block mechanical command

Syntax

block mechanical keyword <range>

Primary keywords:

mass-scale    energy    damping    local

Specify configuration for mechanical calculations.

mass-scale b <timestep f > <maximum-ratio f >

Turn on mass (density) scaling for all blocks to speed convergence of static problems involving very nonuniform blocks or zone meshes, or very nonuniform elastic properties. See below for further discussion.

energy b

Toggle energy summation. All energy calculations are incremental, except strain energies from the block insitu stress command (if it is used with energy on).

damping keyword

Set damping for static and dynamic analyses. local damping is the default for this keyword.

combined <f >

Use combined damping set to f (the damping value is 0.8 by default).

contact keyword ...

Set different stiffness damping factors, depending on the material numbers and contact types. See below for further information.

group1 s

The group name of the first group of block(s) in contact.

group2 s

The group name of the second group of block(s) in contact.

slot s

The slot name of the groups.

betaffn f

Face-face contact normal damping factor.

betaffs f

Face-face contact shear damping factor.

betavvn f

Vertex-vertex contact normal damping factor.

betavvs f

Vertex-vertex contact shear damping factor.

betaven f

Vertex-edge contact normal damping factor.

betaves f

Vertex-edge contact shear damping factor.

betavfn f

Vertex-face contact normal damping factor.

betavfs f

Vertex-face contact shear damping factor.

betaefn f

Edge-face contact normal damping factor.

betaefs f

Edge-face contact shear damping factor.

betaeen f

Edge-edge contact normal damping factor.

betaees f
Edge-edge contact shear damping factor.
global <fac > <f1 > <f2 >

Viscous damping is specified for the blocks. See below for further information.

local <f > (this is the default)

Use local damping set to f (the damping value is 0.8 by default).

maxwell <fd1 > <f1 > <fd2 > <f2 > <fd3 > <f3 > <zone-only b1 > <contact-only b2 > <range...>
Applies Maxwell damping to all zones and sub-contacts in range
fd1 f1 … peak damping ratio and frequency for Maxwell element 1 fd2 f2 … peak damping ratio and frequency for Maxwell element 2 fd3 f3 … peak damping ratio and frequency for Maxwell element 3
Optional keywords:
zone-only b1 applies damping only to zones in range, not to contacts; default: off contact-only b2 applies damping only to contacts in range, not to zones; default: off
rayleigh f1 f2 <keyword>

This form of the command is normally used for dynamic calculations when a certain fraction of critical damping is required over a given frequency range. This type of damping is known as Rayleigh damping, where f1 = the fraction of critical damping operating at the center frequency of f2. See below for further discussion.

mass

Set the mass damping term to 0.

stiffness

Set the stiffness damping term to 0.

Mass Density Scaling

When using mass density scaling for rigid blocks, the user must supply a value for bcmass. (A dummy place for zcmass is therefore required.) For deformable blocks, zcmass and bcmass need not be supplied. If no value is supplied, the code will perform density scaling automatically, using average zone and block masses. Average zone and block masses generally lead to satisfactory solutions.

Values for minimum, average and maximum zone and block masses can be found using the block list limits command. Mass scaling is always on for the default (static) calculation mode. Mass scaling may only be turned off for the dynamic analysis option. Mass scaling must be turned off for correct results during the dynamic calculation.

In a dynamic analysis, global scaling should not be used. For complex jointed systems, small block zones may be created during the automatic meshing procedure. The small zones require small timesteps for numerical stability of the explicit algorithm. This makes some dynamic solutions extremely time-consuming. However, since these zones may be very small, with very small masses, it is possible to introduce some density scaling only for these zones in such a way that the change of the system inertia is negligible. This is referred to as partial density scaling. Partial density scaling was implemented in 3DEC in such a way that the user controls the amount of scaling to be introduced. Given the timestep calculated by the code, the user specifies the desired timestep with the timestep keyword. This command specifies that only the amount of density scaling required to achieve the timestep f is to be applied to the system. When a model cycle command is given, a message indicating the number of gridpoint masses that were scaled, and the amount of additional mass introduced, is printed. The maximum-ratio keyword specifies the maximum ratio of the maximum to minimum gridpoint mass scaling factors.

Global Damping

For static or steady-state problems, the objective is to absorb vibrational energy as rapidly as possible. In this case, the form of the command using the global keyword should be used; this causes energy to be absorbed in proportion to the rate of change of kinetic energy. The optional parameter fac is the ratio of damping dissipation to kinetic energy change. If fac is not given, the default value of 0.5 is used. In most cases, this gives a fast convergence. The multipliers f1 and f2 adjust the damping coefficient by a fractional amount as the ratio changes. The default values of 0.99 for f1 and 1.05 for f2 are optimum in most cases.

Contact Damping

The Beta factors are the normal and shear stiffness damping factors to be used for the different contact types. This method of setting the stiffness damping factors gives greater control for block bounce simulations.

The general equation for the stiffness damping force is

\[f = −β × K × aΔγ\]
where: \(f\) = damping force;
  \(β\) = damping factor;
  \(K\) = stiffness;
  \(a\) = area of contact; and
  \(Δγ\) = change in velocity.

Rayleigh Damping

Input frequencies for the program are in cycles/sec, not radians/sec. The optional modifiers stiffness and mass denote that the damping is to be restricted to stiffness or mass-proportional, respectively. If they are omitted, normal Rayleigh damping is used. Note that by specifying stiffness-damping, the critical timestep for numerical stability will automatically be reduced. It is still possible for instability to result if large deformation occurs. In such a case, lower the timestep with the model mechanical timestep safety-factor command.) Damping considerations for dynamic analysis are discussed further in Section 2 in Optional Features.

Note

Mass scaling (mass-scale) is performed automatically when damping is set to auto or local. Mass scaling is turned off when the command DAMPING 0,0 is invoked. Mass scaling may only be turned off if the dynamic option is active.