Joint creation using LS-DYNA

JOINT CREATION:

AIM:
The objective of this project is to create different types of joints namely translational, revolute, cylindrical, and spherical, or a ball joint. These joints are created between two rigid or deformable bodies.

In LS_DYNA it is not required to model the joint in its physical form i.e the geometry of the joint must not be modeled to replicate the behaviour of the joint. However this behaviour of the joint can be established using keywords which contain the mathematical equation regarding the behaviour of the joint while the location of the joint must be referenced by the user.

INTRODUCTION:

The joints are created by specifying nodes which lie between the two rigid or deformable bodies. These nodes are defined to its respective parent body using the CONSTRAINED_EXTRA_NODES and CONSTRAINED_NODAL_RIGID_BODY keywords.

The distinguishing factor for both cases - rigid and deformable are,

1. MAT - The rigid bodies are modeled using the MAT_020(rigid) card whereas the deformable bodies are modeled using the ELASTIC material card.

2. CONSTRAINED JOINT NODES - In case of rigid bodies, the nodes where the joints are created are defined using the CONSTRAINED_EXTRA_NODES keyword whereas in case of deformable bodies the respective deformable nodes are made into a CONSTRAINED_NODAL_RIGID_BODY to which the respective joint definitions are specified.

KEYWORD SETUP:

DEFORMABLE BODIES

The deformable bodies are modeled either using the Shell or Solid SECTION card based upon the type of body but with a consistent material card of MAT_ELASTIC.

SPHERICAL JOINT

To model the sperical joint, two Cylindrical solid elements are generated using,

Mesh -> ShapeM -> Cylinder Solid

Both the deformable parts are separated with a distance of 10mm.

To specify the nodes where the joint has to be created we can make use of the CONSTRAINED_NODAL_RIGID_BODY keyword. This keyword lets us create a single master node that is interconnected with the specified deformable nodes. Thereby this master node affects the interconnected nodes, in other words any load or joint created on the master node affects the slave interconnected nodes.

The CNRB can be created using, CreEnt-> Constrained-> CNRB

Using this keyword the outer edge nodes of both the parts are selected thereby creating two rigid nodes on the midst of all the selected surrounding nodes.

These CNRB nodes are then translated at the middle of the separation of the two parts since the joint must lie between the parts and the joint must consist a node from the two bodies that are coincident as shown below,

The Spherical joint is then created using the CONSTRAINED_JOINT_SPHERICAL keyword where the nodes (CNRB) are referenced such that a ball joint is created between the parts.

Then a translational movement is applied on one part while the other kept stationary to visualize the presence of the ball joint.

REVOLUTE JOINT

The creation of a revolute joint is different from a ball joint such that in a revolute joint 2 pairs of CNRB are required i.e 2 CNRB in part A and 2 CNRB from part B.

In this case, two shell elements are created using,

Mesh-> ShapeM-> 4N Shell. Using this command two shell parts of 200 mm in length are created separated by a distance of 10 mm from the top plate.

Using CNRB keyword 4 rigid nodal bodies are created and they are translated to lie on the same axis while each node pair of both the parts lie on the same coordinate.

Both the parts - top and the bottom moment is applied to visualize the effect of the revolute joint on both the parts.

We can notice that the revolute joint created using 2 pairs of CNRB is keeping both the parts intact while allowing for revolute motion. The deformation seen in the shell elements is due to the elastic material that was used to model the part which would remain in case the parts are modeled using the MAT_20 material card.

TRANSLATIONAL JOINT

This joint involves about 3 CNRB parts where 2 pairs represent the axis of translation while the 3rd pair is used to reference the rotation axis about which the joint when created prevents rotation about.

Both the parts are made of shell elements with MAT_ELASTIC to model it as a deformable body.

Translational motion is applied to the right part using the INITIAL_VELOCITY keyword while the other is kept stationary to visualize the effect of the joint.

CYLINDRICAL JOINT

This joint requires only two pairs of CNRB which are used to define the axis of rotation of the cylindrical joint.

To demonstrate this joint, a shell and a solid cylindrical part is created using,

Mesh-> ShapeM-> Cylinder Shell

Mesh-> ShapeM-> Cylinder Solid

The radial distance between the two bodies is about 5mm which is compensated using the Thickness of the outer shell which is specified in the SECTION_SHELL card.

Four CNRB are created using the edge nodes of both the shell and solid part which act as the dependent nodes to the master independent CNRB node. These nodes are then transformed to align with respect to the reference axis of rotation and each node of both the pairs are made coincident such that they defined the location of the joint and the axis of rotation of the joint. These nodes are referenced in the CONSTRAINED_JOINT_CYLINDRICAL card as the N1, N2, N3 and N4 parameters.

Angular displacement is defined for the outer shell element such that a relative rotation and movement with respect to the solid part can be visualized due to the presence of the cylindrical joint.

RIGID BODY

The same above-mentioned joints and their procedure are the same in case of rigid bodies with the only difference being in the material card and the nodal type for the joint creation.

In case of rigid bodies a CONSTRAINT_NODAL_RIGID_BODY cannot be used since the elemental nodes are rigid by nature. Hence the nodes where the joint must be located are made a part of the rigid body using the CONSTRAINT_EXTRA_NODES keyword.

CONSTRAINED_EXTRA_NODES: This keyword is used to add any node either present within or outside a body which can be made a part of a body. Hence this node present outside or in our case between the two parts where the joint exists can be a part of the body itself.

In a way this keyword lets the user define a node just as a CNRB in a way that this node is dependent and interlinked with the rigid body.

The two required parameter in a CONSTRAINED_EXTRA_NODES card are the part ID and the node ID. Upon creating the node where the joint needs to be created, this node can be renumbered for significance and then be referenced in the CONSTRAINED_EXTRA_NODES card along with the part that it needs to be associated with.

Hence after creating this node outside the part where the joint is to be located and being referenced, this node acts as a node in the part itself to which as shown above the same joints can be modeled.

CONCLUSION

Thus the different type of nodes are created for both the rigid and deformable bodies using CONSTRAINED_EXTRA_NODES and CONSTRAINED_NODAL_RIGID_BODY respectively and these joints behaved as they were expected to in both the cases.

Thus without actually modeling the geometry of each joints we can create the behaviour of each joints just by specifying their location.