Discrete Contact Research Articles

Limited adult vocal learning maintains call dialects but permits pair-distinctive calls in red crossbills

Vocal imitation can inform receivers about the social background of signallers because shared signals reflect learning experiences. Open-ended imitation can generate shared signals that indicate current social affiliation, whereas limited vocal plasticity in adulthood can permit signals to reflect an individual's origin. I examined the relations between vocal learning, signal sharing and social dynamics in red crossbills, Loxia curvirostra. Two levels of shared variation exist within crossbills' contact calls: discrete call variants distinguish ecologically diverged crossbill forms, and within these variants, some bonded pairs produce calls with nearly identical structures. Assessment of vocal and social behaviour of experimentally housed birds revealed that some adult crossbills converged on calls shared with companions of their own form, but no birds learned new, categorically distinct call variants. Also, while the process of call convergence within the bounds of discrete variants was associated with affiliation, birds that produced different contact call variants interacted less frequently and less amicably. These results suggest that call learning is limited in adult crossbills, generating calls with two levels of shared variation and multiple social functions: discrete contact call variants reflect birds' ecological forms, while pair-distinctive calls within those discrete variants reflect current social affiliation. Thus, a single social call concurrently reflects bonding among conspecifics of common origin and promotes the social isolation of ecologically diverged forms of red crossbills.

Forced Response Prediction of Constrained and Unconstrained Structures Coupled Through Frictional Contacts

In this paper, a forced response prediction method for the analysis of constrained and unconstrained structures coupled through frictional contacts is presented. This type of frictional contact problem arises in vibration damping of turbine blades, in which dampers and blades constitute the unconstrained and constrained structures, respectively. The model of the unconstrained/free structure includes six rigid body modes and several elastic modes, the number of which depends on the excitation frequency. In other words, the motion of the free structure is not artificially constrained. When modeling the contact surfaces between the constrained and free structure, discrete contact points along with contact stiffnesses are distributed on the friction interfaces. At each contact point, contact stiffness is determined and employed in order to take into account the effects of higher frequency modes that are omitted in the dynamic analysis. Depending on the normal force acting on the contact interfaces, quasistatic contact analysis is initially employed to determine the contact area as well as the initial preload or gap at each contact point due to the normal load. A friction model is employed to determine the three-dimensional nonlinear contact forces, and the relationship between the contact forces and the relative motion is utilized by the harmonic balance method. As the relative motion is expressed as a modal superposition, the unknown variables, and thus the resulting nonlinear algebraic equations in the harmonic balance method, are in proportion to the number of modes employed. Therefore the number of contact points used is irrelevant. The developed method is applied to a bladed-disk system with wedge dampers where the dampers constitute the unconstrained structure, and the effects of normal load on the rigid body motion of the damper are investigated. It is shown that the effect of rotational motion is significant, particularly for the in-phase vibration modes. Moreover, the effect of partial slip in the forced response analysis and the effect of the number of harmonics employed by the harmonic balance method are examined. Finally, the prediction for a test case is compared with the test data to verify the developed method.

Efficient simulation of multi‐body contact problems on complex geometries: A flexible decomposition approach using constrained minimization

AbstractWe consider the numerical simulation of non‐linear multi‐body contact problems in elasticity on complex three‐dimensional geometries. In the case of warped contact boundaries and non‐matching finite element meshes, particular emphasis has to be put on the discretization of the transmission of forces and the non‐penetration conditions at the contact interface. We enforce the discrete contact constraints by means of a non‐conforming domain decomposition method, which allows for optimal error estimates. Here, we develop an efficient method to assemble the discrete coupling operator by computing the triangulated intersection of opposite element faces in a locally adjusted projection plane but carrying out the required quadrature on the faces directly. Our new element‐based algorithm does not use any boundary parameterizations and is also suitable for isoparametric elements. The emerging non‐linear system is solved by a monotone multigrid method of optimal complexity. Several numerical examples in 3D illustrate the effectiveness of our approach. Copyright © 2008 John Wiley & Sons, Ltd.

