An explicit is solved by calculating results in small time increments or time steps. The size of the time step depends
on many factors but is automatically calculated by Radioss.

Composite materials consist of two or more materials combined each other. Most composites consist
of two materials, binder (matrix) and reinforcement. Reinforcements come in three forms, particulate,
discontinuous fiber, and continuous fiber.

A rigid body is defined by a set of secondary nodes and a main node. It can be compared to a part with an infinite
stiffness. No relative displacement is allowed between secondary nodes, and the general motion of the rigid body manages
the main node.

A rigid wall is a nodal constraint applied to a set of secondary nodes in order to avoid the node penetration to the
wall. If contact is detected, then the secondary node acceleration and velocity are modified.

Interface TYPE2, also called tied interface is a nodal constraint to rigidly connect a set of secondary nodes to a main surface. The secondary nodes forces and
moments are transferred to the main nodes, and then secondary nodes are positioned kinematically according to the
motion of the main nodes.

A cylindrical joint is like a rigid body, except that one specific direction is defined with the first two secondary
nodes. All nodes are free to move along this direction and to rotate around it.

The rigid link option imposes the same velocity on all secondary nodes for one or more directions. Directions are
defined to a skew or a global frame, velocity is computed with momentum conservation.

Gear type joints are more complex than other kinematic joints. They use the Lagrange Multiplier method and
are compatible with all other Lagrange Multiplier kinematic conditions and incompatible with all classical
kinematic conditions.

As nodal constraints are based on kinematic conditions applied on nodal DOF, therefore it is not allowed to apply
two nodal constraints to the same set of nodes, unless the induced kinematic conditions are perfectly orthogonal (for
example: boundary condition in the X-direction and rigid link in the Y-direction).

Optimization in Radioss was introduced in version 13.0. It is implemented by invoking the optimization capabilities of
OptiStruct and simultaneously using the Radioss solver for analysis.

As nodal constraints are based on kinematic conditions applied on nodal DOF, therefore it is not allowed to apply
two nodal constraints to the same set of nodes, unless the induced kinematic conditions are perfectly orthogonal (for
example: boundary condition in the X-direction and rigid link in the Y-direction).

As nodal constraints are based on kinematic conditions applied on nodal DOF,
therefore it is not allowed to apply two nodal constraints to the same set of nodes, unless
the induced kinematic conditions are perfectly orthogonal (for example: boundary condition
in the X-direction and rigid link in the Y-direction).

Radioss Starter will issue the following warning each time two nodal
constraints are applied to the same set of
nodes.

It is of very important to take all warnings about incompatible kinematic conditions into account. True incompatible kinematic conditions (that is, nodes belonging to several rigid bodies) can generate energy and local instability. In such a case, accuracy of the results will be seriously decreased.

Radioss Starter does not check if the kinematic conditions are really
incompatible. If they are strictly orthogonal, or if they are not applied
simultaneously, just ignore the warning. Figure 1 illustrates
two cases: in the first case, a node is secondary on a rigid wall and has a boundary
condition in a non-orthogonal direction. If the rigid wall is fixed, there are no
possible incompatible conditions (the node cannot impact on the wall). If the wall
is moving, it is impossible after impact to respect both conditions. Therefore, the
boundary condition is not applied and the reaction forces on the wall are incorrect.
In the second case, a node is defined as a secondary for two parallel walls. If the
two rigid walls are fixed, there are no possible incompatible conditions, as the
node cannot impact the two walls at the same time. If one wall is moving, that
results no problem, as long the moving wall is not crossing the fixed wall.

On a multi-processor Radioss version, a run executed twice can give
different results if some kinematic conditions are incompatible. This is still true
if option /PARITH/ON is being used. For example, if a secondary
node impacting on two non-orthogonal rigid walls, as shown in Figure 2, the results obtained on a multi-processor can be arbitrary. If rigid wall 1 is
computed before rigid wall 2, velocity V0 is replaced with velocity V12. If rigid
wall 2 is computed before rigid wall 1, velocity V0 becomes V21. On multi-processor
computers, the order in which rigid walls and other kinematic conditions are applied
is arbitrary and can be changed from one cycle to the next and from one run to
another.

The Langrange Multiplier method allows applying several nodal constraints to the same set of nodes as it resolves a global system of equations with all Langrange Multiplier constrains. However, it is not allowed to mix both methods for the same set of nodes. Nevertheless, both methods can be used successfully in a model, as long as they are applied to different nodes.