Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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Oasys LS-DYNA Environment: User Guide (Version 8.1) become negative. The symptom of this is that the body begins to rotate, often slowly at first, but at growing speed until the analysis crashes. RESTRAINTS Restraints on rigid bodies are defined on the material input, card 2. These allow translations of the centre of mass, and rotations about axes through the centre of mass, to be clamped. Two methods are available: - Restraints in global axes (CMO = +1.0) - Restraints in local axes using SPC restraint codes (CMO = -1.0) While it is also possible to restrain rigid bodies by restraining at least 3 non-collinear nodes, this is not recommended. If restraints are defined on the material cards, these override any nodal restraints. The restraint condition of each rigid body is printed in the .OTF file during the input phase. It is not possible to pin' a rigid body simply by restraining one node; the motion imposed by the rigid body will override the restraints. A pinned condition can be achieved by declaring the centre of mass to be at the pin location and restraining all translational degrees of freedom of the body. Alternatively, a spherical joint to a fully restrained rigid body can be created. It is recommended to check the restrain conditions of rigid bodies in the OFT file: Search for “r i g i d b o d y c o n s t r a i n t s” JOINTS Joints can be used to link two rigid bodies together (see *CONSTRAINED_JOINT). Spherical, revolute and cylindrical types are recommended. All the joints work by a penalty method, i.e. any relative displacement in a direction which the joint restrains is resisted by a force proportional to the displacement. The stiffness of the joints is calculated automatically, and is the maximum value which will not give rise to instability. If desired, the user can reduce the stiffness by entering a relative penalty stiffness less than 1.0 on the joint cards. Spherical joints act as a pin, preventing relative translation but allowing rotation. Revolute joints act as a pair of pins, allowing only relative rotational about the axis through the pins. Cylindrical joints allow relative translation along, and rotation about, an axis which is fixed in one of the two rigid bodies. EXTRA NODES Extra nodes may be added to a rigid body using *CONSTRAINED_EXTRA_NODES. The effect is the same as if the nodes were attached to elements of the body. Page 8.2
Oasys LS-DYNA Environment: User Guide (Version 8.1) RIGID BODY MERGE Two rigid bodies can be merged together so that they act as a single rigid body. The input (see *CONSTRAINED_RIGID_BODIES) consists of two part numbers, both of which must be rigid (i.e. reference type 20 materials). The second body is absorbed into the first, and any subsequent references to the second body become meaningless. Thus if three rigid bodies are to be merged together (say A, B and C) the correct procedure is to merge A with B, then A with C. A will then include all the elements and nodes of B and C. If inertia properties are to be defined for the merged body, these should reference material A. Rigid body merges can be used to avoid the error which arises if two bodies share common nodes. By merging the two bodies the nodes then belong only to the first body. DEFINED VELOCITIES The motion of a rigid body can be prescribed by using Displacement or Velocity Boundary Conditions (see *BOUNDARY_PRESCRIBED_MOTION_RIGID). These should be applied to the body as a whole, not to the individual nodes (i.e. don't use *BOUNDARY_PRESCRIBED_MOTION_NODE for nodes belonging to rigid bodies). Rotational velocities are applied at the centre of mass. OUTPUT The elements which form a rigid body can be plotted in the normal way using I-DEAS, CAEDS, NISA DISPLAY, PATRAN, OASYS D3PLOT, etc. Stresses and strains in the elements and shell thicknesses appear as zero. If the contact stress output file (.CTF) is used (see Section 8) then contact stresses on the rigid bodies can be plotted using OASYS D3PLOT. Time-histories of nodal displacements, velocities and accelerations, and the kinetic energy of each rigid body, can be plotted using OASYS T/HIS. Page 8.3
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Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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