392,251 research outputs found
Contact mechanics in fretting fatigue
This paper studies the contact mechanics in a line contact during fretting fatigue conditions. In literature one can find numerical and analytical solutions of normal and tangential stresses for a variety of loading cases. However, a unified solution valid for all loading cases during fretting fatigue conditions is not available. We present in this paper a strategy to combine existing contact mechanics theories into a unified calculation procedure. Therefore, the relevant contact mechanics theories for an idealized cylinder-on-flat contact are selected and bundled. Two clear flowcharts group the existing theories, which results in a unified strategy that can easily be implemented in a programming language. A Matlab script was programmed and calculates the normal and tangential stress distribution based on the applied forces, the geometry of the contact, the coefficient of friction and the material properties. The present theory can be used to automate the calculation of the stress distributions, or as validation of new numerical techniques. The script is modular and can be extended to calculate the lifetime of a component, by adding lifetime criteria
Neutron matter at next-to-next-to-next-to-leading order in chiral effective field theory
Neutron matter presents a unique system for chiral effective field theory
(EFT), because all many-body forces among neutrons are predicted to
next-to-next-to-next-to-leading order (N3LO). We present the first complete
N3LO calculation of the neutron matter energy. This includes the subleading
three-nucleon (3N) forces for the first time and all leading four-nucleon (4N)
forces. We find relatively large contributions from N3LO 3N forces. Our results
provide constraints for neutron-rich matter in astrophysics with controlled
theoretical uncertainties.Comment: 5 pages, 4 figures; improved version, 3N ring and 2pi-contact
contributions corrected, conclusions unchanged; v3: minor changes, published
versio
Marginal stability in jammed packings: quasicontacts and weak contacts
Maximally random jammed (MRJ) sphere packing is a prototypical example of a
system naturally poised at the margin between underconstraint and
overconstraint. This marginal stability has traditionally been understood in
terms of isostaticity, the equality of the number of mechanical contacts and
the number of degrees of freedom. Quasicontacts, pairs of spheres on the verge
of coming in contact, are irrelevant for static stability, but they come into
play when considering dynamic stability, as does the distribution of contact
forces. We show that the effects of marginal dynamic stability, as manifested
in the distributions of quasicontacts and weak contacts, are consequential and
nontrivial. We study these ideas first in the context of MRJ packing of
d-dimensional spheres, where we show that the abundance of quasicontacts grows
at a faster rate than that of contacts. We reexamine a calculation of Jin et
al. (Phys. Rev. E 82, 051126, 2010), where quasicontacts were originally
neglected, and we explore the effect of their inclusion in the calculation.
This analysis yields an estimate of the asymptotic behavior of the packing
density in high dimensions. We argue that this estimate should be reinterpreted
as a lower bound. The latter part of the paper is devoted to Bravais lattice
packings that possess the minimum number of contacts to maintain mechanical
stability. We show that quasicontacts play an even more important role in these
packings. We also show that jammed lattices are a useful setting for studying
the Edwards ensemble, which weights each mechanically stable configuration
equally and does not account for dynamics. This ansatz fails to predict the
power-law distribution of near-zero contact forces, .Comment: final submitted versio
Spanning the scales of granular materials through microscopic force imaging.