A parameterized numerical method for generating discrete helical gear tooth surface allowing non-standard geometry

Gear analysis is typically performed using calculation based on gear standards. Standards provide a good basis in gear geometry calculation for involute gears, but these are unsatisfactory for handling geometry deviations such as tooth flank modifications. The efficient utilization of finite-element calculation also requires the geometry generation to be parameterized. A parameterized numerical approach was developed to create discrete helical gear geometry and contact line by simulating the gear manufacturing, i.e. the hobbing process. This method is based on coordinate transformations and a wide set of numerical calculation points and their synchronization, which permits deviations from common involute geometry. As an example, the model is applied to protuberance tool profile and grinding with tip relief. A fairly low number of calculation points are needed to create tooth flank profiles where error is <1 μm.

Discrete contact continuous film magnetoresistive biosensor

This paper describes a magnetic biosensor having continuous magnetoresistance film stack with discrete nonmagnetic contact pads. Detection unit is defined by a pair of contact pads. With contact pad miniaturization not limited by magnetic degradation and sensitivity variation as in patterned magnetic films, submicron spatial resolution can be achieved. Semicontinuous differential waveform can be generated with sequential readout of the detection units and the bead presence is detected by pulse pattern recognition. Methods to align the magnetic labeled entities toward the highest sensitivity region of the continuous biosensor with external field gradient and physical confinement are discussed.

Tes, a Specific Mena Interacting Partner, Breaks the Rules for EVH1 Binding

The intracellular targeting of Ena/VASP family members is achieved via the interaction of their EVH1 domain with FPPPP sequence motifs found in a variety of cytoskeletal proteins, including lamellipodin, vinculin, and zyxin. Here we show that the LIM3 domain of Tes, which lacks the FPPPP motif, binds to the EVH1 domain of Mena, but not to those of VASP or Evl. The structure of the LIM3:EVH1 complex reveals that Tes occludes the FPPPP-binding site and competes with FPPPP-containing proteins for EVH1 binding. Structure-based gain-of-function experiments define the molecular basis for the specificity of the Tes-Mena interaction. Consistent with in vitro observations, the LIM3 domain displaces Mena, but not VASP, from the leading edge and focal adhesions. It also regulates cell migration through a Mena-dependent mechanism. Our observations identify Tes as an atypical EVH1 binding partner and a regulator specific to a single Ena/VASP family member.

Asymptotic Results for the MPL Estimators of the Contact Process of Order p

Let X be a discrete time contact process (CP) of order p on Z 2 as defined by Durrett and Levin (1994), that is a CP where the function of birth takes p different values. We study the maximum marginal pseudo-likelihood (MPL) estimator of the model based on space-time evolution of X, that is, T + 1 successive observations of X on a finite subset S of sites. When T → ∞, this estimator is consistent and asymptotically normal for a non vanishing supercritical CP. We also propose a statistical test for the parameters of the model and verify the asymptotic results by some simulation studies.

Space-time estimation of a particle system model

Let X be a discrete time contact process (CP) on ℤ2, as defined by Durrett and Levin (1994, Stochastic spatial models: a user's guide to ecological applications. Philosophical Transactions of the Royal Society of London Series B, 343, 329–350). We study the estimation of the model based on space-time evolution of X, that is, T + 1 successive observations of X on a finite subset S of sites. We consider the maximum marginal pseudo-likelihood (MPL) estimator and show that, when T→∞, this estimator is consistent and asymptotically normal for a non-vanishing supercritical CP. Numerical studies confirm these theoretical ones.

