Product: Abaqus/Explicit
Mass adjustment:
is useful to set the net mass of one or more components in the model to a known value;
is useful to account for any errors in mass due to modeling approximations;
is useful to account for mass from nonstructural features otherwise omitted from the model, such as paint;
can be applied over all element types that have mass;
adjusts the mass of the individual elements in an element set in proportion to their pre-adjusted mass including any nonstructural mass, so as to meet the specified target value for the set;
can be used to redistribute mass among elements in the set to raise the minimum stable time increment to a target value;
can be specified only once in an Abaqus/Explicit analysis during the model definition; and
can be applied in a hierarchical fashion to adjust the mass for individual parts first and then for an assembly of these parts.
The mass of a component in a numerical model may differ from its actual value for a number of reasons including modeling approximations and omission of minor features from the model. You can specify mass adjustment in the numerical model for such components by identifying the element sets defining these components and their respective total mass values. For a given element set, the mass is adjusted at the start of the analysis such that the adjustment in each element in that set is in proportion to the pre-adjusted mass of that element, thus preserving the center of mass and the principal directions of the rotary inertia. The pre-adjusted mass of an element includes the mass due to any associated material density; any mass directly specified on the section definition as in the case of beam, pipe, shell, membrane, rigid, and surface elements; and any nonstructural mass applied directly to that element. “Knee bolster impact with general contact,” Section 2.1.9 of the Abaqus Example Problems Manual, is an example of setting the total mass of an element set using mass adjustment.
When mass is adjusted for an element with active rotational degrees of freedom, the rotary inertia contribution from that element is also modified proportionally to correspond with the scaling in the element mass from mass adjustment, thus preserving the principal directions of the rotary inertia. The adjusted mass value is considered when calculating the stable time increment of an element. Loads such as mass proportional damping (see “Material damping,” Section 25.1.1) and gravity take the adjusted mass into account.
Mass adjustment can be applied in a hierarchical fashion to adjust the mass for individual parts first and then for an assembly of these parts. In this scenario, the mass adjustment defined over the assembly may further modify the adjusted mass of the individual parts. You must associate all of the mass-adjusted element sets in the desired order with a single mass adjustment definition.
Abaqus/Explicit automatically calculates the mass, center of mass, and rotary inertia of each element set and prints the results 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). The contributions from mass adjustment are also listed in these tables.
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 Manual, 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.
To adjust the total mass of one or more components in the model, you first identify the corresponding element sets. If you specify multiple elements sets, the mass is adjusted in the order in which the element sets are specified. For element sets that share elements, you must determine the order in which to specify the element sets to obtain the desired results.
You must specify the total mass for each mass-adjusted element set.
| Input File Usage: | *MASS ADJUST element_set_name, element_set_mass |
You can redistribute the mass of an element set to achieve a target time increment and specify the total mass for each mass-adjusted element set, or you can redistribute the mass without changing the existing total mass of the element set. You can set a default target time increment that is applicable for all of the mass-adjusted sets as well as specific targets for any of the individual sets. When both a default target and a specific target are specified, the specific target is used for that set.
| 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 element_set_name, element_set_mass, element_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 element_set_name, CURRENT, element_set_min_stable_time_increment |