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LIGGGHTS Flexible Fibers

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Bond Style Gran Command

Syntax

bond_type gran
bond_coeff 1 ro ri Ybond Gbond damp_type damp_val(s) break_type break_val(s)

Examples

bond_style gran
bond_coeff 1 1.0 0.0 1.5e9 1.0e9 1 1.0 0 0.01
bond_coeff 1 1.0 0.0 1.5e9 1.0e9 1 1.0 1 1.5e6 1.0e6
bond_coeff 1 1.0 0.0 1.5e9 1.0e9 1 1.0 2 1.5e6 1.0e6 330.0
bond_coeff 1 1.0 0.0 1.5e9 1.0e9 2 0.0 0.01 0.0 1 1.5e6 1.0e6
bond_coeff 1 1.0 0.0 1.5e9 1.0e9 2 0.0 0.01 0.0 5 1.5e6 1.0e6

Warning

This is a beta version... Be Careful...

Description

The gran bond style uses the following force and moment equation

bond equation

The granular bond is adopted from D.O. Potyondy and P.A. Cundall's original paper, "A Bonded-Particle for Rock". The model currently utilizes two different damping styles, one inspired by Guo et al. (2013) and slightly modified by Schramm et al. (2019) (linear damp [1]).

bond equation

and a nonlinear damping version (nonlinear damp [2]) and is currently being tested and submitted for peer review.

bond equation

The bond gran model offers five break styles. The simple break style (Break_Style 1) looks at the distance between two spheres and if the distance is greater than 2*r_break, the bond is considered to be broken. The next method (Break_Style 2) checks the maximum stress inside the bond. If the stress is greater than a critical stress, the bond breaks. The user needs to supply the critical normal stress (Pa) and the critical tangential stress (Pa). The maximum stress is calculated as follows.

bond equation

The third break style (Break_Style 3) expands on the stress dependent method by looking at the average temperature between the two bonded spheres. If this average temperature is greater than a critical temperature, the bond breaks. The user must also add the following fixes to use this break style.

fix ftco all property/global thermalConductivity peratomtype 5.
fix ftca all property/global thermalCapacity peratomtype 10.
fix heattransfer all heat/gran initial_temperature 300.

Break styles 4 and 5 are soft break methods in which the bond does not break until the two atoms have separated completely allowing for heavily overlapped atoms that were bonded to break without exploding. Both styles reset the bond length between the atoms to the distance at which the bond broke and provides a normal force to separated the two atoms. Break style 4 simply uses 1% of the original normal stiffness while break style 5 uses the hertzian contact law .

The following coefficients must be defined for each bond type via the bond_coeff command as in the examples shown above, or in the data file or restart files read by the read_data or read_restart commands:

  • ro is the percentage of the outer radius of the bond in respect to the atom radius and takes values between 0 and 1.0
  • ri is the percentage of the inner radius of the bond in respect to the atom radius and takes values between 0 and 1.0 and must be less than ro
  • Ybond is the bond Young's modulus in Pa
  • Gbond is the bond shear modulus in Pa
  • damp_type is the type of damping to be used in the simulation and takes values of 0 (no damping), 1 (linear damping), or 2 (nonlinear damping)
  • damp_val(s) is the bond coefficients. For damp_type 0, the user must still set a value but it will not be used. For damp_type 1 a single damping coefficient is used and takes values greater than 0.0. For damp_type 2 three damping coefficients must be provided.
  • break_type is the type of breaking to be used in the simulation and takes values 0 or 1 or 2 or 3 or 4 or 5.
  • break_val(s) are the coefficients to be used with a break_type. For break_type 1, a single value is needed. For break_type 2,4, and 5 two values must be provided. For break_type 3 three values must be provided.

Experimental Features

A fix for accurate normal stiffness values has been added when using smooth fibers. This fix can be applied by adding the additional define during the building process (See the how to install section for more info):

-DDO_SMOOTH_FIBER_FIX

Restrictions

This bond style can only be used if LIGGGHTS(R)-PUBLIC was built with the MOLECULAR package (which it is by default). See the How to Install section for more info on packages.

Related Commands

bond_coeff delete_bonds

Defaults

none