LS-DYNA Mass Calculation

The mass calculation for LS-DYNA is accessible from the Summary panel in the Post page.

The mass reported is not simply calculated by Density x Volume for each part. It follows the many LS-DYNA requirements to handle rigid body mass, non-structural mass, and lumped mass.

Contributing Total Mass factors:

totalmass = structuralmass + lumpedmass + nonstructuralmass + rigidbodymass + transferredmass + distributedmass

Structural Mass

The volume x density; except in case of *PART_INERTIA in which it is also the total mass.

Lumped Mass

Accounts for contributions from *ELEMENT_MASS, *ELEMENT_MASS_NODE_SET, and *ELEMENT_INERTIA. This does not take into account the transfer of lumped mass to rigids.

Non structural Mass (NSM)

Accounts for contributions from ELEMENT_MASS_PART, ELEMENT_MASS_PART_SET, and NSM in *SECTION. This does not take into account the transfer of lumped mass to rigids.

RigidBodyMass Mass

The mass of *CONSTRAINED_NODAL_RIGID_BODIES.

Transferred mass

The mass transferred from deformable nodes to rigid materials. This includes lumped mass transferred from rigid or deformable nodes to the rigid materials.

•	For rigid material, this is the mass gained from deformable (+).

•	For deformable parts, this is the mass lost to rigid material (-).

Distributed Mass

The mass distributed from nodal rigid bodies to free nodes.

Engineering Mass

The mass of the part that most closely matches its real engineering meaning. The engineering mass is the most useful for possible mass adjustments. Engineering mass is the sum of structural, non-structural, and lumped mass.

Engineering mass exceptions:

•	For PART with PART_INERTIA:

o	Slave CRB of this part should have 0 mass.

o	Mass of the part should be equal to TM (if Iflag=1 => transfer mass from slave to master).

•	For *CONSTRAINED_NODAL_RIGID_BODY_INERTIA:

o	The mass of the slave nodes on the connected part should not be taken into account and excluded.

LS-DYNA Part Mass

The mass listed in d3hsp, where you also have COG information.

Total mass is also obtained with the following calculations:

Total mass = LS-DYNA part mass + lumped mass (for deformable parts as rigid part already include lumped mass) + CNRB mass.

Center of Gravity (CG) is computed from the total mass (for each part); the inertia are computed from the total mass.

See Also: