bluuebunny/crossref_metadata_2025 huggingface dataset:
Plastic Analysis of Metal Foam Core Sandwich Beam Transversely Loaded by a Flat Punch: Combined Local Denting and Overall Deformation
Authors: Qin Qing Hua , Wang T. J.
Published: March 2012

The objective of this work is to investigate the quasi-static plastic behavior of a fully clamped metal foam core sandwich beam transversely loaded by a flat punch. A rigid-plastic beam-on-foundation model is extended to study the local denting deformation of a metal foam core sandwich beam. The effects of local denting and core strength on the overall deformation are incorporated in the analysis. Analytical solutions are derived for three different regimes of post-yield deformation mechanisms. Additionally, finite element results are obtained. Comparisons of the present analytical predictions with numerical, previous experimental, and analytical results are presented, respectively. It is shown that local denting has a significant effect on the finite deflection response of the metal foam core sandwich structure. The load-carrying and energy absorption capacities of sandwich beams may be overestimated if the effect of local denting is neglected in analysis. It is demonstrated that the present analytial model can reasonably predict the behaviors of post-yield deformation of sandwich beams. Moreover, the present analytical method can be extended to predict the low velocity/energy impact problems of sandwich structures.
Conformal Contact Between a Rubber Band and Rigid Cylinders
Authors: Morimoto Takuya , Iizuka Hiroshi
Published: March 2012

We consider a conformal contact problem between a rubber band and rigid cylinders that involves geometrical and material nonlinearities. The rubber band is assumed to be incompressible, neo-Hookean rubber. From the geometry and elasticity of the band, we present simple formulas to estimate the force–stretch relations and the contact pressure distributions on the cylinder. We show that the theoretical results are in good agreement with those of the finite element analysis when the rubber band is thin enough to be negligible to the bending stiffness. This verifies that the theory can effectively take into account both the material and geometrical nonlinearities of the band under the present conditions.
Response of a Rotating Shaft to Uniaxial Random Excitation
Authors: Dimentberg M. F. , Naess A. , Sperling L.
Published: March 2012

Random vibrations are considered for a Jeffcott rotor subject to uniaxial broadband random excitation by a lateral force along one of its transverse axes. Exact analytical solution for mean square responses is obtained which provide quantitative description of two effects: the magnification of mean square whirl radius due to rotation; and the increasing mean square response along the nonexcited direction with increasing rotation speed, that is, the spread of vibration to all directions around the shaft. The latter effect clearly corresponds to the approaching forward whirl of the shaft approaching its instability threshold; it can be used for the on-line evaluation of the rotor’s stability margin from the simple processing of its measured response signals as demonstrated by direct numerical simulation.
Admissibility of Acceleration Dependent Forces in Newtonian Dynamics
Authors: Ghosh Amitabha
Published: March 2012

L. A. Pars (1964, A Treatise on Analytical Dynamics, Heinemann, London) has shown in his comprehensive treatise that acceleration dependent forces are not admissible in Newtonian mechanics. More recently, Zhechev (2007, On the Admissibility of Given Acceleration – Dependent Forces in Mechanics, Jr. of Applied Mechanics, ASME, 74, Jan, pp. 107–111; 2007, Peculiarities of the use of Acceleration – Dependent Forces in Mechanical Problems, Proc. I. Mech. E, 221 Part K, pp. 497–503) has shown that the proof given by Pars is faulty and has concluded that acceleration dependent forces are admissible in Newtonian mechanics and in many cases such forces are useful in controlling mechanical systems. This brief technical note attempts to show that the matter is more complex and needs further discussion.
Computational Model of Hydrostatically Coupled Dielectric Elastomer Actuators
Authors: Wang Huiming , Cai Shengqiang , Carpi Federico , Suo Zhigang
Published: March 2012

A hydrostatically coupled dielectric elastomer (HCDE) actuator consists of two membranes of a dielectric elastomer, clamped with rigid circular rings. Confined between the membranes is a fixed volume of a fluid, which couples the movements of the two membranes when a voltage or a force is applied. This paper presents a computational model of the actuator, assuming that the membranes are neo-Hookean, capable of large and axisymmetric deformation. The voltage-induced deformation is described by the model of ideal dielectric elastomer. The force is applied by pressing a rigid flat punch onto one of the membranes over an area of contact. The computational predictions agree well with experimental data. The model can be used to explore nonlinear behavior of the HCDE actuators.
Elastodynamic Analysis of Underground Structural Failures Induced by Seismic Body Waves
Authors: Uenishi Koji
Published: March 2012