Continuous-Thread Model of Load Distribution and Nut Wear in Grease-Lubricated Lead Screws

Even under constant operating conditions, greases lubricating lead screw assemblies, as well as materials comprising their nut and screw threads, must perform over a range of contact pressures that not only varies from thread to thread but also evolves with the sliding distance as most heavily loaded threads wear most rapidly. An example modeled here, consisting of a nut whose bulk body is rigid and an elastic screw body having continuous elastic meshed threads, reveals the broad distribution of load along the length of the nut threads in an unworn lead screw assembly, as well as the distribution of wear depth that must accompany an eventual steady-state of uniform rate of nut thread wear. Thread load redistributions brought on by changes in operating conditions such as applied load, or even temperature in the case of a nut/screw pair of dissimilar materials, are similarly modeled. Such load redistributions in worn lead screws may cause either the top or the bottom positions of the nut thread to become most heavily loaded, depending on whether an operating condition is increased or decreased. Most importantly, the extent of these redistributions using this continuous thread model can significantly exceed those predicted by an earlier preliminary model where threads were considered as discrete turns and contact pressures could only be described as an average value over each turn. The operating condition changes are shown capable of causing worn threads at either extreme of the nut to become momentarily unloaded and removed from the contact, and this continuous-thread model indicates that even smaller changes in the operating condition can initiate such unloading, as compared to predictions from the discrete-thread model.

An Error Estimate for the Signorini Problem with Coulomb Friction Approximated by Finite Elements

The present paper is concerned with the unilateral contact model and the Coulomb friction law in linear elastostatics. We consider a mixed formulation in which the unknowns are the displacement field and the normal and tangential constraints on the contact area. The chosen finite element method involves continuous elements of degree one and continuous piecewise affine multipliers on the contact zone. A convenient discrete contact and friction condition is introduced in order to perform a convergence study. We finally obtain a first a priori error estimate under the assumptions ensuring the uniqueness of the solution to the continuous problem.

Discrete element method contact model for dynamic simulation of inelastic rock impact

A contact model for inelastic impact-rebound events such as those that occur when a rock bounces down a slope is developed for dynamic simulations using the discrete element method. Upon impact, the normal contact force follows a linear elastic relationship as a function of relative displacement up to a pre-set transition force, after which the contact force is fixed at the transition force until the relative velocity between the objects in contact reaches zero. Upon unloading (rebound), the contact force follows a power function that enables the contact model to remove, in a time-stepping manner, significant stored elastic energy from the contact thus reducing the rock's kinetic energy. The model automatically generates lower values of coefficient of normal restitution as the impact velocity increases.

Unilateral non‐linear dynamic contact of thin‐walled structures using a primal‐dual active set strategy

AbstractThe efficient modelling of three‐dimensional contact problems is still a challenge in non‐linear implicit structural analysis. We use a primal‐dual active set strategy (SIAM J. Optim. 2003; 13:865–888), based on dual Lagrange multipliers (SIAM J. Numer. Anal. 2000; 38:989–1012) to handle the non‐linearity of the contact conditions. This allows us to enforce the contact constraints in a weak, integral sense without any additional parameter. Due to the biorthogonality condition of the basis functions, the Lagrange multipliers can be locally eliminated. We perform a static condensation to achieve a reduced system for the displacements. The Lagrange multipliers, representing the contact pressure, can be easily recovered from the displacements in a variationally consistent way.For the application to thin‐walled structures we adapt a three‐dimensional non‐linear shell formulation including the thickness stretch of the shell to contact problems. A reparametrization of the geometric description of the shell body gives us a surface‐oriented shell element, which allows the application of contact conditions directly to nodes lying on the contact surface. Shell typical locking phenomena are treated with the enhanced‐assumed‐strain‐method and the assumed‐natural‐strain‐method.The discretization in time is done with the implicit Generalized‐α method (J. Appl. Mech. 1993; 60:371–375) and the Generalized Energy–Momentum Method (Comp. Methods Appl. Mech. Eng. 1999; 178:343–366) to compare the development of energies within a frictionless contact description.In order to conserve the total energy within the discretized frictionless contact framework, we follow an approach from Laursen and Love (Int. J. Numer. Methods Eng. 2002; 53:245–274), who introduced a discrete contact velocity to update the velocity field in a post‐processing step.Various examples show the good performance of the primal‐dual active set strategy applied to the implicit dynamic analysis of thin‐walled structures. Copyright © 2006 John Wiley & Sons, Ltd.

