The Mass Properties forms provide the capability to calculate mass properties for all or any part of a model. Both FEM and geometric entities are supported, as are most of the widely used analysis model types. These include 2D Axisymmetry, 2D Plane Stress, 2D Plane Strain, and 3D. The primary Mass Properties form is accessed from the Tools menu which is located on the main form.

Patran generates a complete set of mass properties for the user-specified region and outputs all data to a spreadsheet. This set includes the mass and volume, the center of gravity (CG), the inertia tensor at the origin of the reference frame, the inertia tensor at the CG, the principal inertias at the CG, the radii of gyration corresponding to the principal inertias at the CG, and the principal directions for the inertia tensor at the CG. The principal directions at the CG are given in three forms: (1) as three orthogonal unit vectors, (2) as a triad of space-fixed rotation angles in a 3-2-1 sequence, and (3) as a triad of body-fixed rotation angles in a 3-1-3 sequence. Where applicable, the mass properties are provided in both the reference Cartesian frame and in the user-specified coordinate frame. For the special case of displaying mass properties on an individual element basis, additional output of plate thickness, plate surface area, bar area, bar length, and the property type name (ie. Stan. Membrane (CQUAD4)) are given. Output options include the graphical display of the principal axes, the creation of a coordinate frame aligned with the principal axes (except for display by entity mode), and the creation of a report file. The principal axes are plotted in proportion to the magnitudes of the radii of gyration of the corresponding principal inertias, as shown below:

Newly created principal inertia coordinate frames are assigned the next available coordinate frame ID in the database. Mass Properties report files are written in standard Patran report file format. Each Mass Properties report contains all of the mass properties along with a list of all included entities and a list of all rejected entities.

The densities, shell thicknesses, beam cross-sectional areas, non-structural mass, and concentrated mass values used to calculate mass properties come from element property definitions by default, but you can override the element property records by using 1.0 for densities, thicknesses, and areas; and using 0.0 for non-structural mass and concentrated mass. The mass properties are generated in units that are consistent with those used in your referenced geometry, element properties, and material properties. For the entity display method, the actual plate thickness, areas, and bar length will be displayed regardless of the “Thicknesses/Areas/NSM” setting. If no properties exist for an element then a value of “--” will be displayed if the “Thickness/Areas/NSM” menu is set to unity.

All offsets are ignored. Mass property calculations are based on the assumption that all element offsets, including beam, shell, and concentrated mass offsets, are zero. If a referenced entity has an offset, a warning is issued. Non-structural mass is included in the calculation, while non-structural inertia is ignored without a warning. Constant and variable-thickness plates and shells are supported. General beams and rods with constant or variable cross-sectional areas are supported, but only tapered beams with constant cross-sectional areas are supported. Furthermore, beams defined using cross-sectional dimensions instead of section properties (e.g., I-beams requiring the input of web and flange dimensions) are supported. Concentrated masses are supported, but direct-input mass matrices are not. Composite materials are supported. If a composite property such as laminate thickness is defined in both the element property record and in the material record, then the value in the element property record will be used. Material densities defined with fields are not supported. If you are not sure whether an entity is supported, you can put it into a new group containing no other entities and attempt to generate mass properties for that group. Patran will then tell you if that entity is supported.

Mass properties for shells are calculated by treating the thickness as a weighting function and assuming that all mass lies in the surface of the shell. Likewise, beam mass property calculations treat the cross-sectional area as a weighting function with all mass assumed to lie in the locus of the 1D beam. Thus the mass properties calculated for these entities differ slightly from those based on the corresponding 3D solids.

If you specify that your Mass Properties Region includes only Geometry, and you further specify that densities, concentrated masses, thicknesses, or areas are to be taken from Element Property records, then those records must be defined on the geometry and not on the FEM entities within the geometry. If, however, you change the Mass Properties Region to include only FEM entities, then any needed Element Property records may be defined on either the FEM entities themselves, or the geometry containing the FEM entities.

Field defining thickness, cross-sectional area, and non-structural mass are taken into detailed analysis. For a geometry entity in the mass property region, a field property is integrated over the entity regardless of the property’s value type. For a FEM entity, a field property is evaluated at the centroid of the entity if the property’s value type is real scalar, and is integrated over the entity if the property’s value type is element nodal. Discrete FEM fields are supported only for real scalar properties of FEM entities.

Patran attempts to treat all entities as though they are consistent with the selected analysis model type, regardless of the element type of the entity’s element property record. For example, if a surface is assigned an element property record corresponding to that of a non-axisymmetric shear panel, and the surface is subsequently referenced for a 2D axisymmetric mass property calculation, Patran will calculate the mass properties for the solid of revolution generated by that surface as long as it can find the properties it needs to do the calculation (and as long as the surface lies in the user-specified modeling plane). In this case it only needs to find the density defined in the material referenced by the element property record.

Naturally, however, all entities are removed from the user-specified region that are geometrically inconsistent with the analysis model type. Two entities that are geometrically inconsistent with 2D Axisymmetric analyses, for example, are surfaces that do not lie in the axisymmetric modeling plane, and solids. Entities lacking a needed property such as density are also discarded. Upon completion, the user receives a warning if any specified entities were ignored. The warning includes a list of all rejected entities.

The 3D Analysis Type option is the default choice, and it is the only Analysis Type option that does not exclude any entities based on their dimensionality, i.e., all 0D, 1D, 2D, and 3D entities can be included. This is the only option that supports display on a per group or per entity basis.

The 2D Axisymmetric option is used to generate the mass properties of the 3D body given the 2D axisymmetric model. It is intended for axisymmetric shells, solids, and concentrated masses. Accordingly, only 0D, 1D and 2D entities may be included. Specified entities not lying in the axisymmetric modeling plane are omitted from the mass property calculations and added to the output list of rejected entities. Masses assigned to concentrated mass elements in axisymmetric problems are treated as linear mass densities. Patran calculates the mass of the equivalent 1D hoop by multiplying the input mass by 2πr. Likewise, the moments of inertia applied to concentrated masses in axisymmetric models are treated as linear moment of inertia densities. The inertia tensor of the equivalent 1D hoop due to the input moments of inertia is obtained by multiplying the input moments of inertia by 2πr. The careless use of input product of inertia terms in axisymmetric concentrated mass elements can result in calculated mass properties that are not axisymmetric.

The 2D Axisymmetric option cannot be used to calculate the mass properties of non-axisymmetric cyclic symmetry models. These are supported in 3D mode only, in which case the mass properties are calculated for the model only, and not for the entire structure which would be generated by the prescribed rotational and reflective transformations.

The 2D Plane Stress and 2D Plane Strain options include coplanar 0D and 2D entities only. The 2D entities are assumed to have unit thickness. The modeling plane is assumed to be the plane of the first 2D entity. If there are no 2D entities, then it is assumed to be the plane of the first three non-collinear 0D entities. Specified entities not lying in this plane are omitted from the mass property calculations and added to the output list of rejected entities.