Scattering of elastic waves by structural inhomogeneities such as cylindrical cavities has been a subject of intensive study for decades. The time-harmonic elastodynamic analysis making use of the wave function expansions is one of the typical approaches for such problems, and since it gives semianalytical solutions that may show the effect of parameters of the problem rather explicitly, it is still repeatedly used in the study of dynamic response of elastic structures including inhomogeneities. Here, motivated by the observation of the unique underground structural failure patterns caused by the 1995 Hyogo-ken Nanbu (Kobe), Japan, earthquake, we analyze scattering of a plane harmonic body wave by a uniformly lined circular tunnel (cylinder), and from the structural failure patterns we evaluate possible mechanical characteristics of the associated incident seismic waves. In the two-dimensional, in-plane time-harmonic elastodynamic model employed, the lined circular tunnel may be located at a finite depth from an approximate flat free surface of a homogeneous isotropic linear elastic medium (half-space), and the plane wave impinges upon the tunnel at an arbitrary incident angle. We compare the effect of P and SV wave incidences by calculating the dynamic amplification of stresses and displacements around this simplified tunnel, and also show the influence of the wavelength and the incident angle of the plane wave, the overburden thickness, and the relative compliance of the linear elastic lining with respect to the surrounding medium. The results suggest that the observed underground structural failures, the exfoliation of the lining concrete and buckling of the reinforcing steel bars on the sidewall as well as the detachment of the subgrade from the invert, might have been induced by the incidence of P waves in a relatively high frequency range.
The LEFM Concept for Interfacial Cracks Application to the Problem of Coating Delamination on Cylindrical Substrates
Authors: Nikolic Ruzica R. , Djokovic Jelena M.
Published: March 2012

In this paper the delamination of coating subjected to compressive stress on a cylindrical substrate is considered. This problem is particularly interesting in oxide coatings on wire elements exposed to extreme temperatures and in ceramic coatings on turbine engine blades or other components that operate at high temperatures. Using the results of Hutchinson (Hutchinson, 2001, “Delamination of Compressed Films on Curved Substrates,” J. Mech. Phys. Solids, 49, pp. 1847–1864) the aforementioned problem is discussed from the aspect of application of the linear elastic fracture mechanics (LEFM) concept for an interfacial crack. The energy release rate and mode mixity for the case of the coating delamination in the axial and radial directions are determined. It is shown that the results also depend on whether the substrate is convex or concave. Delamination in the radial direction in the case of the concave substrate is harder, but it is more likely when the substrate is convex. Delamination in the axial direction is equally likely in both cases. The results presented in this paper justify the application of the concept of linear elastic fracture mechanics for an interfacial crack for explaining the influence of the elastic characteristics of the substrate on the buckling delamination of the coating.
The Role of Thermal Pressurization and Dilatancy in Controlling the Rate of Fault Slip
Authors: Segall Paul , Bradley Andrew M.
Published: March 2012

Geophysical observations have shown that transient slow slip events, with average slip speeds v on the order of 10−8 to 10−7 m/s, occur in some subduction zones. These slip events occur on the same faults but at greater depth than large earthquakes (with slip speeds of order ∼ 1 m/s). We explore the hypothesis that whether slip is slow or fast depends on the competition between dilatancy, which decreases fault zone pore pressure p, and thermal pressurization, which increases p. Shear resistance to slip is assumed to follow an effective stress law τ=f(σ-p)≡ fσ¯. We present two-dimensional quasi-dynamic simulations that include rate-state friction, dilatancy, and heat and pore fluid flow normal to the fault. We find that at lower background effective normal stress (σ¯), slow slip events occur spontaneously, whereas at higher σ¯, slip is inertially limited. At intermediate σ¯, dynamic events are followed by quiescent periods, and then long durations of repeating slow slip events. In these cases, accelerating slow events ultimately nucleate dynamic rupture. Zero-width shear zone approximations are adequate for slow slip events but substantially overestimate the pore pressure and temperature changes during fast slip when dilatancy is included.
An Associative and Non-Associative Anisotropic Bounding Surface Model for Clay
Authors: Jiang Jianhong , Ling Hoe I. , Kaliakin Victor N.
Published: March 2012

An anisotropic elastoplastic bounding surface model with non-associative flow rule is developed for simulating the mechanical behavior of different types of clays. The non-associative flow rule allows for the simulation of not only strain-hardening but also strain-softening response. The theoretical framework of the model is given, followed by the verification of the model as applied to the experimental results of a strain-hardening Kaolin tested under different undrained stress paths. The undrained behavior of Boston Blue clay, which exhibits a strain-softening behavior, is also simulated. It is shown that the non-associative nature of the model gives more accurate results than those of the same model employing an associative flow rule, especially for normally consolidated Kaolin specimens. The results show that the model is also capable of simulating the strain-softening behavior of Boston blue clay with reasonable accuracy.
Wrinkling Phenomena in Neo-Hookean Film/Substrate Bilayers
Authors: Cao Yanping , Hutchinson John W.
Published: March 2012

Wrinkling modes are determined for a two-layer system comprised of a neo-Hookean film bonded to an infinitely deep neo-Hookean substrate with the entire bilayer undergoing compression. The full range of the film/substrate modulus ratio is considered from the limit of a traction-free homogeneous substrate to very stiff films on compliant substrates. The role of substrate prestretch is considered wherein an unstretched film is bonded to a prestretched substrate with wrinkling arising as the stretch in the substrate is relaxed. An exact bifurcation analysis reveals the critical strain in the film at the onset of wrinkling. Numerical simulations carried out within a finite element framework uncover advanced post-bifurcation modes including period-doubling, folding and a newly identified mountain ridge mode.
Reconstructing Free-Flight Angular Velocity from a Miniaturized Wireless Accelerometer
Authors: McGinnis Ryan , Perkins N. C. , King Kevin
Published: March 2012

