Week 6 - Creating connection for Rear door

Aim: To implement connections for a Rear Door FE model.

Objective:

• Understand car rear door assembly
• Understand different types of connections and 1D elements.
• Perform connections for a FE model.

Methodology for giving connections

• Import the FE model
• Understand the geometry and mechanism
• Give connection to all the bolt holes
• Given the connections to the overlapping surface.
• Give the necessary connection to the areas where it feels necessary as per your analysis requirement. eg. weather-seal strip, handle, spot welds, etc.

There are mainly 5 types of connection

1. Weld
2. Adhesives
3. Bolt
4. Area contact
5. Replace part

1D Element

If a large number of bolts are to be modeled, then the use of solid element representation can be far too burdensome in terms of setup time, contact complexity, and computational cost. An alternative is to use a 1D beam element representation. This can also use a similar preload method.

There are many different implementations of this approach, but we will focus on a method that uses a rigid element to spider out the head and nut tend of the bolt to the top and bottom plate respectively, and can connect to 2D or 3D element representation

They are simply the connection between nodes. There are a few working elements majorly used:

1. a) Non-rigid Links

1. CROD
2. CBAR
3. CBEAM

1. b) Rigid Links

1. RB2
2. RB3

CROD:

• This element only 'understand' "tension & compression" loads (only axial force only).
• They only have translational DOF.
• But still, this element has a torsional stiffness.

CBAR:

• Simple beam with constant properties (symmetrical and constant cross-section).
• shear center and neutral axis should consider, any offset between the neutral axis and the shear center is not accounted
• Not useful to model the beam that warps (open channel section).

CBEAM:

• The cross-sectional properties are tapered.
• the neutral axis and shear axis does not coincide
• the effect of cross-sectional warping on torsional stiffness is critical
• the difference in the mass center of gravity and shear center is significant.

Rigid Elements

A rigid element is a link from one node to one or more other nodes, where the motion of the node(s) is governed by the "degrees of freedom" you choose to connect. A rigid element is actually a constraint equation rather than an actual element.

• Bolt pre-load at a critical interface needs to be modeled but does not need the bolt to be modeled with any more detail than as a "bar" or "beam" element. An RBE2 can be used to connect the end node of the "bolt" element to the multiple nodes which represent the washer area over which the bolt load acts.
• To model the dynamic behavior of a vehicle, the engine can be modeled as a mass element connected by an RBE2 rigid element to the engine's multiple mounting locations. This makes the reasonable assumption that the engine is "very stiff" compared to the body structure and so the engine does not need to be modeled in detail. The mass is located at the center of gravity of the engine, and the rigid element links the engine mass to the rubber mounts (eg. springs or bushes) whose stiffnesses are typically much more relevant.
• 

Note:

• Both elements "look" the same, in that they have a single node connected to one or (usually) multiple other nodes. The difference is that the RBE2 element adds (infinite) stiffness to the structure, while RBE3 elements "distribute forces" around the connected nodes, without adding any stiffness.

Connectors

These are the geometrical representations of the connection between the entities. The advantage of connectors is the ability to create multiple connections at a single time.

Spot

• connection at a point
• spot weld, rivet, etc.

Bolt

• connection at the hole locations.
• eg. Bolts

Seam

• connection along the length
• seam weld
• connect geometry only

Area

• Connection of areas
• Adhesives

Role of a seal in the NVH behavior of a car

Automotive weather seals are typically dual extrusion bulbs of sponge and dense rubber that are attached to either the door or the car body in order to seal the passenger compartment. The seals can be held in place in several manners, e.g. intermittent pushpins, continuous carriers, etc., and have a vast variety of shapes with a height of approximately 10-30 mm. The seal wall thickness is typically a few millimeters to provide a maximum sealing area at a low compression force.

The seal strip induces some residual stiffness and viscoelastic contribution to the door support conditions and plays a role in the way exterior noise and vibration can be transmitted to the interior of the car. The initial use of seals in automotive applications was aimed at accommodating construction variations.

Lately, the isolation issue became important, and the design was oriented to better isolate the passenger compartment from dust, water, and air leakage. Nowadays the general trend in seal design is mainly oriented on the isolation of the passenger compartment from noise and vibration. For automotive applications, the material requirements aspect, such as resilience, weather resistance (including ultraviolet radiation effects), bonding strength, tear, abrasion resistance, etc. also need to be considered when designing for optimal weather seals.

