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We present a stability map which predicts the domains of shear instability due to grain rotation in nanocrystalline materials. The onset of this mode of instability is influenced by grain-size-dependent mechanisms and the length-scale of intergranular interaction. The map shows the grain size regimes that are inherently susceptible to this mode for a range of materials. In the amorphous limit, the model predicts embryonic nuclei sizes of about 10-50 nm, which agrees well with the shear band thicknesses for many metallic glasses.

Realistic modeling of medical interventions involving tool-tissue interactions has been considered to be a key requirement in the development of high-fidelity simulators and planners. Organ geometry, the soft-tissue constitutive laws, and boundary conditions imposed by the connective tissues surrounding the organ are some of the factors that govern the accuracy of medical intervention planning. In this study it is demonstrated that (for needle path planning) the organ geometry and boundary constraints surrounding the organ are the most important factors inuencing the deformation. As an example, the procedure of needle insertion into the prostate (e.g. for biopsy or brachytherapy) is considered. Image segmentation is used to extract the anatomical details from magnetic resonance images, while object-oriented finite element analysis (OOF) software is used to generate finite element (FE) meshes from the segmented images. Two-dimensional FE simulations that account for complex anatomical details along with relative motion between the prostate and its surrounding structure using cohesive zone models are compared with traditional simulation models having simple organ geometry and boundary constraints. Nodal displacements for these simpler models were observed to be up to 14 times larger than those obtained from the anatomically accurate models.

Experimental techniques to study and characterize the dynamic behavior of ceramics and ceramic composites are described. A new technique based on the split Hopkinson pressure bar is used to obtain the deformation and failure characteristics of brittle materials at moderately high strain rates. A normal plate impact recovery technique is used to study the evolution of damage under controlled loading conditions and understand the micromechanisms of damage in materials. The normal plate impact recovery technique has the advantage of applying sufficiently high stress levels to initiate damage and yet providing controlled amounts of energy input to avoid catastrophic failure of the material. Hence, the samples can be recovered intact and evaluated for damage using standard characterization techniques such as microscopy and ultrasonics. The damage assessment in the material together with the real-time interferometric measurements serves as a powerful experimental procedure to study processes such as damage initiation, damage evolution, and the failure modes of very hard and brittle ceramics and ceramic composites under stress wave loading. Results are presented for a boron carbide aluminum cermet, a ceramic composite and the role of its microstructure on the failure mechanisms is studied. The cermet exhibits high strength and large strain to failure, in comparison to monolithic ceramics. The basic failure mechanisms in the cermet appear to be axial microcracking due to compression, and transverse cracking generated by compression release and by tension. The cracks are also found to be bridged by ductile aluminum or intermetallics, which together with the microcracking could possibly increase the toughness of the cermet at high loading rates.

Realistic modeling of the interaction between surgical instruments and human organs has been recognized as a key requirement in the development of high-fidelity surgical simulators. Primarily due to computational considerations, most of the past simulation research within the haptics community has assumed linear elastic behavior for modeling tissues, even though human soft tissues generally possess nonlinear viscoelastic properties. Hence, this paper quantitatively compares linear and nonlinear elasticity-based models. It is demonstrated that, for a nonlinear model, the well-known Poynting effect developed during shearing of the tissue results in normal forces not seen in a linear elastic model. The difference in force magnitude and force direction for linear and nonlinear models are larger than the just noticeable difference for contact force and force-direction discrimination thresholds published in the psychophysics literature, respectively. This work applies a proposed framework for examining the effect of tool-tissue interaction modeling techniques on human perception of surgical simulators with haptic feedback

We present results on (a) relatively new “nano-micro” composites that are consciously developed by controlling structure at two different length scales, and (b) on metallic glasses (which may be viewed as an extreme case of “nanostructure”).

