Week 6

Objective :

To create proper connections for the given rear door model which will go for nvh simulation.

• Import the model which is in nas format to ansa.
• Give proper connection to the model.

Theory :

What is NVH?

It stands for Noise, Vibration and Harshness, and is basically a measure of how much unpleasant aural and tactile feedback the car delivers as you drive.

Put simply, the noise is what you can hear, the vibration is what you can feel and the harshness is how much of an effect thumps, bumps, noise and vibration have on the cabin and its occupants.

Removing and isolating passengers from NVH is a field that's seen monumental advances over the past few decades. Originally the preserve of the most luxurious marques - and an acronym that was only even discussed by premium brands - NVH reduction is a global push, spurred on by consumers who want increasing insulation from the environment they're driving through. Perhaps to avoid the sound of other humans ROFLing.

Today, we even expect sporty convertibles to be NVH minimal.

This presents a quandary for engineers. Traditional sound and vibration-deadening material adds weight, which can spoil acceleration, handling and fuel economy, so they've had to come up with lighter and more effective methods than the traditional bit of tar paper.

In some cases, though, technology that offers a better drive can also offer a quieter ride. Unfortunately, it doesn't come cheaply.

What causes NVH?

Noise, vibration and harshness are caused by the car's mechanical and electrical systems, as well as the car's interaction with road surfaces and its passage through the air.

The main sources of NVH in a car are its engine, drivetrain and tyres, as well as the sound of airflow along its body.

FE connections :

Most structures involve some form of jointing or connection. Using rivets to connect structural plates is almost as old as the introduction of bronze and then iron into early civilization. Bolted connections became possible with the advent of screw cutting methods and usage was accelerated by the standardization of pitch and thread, for example. Even today, fabricated structures such as aircraft and ships use many thousands of bolts and rivets to connect components together. Other large-scale connection technologies include welding and spot welding. The improvement of adhesive technology coupled with much wider use of composites has meant a major resurgence in the use of bonding.

Connections can be of a continuous nature, such as in large surface regions of plates, flanges and where other abutments exist. Alternatively, discrete load paths may be formed by lugs and pins, clips or similar connectors.

The engineer is faced with a difficult decision when attempting to simulate such connections and joints within a finite element analysis (FEA). In many cases the details of each individual connection can be ignored if an overall stiffness or strength assessment is to be made and the connection is assumed reasonably continuous. However there may be doubts about the local flexibility and load paths developed with this assumption. It may be that the assessment of local behavior of the connector is essential to a safety case, for example with main attachment fittings. In some cases the interaction between the connectors and the surrounding structure is critical, as in the case of pre-loaded bolts and inter-rivet buckling.

What is rb2 and rb3 elements?

The Nastran group of elements often described as the "Rigid elements" are very useful
engineering analysis tools. "Rigid Element" is a slight misnomer, but the term is still used to
describe the most commonly used items: the "RBE2" and the "RBE3".

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, but for practical purposes that
matters little, except for the tips to avoid common errors with rigid elements

What is the difference between RBE2 and RBE3 elements?

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.
A rigid element can be as simple as two nodes connected at coincident locations (although it
would be very unusual to use an RBE3 in that configuration), through to the more common
appearance of a "spider" or "sea urchin" connection, or any arbitrary connection of one to
multiple nodes.
RBE2 Rigid element used to approximate the effect of a bolt head / washer.
For an RBE2 rigid element, the single node is the "independent node". It has 6 degrees of
freedom, regardless of what your FEA graphical interface looks like. The other node(s) you select
are the dependent nodes. You decide which of their degrees of freedom you wish to connect.
Note that it is important to understand the significance of your choices.

Procedure :

Step 1 : Import the model to ansa, here importing the model is different we have to import through input>nastran deck and open the file.

Step 2 : After importing the model we can see that model is fully meshed and that means model is in assembly stage and we have to give some connections for model before assembly.

Step 3 : First we have to start with the small connections like bolt,fasterners,studs e.t.c, these all can be represent through rb2 element. So we have to go nastran deck and select rb2 many nodes option and give it where ever needed (like holes). Here we can go for automatic connections by using the connection manager, this option is used when three or more holes are joined with one bolt. For bolt 6DOF are restricted in order to make rigid.

Step 4 : Now we have to give connection for the door hinge here we have to give perfect connection that door wants to move only direction to do that first we have to create to rb2 elements near the holes and by using two node rb2 element we have to give a 5DOF to the stem like element.

Step 5 : Now we have to give connection for remaining holes before that we have to give washer to that holes, because washer give proper connections. To give washer go to mesh deck and select zonecut option and give the 2mm and select the hole it will automatically gives the mesh.

Step 6 : After that we have to give handle connection for that first we have to give the rb2 element and then we have to give give connection between both the rb2 element and we have to define the mass using the conmi and here mass unit is tonnes (1e-4 ton).

Step 7 : Now we have to go with the spot weld connection for some parts in oder to do that first create a 3d point using the conbertor and convert it as a spot weld point and go to the connection manager and select the point and give the p1 and p2 property and then select RBE3 - HEXA - REB3 and apply it. Thus a spot weld connection is created.

Step 8 : Now we have to create a bush connection for the glass. To do that first we need to curve along the glass and convert it as a spotline and go to the connection manager and select the curves and select the p1,p2 for the connection and select the RBE3 - CBUSH - RBE3 and apply it. Like this do for the remaining curves and create a bush connection between the glass and door.

Step 9 : Thus all are connections are given to the car door model and it is ready for the NVH analysis.

Result :

Thus we have created the connections for the car door model using the bolts, spot weld, rb2, rb3, cbush.https://drive.google.com/file/d/13SNfWEwEnJ1wP1BT83F03yNlBhn44gV1/view?usp=sharing this is the link for above model.