K.T. Ramesh

Effects of nanocrystalline and ultrafine grain sizes on constitutive behavior and shear bands in iron

The mechanical behaviors of consolidated iron with average grain sizes from tens of nanometers to tens of microns have been systematically studied under uniaxial compression over a wide range of strain rates. In addition to the well-known strengthening due to grain size refinement, grain size dependence is observed for several other key properties of plastic deformation. In contrast with conventional coarse-grained Fe, high-strength nanocrystalline and submicron-grained Fe exhibit diminished effective strain rate sensitivity of the flow stress. The observed reduction in effective rate sensitivity is shown to be a natural consequence of low-temperature plastic deformation mechanisms in bcc metals through the application of a constitutive model for the behavior of bcc Fe in this strain rate and temperature regime. The deformation mode also changes, with shear localization replacing uniform deformation as the dominant deformation mode from the onset of plastic deformation at both low and high strain rates. The evolution and multiplication of shear bands have been monitored as a function of plastic strain. The grain size dependence is discussed with respect to possible enhanced propensity for plastic instabilities at small grain sizes.

Deformation behavior and plastic instabilities of ultrafine-grained titanium

Ultrafine-grained (UFG) Ti samples have been prepared using equal channel angular pressing followed by cold rolling and annealing. The deformation behavior of these materials, including strain hardening, strain rate dependence of flow stress, deformation/failure mode, and tensile necking instability, have been systematically characterized. The findings are compared with those for conventional coarse-grained Ti and used to explain the limited tensile ductility observed so far for UFG or nanocrystalline metals.

Evolution and microstructure of shear bands in nanostructured Fe

Shear band development in consolidated nanocrystalline and ultrafine-grained Fe has been monitored as a function of overall strain from the onset of plastic deformation. The deformation mechanisms of the grains inside the shear bands, the origin of the inhomogeneous deformation, and the propensity for shear localization in nanostructures are explained based on microstructural information acquired using transmission electron microscopy.

Mechanics of Flexible Needles Robotically Steered through Soft Tissue

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.

Deformation and Failure of Zr 57 Ti 5 Cu 20 Ni 8 Al 10 Bulk Metallic Glass Under Quasi-static and Dynamic Compression

We have examined the mechanical behavior of Zr 5 7 Ti 5 Cu 2 0 Ni 8 Al 1 0 bulk metallic glass under uniaxial compression at strain rates from 10 - 4 to 3 x 10 3 s - 1 . The failure stress decreases with increasing strain rate, and shear-band formation remains the dominant deformation mechanism. A consideration of basic properties of adiabatic shear bands makes it appear unlikely that shear bands formed under quasi-static loading are adiabatic; in the dynamic case, the time scales of deformation and thermal conduction are similar, indicating that a more sophisticated calculation is required. In the dynamic tests, however, high-speed cinematography reveals evidence that the mechanism of failure involves an adiabatic component.

Modeling of tool-tissue interactions for computer-based surgical simulation: A literature review

Surgical simulators present a safe and potentially effective method for surgical training, and can also be used in robot-assisted surgery for pre- and intra-operative planning. Accurate modeling of the interaction between surgical instruments and organs has been recognized as a key requirement in the development of high-fidelity surgical simulators. Researchers have attempted to model tool-tissue interactions in a wide variety of ways, which can be broadly classified as (1) linear elasticity-based, (2) nonlinear (hyperelastic) elasticity-based finite element (FE) methods, and (3) other techniques not based on FE methods or continuum mechanics. Realistic modeling of organ deformation requires populating the model with real tissue data (which are difficult to acquire in vivo) and simulating organ response in real time (which is computationally expensive). Further, it is challenging to account for connective tissue supporting the organ, friction, and topological changes resulting from tool-tissue interactions during invasive surgical procedures. Overcoming such obstacles will not only help us to model tool-tissue interactions in real time, but also enable realistic force feedback to the user during surgical simulation. This review paper classifies the existing research on tool-tissue interactions for surgical simulators specifically based on the modeling techniques employed and the kind of surgical operation being simulated, in order to inform and motivate future research on improved tool-tissue interaction models.

The mechanical response of a 6061-T6 A1/A12O3 metal matrix composite at high rates of deformation

Abstract The mechanical properties of a 6061-T6 aluminum alloy reinforced with a 20 vol.% fraction of alumina particles and of an unreinforced 6061-T6 alloy are studied over a range of strain rates (10-4to 6 x 105s-1) using quasistatic compression, compression and torsion Kolsky Bars, and high strain rate pressure-shear plate impact. At a given strain rate the composite displays increased strength but essentially the same strain hardening as the matrix. However, the composite displays a stronger rate-sensitivity than does the unreinforced alloy at high rates of deformation (>103s-1). The rate-sensitivity of the unreinforced alloy is shown to be largely the result of the imposed strain rate rather than of the rate history. For quasistatic deformations, a model proposed by Bao et al. (1991) describes the behavior of the composite fairly accurately given the behavior of the unreinforced alloy. This paper presents an extension of the model that is able to predict the dynamic behavior of the composite given the dynamic response of the monolithic alloy.

Thermal fatigue as the origin of regolith on small asteroids

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.

Microstructure and mechanical properties of tantalum after equal channel angular extrusion (ECAE)

We have investigated the microstructure and mechanical properties of equal channel angular extruded (ECAE) Ta. Mechanical properties were measured both under quasi-static loading and dynamic loading (in the latter case, the compression Kolsky bar technique was employed to attain strain rates of ∼10 3 s −1 ). It is shown that four passes of ECAE with route C at room temperature, which results in an equivalent strain of ∼4.64, increases the strength of Ta by a factor of 2–3 under quasi-static loading, and by a factor of more than 1.5 under dynamic loading. Under quasi-static loading, the ECAE processed samples exhibit almost elastic-perfect plastic behavior; under dynamic loading, slight softening is observed, presumably due to adiabatic heating. It is found that ECAE decreases the strain rate sensitivity. Comparison of the X-ray diffraction (XRD) between the un-processed and ECAE processed Ta indicates significant broadening of the XRD peaks in the ECAE processed sample. Transmission electron microscopy reveals textured, elongated substructures with an average size of about 200 nm, and the substructures are separated by small angle grain boundaries. This work shows the potential for the production of ultra-fine grained or even nano-structured refractory metals with high melting points by using severe plastic deformation. Signs indicating increased shear localization tendancy were observed at high strain rates.

Finite deformations and the dynamic measurement of radial strains in compression Kolsky bar experiments

A novel technique has been developed for the direct non-contact measurement of the radial deformations of a specimen during a compression Kolsky bar (split-Hopkinson pressure bar) experiment. Application of the new technique makes possible an analysis of the compression Kolsky bar experiment in terms of finite deformations, since the technique provides a complete experimental determination of the deformation gradient tensor during dynamic loading. Using the new technique, we have determined the relative validity of the incompressibility and Bell constraints for finite deformation dynamic plasticity. The experimental results show that the plastic incompressibility constraint is more appropriate for the dynamic compression of 6061-T6 aluminum. It is also shown that the traditional measure of axial strain rate derived from Kolsky bar experiments should be replaced by the axial rate of deformation that is valid for finite deformations. Finally, the technique has been used to investigate the dynamic compression of porous pure iron. It is shown that the new technique extends the capabilities of the compression Kolsky bar technique to include the investigation of plastically compressible materials.