Although a number of cytoskeletal derangements have been described in the setting of traumatic axonal injury (TAI), little is known of early structural changes that may serve to initiate a cascade of further axonal degeneration. Recent work by the authors has examined conformational changes in cytoskeletal constituents of neuronal axons undergoing traumatic axonal injury (TAI) following focal compression through confocal imaging data taken in vitro and in situ. The present study uses electron microscopy to understand and quantify in vitro alterations in the ultrastructural composition of microtubules and neurofilaments within neuronal axons of rats following focal compression. Standard transmission electron microscopy processing methods are used to identify microtubules, while neurofilament identification is performed using antibody labeling through gold nanoparticles. The number, density, and spacing of microtubules and neurofilaments are quantified for specimens in sham Control and Crushed groups with fixation at

The current research extends our previous work on brittle fragmentation of a one-dimensional bar [F. Zhou, J.-F. Molinari, K.T. Ramesh, A cohesive-model based fragmentation analysis: effects of strain rate and initial defects distribution, International Journal of Solids and Structures 42 (2005) 5181-5207] to a circular ring that is dynamically expanded (e.g. by explosive loading). The expanding ring test is a convenient and effective technique to study the dynamic fracture and fragmentation properties of materials under high strain rate tensile loading. In this paper we describe the basic elastodynamic equations that govern the expansion of the ring, and develop a numerical integration scheme based on characteristic line differential relationships to solve these equations. A dynamic crack initiation criterion and a cohesive crack growth model are used to describe the failure and fracture process. With this methodology we investigate the fragmentation processes of rings under two typical loading conditions: forced constant velocity expansion and free expansion. The effects of ring size and the fracture properties of the material are investigated. The Rayleigh distribution function is used to describe the statistical characteristics of the fragment populations. Finally we discuss the energy conversions during the fragmentation process. (c) 2005 Elsevier B.V. All rights reserved.

We have systematically investigated the quasi-static and dynamic mechanical behavior (especially dynamic failure) of ultra-fine grained (UFG) tungsten (W) under uniaxial compression. The starting material is of commercial purity and large grain size. We utilized severe plastic deformation to achieve the ultrafine microstructure characterized by grains and subgrains with sizes of ∼500 nm, as identified by transmission electron microscopy. Results of quasi-static compression show that the UFG W behaves in an elastic–nearly perfect plastic manner (i.e., vanishing strain hardening), with its flow stress approaching 2 GPa, close to twice that of conventional coarse grain W. Post-mortem examinations of the quasi-statically loaded samples show no evidence of cracking, in sharp contrast to the behavior of conventional W (where axial cracking is usually observed). Under uniaxial dynamic compression (strain rate ∼103 s−1), the true stress–true strain curves of the UFG W exhibit significant flow softening, and the peak stress is ∼3 GPa. Furthermore, the strain rate sensitivity of the UFG W is reduced to half the value of the conventional W. Both in situ high-speed photography and post-mortem examinations reveal shear localization and as a consequence, cracking of the UFG W under dynamic uniaxial compression. These observations are consistent with recent observations on other body-centered cubic metals with nanocrystalline or ultrafine microstructures. The experimental results are discussed using existing models for adiabatic shear localization in metals.

The dynamic response of soft human tissues in hydrostatic compression and simple shear is studied using the Kolsky bar technique. We have made modifications to the technique that allow loading of a soft tissue specimen in hydrostatic compression or simple shear. The dynamic response of human tissues (from stomach, heart, liver, and lung of cadavers) is obtained, and analyzed to provide measures of dynamic bulk modulus and shear response for each tissue type. The dynamic bulk response of these tissues is easily described by a linear fit for the bulk modulus in this pressure range, whereas the dynamic shearing response of these tissues is strongly non-linear, showing a near exponential growth of the shear stress.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Computational solid mechanics (CSM) for ballistics, as it exists today, does not have predictive capabilities that are comparable to those of computational fluid dynamics (CFD). The most important reason for this is the lack of high quality physical models for the damage and failure processes that occur during high‐speed impact. An approach for modeling impact damage in metals, based on physical modeling of adiabatic shear bands and voids, will be presented. Scaling laws for damage from shear bands have been developed over the past decade from detailed analysis of the thermomechanical processes, and are being adapted for efficient use in large‐scale computations. A similar approach for ductile void growth and spall has also been initiated. This approach is expected to lead to models that are based on the essence of the physics, rather than on fitting of phenomenological models to large databases.

