 Effective contact radius
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% $E_{eff}$: Effective Young modulus
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% $m_{eff}$: Effective mass
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% $e$: Normal coefficient of restitution
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% $\beta = \frac{\pi}{ln(e)}$
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% $S$: Variable unload stiffness coefficient parameter
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% *References*:
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% * <https://doi.org/10.1122/1.549893
% O.R. Walton and R.L. Braun.
% Viscosity, granular temperature, and stress calculations for shearing assemblies of inelastic, frictional disks, _J. Rheol._, 30:949, 1986>
% (proposal)
%
classdef ContactForceN_ElastoPlasticLine