If you walk on sand, it supports your weight. How do the disordered forces between particles in sand organize, to keep you from sinking? This simple question is surprisingly difficult to answer experimentally: measuring forces in three dimensions, between deeply buried grains, is challenging. Here we describe experiments in which we have succeeded in measuring forces inside a granular packing subject to controlled deformations. We connect the measured micro-scale forces to the macro-scale packing force response with an averaging, mean field calculation. This calculation explains how the combination of packing structure and contact deformations produce the observed nontrivial mechanical response of the packing, revealing a surprising microscopic particle deformation enhancement mechanism
Modelling High Speed Machining with the SPH Method
The purpose of this work is to evaluate the use of the Smoothed Particle Hydrodynamics (SPH) method within the framework of high speed cutting modelling. First, a 2D SPH based model is carried out using the LS-DYNA® software. SPH is a meshless method, thus large material distortions that occur in the cutting problem are easily managed and SPH contact control allows a “natural” workpiece/chip separation. The developed SPH model proves its ability to account for continuous and shear localized chip formation and also correctly estimates the cutting forces, as illustrated in some orthogonal cutting examples. Then, The SPH model is used in order to improve the general understanding of machining with worn tools. At last, a milling model allowing the calculation of the 3D cutting forces is presented. The interest of the suggested approach is to be freed from classically needed machining tests: Those are replaced by 2D numerical tests using the SPH model. The developed approach proved its ability to model the 3D cutting forces in ball end milling
Formulation to Predict Lower Limb Muscle Forces during Gait
The human body has more muscles than Degrees of Freedom (DoF), and that leads to indeterminacy in the muscle force calculation. This study proposes the formulation of an optimization problem to estimate the lower-limb muscle forces during a gait cycle of a patient wearing an instrumented knee prosthesis. The originality of that formulation consists of simulating muscle excitations in a physiological way while muscle parameters are calibrated. Two approaches have been considered. In Approach A, measured contact forces are applied to the model and all inverse dynamics loads are matched in order to get a physiological calibration of muscle parameters. In Approach B, only the inverse dynamics loads not affected by the knee contact loads are matched. With that approach, contact forces can be predicted and validated by comparison with the experimental ones. Approach B is a test of the optimization method and it can be used for cases where no knee contact forces are available
Spanning the Scales of Granular Materials: Microscopic Force Imaging
If you walk on sand, it supports your weight. How do the disordered forces
between particles in sand organize, to keep you from sinking? This simple
question is surprisingly difficult to answer experimentally: measuring forces
in three dimensions, between deeply buried grains, is challenging. We describe
here experiments in which we have succeeded in measuring forces inside a
granular packing subject to controlled deformations. We connect the measured
micro-scale forces to the macro-scale packing force response with an averaging,
mean field calculation. This calculation explains how the combination of
packing structure and contact deformations produce the unexpected mechanical
response of the packing, and reveals a surprising microscopic particle
deformation enhancement mechanism.Comment: Data and code available at http://www.phy.duke.edu/~nb108
A fast time-domain model for wheel/rail interaction demonstrated for the case of impact forces caused by wheel flats
The prediction of impact forces caused by wheel flats requires the application of time-domain models that are generally more computationally demanding than are frequency-domain models. In this paper, a fast time-domain model is presented to simulate the dynamic interaction between wheel and rail, taking into account the non-linear processes in the contact zone. Track and wheel are described as linear systems using impulse-response functions that can be precalculated. The contact zone is modelled by non-linear contact springs, allowing for loss of contact. This general model enables the calculation of the vertical contact forces generated by the small-scale roughness of rail and wheel, by parametric excitation on a discretely supported rail and by discrete irregularities of rail and wheel. Here, the model is applied to study the excitation caused by wheel flats by introducing a flat on a rotating wheel whose profile in the contact zone is updated in every time step. To demonstrate the functioning of the model, simulation results are compared to field measurements of impact forces and a brief parameter study is presented
Investigation of inner contact and friction conditions of a spherical roller bearing using multi-body simulation
At the Institute of Machine Elements, Gears, and Transmission (MEGT, University of Kaiserslautern) a spherical roller bearing has been modeled in an multi body system (MBS) environment. The use of the commercial MBS (multi-body system) software (MSC ADAMS) allows the development of user-written subroutines for contact recognition and the calculation of contact forces. Those subroutines can help understanding the principles of friction phenomena inside spherical roller bearings, while the measurement of those effects is difficult.Measurements on a friction torque test rig for roller bearings are used to validate the MBS models. Since the sum of all contact forces equals the friction of the bearing, this test stand provides a way for validation of the contact and friction calculations
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