This paper describes a simple yet accurate technique for the continuous measurement of projectile velocities in plate impact experiments (the impact velocity is a critical parameter in the interpretation of such experiments). The technique, known as the laser line velocity sensor (LLVS), uses a laser line generator to generate a laser sheet of uniform intensity that is incident on a photodiode at the focus of a collecting lens. The amount of light reaching the photodiode diminishes as the projectile intersects the laser sheet, and so a simple calibration procedure in conjunction with a high‐bandwidth optoelectronic system provides a measure of the projectile velocity. Since the measurement is continuous, both the projectile velocity and acceleration can be obtained. The LLVS is very stable, does not need reconstruction after impact, does not need frequent recalibration, and is insensitive to rigid body motions. An example of the application of the technique is provided.

Microstructural evolution of bond coat with thermal cycling was characterized with transmission electron microscopy (TEM) and high temperature X-ray diffraction (HT-XRD) analysis. Before thermal cycling, the structure of asfabricated bond coat was confirmed to be a long-range ordered B2 β-phase. After thermal cycling to ∼28% of the cyclic life, the bond coat was found to transform into a Nirich L10 martensite (M) from its original B2 structure. The transformations, M ↔ B2, were demonstrated to be reversible and to occur on heating and cooling in each cycle. Quantitative high temperature XRD measurements verified the phase transformations produce about 0.7 % transformation strain. Finite element calculations incorporating the transformation strain indicate that the mertensitic transformation significantly influences the development of stresses and strains in TBC systems.

Twinning is an important mode of deformation in magnesium (Mg) and its alloys at high strain rates. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension-compression asymmetry, and sometimes non-Schmid effects. Extension twins in Mg can accommodate significant plastic deformation as they grow, and thus twinning affects the overall rate of plastic deformation. We use an experimental approach to study the deformation twinning mechanism under dynamic loading. We perform normal plate impact recovery experiments (with microsecond pulse durations) on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several m s −1 . These velocities are several orders of magnitude higher than those so far measured in Mg under quasi-static loading conditions. Electron back-scattered diffraction (EBSD) is then used to characterize the nature of the twins and the microstructural evolution. Detailed crystallographic analysis of the twins enables us to understand twin nucleation and growth of twin variants under dynamic loading.

In order to investigate the strain-rate dependence of the mechanical behavior over a wide range of strain rates a small version of the Kolsky Bar, called the Desktop Kolsky Bar, has been developed for testing small samples. The results of the experimental and numerical examinations show that this technique can be utilized for measuring the high-rate behavior at strain rates from approximately 10(+3)s(-1) up to approximately 5 x 10(+4)s(-1). The technique can bridge the capabilities of the conventional Kolsky bar and pressure-shear plate impact techniques, and thus improve our understanding of material behavior at high rates. Our experimental work has all been on the aluminum alloy 6061-T651. While we have been able to obtain both desktop Kolsky bar data at 5x10(4)s(-1) and pressure-shear plate impact data on this material, we have not as yet been able to obtain overlapping data. The lowest strain rate that we have achieved in the pressure-shear experiment is 8x10(4)s(-1), rather than the 5x10(4)s(-1) that needed for direct comparison.

We have investigated failure in bulk metallic glass-forming alloys under dynamic compression. We implemented a recovery technique for the compression Kolsky bar to obtain dynamically deformed, intact specimens at various stages of deformation; this allows us to characterize the development of failure. We have also used high-speed photography to examine the failure process during the recovery experiments. The experimental results indicate that the failure under dynamic loading is somewhat different from that under quasi-static loading. Specimens subjected to quasistatic deformation developed multiple shear bands and substantial plastic deformations, while specimens subjected to dynamic (—strain rate ∼103 s-1) compressive loading fail by fracture along one dominant shear band. The mechanisms of dynamic failure in bulk metallic glasses are discussed on the basis of these experimental results.

