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Redistribution of mass to raise the minimum stable time increment to a target value
You can increase the minimum stable time increment in the initial configuration for an element set to a specified target value by redistributing mass among the elements in that set. The redistribution of mass to affect the stable time increment and adjustment of mass to achieve a target total mass can be requested independently of each other. If both options are requested for a given element set, the mass is first adjusted to meet the target total mass for the set and then redistributed among the elements to achieve the target time increment.
You can set a default target time increment that is applicable for all of the mass-adjusted element sets as well as specific targets for any of the individual element sets. Within each set, the mass is transferred to the elements with time increments below the target value from the remaining elements. Abaqus/Explicit prints the amount of mass available for redistribution along with the percentage of this amount that is redistributed to the data (.dat) file if model definition data are requested (see “Controlling the amount of analysis input file processor information written to the data file” in “Output,” Section 4.1.1). If a sufficient amount of mass is not available to meet the specified target time increment, the analysis terminates with an error message. “Impact of a water-filled bottle,” Section 2.3.2 of the Abaqus Example Problems Guide, is an example of maintaining the target stable time increment of an element set using mass adjustment.
When compared to the fixed mass scaling functionality, the redistribution feature above does not alter the total mass of the set. However, both features affect the center of mass and the principal directions of rotary inertia. The redistribution feature is performed only in the initial configuration at the start of the analysis; whereas the fixed mass scaling is performed in the configuration at the start of the step requesting that mass scaling. When you specify mass adjustment and mass scaling, the mass scaling adds mass as necessary on top of the adjusted mass.
Input File Usage:
Use the following option to raise the time increment and specify the total mass:
*MASS ADJUST, TARGET DT=min_stable_time_increment elem_set_name, elem_set_mass, elem_set_min_stable_time_increment
Use the following option to raise the time increment without altering the total mass:
*MASS ADJUST, TARGET DT=min_stable_time_increment elem_set_name, CURRENT, elem_set_min_stable_time_increment
Redistribution of mass using a scale factor
An alternative method to adjust the mass of an element set is to specify a scale factor. When the mass adjustment is defined using a scale factor, you can maximize the minimum stable time increment in the initial configuration for an element set by redistributing either the total mass or only the added mass among the elements in that set. The redistribution of the total mass to maximize the time increment is done iteratively by taking mass from each element with a stable time increment greater than the average and adding it to the elements with lower time increments—this iterative process stops when all elements have the same time increment within a tolerance.
The redistribution of only the added mass to maximize the time increment is done in a similar fashion, but only the added mass is transferred between elements. The elements may not have the same time increment after the redistribution is complete. In this case the scale factor should be greater than one. Both choices of mass redistribution described here affect the center of mass and the principal directions of rotary inertia, but redistributing only the added mass with a relatively small scale factor may have a lesser impact.
Alternatively, you can scale the mass to redistribute it uniformly (in proportion with the pre-adjusted mass) without affecting the center of mass and the principal directions of rotary inertia.
Input File Usage: Use one of the following options to maximize the time increment by redistributing the total mass of the element set:
*MASS ADJUST, TARGET DT=MAXIMIZE
elem_set_name, , , scale_factor, REDIST
*MASS ADJUST
elem_set_name, , , MAXIMIZE, scale_factor, REDIST
Use one of the following options to redistribute only the added mass:
*MASS ADJUST, TARGET DT=MAXIMIZE
elem_set_name, , , scale_factor, ADD
*MASS ADJUST
elem_set_name, , , MAXIMIZE, scale_factor, ADD
Use the following option to scale the mass for an element set without altering its center of mass:
*MASS ADJUST
elem_set_name, , UNIFORM, scale_factor
2.7 Nonstructural mass definition
• “Nonstructural mass definition,” Section 2.7.1
2.7.1 NONSTRUCTURAL MASS DEFINITION
Products: Abaqus/Standard Abaqus/Explicit Abaqus/CAE
References
• “Point masses,” Section 30.1.1
• “Density,” Section 21.2.1
• “Adjust and/or redistribute mass of an element set,” Section 2.6.1
• *NONSTRUCTURAL MASS
• “Defining nonstructural mass,” Section 33.4 of the Abaqus/CAE User’s Guide, in the HTML version of this guide
Overview
A nonstructural mass:
• is a contribution to the model mass from features that have negligible structural stiffness (such as paint on sheet metal panels in a car);
• can be used to bring the net mass of one or more components in the model up to a known value;
• can be positive to add mass to the model and negative to remove mass from the model, with the corresponding increase or decrease in the element stable time increment in an Abaqus/Explicit analysis;
• can be specified in the form of a total mass of the nonstructural features to be distributed over one or more components in the model;
• can be specified in the form of an increase in density over the smeared region;
• can be specified in the form of mass per unit area to be applied over a smeared region consisting of shells, membranes, and/or surface elements; and
• can be specified in the form of mass per unit length to be applied over a smeared region consisting of beam, pipe, and/or truss elements.
Nonstructural mass
The mass contribution from nonstructural features can be included in the model even if the features themselves are omitted. The nonstructural mass is smeared over an element set that is typically adjacent to the nonstructural feature. This element set can contain solid, shell, membrane, surface, beam, pipe, or truss elements. The nonstructural mass can be specified in the following forms:
• a total mass value,
• a mass per unit volume,
• a mass per unit area (for element sets that contain conventional shell, membrane, and/or surface elements), or
• a mass per unit length (for element sets that contain beam, pipe, and/or truss elements).
When a total mass is spread over an element set region, it can be distributed either in proportion to the underlying element “structural” mass or in proportion to the element volume in the initial configuration.