The theory governing the torque-free motion of a rigid body is well established, yet direct experimental measurement in the laboratory remains an obvious challenge. This paper addresses this challenge by presenting a novel miniature wireless inertial measurement unit (IMU) that directly measures the motion of a rigid body during free-flight. The IMU incorporates three-axis sensing of acceleration and three-axis sensing of angular velocity with a microcontroller and an RF transceiver for wireless data transmission to a host computer. Experiments consider a rigid body that is spun up by hand and then released into free-flight. The measured rotational dynamics from the IMU are carefully benchmarked against theoretical predictions. This benchmarking reveals that the angular velocity directly measured by the angular rate gyros lies within 6% of that predicted by the (Jacobi elliptic function) solutions to the Euler equations. Moreover, experimentally constructed polhodes elegantly illustrate the expected stable precession for rotations initiated close to the major or minor principal axes and the unstable precession for rotations initiated close to the intermediate axis. We then present a “gyro-free” design that employs a single, triaxial accelerometer to reconstruct the angular velocity during free-flight. A measurement theory is presented and validated experimentally. Results confirm that the angular velocity can be reconstructed with exceedingly small errors (less than 2%) when benchmarked against direct measurements using angular rate gyros. The simpler gyro-free design addresses restrictions imposed by rate gyro cost, size, and measurement range and may enable high-volume commercial applications of this technology in instrumented baseballs, basketballs, golf balls, footballs, soccer balls, softballs, and the like.
Riccati Discrete Time Transfer Matrix Method for Dynamic Modeling and Simulation of an Underwater Towed System
Authors: Wang Guoping , Rong Bao , Tao Ling , Rui Xiaoting
Published: March 2012

Efficient, precise dynamic modeling and control of complex underwater towed systems has become a research focus in the field of multibody dynamics. In this paper, based on finite segment model of cable, by defining the new state vectors and deducing the new transfer equations of underwater towed systems, a new highly efficient method for dynamic modeling and simulation of underwater towed systems is presented and the pay-out/reel-in process of towed cable is studied. The computational efficiency and numerical stability of the proposed method are discussed. When using the method to study the dynamics of underwater towed systems, it avoids the global dynamic equations of system, and simplifies solving procedure. Irrespective of the degree of freedom of underwater towed system, the matrices involved in the proposed method are always very small, which greatly improve the computational efficiency and avoids the computing difficulties caused by too high matrix orders for complex underwater towed systems. Formulations of the method as well as numerical simulations are given to validate the proposed method.
Exact Two-Dimensional Contact Analysis of Piezomagnetic Materials Indented by a Rigid Sliding Punch
Authors: Ting Zhou Yue , Yong Lee Kang
Published: March 2012

The aim of the present paper is to investigate the two-dimensional moving contact behavior of piezomagnetic materials under the action of a sliding rigid punch. Introduction of the Galilean transformation makes the constitutive equations containing the inertial terms tractable. Eigenvalues analyses of the piezomagnetic governing equations are detailed, which are more complex than those of the commercially available piezoelectric materials. Four eigenvalue distribution cases occur in the practical computation. For each case, real fundamental solutions are derived. The original mixed boundary value problem with either a flat or a cylindrical punch foundation is reduced to a singular integral equation. Exact solution to the singular integral equation is obtained. Especially, explicit form of the stresses and magnetic inductions are given. Figures are plotted both to show the correctness of the derivation of the exact solution and to reveal the effects of various parameters on the stress and magnetic induction.
A Spectral-Tchebychev Solution for Three-Dimensional Vibrations of Parallelepipeds Under Mixed Boundary Conditions
Authors: Filiz Sinan , Bediz Bekir , Romero L. A. , Ozdoganlar O. Burak
Published: March 2012

Vibration behavior of structures with parallelepiped shape—including beams, plates, and solids—are critical for a broad range of practical applications. In this paper we describe a new approach, referred to here as the three-dimensional spectral-Tchebychev (3D-ST) technique, for solution of three-dimensional vibrations of parallelepipeds with different boundary conditions. An integral form of the boundary-value problem is derived using the extended Hamilton’s principle. The unknown displacements are then expressed using a triple expansion of scaled Tchebychev polynomials, and analytical integration and differentiation operators are replaced by matrix operators. The boundary conditions are incorporated into the solution through basis recombination, allowing the use of the same set of Tchebychev functions as the basis functions for problems with different boundary conditions. As a result, the discretized equations of motion are obtained in terms of mass and stiffness matrices. To analyze the numerical convergence and precision of the 3D-ST solution, a number of case studies on beams, plates, and solids with different boundary conditions have been conducted. Overall, the calculated natural frequencies were shown to converge exponentially with the number of polynomials used in the Tchebychev expansion. Furthermore, the natural frequencies and mode shapes were in excellent agreement with those from a finite-element solution. It is concluded that the 3D-ST technique can be used for accurate and numerically efficient solution of three-dimensional parallelepiped vibrations under mixed boundary conditions.
The Effect of Rotation and Inhomogeneity on the Transient Magneto-Thermoviscoelastic Stresses in an Anisotropic Solid
Authors: Fahmy Mohamed Abdelsabour
Published: March 2012