In this paper, we present an analytical model to investigate stochastic size effects on the critical stress for the activation of non-basal slip and contraction twinning (CT) in miniaturized hexagonal-close packed (HCP) single crystals. The analysis is based on the basic model developed by Parthasarathy et al (Scr. Mater. 56 313–6), but extends it to account for the mechanism-based description of pyramidal slip and CT in HCP metals. The results demonstrate that for a given initial dislocation density, a probabilistic competition exists among CT nucleation, CT growth and pyramidal slip. Based on the stochastic model, we construct a mechanism map that provides regimes of dominant deformation modes that govern the macroscopic yield stress in HCP single crystals.

We have developed two techniques for time-resolved x-ray diffraction from bulk polycrystalline materials during dynamic loading. In the first technique, we synchronize a fast detector with loading of samples at strain rates of ~10(3)-10(4) s(-1) in a compression Kolsky bar (split Hopkinson pressure bar) apparatus to obtain in situ diffraction patterns with exposures as short as 70 ns. This approach employs moderate x-ray energies (10-20 keV) and is well suited to weakly absorbing materials such as magnesium alloys. The second technique is useful for more strongly absorbing materials, and uses high-energy x-rays (86 keV) and a fast shutter synchronized with the Kolsky bar to produce short (~40 μs) pulses timed with the arrival of the strain pulse at the specimen, recording the diffraction pattern on a large-format amorphous silicon detector. For both techniques we present sample data demonstrating the ability of these techniques to characterize elastic strains and polycrystalline texture as a function of time during high-rate deformation.

Thermal barrier coatings (TBCs) are known to spall as a result of the residual stresses that develop during thermal cycling. TBC's are multi-layered coatings comprised of a metallic bond coat, thermally grown oxide and the ceramic top coat, all on top of a Ni-base superalloy substrate. The development of residual stresses is related to the generation of thermal, elastic and plastic strains in each of the layers. The focus of the current study is the development of a finite element analysis (FEA) that will model the development of residual stresses in these layers. Both interfacial roughness and material parameters (e.g., modulus of elasticity, coefficient of thermal expansion and stress relaxation of the bond coat) play a significant role in the development of residual stresses. The FEA developed in this work incorporates both of these effects and will be used to study the consequence of interface roughness, as measured in SEM micrographs, and material properties, that are being measured in a parallel project, on the development of these stresses. In this paper, the effect of an idealized three-dimensional surface roughness is compared to residual stresses resulting from a grooved surface formed by revolving a sinusoidal wave about an axis of symmetry. It is shown that cylindrical and flat button models give similar results, while the 3-D model results in stresses that are significantly larger than the stresses predicted in 2-D.

We have studied plastic deformation, including “serrated flow,” of bulk metallic glasses under quasi-static uniaxial compression. The deformation response is essentially elastic-perfectly plastic, but the “plastic” deformation actually consists of sections of elastic loading separated by abrupt load drops. The load drops are due to the formation of shear bands, which represent the primary mechanism of plastic deformationIn Zr-Ti-Cu-Ni-Al bulk metallic glasses, fracture occurs after about 1-2% plastic strain, but in Zr-Ta-Cu-Ni-Al metallic glass the plastic strain to failure can be as large as 6-7%. The difference appears to be due a strong tendency for the shear bands in this alloy to branch. The branching presumably reduces the stress concentration on the shear bands, retarding the onset of fracture. No evidence is seen for the formation of crystalline phases in this alloy.

The ability of some polymeric gels to shrink and swell with changes in their environment makes them of interest in many applications such as artificial muscles and drug delivery systems. While much work has been done to study the behavior and properties of these gels, little information is available regarding the full constitutive description of the mechanical and actuation properties. This work is focused on developing constitutive descriptions of the mechanical properties of such gels, and to determine how these properties change due to changes in the environment. Since these gels can undergo finite elastic deformations, uniaxial tests do not provide sufficient property information and a combination of loading conditions must be used. A biaxial testing system has been developed to test thin sheets of these films, and includes the ability to monitor and change the environmental conditions around the specimen. Initial tests were performed on latex to determine the quality of the testing apparatus. Preliminary results on a polyacrylonitrile gel are presented.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Active polymer gels can achieve large, reversible deformations in response to environmental stimuli, such as the application of an electric field or a change in pH level. Consequently, great interest exists in using these gels as actuators and artificial muscles. The goal of this work is to characterize the mechanical properties of ionic polymer gels and to describe how these properties evolve as the gel actuates. Experimental results of uniaxial tests on poly(vinyl alcohol)-poly(acrylic acid) gels are presented for both acidic and basic environments. These materials are shown to be to be slightly viscoelastic and compressible and capable of large recoverable deformations. The gels also exhibit similar stress in response to mechanical deformation in both the acid and the base.