Mechanics of Climbing and Attachment in Twining Plants

Twining plants achieve vertical growth by revolving around supports of different sizes on which they exert a pressure. This observation raises many intriguing questions that are addressed within the framework of elastic filamentary structures by modeling the stem close to the apex as a growing elastic rod. The analysis shows that vertical growth is achieved thanks to discrete contact points and regions with continuous contact, that the contact pressure creates tension in the stem as observed experimentally, and that there is a maximal radius of the pole around which a twiner can climb.

Modeling and identification of tool holder–spindle interface dynamics

The majority of the chatter vibrations in high-speed milling originate due to flexible connections at the tool holder–spindle, and tool–tool holder interfaces. This article presents modeling of contact stiffness and damping at the tool holder and spindle interface. The holder–spindle taper contact is modeled by uniformly distributed translational and rotational springs. The springs are identified by minimizing the error between the experimentally measured and estimated frequency response of the spindle assembly. The paper also presents identification of the spindle's dynamic response with a holder interface, and its receptance coupling with the holder–tool stick out which is modeled by Timoshenko beam elements. The proposed methods allow prediction of frequency-response functions at the tool tip by receptance coupling of tools and holders to the spindle, as well as analyzing the influence of relative wear at the contact by removing discrete contact springs between the holder and spindle. The techniques are experimentally illustrated and their practical use in high speed milling applications is elaborated.

Probabilistic prospect assessment in the modern exploration era

Darrel Norman, GeoKnowledge USA, suggests that developments in exploration technology and assessment methodologies provide new opportunities and challenges. The modern probabilistic prospect assessment must incorporate multiple zones, risk dependencies and volume correlations, discrete contact scenarios, and contact uncertainty integrated across depth-volume functions in order to honour the uncertainties defined by the evaluation. The modern oil exploration business is a business of decisions. The managers of exploration efforts, from the largest majors down to the smallest independents, are continually choosing between alternatives. Where should we deploy our people? What plays should we evaluate? Should we acquire acreage? Which opportunity should we drill? The decisions are usually made based on the evaluation of a prospect or a group of prospects. Even pure frontier plays are judged based on the potential of real or postulated leads. The prospect evaluation attempts to answer the fundamental question of oil and gas exploration: ‘What is the chance that this prospect will be a commercial success?’ To answer that question, we must describe the range of possible recoverable volumes, and the chances of finding those volumes. The process of estimating the range of potential volumes and their associated probabilities is called probabilistic prospect assessment. It involves quantifying geologic risks and uncertainties. Risk and uncertainty are largely matters of perception. They cannot be measured or calculated directly. Our perception of a prospect’s risk and uncertainty is a function of the geoscientist’s evaluation and the confidence that we have in the evaluation. ‘Evaluation’ should not imply a single map or model. The geoscientist’s evaluation of a prospect should include full disclosure of all possible outcomes and their relative likelihood. The systematic quantification of risk and uncertainty is a numeric expression of the evaluation. Over the past 15 years our ability to quantify the geologic risks and uncertainties associated with the typical exploration prospect has improved tremendously. The primary source of the improvement has been the use of 3D seismic and the detailed evaluations that are now possible prior to testing a prospect. In some trends, 3D seismic provides a level of detail that previously could be obtained only with dense well control. Our ability to make detailed evaluations of undrilled prospects has necessitated a change in our probabilistic assessment methods. As our descriptions of the untested prospect’s risk and uncertainties become more detailed, our methods for quantifying those perceptions must also become more detailed. Otherwise, the probabilistic assessment will not support the geologic evaluation (and vice versa). Modern geologic evaluations deserve modern assessments of uncertainty and risk.

