To Mesh the Interior Plastic Component Of A Side Door of An Automobile For Structural Analysis Using ANSA

OBJECTIVE: To perform meshing operation for the interior plastic component of a side door of an automobile for structural analysis using ANSA.

2. PROCEDURE:

2.1 IMPORTING MODEL AND GEOMETRY CLEANUP:

i) First the model has to be imported to the ANSA GUI window using FILE -> OPEN.

ii) Then we shall check for the geometry errors in the model using checks option under tools section.

iii) These geometrical errors have to be rectified by the user using some of the tools available in ANSA from the modules buttons namely:

a) Faces -> New -> Coons (or) Fitted

b) universal Delete Option

c) Faces -> Topo

d) Faces -> Cut & Project Cut

e) Faces -> Intersect

f) Cons -> Paste & Release

iv) After rectifying all the geometrical errors, once again geometry check has to be executed to confirm that there are no errors.

2.2 EXTRACTING MID-SURFACE:

Why Should we Extract Mid-Surface?

To perform Structural Anlaysis for the components having thickness less than 6mm, it is always recommended to extract mid-surface from the component and proceed for Meshing. Meshing the whole component which has very less thickness is computationally expensive hence mid-surface is extracted from the component for which thickness is assigned and then the model is meshed.

Mid-surface of a component can be obtained using three methods, which are:

I. Manual extraction method

II. Auto extraction method

III. Auto - Manual extraction method (i.e. Combination method)

Here in this model which is the side door component all the above three methods are employed in obtaining the mid-surface.

I. Manual Extraction Method:

i) In this method, tools such as Faces -> Offset & Middle are used to get the mid-surface. To measure the thickness for the offset, measure tool is used which is available in utilities section.

If the thickness is not clearly visible in the model then it can be measured by inserting weld spots on the required surface and projecting them on to the other surface and measuring them.

ii) The main body which is having 2.8mm thickness is given an offset of 2.8/2 and according to the direction of the offset the sign of the value is changed. For instance, in the below figure negative offset value is given to obtain offset surface in the opposite direction.

iii) For the case of ribs which act as a supporting member to the component have a varying thicknesses such as 2mm,4mm,4.5mm,5mm,6mm.

On extracting the mid surface of the ribs, they are projected on to the surface using Faces ->Extend to get a triple con.

After creating the mid-surface using the mentioned tools we create separate PID's for individual components.

PID LIST:

MID SURFACE MANUAL:

2.3 MESHING:

Why Do We Carry Out Meshing?

The Basic idea of Finite Element Analysis (FEA) is to make calculations at only limited (finite) number of points and then interpolate the results for the entire domain (surface or volume). Any continuous object has infinite degrees of freedom and its just not possible to solve this problem in this format. Finite element method reduces the degree of freedom from infinite to finite with the help of discretization or meshing (consisting of nodes and elements).

i) Before proceeding to meshing we have to set the target length of the mesh. Here the target length taken is 4mm.

For this, shift from Topo mode to Mesh mode from the modules section and select Perimeters -> Length option and selct the complete model by dragging over the model using left mouse button and click on middle mouse button and enter length as 4mm.

ii) Now inorder to mesh the component ANSA provides many algorithms for meshing such as Free, Spot, Batch, Best, Adv. Front etc. Which ever algorithm gives good mesh can be chosen. Here Best mesh algorithm is chosen.

iii) To get a good mesh one has to mesh the component surface by surface and final meshed component has the following mesh elements.

iv) Now set the quality criteria for the meshed component. The quality is as follows:

vi) Now click on the hidden elements option under drawing styles section and off elements can be seen. These elements are the one which are exceeding the given quality criteria.

These off elements can be rectified using some of the tools such as:

a) shell mesh -> Reconstruct , Smooth

b) Elements -> Split , Swap , Join , New , Delete

c) Grids -> MV Surf , Paste , Align

v) By using the above tools, the rectified component is shown below:

II. Auto Extraction Method:

i) Auto mid-surface is obtained by using an option Faces -> Mid. Surface casting. Set the parameters and click on ok. Now we get the mid-surface of the component. The advantage of mid-surface casting is that it provides a meshed component.

ii) After obtaining the mid-surface, thickness has to be assigned to it under the same option Mid.Surface -> Calculate thickness.

Model after assigning thickness:

PID LIST:

iii) Now after assigning the thickness, similar to the manual method here also quality criteria is set and hidden elements which are exceeding the criteria are rectified.

Model after hidden elements are rectified:

III. Auto - Manual Extraction Method (Combination):

i) To use the combination method, first the mid surface is extracted using Mid.Surface -> Casting.

ii) All the geometry parts obtained using this method may not be in accordance with the original geometry. Hence the parts that have been distorted will be again extracted using manual method i.e. using offset and middle option. Hence this method is called as combination method.

iii) First mid surface using auto method is extracted as discussed earlier.

iv) Now when the component is examined , it is seen that the ribs present in the original geometry are not extracted properly and hence these have to be extracted using manual method.

v) To do this, first the FE model has to be converted to surface. This can be done in mesh mode.

Hence change from Topo mode to Mesh mode. Then select Elements -> To surface from module buttons and select the whole component and click on middle mouse button. By doing this, the FE model is converted to surface and now the necessary changes can be done.

PID LIST:

vi) The rectified geometry component is then meshed using target length of 4mm as shown below:

vii) After meshing is done we set the quality criteria and the hidden elements are rectified.

The rectified model using combination method is as below:

Why to have less no.of traingle elements?

In all the above models we see that the triangle percentage is maintained less than 5%. This is because triangle elements are stiffer than the quad elements i.e. Triangular Finite Elements are stiffer in bending problems because they have constant strain across the element. This simply makes it impossible for an FE solver to calculate the bending response properly.

(OR)

The triangular element is called as a constant strain triangle(CST) element. Value of the stress tensor in the whole element will be constant . This may not be true in the real situation. So this approximation may create a lot of error.

That is the reason it always advised to have less no.of tria elements in the component.

3. LEARNING OUTCOMES:

From the above model, following were the learning outcomes:

i) clearing geometrical errors using different ANSA tools.

ii) extracting the mid-surface using manual,auto and combination methods.

iii) improving the mesh quality and mesh flow using different ANSA tools.

4. CONCLUSION:

i) The interior component of side door panel was meshed by extracting the mid-surface using manual, auto and auto-manual methods.

ii) When all the three methods were compared then mid-surface extraction using manual method seemed very accurate with respect to the original geometry. Getting a good mesh flow was also possible in this method as meshing was done surface by surface. But only limitation with this method is that it is very time consuming. If time factor is important then this method will not be a good choice.

iii) In automatic mid-surface generation method, the software gives us the mid-surface along with the meshed FE model which takes very less time in giving the result. But the geometry capturing was not upto the mark as the software was failing in capturing rib areas.

iv) Hence to solve for time and accuracy, we can rely on auto-manual method were we can obtain the mid-surface very quickly and make changes to the geometry wherever the software failed to capture the areas. This method is not as acccurate as the manual method but better than the auto generation method.

So if timefactor plays a major role then in that situation we can compromise with the accuracy and proceed with auto-manual method.

LINK FOR THE MODEL: https://drive.google.com/file/d/1CNfqY2D72jCapWoCplRTvFQUf-1K9eR4/view?usp=sharing