The main objective of this paper is to study the transient magneto-thermoviscoelastic stresses in a nonhomogeneous anisotropic solid placed in a constant primary magnetic field acting in the direction of the z-axis and rotating about it with a constant angular velocity. The system of fundamental equations is solved by means of a dual-reciprocity boundary element method (DRBEM). The results indicate that the effects of inhomogeneity and rotation are very pronounced.
Effects of Transpiration and Internal Heat Generation/Absorption on the Unsteady Flow of a Maxwell Fluid at a Stretching Surface
Authors: Mukhopadhyay Swati , Vajravelu Kuppalapalle
Published: March 2012

The effect of transpiration on unsteady two-dimensional flow of an MHD non-Newtonian Maxwell fluid over a stretching surface in the presence of a heat source/sink is investigated. The upper convected Maxwell fluid model is used to characterize the non-Newtonian fluid behavior. Using a similarity transformation the governing partial differential equations of the problem are reduced to a system of ordinary differential equations (ODEs), and the ODEs are solved numerically by a shooting method. The flow features and the heat transfer characteristics are analyzed and discussed in detail for several sets of values of the governing parameters. Though the velocity of the fluid initially decreases with increasing unsteady parameter but it increases finally. Quite the opposite is true with the temperature. Furthermore, the velocity of the fluid decreases with an increasing magnetic or Maxwell parameter. But the temperature is enhanced with an increasing Maxwell parameter. It is observed that the effect of the transpiration is to decrease the fluid velocity as well as the temperature. The results obtained reveal many interesting behaviors that warrant further study of the equations related to non-Newtonian fluid phenomena, especially the shear-thinning phenomena. Shear thinning reduces the wall shear stress.
Mixed-Mode Crack-Tip Fields in an Anisotropic Functionally Graded Material
Authors: Zhang Linhui , Kim Jeong-Ho
Published: March 2012

This paper provides asymptotic full crack-tip stress field solutions for an in-plane mixed-mode stationary crack in an anisotropic functionally graded material. A monoclinic graded material that has a material symmetry plane is considered. The complex variable approach and the asymptotic scaling factor are used to solve the governing fourth-order partial differential equation for exponentially graded anisotropic materials with gradation either parallel or perpendicular to the crack. Full crack-tip stress fields under mode-I and mode-II loading are visualized and discussed for homogeneous and exponentially graded anisotropic materials. We observe that higher-order terms are affected by material gradation and play an important role on crack-tip stress fields in functionally graded materials.
Modeling the Deformation Response of High Strength Steel Pipelines—Part I: Material Characterization to Model the Plastic Anisotropy
Authors: Neupane Sunil , Adeeb Samer , Cheng Roger , Ferguson James , Martens Michael
Published: March 2012

The design equations for pipelines subjected to both internal pressure and longitudinal loading are based on the isotropic hardening plasticity model. However, high strength steel (HSS) pipelines exhibit plastic anisotropy, which cannot be incorporated in the traditional isotropic hardening plasticity model. The stress strain behaviors of HSS in the longitudinal and the circumferential directions are different. Thus, it would not be desirable to adopt the same design equations based on the isotropic hardening plasticity model for HSS pipelines. The design equations of HSS steel pipelines have to be developed by solving numerical models incorporating a suitable material plasticity constitutive model for the HSS that can deal with the exhibited plastic anisotropy. In this paper, various plasticity models are studied and an appropriate plasticity model is adopted and calibrated to model the plastic anisotropy exhibited by the HSS.
Modeling the Deformation Response of High Strength Steel Pipelines—Part II: Effects of Material Characterization on the Deformation Response of Pipes
Authors: Neupane Sunil , Adeeb Samer , Cheng Roger , Ferguson James , Martens Michael
Published: March 2012

The material model proposed in Part I (Neupane et al., 2012, “Modeling the Deformation Response of High Strength Steel Pipelines—Part I: Material Characterization to Model the Plastic Anisotropy,” ASME J. Appl. Mech., 79, p. 051002) is used to study the deformation response of high strength steel. The response of pipes subjected to frost upheaval at a particular point is studied using an assembly of pipe elements, while buckling of pipes is examined using shell elements. The deformation response is obtained using two different material models. The two different material models used were the isotropic hardening material model and the combined kinematic hardening material model. Two sets of material stress-strain data were used for the isotropic hardening material model; data obtained from the longitudinal direction tests and data obtained from the circumferential direction tests. The combined kinematic hardening material model was calibrated to provide an accurate prediction of the stress-strain behavior in both the longitudinal direction and the circumferential direction. The deformation response of a pipe model using the three different material data sets was studied. The sensitivity of the response of pipelines to the choice of a material model and the material data set is studied for the frost upheaval and local buckling.
On the Effect of Particle Size Distribution in Cold Powder Compaction
Authors: Olsson Erik , Larsson Per-Lennart
Published: March 2012

