeneous labeling of proteins. ...Here, we report that a linchpin can create a unique chemical space to enable site-selectivity for histidine and aspartic acid modifications overcoming the pre-requisite of chemoselectivity.
Linchpin-enabled promiscuous electrophile uncovers an unchartered reactivity landscape for the precision engineering of proteins.
The tip asymmetry of a bevel-tip needle results in the needle naturally bending when it is inserted into soft tissue. This enables robotic needle steering, which can be used in medical procedures to ...reach subsurface targets inaccessible by straight-line trajectories. However, accurate path planning and control of needle steering require models of needle-tissue interaction. Previous kinematic models required empirical observations of each needle and tissue combination in order to fit model parameters. This study describes a mechanics-based model of robotic needle steering, which can be used to predict needle behavior and optimize system design based on fundamental mechanical and geometrical properties of the needle and tissue. We first present an analytical model for the loads developed at the tip, based on the geometry of the bevel edge and material properties of soft-tissue simulants (gels). We then present a mechanics-based model that calculates the deflection of a bevel-tipped needle inserted through a soft elastic medium. The model design is guided by microscopic observations of needle-gel interactions. The energy-based formulation incorporates tissue-specific parameters, and the geometry and material properties of the needle. Simulation results follow similar trends (deflection and radius of curvature) to those observed in experimental studies of robotic needle insertion.
The mechanical response of single crystal silicon carbide (SiC) of two hexagonal polytypes (six layer, 6H- and four layer, 4H) was investigated using nanoindentation. Indentations were performed on ...two specific crystallographic orientations of single crystals i.e., normal to the basal, (0001) and prismatic, (101¯0) planes, in the load range between 25 mN and 500 mN. A significant anisotropy in the hardness is observed with the basal orientations showing a higher hardness compared to prismatic orientations. In both orientations, the 6H-SiC polytype exhibits higher hardness than the 4H-SiC polytype. It is also observed that the hardness decreases with increasing indentation load, suggesting that SiC crystals exhibit indentation size effect. However, unlike hardness, elastic modulus is independent of indentation load and the elastic anisotropy is insignificant. Severe cracking, particularly at higher indentation loads is noticed near the edges of the indentation imprints. The indentation fracture toughness, KICi computed from the imprints shows slightly higher values for 6H-SiC compared to the 4H-SiC. However, for both the polytypes, a slightly higher KICi is observed for basal indentations compared to the prismatic ones.
•4H- and 6H-SiC single crystal polytypes exhibit a strong indentation size effect.•Nanohardness of basal (0001) orientations is higher than the prismatic (101¯0) ones.•The indentation fracture toughness of (0001) orientations is higher than the (101¯0).•Hardness and indentation fracture toughness of 6H-SiC is higher than the 4H-SiC single crystals.
In-situ ultra high speed optical imaging with 200 ns temporal resolution is used to gain insight into the evolution of twinning in single crystal magnesium under dynamic loading. Under compression ...along the a-axis, nucleation of twins is observed to occur on two conjugate {101¯2} twin planes. Twin nucleation appears to be stress-driven with the first twins nucleating at resolved stresses of 5–7 MPa. These first twins propagate across the specimen at very high speeds of the order 1 km/s. After the first twins stop growing, twin boundary growth is observed to be very small and relatively slow. The nucleation of additional twins from the boundaries of pre-existing twins is found to be preferred over twin boundary growth at these rates of loading. As a result, twin nucleation is found to have a dominant contribution to twin volume fraction evolution at later times. Lastly, twinning is found to have a dominant contribution to the net plastic strain at these strain rates.
The mechanical behavior of extruded pure magnesium was studied experimentally under high strain rate ) compression loading in the extrusion direction. Electron back scattered diffraction was used to ...examine the changes in the texture and transmission electron microscopy was used to investigate the dislocation structures in the material. Extensive grain reorientation due to extension twinning is observed. Dislocation activity is observed inside the parent region ( slip) as well as the twinned region ( slip). The high degree of strain hardening observed is postulated to be due to the texture hardening associated with extension twinning coupled with significant increase in the dislocation density with strain. In compression along the extrusion direction, extension twinning and dislocation activity are both needed to accommodate plastic deformation.
– Dynamic and quasistatic unconfined compression experiments were performed on samples of the L5 ordinary chondrite MacAlpine Hills (MAC) 88118. The dynamic compressive strength measurements are the ...first of its kind for a stony meteorite, and indicate that the material tested here exhibits significant increase in compressive strength with increasing strain rate. This increase in compressive strength is larger than the increases typically observed in terrestrial rock specimens compressed over a similar range of strain rates. The details of the failure process are investigated by recording images of the experiments in real time. Specifically, the crack growth speeds in the dynamic compression experiments are measured from ultrahigh‐speed images. These speeds are significantly lower than the speeds observed in terrestrial analogs that are typically used in numerical investigations of large‐scale impacts. The implications of these measurements of strength and failure process are explored in the contexts of atmospheric disruption of meteors and impact cratering/disruption of asteroids.
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