Mass Adjustment

To adjust or scale 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 or scaled 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.

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 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) 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). The contributions from mass adjustment are also listed in these tables. Element output variable MASSADJUST can be requested as output to the output database (.odb) file, and it will indicate how the mass of the set is adjusted or redistributed to each element included in the set (see Abaqus/Explicit Output Variable Identifiers). This output variable is available as field output (contour plots) in the first output frame of the first analysis step.

Mass adjustment contributions applied to an element set are always included when transferring model data between Abaqus/Explicit analyses (see Transferring Results from One Abaqus/Explicit Analysis to Another). There is no need to redefine these contributions in the import analysis unless different mass adjustment is required for the element set.

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). 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 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.

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.