Thin film composite sensors have been fabricated which incorporate piezoelectric ceramic particles in apolymer matrix. These composites are more compliant than pure piezoelectric ceramics and can beembedded in thick structures to monitor internal mechanical conditions, such as the evolution of damage. Composite films consisting of Ca-modified lead titanate particles in a poly(vinylidene fluoride- trifluoroethylene) matrix are examined in this paper. The viscoelastic properties of these composites (withvarious volume fractions, up to 60% ceramic) have been measured over a range of frequencies (0.01 to 100rad/sec). The complex h1 piezoelectric coefficient has also been measured for these composites for various volume fractions and over a frequency range from 5 to 100 rad/sec. The magnitudes and phase angles of thepiezoelectric coefficients are shown to be highly frequency dependent.Keywords: piezoelectric, composite, sensor, lead titanate, P(VDF-TrFE) 1. INTRODUCTION Much information regarding the mechanical status of a structure can be obtained through surface monitoringsensors such as strain gages. However, as thick polymeric composite structures become increasingly utilized,many mechanical issues (such as residual stresses and the progressive evolution of internal damage) would bebetter sensed from within the structure. Currently, embedded sensors are being used to detect stress waves in

The potential applications of pure piezoelectric ceramics as embedded sensors are limited by the stiffness and brittleness of these materials. To achieve a more compliant sensor, composites have been developed which incorporate calcium-modified lead titanate particles in an polymer matrix. Such compliant sensors, in thin film form, may be useful within thick composite structures such as tank hulls and helicopter blades. The mechanical response of 0 - 3 composite films of Ca-modified lead titanate in polyvinylidene fluoride-trifluoroethylene and Epon828 epoxy matrices is investigated in this work. The electrical response of these composites is currently being studied by Wenger et al. The viscoelastic properties of these thin film composites with various volume fractions have been measured over a wide range of frequencies and temperatures. The observed mechanical response of these heterogeneous materials is compared with the predictions of several simple models for such composites. Preliminary piezoelectric results are also presented.

Le comportement mecanique de polycristaux de Vanadium (purete commerciale : 99,8%) a ete caracterise en compression pour des taux de deformation allant de 10 -4 a 6.10 3 s -1 . L'evolution de la limite d'elasticite en fonction de la temperature a par ailleurs ete analysee sur une plage de 25 a 700 °C. L'observation en microscopie optique des echantillons deformes a montre l'absence de mâcle, de bande de deformation ou de micro-fissure, ce qui suggere un mode de deformation par glissement cristallographique. A l'exception d'une anomalie de limite elastique vers 400 °C, le taux de deformation et l'evolution de la limite d'elasticite en fonction de la temperature est caracteristique des metaux cubiques centres.

As received 1100-O aluminum was cold rolled (CR) to 30%, 70%, and 80% reduction, respectively, to study the effects of microstructural evolution on the spall response using plate impact experiments. Previous results show a sharp increase in pullback velocity for 1100-O aluminum with increase in peak shock stress between 4.0 and 8.3 GPa due to hardening, followed by a decrease for peak shock stresses up to 12.0 GPa possibly due to softening. This maximum was not observed for the 30% CR, which showed only an increase in pullback velocity over the shock stress range of 4.0–12.0 GPa due to hardening (net increase in dislocation density). For the 70% CR aluminum, no change was observed in the pullback velocity over the range tested (4.0–11.0 GPa) probably due to saturation in dislocation density. Similar observations were made for the 80% CR, that is, no change was observed in the spall response between 4.0 GPa and 11.0 GPa. However, variations were observed in the spall response for the 80% CR, and these variations are attributed to material inhomogeneity possibly caused by increased cold rolling beyond saturation. The results also show a significant increase in Hugoniot Elastic Limit with increase in percent cold rolling.