At a certain driving speed, the pressure difference between the interior and the exterior of the car pushes the seal outward. If the resultant pressure is higher than the frictional forces at the contact area, separation occurs. The occurrence of separation is influenced by the compression rate and the friction coefficient of the rubber to the metal contact area. When separation occurs, an open path for direct noise is created and turbulence at the door edge is generated. The turbulence is due to the interaction between the escaping air fluxes from the passenger compartment, encountering the air fluxes at the exterior of the car.

The damping, stiffness, and mass distribution influence the car door vibration levels and the mechanism of energy transmission from the door to the car body. The support and isolation characteristics of the seal become important performance factors that should be taken into account in seal design and analysis.

PROCEDURE:

Import FE model

Once you open the model understand the mechanism and necessary areas that need to be rigidised or made connected.

Firstly, we will cover all bolt connections through the RB2 stem element. we will perform this connection by using the auto-mode for the selection of nodes.

1. Under the Topo module select the below point section to create a COG 3D point on any of the co-axial bolt holes.
2. convert the 3D point into the bolt.
3. Bolt representation of 3D point.

Open the connection manager to select the type of element and surfaces to be made connected.

Assign the values and P1, P2, P3, P4 surface to be made connections with.

Assign search distance as 20, zone: In/out X check the Body column to keep RB2 stem body(see below).

once you are done with assigning values then apply and realize the connection.

Do the same with all other bolt holes and this can be done for all the holes by selecting them in one GO process.

Note: surface selection(P1, P2,etc.) by modify-> then use F1 key-> then after selecting press twice Enter key

Hinge connection

connects the surface of the overlapping surface with the help of RB2 cluster element

Once the cluster RB2 elements are assigned the rotating link is made using a two-node RB2 element where the CM parameter represents the DOF here we assign 12345. As it is to be noted that 6 is intentionally not provided to allow rotation DOF about Z-axis. Therefore DOF in all directions is arrested to expect rotational motion about Z_axis, so as to allow the opening movement of the door.

A similar process can be done for the lower hinge joint.

Open holes and slots connections

We will use RB2 many nodes option to rigidise those nodes. one by one we will select the open slots and holes and assign them many nodes RB2 rigid elements. (shown below)

Door inside mechanism

There is a glass slide mechanism for the window movable glass. so we need to make spot weld connections as well as RB2 connections so as to complete the motion transfer.

Few plates are free that have no attachments needed to be spot welded to avoid any continuity in the force transmission.

Glass sliding attachment spot weld and Rb2 many nodes connection

Note:

• use the COG 3D point and convert that into a spot weld.
• For the spot weld, we use RB3-HEXA-RB3, and there in the connection manager used the 'shift'+'?' key to select the weld PID.

Handle connection

• Give many nodes RB2 handle slots and then connect the slot with two nodes
• Assign mass to the element at the COG

We need to assign the mass to the COG of the element connecting two slots with cluster nodes connection. For doing so we will use COMM2 & assign value for mass ('M' as 0.0001 tons).

Weather seal with the window glass

Now we have to finally make connections for the weather seal strip and window glass. To do so we need to follow certain steps.

1) Hide the FE model except for the glasses.

2) construct the curve all around the glass edge using the Fea2curve tab. Select each edge of the glass one by one to do so. (as shown below)

3)Covert the curve to spot lines. (one curve at a time)

4)Go to the connection manager to assign a connection. we use the RB3-CBUSH-RB3 element to assign connections. Also, set the PBUSH ID (door links) to assign the stiffness values.

note:

• set 'S' value as 10 and give search distance high to 50 as the distance is quite more between the glass and attaching surface.
• Transform copy the base curve to 10 units so as to apply connection on both sides of the window base.
• the value k values will be given as per the solver requirements.

5) Select the P1 P2 carefully then apply and realize.

6) Change all the CBUSH element orient as 'cord' and CID as 0. To change the orientation of the element from local to the global coordinate system.

CONCLUSION:

After the completion of the challenge, it can be concluded that for any type of model the understanding of the mechanism is very important. As the mechanism will decide in analysis ways to make decisions regarding assigning connections. The connection procedure is moreover the same for any type of model but the requirement to assign the connection may differ from model to model with the requirement of the analysis. So for the NVH analysis, the weather seal was important to capture and is captured in a pretty good way with help of the CBUSH element. Most of the areas are covered and are been rigidised as well as connected as per the requirements.

LEARNING OUTCOME:

1. 1D element and types of connection
2. Different types of methods to make connections
3. Assigning mass to the Rigid body
4. Aware of the working mechanism of the car door.
5. assigning spot and seam weld to the surface
6. Arresting required DOF of the element