A “structural” mass is defined as the sum of all the mass contributions to an element outside of the nonstructural features. This may include the mass due to any material definitions associated with the element; any “mass per unit area” given on the section definition for shell, membrane, and surface elements; mass from any rebars included in shell, membrane, and surface elements; and any additional inertia given on the section definition of beam/pipe elements. A nonstructural mass contribution to an element is not allowed if that element has no structural mass.
A given element in the model can have contributions from multiple nonstructural mass specifications. The nonstructural mass in a given element will participate in any mass proportional distributed loads, such as gravity loading, defined on that element. When a nonstructural mass is added to a shell, beam, or pipe element with active rotational degrees of freedom, the nonstructural contribution affects both the element mass and the element rotary inertia. The element stable time increment increases with a positive nonstructural mass and decreases with a negative nonstructural mass. In general, it is easier to use a nonstructural mass definition to bring an additional mass into the model than to do the same with a group of point masses. It is also more beneficial in an Abaqus/Explicit analysis due to a possibly higher time increment.
Any mass proportional damping specified as part of the material definition (see “Material damping,” Section 26.1.1) will also apply to the nonstructural mass contribution assigned to the element or element set using that material definition.
Nonstructural mass contributions associated with an element set are not imported when transferring model data between Abaqus analyses (see “Transferring results between Abaqus analyses: overview,” Section 9.2.1). These contributions need to be redefined in the import analysis if they are to be included in the model.
Defining nonstructural mass
To define a nonstructural mass contribution to the model mass, you must first identify the region over which the contribution must be added. You then specify the value of the nonstructural mass using the appropriate units and, if the total mass from the nonstructural features is known, determine how the nonstructural mass is distributed over the region.
Input File Usage: *NONSTRUCTURAL MASS, ELSET=element_set_name
Abaqus/CAE Usage: Property or Interaction module: Special→Inertia→Create:
Nonstructural mass: select region
Specifying the units of the nonstructural mass
The nonstructural mass can be specified in different types of units, depending on the types of elements contained in the specified region.
Specifying units of mass
A total nonstructural mass with units of “mass” can be spread over a region containing solid, shell, membrane, beam, pipe, and/or truss elements.
Input File Usage: *NONSTRUCTURAL MASS, UNITS=TOTAL MASS total mass of the nonstructural feature
Abaqus/CAE Usage: Property or Interaction module: Special→Inertia→Create: Nonstructural mass: select region: Units: Total Mass: Magnitude: total mass of the nonstructural feature
Specifying units of mass per unit volume
A nonstructural mass with units of “mass per unit volume” can be spread over a region containing solid, shell, membrane, beam, pipe, and/or truss elements.
Input File Usage: *NONSTRUCTURAL MASS, UNITS=MASS PER VOLUME added density due to the nonstructural feature
Abaqus/CAE Usage: Property or Interaction module: Special→Inertia→Create: Nonstructural mass: select region: Units: Mass per Volume: Magnitude: added density due to the nonstructural feature
Specifying units of mass per unit area
A nonstructural mass with units of “mass per unit area” can be spread over a region containing conventional shells, membranes, and/or surface elements.
Input File Usage: *NONSTRUCTURAL MASS, UNITS=MASS PER AREA added mass per unit area due to the nonstructural feature
Abaqus/CAE Usage: Property or Interaction module: Special→Inertia→Create: Nonstructural mass: select region: Units: Mass per Area: Magnitude: added mass per unit area due to the nonstructural feature
Specifying units of mass per unit length
A nonstructural mass with units of “mass per unit length” can be spread over a region containing beam, pipe, and/or truss elements.
Input File Usage: *NONSTRUCTURAL MASS, UNITS=MASS PER LENGTH added mass per unit length due to the nonstructural feature
Abaqus/CAE Usage: Property or Interaction module: Special→Inertia→Create: Nonstructural mass: select region: Units: Mass per Length: Magnitude: added mass per unit length due to the nonstructural feature
Controlling the distribution of the total mass from nonstructural features
There are two methods available for distributing the nonstructural mass over the region when the total mass from the nonstructural features is known.
Distributing the nonstructural mass in proportion to the element structural mass
If you do not want to change the center of mass for the region, distribute the nonstructural mass in proportion to the element structural mass. This method results in a uniform scaling of the structural density of the region. Abaqus uses mass proportional distribution by default.
The element structural mass in shell, membrane, and surface elements includes any mass contribution from rebar provided that the rebar are defined as a rebar layer (see “Defining reinforcement,” Section 2.2.3).
Input File Usage: *NONSTRUCTURAL MASS, UNITS=TOTAL MASS, DISTRIBUTION=MASS PROPORTIONAL total mass of the nonstructural feature
Abaqus/CAE Usage: Property or Interaction module: Special→Inertia→Create: Nonstructural mass: select region: Units: Total Mass: Magnitude: total mass of the nonstructural feature: Distribution: Mass Proportional
Distributing the nonstructural mass in proportion to the element volume
Alternatively, you can distribute the nonstructural mass in proportion to the element volume in the initial configuration. This method results in a uniform value added to the underlying structural density over the region. Therefore, the center of mass for the region may be altered if the region has nonuniform structural density.
Input File Usage: *NONSTRUCTURAL MASS, UNITS=TOTAL MASS, DISTRIBUTION=VOLUME PROPORTIONAL total mass of the nonstructural feature
Abaqus/CAE Usage: Property or Interaction module: Special→Inertia→Create: Nonstructural mass: select region: Units: Total Mass: Magnitude: total mass of the nonstructural feature: Distribution: Volume Proportional
2.8 Distribution definition
• “Distribution definition,” Section 2.8.1