Plate impact experiments were conducted to study the effects of peak shock stress and pulse duration on the spall response of fully annealed 1100 aluminum. The spall strength was observed to decrease as the pulse duration was increased from approximately 0.58 μs to 1.17 μs. Also, an increase in tensile unloading strain rate increases the spall strength. However, our results also show an increase in spall strength with increase in peak shock stress up to approximately 8.3 GPa, followed by a decrease in spall strength for higher shock stresses.

We describe a novel valve-based microfluidic axon injury micro-compression (AIM) platform that enables focal and graded compression of micron-scale segments of single central nervous system (CNS) axons. The device utilizes independently controlled "push-down" injury pads that descend upon pressure application and contact underlying axonal processes. Regulated compressed gas is input into the AIM system and pressure levels are modulated to specify the level of injury. Finite element modeling (FEM) is used to quantitatively characterize device performance and parameterize the extent of axonal injury by estimating the forces applied between the injury pad and glass substrate. In doing so, injuries are normalized across experiments to overcome small variations in device geometry. The AIM platform permits, for the first time, observation of axon deformation prior to, during, and immediately after focal mechanical injury. Single axons acutely compressed (~5 s) under varying compressive loads (0-250 kPa) were observed through phase time-lapse microscopy for up to 12 h post injury. Under mild injury conditions (55 kPa) ~73% of axons continued to grow, while at moderate (55-95 kPa) levels of injury, the number of growing axons dramatically reduced to 8%. At severe levels of injury (95 kPa), virtually all axons were instantaneously transected and nearly half (~46%) of these axons were able to regrow within the imaging period in the absence of exogenous stimulating factors.

The one-dimensional fragmentation of a dynamically expanding ring (Mott's problem) is studied numerically to obtain the fragment signatures under different strain rates. An empirical formula is proposed to calculate an average fragment size. Rayleigh distribution is found to describe the statistical properties of the fragment populations. (c) 2006 American Institute of Physics.

High-speed photography has been used to observe the dynamic failure of uncoated and Cr-coated transparent polycrystalline aluminum oxynitride (AlON) material, undergoing uniaxial, high-strain rate compression with a modified compression Kolsky bar technique. The high-speed photographs are correlated in time with measurements of the stresses in the specimen. In the fully transparent samples, the dynamic activation, growth, and coalescence of cracks and resulting damage zones from spatially separated internal defects have been directly observed and correlated to the macroscopic loss of the load-carrying capacity and ultimate catastrophic failure of this material. Identical experiments on the coated material only show the dynamic progressive failure on the specimen surface and do not show the origin of the failure images at the internal defects.

Shear localization is demonstrated in bulk tungsten (W) of commercial purity under dynamic uniaxial compression. Microstructure refinement via severe plastic deformation was the strategy used to induce this unusual deformation mode for W. The ultrafine microstructure achieved in bcc materials leads to elevated strength and ductility, as well as reduced strain hardening and strain rate hardening, thus enhancing the propensity for adiabatic plastic flow localization.

A new plate impact configuration is described for subjecting lubricants to simple shearing motion under uniform hydrostatic pressure. Thin layers (∼50 μm thick) of the lubricant are confined between two hard metallic plates which are subjected to impact by a parallel plate that is inclined relative to the direction of approach in order to induce both pressure and shear loading. Stress waves in the hard plates are monitored by laser interferometry; all measurements are made before unloading waves arrive from the periphery. For approximately one microsecond the compressed lubricant is subjected to a simple shearing motion and a continuous record of the shear stress and shear rate is obtained. Results presented for the lubricant 5P4E at pressures of 1-5 GPa and shear rates of approximately 9 × 105 s−1 show good agreement with available data on the dependence of the limiting shear stress on the confining pressure at lower pressures and lower shearing rates.