The effect of particle size distribution in powder compaction has been studied using the discrete element method. Both isostatic compaction and closed die compaction are studied together during the entire loading process. Particle rotation and frictional effects are accounted for in the analysis. The particles are, constitutively described by rigid plasticity, assumed to be spherical with the size of the radii that follows a truncated normal distribution. The results show that size distribution effects are small on global compaction properties like compaction pressure if the size distribution is small. Furthermore, the size distribution had no influence at all on the macroscopic behavior at unloading. To verify the model, comparisons were made on two different sets of experiment found in the literature where the particles were of varying sizes. Good agreement was found both on fundamental properties like the average number of contacts per particle and on more important properties from a practical point of view, like the compaction pressure.
Structural Mechanics Based Model for the Force-Bearing Elements Within the Cytoskeleton of a Cell Adhered on a Bed of Posts
Authors: Pathak Amit , Chen Christopher S. , Evans Anthony G. , McMeeking Robert M.
Published: March 2012

Mechanical forces play a vital role in the activities of cells and their interaction with biological and nonbiological material. Various experiments have successfully measured forces exerted by the cells when in contact with a substrate, but the intracellular contractile machinery leading to these actions is not entirely understood. Tan et al., (2003, “Cells Lying on a Bed of Microneedles: An Approach to Isolate Mechanical Force,” Proc. Natl. Acad. Sci. USA, 100(4), pp. 1484–1489) use a bed of PDMS posts as the substrate for cells and measure the localized mechanical forces exerted by the cell cytoskeleton on the posts. In live cell experiments for this setup, post deflections are measured, and from these results the forces applied by the cell are calculated. From such results, it is desirable to quantify the contractile tensions generated in the force-bearing elements corresponding to the stress fibers within the cell cytoskeleton that generate the loads applied to the posts. The purpose of the present article is to consider the cytoskeleton as a discrete network of force-bearing elements, and present a structural mechanics based methodology to estimate the configuration of the network, and the contractile tension in the corresponding stress fibers. The network of stress fibers is modeled as a structure of truss elements connected among the posts adhered to a single cell. In-plane force equilibrium among the network of stress fibers and the system of posts is utilized to calculate the tension forces in the network elements. A Moore-Penrose pseudo-inverse is used to solve the linear equations obtained from the mechanical equilibrium of the cell-posts system, thereby obtaining a least squares fit of the stress fiber tensions to the post deflections. The predicted network of force-bearing elements provides an approximated distribution of the prominent stress fibers connected among deflected posts, and the tensions in each fibril.
Comparison of the In Vitro Hemodynamic Performance of New Flow Diverters for Bypass of Brain Aneurysms
Authors: Trager Asher L. , Sadasivan Chander , Lieber Baruch B.
Published: March 2012

One possible treatment for cerebral aneurysms is a porous tubular structure, similar to a stent, called a flow diverter. A flow diverter can be placed across the neck of a cerebral aneurysm to induce the cessation of flow and initiate the formation of an intra-aneurysmal thrombus. This excludes the aneurysm from the parent artery and returns the flow of blood to normal. Previous flow diverting devices have been analyzed to determine optimal characteristics, such as braiding angle and wire diameter. From this information, a new optimized device was designed to achieve equivalent hemodynamic performance to the previous best device, but with better longitudinal flexibility to preserve physiological arterial configuration. The new device was tested in vitro in an elastomeric replica of the rabbit elastase induced aneurysm model and is now in the process of being tested in vivo. Particle image velocimetry was utilized to determine the velocity field in the plane of symmetry of the model under pulsatile flow conditions. Device hemodynamic performance indices such as the hydrodynamic circulation were evaluated from the velocity fields. Comparison of these indices with the previous best device and a control shows that the significant design changes of the device did not change its hemodynamic attributes (p > 0.05).
Development of array‐type prompt gamma measurement system for in vivo range verification in proton therapy
Authors: Min Chul Hee , Lee Han Rim , Kim Chan Hyeong , Lee Se Byeong
Published: March 2012

Purpose:

In vivo range verification is one of the most important parts of proton therapy to fully utilize its benefits delivering high radiation dose to tumor, while sparing the normal tissue with the so‐called Bragg peak. Currently, however, range verification method is not used in clinics. The purpose of the present study is to optimize and evaluate the configuration of an array‐type prompt gamma measurement system on determining distal dose edge for in vivo range verification of proton therapy.

Methods:

To effectively measure the prompt gammas against the background gammas, the Monte Carlo simulations with the MCNPX code were employed in optimizing the configuration of the measurement system, and the Monte Carlo method was also used to understand the effect of the background gammas, mainly neutron capture gammas, in the measured gamma distribution. To reduce the effect of the background gammas, the optimized energy window of 4–10 MeV in measuring the prompt gammas was employed. A parameterized source was used to maximize computation speed in the optimization study. A simplified test measurement system, using only one detector moving from one measurement location to the next, was constructed and applied to therapeutic proton beams of 80–220 MeV. For accurate determination of the distal dose edge, the sigmoidal curve‐fitting method was applied to the measured distributions of the prompt gammas, and then, the location of the half‐value between the maximum and minimum value in the curve‐fitting was determined as the distal dose edge and compared with the beam range assessed by the proton dose distribution.