Your Synapse or Mine? Promiscuous Protein Exchanges between Neighboring Synapses

Synapses, the connections that link neurons into circuits, can be plastic or stable in the mammalian brain. Right after birth, synapses form and dissolve among nascent neurons at breakneck speed as the animal adapts to its new surroundings. But, over time, while some plasticity remains and allows for learning, most synapses stabilize and some may last a lifetime. How synapses are maintained over such long periods is somewhat of a mystery, especially in light of the fact that structural proteins constantly move in and out of synapses. In theory, the active turnover of synaptic components might simply reflect the balance between protein synthesis and degradation. But, in a recent study, Shlomo Tsuriel, Ran Geva, Noam Ziv, and their colleagues find that two prominent synaptic proteins, Synapsin I and ProSAP2, turn over primarily through rapid exchanges between neighboring synapses, rather than via synthesis and degradation. These observations add an interesting twist to the already complex picture of synapse biology. Synapses are specialized devices that serve to transfer electrical impulses between neurons. They form at discrete contact points between the neuron’s main branch (the axon) and the complex arborizations (dendrites) that sprout from its target neuron’s cell body. A number of specialized structures and molecules accumulate at synapses, including synaptic vesicles chock-full of neurotransmitters on the axonal (presynaptic) side, and neurotransmitter receptors on the dendritic (postsynaptic) side. Synapsin I and ProSAP2 play important structural roles: Synapsin I tethers synaptic vesicles underneath the presynaptic lipid membrane and ProSAP2 organizes the postsynaptic architecture. To follow the whereabouts of Synapsin I and ProSAP2, the researchers tagged each protein with fluorescent dyes and coaxed cultured neurons from the hippocampus (a brain region involved in learning) of newborn rats to synthesize these fluorescently tagged proteins. As the neurons grew in culture, they established synapses that incorporated the tagged Synapsin I or ProSAP2. The synapses were easily visualized as bright fluorescent spots studding dendrites and axon branches. The first dye, called green fluorescent protein (GFP, a small protein that was originally isolated from jellyfish), fluoresces readily but can be extinguished with intense illumination, a phenomenon called photobleaching. The researchers photobleached individual synapses containing GFP-tagged Synapsin I or ProSAP2 with an intense laser beam. Over time, a fluorescent signal reappeared at the bleached synapses, indicating that bleached proteins were replaced with tagged proteins from unbleached areas. Tagged Synapsin replenished bleached synapses in about 40 minutes, and tagged ProSAP2 in two to four hours. But these experiments did not show where the replenishing proteins came from. To answer this question, the researchers took advantage of a second dye, photoactivatable variant of GFP (PA-GFP), whose fluorescence is activated, rather than extinguished, with intense illumination. The researchers photoactivated PA-GFP-tagged Synapsin I or ProSAP2 over small portions of dendrites or axons. Over the course of 10 to 40 minutes, fluorescence gradually declined at the illuminated synapses, and concomitantly increased in neighboring synapses. These results indicate that pre- and postsynaptic proteins routinely hop from one synapse to the next with timescales of tens of minutes, a behavior that might account for the rapid replenishment of photobleached synapses. Still, some of the replenishing material could also have come from new protein synthesis. By tracking PA-GFP-tagged proteins from cell bodies, where most synthesis typically occurs, into dendrites and axons, the researchers determined that newly synthesized Synapsin I and ProSAP2 moved too slowly to explain the rapid replenishment of bleached synapses. In addition, inhibitors of protein synthesis and degradation did not significantly affect the synapses’ replenishment rates, confirming that the high turnover rate of Synapsin I and ProSAP2 owes mostly to local exchanges among neighboring synapses. How the promiscuous exchange of structural proteins such as Synapsin I and ProSAP2 affects synaptic stability is still unclear. Competition for a local pool of synaptic components could eventually determine which synapse is stabilized. Curiously, synaptic signaling may be a destabilizing factor in the young hippocampal neurons, as electric stimulations to the cultures greatly increased Synapsin I and ProSAP2 trafficking. Whether local promiscuity is a characteristic of youthful synapses or also holds true for more mature ones remains to be seen.

