# Bolt Modeling for Crash

There are different ways for modeling bolt connection, and more detailed model accuracy of the results:
• Rigid bolt
• Rigid bolt with spring (and pretension)
• Deformable bolt with spring and pretension
• Deformable bolt with initial stress

## Rigid Bolt

Use one rigid body (/RBODY) connected to the parts where the bolt is linked is the simplest method. Although this method is very stable and easy to modeling, you cannot consider pretension, elasto-plastic and rupture behavior for bolt, and there is no force output and model the bolt head passing through the hole is not possible.
Note: The main node should be a free node.

## Rigid Bolt with Spring (and Pretension)

Use two rigid bodies (/RBODY) connected to each set of parts which are linked with the bolt and then set one spring element (/PROP/SPR_BEAM) between both rigid bodies. With this method it is possible to describe the elasto-plastic behavior rupture behavior in any direction, and the force passing through the bolt (normal force, shear force, moments) could be output, using /TH/SPRING. This method is often used in the automotive crash analysis, but it has limited capability to model the pretension.
Note:
1. The main node should be a free node.
2. The spring element is attached to the rigid bodies as secondary node.
To describe pretension with this method another two spring elements (/PROP/SPR_PRE and /PROP/SPR_BEAM) need to be added. /IMPDISP or /CLOAD could be used to simulate preloading. Auto-balance state is calculated by Radioss.

## Deformable Bolt with Spring and Pretension

Meshed bolt with solid elements (/BRICK or any other solid element type) and cut in the middle. Two rigid bodies connected to each side of meshed bolt and spring series (two springs with /PROP/SPR_BEAM and one spring with /PROP/SPR_PRE) like above between rigid bodies are used to describe the pretension. With this more detailed model, the bolt head passing through the hole could be considered in simulate.

## Deformable Bolt with Initial Stress

With this method, bolt is fully-meshed with solid elements (/BRICK), which is more realistic, and the material and failure character could be easily defined. Use /SECT to get section force/moment (/TH/SECT) in bolt. To define the pretension, initial stress (/INIBRI/STRS_F) is needed to get an initial analysis (tensile test on the bolt).
Note:
1. The rupture and stress accuracy depend of the mesh size.
2. Section force in the bolt is necessary to measure the forces passing through the hole.