Bodies

Bodies are the model elements that have mass and inertia. Bodies can be rigid or flexible.

A rigid body has only mass and inertia, and does not deform during the simulation. An initial velocity can be assigned. Mass and inertia information can be omitted for kinematic, static, and quasi-static simulations. It does not affect displacement, velocity, and acceleration results of kinematic simulation or displacement results of a static or a quasi-static simulation. Mass and inertia information must be correctly specified if joint-reaction forces are of interest in kinematic, static, or quasi-static simulations.

A flexible body deforms during the simulation. Mass and inertia are determined by the geometry and material of the structure defining the body. An initial velocity and damping can be assigned. Flexible bodies are formulated using an orthogonal set of modes A that represent the displacements u of the flexible body such that

Where, q are the modal coordinates which are to be determined by the multi-body dynamics analysis. The set of orthogonal modes is determined in a Component Mode Synthesis (CMS). Depending on the model, CMS can be performed as a pre-processing step using a special simulation (Direct Matrix Input). Besides displacements, velocities, and accelerations, stresses and strains can also be computed for flexible bodies.

One special rigid body is the ground body. It describes the reference environment, and does not add any degrees of freedom to the system. It is at absolute rest. Any grounded body is merged into one.

Bodies are defined in terms of a finite element model. A body is formed by a group of properties, elastic, rigid, and mass elements as well as grid points.

Rigid bodies are defined on a PRBODY entry. Mass and inertia are either determined from the geometric entities or can be entered on PRBODY.

The ground body is defined using the GROUND bulk data entry.

Flexible bodies are defined using the PFBODY bulk data entry. The interface grid points are automatically determined or are defined on PFBODY using the FLXNODE flag. The procedure, as described in Direct Matrix Approach, is applied to each PFBODY definition. The procedure is fully integrated in the multi-body dynamics solution sequence, where flexh3d files are generated for multiple flexible bodies in the same model. The parameter PARAM, FLEXH3D may be used to control the regeneration of flexh3d files for subsequent runs.