Three-Dimensional Modeling and Simulation of a Falling Electronic Device

This article focuses on a realistic mathematical model for multiple impacts of a rigid body to a viscoelastic ground and its comparison to theoretic results. The methodology is used to study impact on an electronic device. When an electronic device drops to the floor at an uneven level, the rapid successions of impact sequence are important for their shock response to internal structure of the devices. A three-dimensional, continuous contact, computational impact model has been developed to simulate a sequence of multiple impacts of a falling rigid body with the ground. The model simulates the impact procedure explicitly and thus is capable of providing detailed information regarding impact load, impact contact surface, and the status of the body during the impact. For the purposes of model verification, we demonstrate the numerical simulation of a falling rod problem, in which the numerical results are in good agreement with the analytic solutions based on discrete contact dynamics impact models. It is indicated by the numerical experiments that simultaneous impacts occurred to multiple locations of the body and that subsequent impacts might be larger than initial ones due to different angles of impact. The differential equation-based computational model is shown to be realistic and efficient in simulating impact sequence and laid a foundation for detailed finite element analysis of the interior impact response of an electronic device.

Correlation of a discrete friction (Iwan) element and continuum approaches to predict interface sliding behavior

Abstract This paper makes quantitative comparisons between three types of models for predicting frictional interface behavior: (i) Iwan-type friction elements that describe contact in terms of discrete contact points and spring elements, (ii) analytical models based on continuum elasticity and Coulomb friction, and (iii) finite element models which incorporate a simple interface constitutive description. A simplified “fiber pullout” geometry is considered which consists of a clamped rectangular bar being pulled between friction grips. The analysis illustrates that interface slip and load transfer behavior is well described by both analytical shear models and bilinear Iwan-type elements which differentiate elastic and friction effects. Two key contribution of this work are the identification of explicit connections between discrete friction parameters (i.e. spring stiffnesses) and continuum elasticity properties, and a straightforward calibration procedure to correlate lower order models with continuum results. This represents an important step toward lower order frictional interface models that can be used to efficiently yet accurately simulate dynamic stick-slip behavior in dynamically loaded contacts.

Response Surface Optimization for Joint Contact Model Evaluation

When optimization is used to evaluate a joint contact model's ability to reproduce experimental measurements, the high computational cost of repeated contact analysis can be a limiting factor. This paper presents a computationally-efficient response surface optimization methodology to address this limitation. Quadratic response surfaces were fit to contact quantities (contact force, maximum pressure, average pressure, and contact area) predicted by a discrete element contact model of the tibiofemoral joint for various combinations of material modulus and relative bone pose (i.e., position and orientation). The response surfaces were then used as surrogates for costly contact analyses in optimizations that minimized differences between measured and predicted contact quantities. The methodology was evaluated theoretically using six sets of synthetic (i.e., computer-generated) contact data, and practically using one set of experimental contact data. For the synthetic cases, the response surface optimizations recovered all contact quantities to within 3.4% error. For the experimental case, they matched all contact quantities to within 6.3% error except for maximum contact pressure, which was in error by up to 50%. Response surface optimization provides rapid evaluation of joint contact models within a limited range of relative bone poses and can help identify potential weaknesses in contact model formulation and/or experimental data quality.

A comparison of solvers for linear complementarity problems arising from large-scale masonry structures

We compare the numerical performance of several methods for solving the discrete contact problem arising from the finite element discretisation of elastic systems with numerous contact points. The problem is formulated as a variational inequality and discretised using piecewise quadratic finite elements on a triangulation of the domain. At the discrete level, the variational inequality is reformulated as a classical linear complementarity system. We compare several state-of-art algorithms that have been advocated for such problems. Computational tests illustrate the use of these methods for a large collection of elastic bodies, such as a simplified bidimensional wall made of bricks or stone blocks, deformed under volume and surface forces.