Results:

The parameterized source term employed in optimization process improved the calculation speed by up to ∼300 times. The optimization study indicates that an array‐type measurement system with 3, 2, 2, and 150 mm for scintillator thickness, slit width, septal thickness, and slit length, respectively, can effectively measure the prompt gamma distributions minimizing the contribution of background gammas. The present results show that a few hundred counts of prompt gammas can be easily obtained by measuring 10 s at each measurement location for proton beams of ∼4 nA. The distal dose edges determined by the prompt gamma distribution are 5.45, 14.73, and 27.74 cm for proton beams of 5.17 (80 MeV), 14.99 (150 MeV), and 27.38 (220 MeV) cm, respectively.

Conclusions:

The results show that the array‐type measurement system can measure prompt gamma distributions from a therapeutic proton beam within a short measurement time, and that the distal dose edge can be determined within a few millimeters of error without using any sophisticated analysis.
The Aftermath of Megafaunal Extinction: Ecosystem Transformation in Pleistocene Australia
Authors: Rule Susan , Brook Barry W. , Haberle Simon G. , Turney Chris S. M. , Kershaw A. Peter , Johnson Christopher N.
Published: March 2012

Human Impact?

Following the arrival of humans in Australia 40- to 50,000 years ago, many species of large vertebrates rapidly became extinct. By analyzing sediment cores from a site in northeastern Australia,

Rule
et al.

(p.
1483
; see the Perspective by

McGlone

) show that the extinction of the Australian megafauna caused important ecosystem shifts. Prominent among these were a shift from rainforest vegetation to sclerophyllous vegetation and a sustained increase in the incidence of fire. The cores also provide evidence of the cause of megafaunal extinction in Australia, ruling out climate and anthropogenic fire as possible causes while confirming that the extinctions closely followed human arrival. These findings show how landscapes sometimes have been fundamentally changed by the indirect effects of early humans—which underscores the impact that even prehistoric human societies had on natural systems.
MARF1 Regulates Essential Oogenic Processes in Mice
Authors: Su You-Qiang , Sugiura Koji , Sun Fengyun , Pendola Janice K. , Cox Gregory A. , Handel Mary Ann , Schimenti John C. , Eppig John J.
Published: March 2012

Mistress of Meiosis

Meiosis is essential for proper distribution of maternal chromosomes to eggs during oogenesis.

Su
et al.

(p.
1496
) identified a gene, meiosis arrest female 1 (
Marf1
), which is indispensable for meiosis and other oogenic processes. In mice,
Marf1
mutations resulted in meiotic arrest and an increase in nuclear DNA double-strand breaks, phenotypes linked to up-regulated levels of specific messenger RNAs (mRNA). These findings place MARF1 as a key regulator of mammalian female fertility through its integration of oocyte mRNA homeostasis, meiosis, and maintenance of genomic integrity.
Generation of a Synthetic Memory Trace
Authors: Garner Aleena R. , Rowland David C. , Hwang Sang Youl , Baumgaertel Karsten , Roth Bryan L. , Kentros Cliff , Mayford Mark
Published: March 2012

Adding Artificial Associations

In the mammalian cortex, there is significant spontaneous neural activity that is internally generated, rather than arising from sensory inputs, and this activity influences the processing of natural sensory stimuli. What role does this internally generated activity play in forming and accessing new memory representations? Using transgenic mice combined with controlled artificial activation of widely distributed neuronal ensembles associated with specific contexts,

Garner
et al.

(p.
1513
; see the Perspective by

Morris and Takeuchi

) investigated how spontaneous neuronal activity is integrated into a given context representation. Animals underwent fear conditioning in two distinct contexts. When a circuit in one context was artificially activated during fear conditioning in a separate context, a so-called “hybrid” memory was formed. This artificially stimulated network became a necessary component of the memory trace. These results are consistent with the emerging view that internally generated brain activity is not noise, but a coherent representation that can be incorporated into new associations and memories.
Comment on “Productivity Is a Poor Predictor of Plant Species Richness”
Authors: Fridley Jason D. , Grime J. Philip , Huston Michael A. , Pierce Simon , Smart Simon M. , Thompson Ken , Börger Luca , Brooker Rob W. , Cerabolini Bruno E.L. , Gross Nicolas , Liancourt Pierre , Michalet Richard , Le Bagousse-Pinguet Yoann
Published: March 2012

Adler
et al
. (Reports, 23 September 2011, p. 1750) reported “weak and variable” relationships between productivity and species richness and dispute the “humped-back” model (HBM) of plant diversity. We show that their analysis lacks sufficient high-productivity sites, ignores litter, and excludes anthropogenic sites. If corrected, the data set of Adler
et al
. would apparently yield strong HBM support.
Trim28
Is Required for Epigenetic Stability During Mouse Oocyte to Embryo Transition
Authors: Messerschmidt Daniel M. , de Vries Wilhelmine , Ito Mitsuteru , Solter Davor , Ferguson-Smith Anne , Knowles Barbara B.
Published: March 2012

Trimprinting the Genome

Reprogramming the parental genomes during the oocyte-to-embryo transition requires highly controlled epigenetic mechanisms. Although resetting the genome to a ground state is essential, conservation of inheritable marks is equally important. Now,

Messerschmidt
et al.

(p.
1499
) demonstrate that maternal deletion of the epigenetic modifier Trim28 in mice results in a strongly variable, yet ultimately embryonic, lethal phenotype. Aberrant loss of DNA methylation at imprinting control regions and thus partial loss of epigenetic memory was responsible for the phenotype. The stochastic time and mode of embryonic death reflect the exquisitely balanced interplay of maternal and zygotic factors in the early mammalian embryo.
Actinide Topological Insulator Materials with Strong Interaction
Authors: Zhang Xiao , Zhang Haijun , Wang Jing , Felser Claudia , Zhang Shou-Cheng
Published: March 2012

Interacting Topological Insulator

Topological insulators (TIs) hold great promise as a setting for exotic fundamental phenomena, as well as for more practical applications such as quantum computing. This new state of matter has been discovered in materials such as Bi
2
Se
3
, where electrons' spins are correlated with their orbital motion, but interactions between the electrons themselves are negligible. For a TI to fulfill its potential, a material with interacting electrons is desirable.

Zhang
et al.

(p.
1464
) used density functional calculations to predict that strong interactions in actinide compounds such as AmN may drive a transition into a TI state, with a large insulating gap favorable for device applications.
Evolutionary Conservation of Species’ Roles in Food Webs
Authors: Stouffer Daniel B. , Sales-Pardo Marta , Sirer M. Irmak , Bascompte Jordi
Published: March 2012

Untangling the Web

Interspecific interactions link species within complex trophic and nontrophic webs (see the Perspective by

Lewinsohn and Cagnolo

). Theoretical work has suggested that certain characteristics of species, or even interactions, may predispose them to extinction from a network.

Aizen
et al.

(p.
1486
) provide empirical evidence that plant-pollinator interactions are lost nonrandomly following habitat reduction in isolated hills in the Argentine pampas. Some types of interaction were more vulnerable to disruption than others, particularly when the specialization of the interacting was high and when the interactions were infrequent.

Stouffer
et al.

(p.
1489
) applied network theory to predict the dynamical importance of species across different food webs. Characteristic three-node motifs were identified, and species were characterized according to the relative frequencies with which they occupied unique positions within the motifs. These relative frequencies and the dynamic importance of the motifs were then used to identify a species-level importance within a food web.
Entrepreneurship Training for the Developing World
Authors: Quadir Iqbal Z.
Published: March 2012

Education focused on commercial dynamism, not just policy and bureaucracy, can contribute to economic growth in low-income countries and the world.
Iron-Catalyzed Cyclopropanation in 6 M KOH with in Situ Generation of Diazomethane
Authors: Morandi Bill , Carreira Erick M.
Published: March 2012

Tamer Triangulations

Diazomethane is a broadly useful precursor to carbene (CH
2
), which, in turn, reacts with olefins to form triangular carbon cycles known as cyclopropanes. These small rings are of fundamental interest for their strained bonds and turn up periodically in natural products, as well as in pharmaceutical and agrochemical research. Unfortunately, the flip side of diazomethane's facile reactivity is its dangerous tendency to explode—a hazard exacerbated by the need to isolate the compound after its preparation in highly basic water before it can be reacted with hydrophobic olefins.
Morandi and Carreira
(p.
1471
) now show that an iron cyclopropanation catalyst can circumvent the need for the isolation step, inducing reaction of the hazardous compound in a biphasic aqueous/organic medium immediately after its generation.
Keystones in a Tangled Bank
Authors: Lewinsohn Thomas M. , Cagnolo Luciano
Published: March 2012

Ecological network studies highlight the importance of individual species to community conservation.
How Plants See the Invisible
Authors: Gardner Kevin H. , Correa Fernando
Published: March 2012

Plants can sense ultraviolet light with a photoreceptor that only contains standard amino acid side chains.
A Flapping of Wings
Authors: Mackenzie Dana
Published: March 2012

Robot aircraft that fly like birds could open new vistas in maneuverability, if designers can forge a productive partnership with an old enemy: unsteady airflow.
Materials Scientists Look to a Data-Intensive Future
Authors: Service Robert F.
Published: March 2012

Supercomputing power now makes it possible to compute the properties of thousands of crystalline materials in a flash and is expected to guide experimentalists where to search for the next best things.
Learning How to NOT Make Your Own Earthquakes
Authors: Kerr Richard A.
Published: March 2012

As fluid injections into Earth's crust trigger quakes across the United States, researchers are scrambling to learn how to avoid making more.
INTERACTION OF SHOCK WAVE WITH MULTI-FLUIDS INTERFACE USING QUADRILATERAL-BASED ADAPTIVE MESH
Authors: ZHENG H. W. , LI X. J. , YANG G. W. , SHU C.
Published: March 2012

In this paper, the interaction of shock waves with multi-fluids interfaces is investigated by numerical simulations using unstructured quadrilateral adaptive meshes. In order to obtain a detailed structure of the interface, a solution adaptive method for compressible multi-fluid flows developed by Zheng et al. is employed. Firstly, the method is verified by a planar shock and interface interaction problem, which is compared with the front tracking method for the Richtmyer–Meshkov instability problem. Following the verification, the interaction between a circular shock and a sinusoidally perturbed circular interface in cylinder vessel is firstly investigated in our paper. The results show that the solution adaptive method can be employed to study the compressible multi-fluid cases with relatively complex geometry as well as capturing the fine details of interfacial structures of the interaction.
THE BIFURCATION PHENOMENA OF BINARY-STATE OPINION FORMATION ON NETWORKS
Authors: GUO LONG
Published: March 2012