Results are presented of pressure-shear plate impact experiments performed on a traction fluid. The overall behavior of this material appears to be very similar to that of elastohydrodynamic (EHD) lubricants such as 5P4E: the material exhibits a saturation of the flow stress with strain rate, and a nearly linear dependence of the critical shear stress on the applied pressure. The plate impact results are compared with those of other workers using completely different experimental techniques and operating at lower strain rates and much lower pressures. The results obtained from the various techniques are in general agreement.

The tensile strength of single-walled carbon nanotubes (CNT) is examined using a continuum-atomistic (CA) approach. The strength is identified with the onset of the CNT instability in tension. The focus of this study is on the effects of multi-body atomic interactions. Multiscale simulations of nanostructures usually make use of two- and/or three-body interatomic potentials. The three-body potentials describe the changes of angles between the adjacent bonds – bond bending. We propose an alternative and simple way to approximately account for the multi-body interactions. We preserve the pair structure of the potentials and consider the multi-body interaction by splitting the changing bond length into two terms. The first term corresponds to the self-similar deformation of the lattice, which does not lead to bond bending. The second term corresponds to the distortional deformation of the lattice, which does lead to bond bending. Such a split of the bond length is accomplished by means of the spherical–deviatoric decomposition of the Green strain tensor. After the split, the continuum-atomistic potential can be written as a function of two bond lengths corresponding to the bond stretching and bending independently. We apply an example exponential continuum-atomistic potential with the split bond length to the study of tension instability of the armchair and zigzag CNTs. The results of the study are compared with those obtained by Zhang et al. (2004. J. Mech. Phys. Solids 52, 977–998) who studied tension instability of carbon nanotubes by using the Tersoff–Brenner three-body potential, and with recent experimental results on the tensile failure of single walled carbon nanotubes.

Improvements in computational and analytical modelling of large-scale impact and catastrophic disruption events will come from a better understanding of the failure processes that are active during high strain-rate events. In this study we investigate the dynamic compressive failure and fragmentation of basalt, paying particular attention to the role each constituent mineral phase has in these processes. Our results indicate the existence of two fragmentation mechanisms: I. a mechanism that creates small fragments that is associated with the spacing of critically activated defects. These fragments are primarily comprised of pyroxene (which has the lowest fracture toughness in this material). II. a mechanism related to larger fragments that is associated with the structural failure of the sample. These fragments are primarily polyphase and polygrain in composition. In the second part of the paper, we investigate the strength of fragmented basalt material for different initial fragment size distributions: 1. between 10 and 100 microns, 2. between 200 and 800 microns, and 3. between 300 and 1,800 microns. The porosity of each of the three samples was maintained between 30 and 35%. Understanding the composition of the fragments beforehand allows us to better interpret our experimental results, which indicate that the strengths of the fragmented material increased with decreasing fragment sizes, from 4 to 25 MPa, and then to 175 MPa. An increase in strength with smaller fragment sizes is expected because of the associated increase in frictional dissipation, and decrease in the relative contribution of compaction and fracturing mechanisms. However, we do note that fragments less than 100 microns fail as a result of the activation of a different critical defect type than in the bulk material and for fragments larger than 100 microns, where olivine grains are the key contributors to fracture. Altogether, these results highlight the influence of the composition and defect population of planetary materials on the associated length scales that arise from dynamic failure and fragmentation.

Cermets are hard ceramic-metal composites with predominant ceramic phases and are finding increasing applications. It is important to understand the evolution of damage and the failure modes in these materials under dynamic loading conditions in order to design them for impact resistant applications. In this investigation a plate impact recovery technique [1,2] is used to subject cermet specimens to a single step compressive pulse of known amplitude and duration. This technique has the advantage of applying sufficiently high stress levels to initiate damage and yet providing controlled amounts of energy input to avoid catastrophic failure of the material. Ultrasonic velocity and attenuation studies are carried out on the recovered specimens to asses the evolution of damage under stress wave loading. Optical microscopy and image processing techniques are used to quantify the extent of damage in the recovered samples. Techniques such as ultrasonic measurements could prove to be useful in predicting damage levels in materials subjected to dynamic loading. The material chosen for study is a boron carbide-aluminum cermet [3].