The binary-state opinion model with the conservation law of public information is studied. The public information Inf i provided by node i is an exponential function of its degree with the tunable parameter β, which reflects the impact factor of the node i to his/her local neighbors. We realize our model on the top of complex networks with another tunable parameter α, which shows the degree heterogeneous as the form of the power-law degree distribution with the exponent γ = (1 + α)/α. We find that much more public information (β > 2) and less public information (β < -1) cannot let either of the two opinions win during the opinion formation. In the (α,β) space, the system undergoes the doubling bifurcation and then after a point becomes chaotic when the α decreases from 1 to 0. Maybe, our present work can provide some perspectives and tools to understand the conversation of opinion formation in our society.
NUMERICAL SIMULATION OF A MACRO-QUANTUM EXPERIMENT: OSCILLATING WAVE PACKET
Authors: NOTTALE LAURENT , LEHNER THIERRY
Published: March 2012

We perform numerical simulations of a new proposal of laboratory experiment that would allow the transformation of a classical fluid into a quantum-type (super)fluid through the application of a generalized quantum potential. This quantum potential is simulated by using a real time retroactive loop involving a measurement of density, a calculation of the potential in function of the measured density, then an application of the calculated potential through a classical force. This general experimental concept is exemplified here by the case of a nonspreading oscillating wave packet in a harmonic oscillator potential. We find signatures of a quantum-like behavior which are stable against various perturbations. Finally, the feasability of a realization of this concept in an actual plasma experiment is analyzed.
RECENT PROGRESS IN THE STUDY OF FISSION BARRIERS IN COVARIANT DENSITY FUNCTIONAL THEORY
Authors: AFANASJEV A. V. , ABUSARA H. , RING P.
Published: March 2012

Recent progress in the study of fission barriers of actinides and superheavy nuclei within covariant density functional theory is overviewed.
REFLECTION OF DISCONTINUITIES FOR NONLINEAR WEAKLY STABLE BOUNDARY VALUE PROBLEMS
Authors: MARCOU ALICE
Published: March 2012

This paper is concerned with the reflection of nonlinear discontinuous waves, for weakly well-posed hyperbolic boundary value problems, satisfying the (WR) condition, that is in a case where the IBVP is neither strongly stable, nor strongly unstable. We study how the singularities of a striated solution are reflected when the solution hits the boundary. We prove striated estimates and L∞ estimates and observe the loss of one derivative: we show that a discontinuity of the gradient of the solution across a hyperplane can be reflected in a discontinuity across a hyperplane of the solution itself.
STATISTICAL DETECTION OF SPATIAL PLANT PATTERNS UNDER THE EFFECT OF FOREST USE
Authors: LÓPEZ I. , STANDOVÁR T. , GARAY J. , VARGA Z. , GÁMEZ M.
Published: March 2012

The analysis of the consequences of land use (in particular forest use) may be considered as a partial step towards an integrated modeling of a land system. In the paper a forest territory is considered, where a gap-cut is made, and after a given time period the eventual change in the spatial distribution of undergrowth plants and tree seedlings is to be detected. Floristic data are collected along a line transect. A method for the detection of the change in the plant distributions along the transect is proposed to see whether this occurs at the geometric frontier of the human intervention.

Since in the considered case the distribution of the change-point estimate is not known, as a substitute of its confidence interval, the so-called change-interval is calculated, using an adaptation of the bootstrap method. As an illustration, for a concrete plant species, the maximum likelihood estimation of the change-point and the calculation of the above mentioned change-interval is presented. Finally, the validation of the proposed method against some typical ecological situations is also presented, which provides a justification of the used algorithms.
ON COUNTING THE FREQUENCY DISTRIBUTION OF STRING MOTIFS IN MOLECULAR SEQUENCES
Authors: PROSPERI MATTIA C. F. , PROSPERI LUCIANO , GRAY REBECCA R. , SALEMI MARCO
Published: March 2012

This work investigates frequency distributions of strings within a text. The mathematical derivation accounts for variable alphabet size, character probabilities, and string/text lengths, under both the Bernoullian and the Markovian model for string generation. The analysis is limited to the set of non-clumpable strings, that cannot overlap with themselves. Two formulae (exact and approximated) are derived, calculating the frequency distribution of a string of length m found inside a text of length n (with m < n). The approximated formula has a constant complexity (in contrast to an exponential complexity of the exact) and makes it applicable to very long texts. The proposed formulae were applied to analyze string frequencies in a portion of the human genome, and to recalculate frequencies of known repeated motif within genes, associated to genetic diseases. A comparison with state-of-the-art methods was provided. The formulae presented here can be of use in the statistical evaluation of specific motif frequencies within very long texts (e.g. genes or genomes) and help in characterizing motifs in pathologic conditions.