Some preliminary results of a series of plate impact recovery experiments on boron carbide-aluminum cermets are reported. The experimental procedure is discussed, and the possible mechanisms of microcracking during loading and unloading of the specimen are explored.

An experimental investigation has been conducted into the response of elastohydrodynamic (EHD) lubricants subjected to the high shear rates and pressures encountered in EHD lubrication and over comparable time scales. New techniques have been developed to measure the dynamic compressibility of EHD lubricants at pressures as high as 1 GPa, and the response to dynamic shearing at shear rates as high as 4.0 x 104 sec-1, over timescales of about 10-4 sec, and for a range of initial temperatures.

Thin film composite sensors have been fabricated which incorporate piezoelectric ceramic particles in a polymer matrix. Such composites are more compliant than pure piezoelectric ceramics and can be embedded in thick structures to monitor internal mechanical conditions such as the evolution of damage. The mechanical properties and electromechanical interaction of composite films consisting of Ca-modified lead titanate particles in a poly(vinylidene fluoride-trifluoroethylene) matrix are examined in this paper. The viscoelastic properties of these composites (with various volume fractions, up to 60% ceramic) have been measured over a range of frequencies (0.01 to 100 rad ). These composite properties are primarily controlled by the viscoelastic properties of the polymer matrix, and are shown to depend greatly on frequency and ceramic content. The complex piezoelectric coefficients and have been measured for these composites for various volume fractions and over a frequency range from 5 to 100 rad . The magnitudes and phase angles of the piezoelectric coefficients are also shown to be highly frequency dependent.

Ionic polymer gels shrink and swell in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as actuators and artificial muscles. This work focuses on developing a means of characterizing the mechanical properties of such ionic gels and describing how these properties evolve as the gel actuates. A thermodynamically consistent finite elastic constitutive model of an active polymer gel is developed to describe this behavior. The mechanical properties of the gel are characterized by a strain-energy function and the model utilizes an evolving internal variable to describe the actuation state. Applications of the mode to poly(vinyl alcohol)-poly (acrylic acid) gels are presented.

Needles with asymmetric bevel tips naturally bend when they are inserted into soft tissue. In this study, we present an analytical model for the loads developed at the bevel tip during needle-tissue interaction. The model calculates the loads based on the geometry of the bevel edge and gel material properties. The modeled transverse force developed at the tip is compared to forces measured experimentally. The analytical model explains the trends observed in the experiments. In addition to macroscopic studies, we also present microscopic observations of needle-tissue interactions. These results contribute to a mechanics-based model of robotic needle steering, extending previous work on kinematic models.

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.

We have examined the yielding and fracture behavior of Zr57.4Cu16.4Ni8.2Ta8Al10 metallic-glass-matrix composites with a small volume fraction (similar to 4 pct) of ductile crystalline particles under quasi-static uniaxial tension and compression and dynamic uniaxial compression. The yield stress of the composite is the same for quasi-static tension and compression, consistent with a von Mises yield criterion. The measured average angle between the shear bands and the loading axis in quasistatic compression is 47 +/- 2 deg, significantly larger than the value of similar to 42 deg typically reported for single-phase metallic glasses. Finite element modeling (FEM) shows that the measured value is consistent with both the von Mises criterion (48 +/- 4 deg) and the Mohr-Coulomb criterion (46 +/- 5 deg). The fracture surface angles, however, are 41 +/- 1 deg (compression) and 54 +/- 2 deg (tension), in good agreement with observations of single-phase metallic glasses. At low strain rates (< 10(-1) s(-1)), the yield stress is independent of strain rate, while at higher strain rates (> 10(0) s(-1)), the failure stress decreases with increasing strain rate, which again is similar to the behavior of single-phase glasses. These results indicate that while the presence of the particles has a significant effect on the yield behavior of the composites, the fracture behavior is largely governed by the properties and behavior of the amorphous matrix.