Midsurfacing and Creation of Different 1D Elements

Objective:
To create different types of 1D element specified below in the geometry of Parallel Brackets, and assign the material and properties.

Introduction:

• 1D Elements are the elements having dimension in only one direction i.e. X, Y, or Z. 1D elements are represented by a line.
• A 1d element has 2 nodes, each at both the ends. Loads are applied on these nodes for analysis of deformation.
• There are different types of 1D elements that can be used in FEA like bars, rods, beams, springs, rigids, etc. Pre-processing of these 1D elements take less time and also are simple in structure.
• It is very much easier and advisable to create the 1D elements on a surface rather than a 3D model. Therefore, a midsurface is created in between the two surfaces of the 3D model on which the 1D elements are created.
• A Midsurface, as the term suggests, is a middle surface between two surfaces, which reduces the complexity of the calculations and in turn reduces the processing time.
• A midsurface can only be applied to elements or models with even thickness. It creates 2D shell elements in a 3D geometry.

The following image shows the model on which the 1D elements are to be created:

The 1D elements to be created are:

1. Rod element: with only translational DOF and RBE2 link
   Cross-section: BOX- Dimension a = 12mm, b=10mm, thickness t=0.75mm
2. Beam element: with all DOF and RBE3 link
   Cross-section: I section- Dimension a= 8mm, b= 10mm, c= 8.5mm, Thickness t1= 0.75mm, t2=0.6mm, t3= 0.6mm
3. Bar Element: with only x and y DOF and RBE3 link
   Cross-section: Cross (X)-Dimension DIM1= 5mm, thickness DIM2= 1.2mm, dimension DIM3=10mm, thickness DIM4= 1.2mm
4. Mass of magnitude 10 with RBE2 link
5. Mass of magnitude 10 with RBE2 link
6. Spring element

Procedure:

• Open HyperWorks OptiStruct, and import the model from “Import Geometry” option.
  
  

• Select ‘midsurface’ from main menu bar, go to ‘extraction options’, and change ‘offsets+planes+sweeps’ to ‘skin offset’. ‘Skin offset’ allows creating a better midsurface at the curves or the filleted edges.
• Select the upper bracket and create the midsurface.
• Go to ‘components’ in the tab bar, and rename the ‘middle surface’. Create a new component and rename it as well for our convenience. Creating a new component helps to separate the two midsurfaces of the upper and lower brackets.
• Select the ‘midsurface’ again and similarly select the lower bracket. Thus two midsurfaces would be created in both the brackets.
  
  

• After creating the midsurfaces, the solids in the model are deleted which is done by selecting ‘delete’ command displayed by a ‘X’ in the tool bar. After this, the three components, ‘wireframe’, ‘parallel bracket 1’, and ‘parallel bracket 2’ are deleted, and only the surface are kept.
• Next, the lower middle surface is merged with the lower bracket by the use of ‘organize’ option in the ‘tools’ section of the main menu page. In the ‘organize’ option, the lower surface is selected and it is ‘moved’ into the lower bracket. And the final result of the operation would look like the image below.
  
  

• In order to give certain properties to the model, a material is assigned to it. This is done by ‘create’ > ‘materials’ in the tab bar. The material is renamed as ‘Steel’. Later, some properties are associated to the model. For this new ‘property’ is to be created similarly and the material is associated with that property. A thickness of 2mm is specified in the property.
• Each of the brackets is selected separately, and the property created before is mentioned in it. By mentioning the property, the material also gets mentioned directly and there is no need to do it again.
• Make three different ‘properties’ by right clicking in the tab bar as before, as specify each of the cross-sections and rename them accordingly.
  
  

• Now, the model has to be 2D meshed. But for that, the geometry of the model should be refined by adding ‘washer splits’ from the ‘quick edits’ option in the ‘geom’ menu. Washer splits of 5mm each are added by selecting every hole in the model.
• After adding the splits, select the ‘automesh’ command from the ‘2D’ option and mesh the whole component. Here, the element size used is 5mm and the element type is set as ‘mixed’. The mesh has to be refined at the holes and at the filleted edged of the geometry. The image below shows the meshed component.
  
  
  
  
• After meshing the component, create the above mentioned links at the respective holes. For this, go to ‘1D’ menu and select ‘rigid’ link for the ‘a’ section. Clear all the rotational DOFs, select all the nodes of the hole, and ‘create’ RBE2 links on both the brackets.
  
  

• Similarly, create the RBE3 links for the sections ‘b’ and ‘c’, and RBE2 links for sections ‘d’ and ‘e’ with the mentioned DOFs.
• Now, select the ‘Hyperbeam’ from the ‘1D’ menu. In that, click on ‘standard sections’ tab and select ‘Hyperbeam’ and ‘thinwalled box’ option and hit ‘create’. This will open a new window of Hyperbeam, and in that window, a box section of certain default dimension will be created. Change the dimensions as per the values mentioned and a ‘BOX’ cross-section will be created.
  
  
• For creating a I-beam cross-section, right click and select ‘create’. In this, select ‘standard section’ > ‘Hypermesh’ > ‘Standard I section’, and give the dimensions accordingly. To create a ‘CROSS’ cross-section, select ‘standard section’ > ‘Optistruct’ > ‘CROSS’. After creating all the sections, click on model view in the top left corner which will show the model again.
  
  
  
  

• Now in order to create different 1D elements, select ‘1D’ and in that select ‘rod’, set the ‘elem type’ as ‘CROD’ and the property created for rod. Select the middle nodes of both the elements created in ‘a’ which will create a 1D rod element in between the two.
  
  
• To create an I-beam in section ‘b’, select ‘bars’. Select the middle nodes of the elements on both the brackets, and set the vectors ‘N1’ and ‘N2’ as the adjacent points on the inner and outer circles in either upper or lower part of the section. Set the property according to the one created for the beam, and the ‘elem types’ as ‘CBEAM’, which will create a beam.
  
  
  
  

• Similarly, to create a bar element of ‘CROSS’ cross-section, follow the similar process for section ‘c’ and set the ‘elem type’ as ‘CBAR’ instead of ‘CBEAM’.
• To add a mass on section ‘d’ and ‘e’, select ‘masses’, click on the middle nodes of the elements of both the sections, select the ‘elem type’ as ‘CMASS1’, and set the magnitude to 10 and ‘create’ the masses.
• Add ‘spring’ elements on any of the nodes of the sections ‘f’ on both upper and lower parts of the bracket and ‘create’ the spring elements. The ‘elem type’ in the ‘springs’ is ‘CELAS1’.
  
  
  
  

• In order to get a 3D representation of the 1D elements, point the cursor to ‘1D traditional element representation’ (represented by a line) in the tools and ‘Select from list’, ‘1D detailed element representation. Similarly for both the surfaces, select the option ‘2D traditional element representation’ (represented by a plane) and set it to ‘2D detailed element representation’.

Results and Conclusion:

• The following image shows the final model and all the different types of 1D elements created in it.
  
  
  
  

• This challenge helped to understand all the 1D elements present in FEA and HyperMesh, the application of those elements and the process to create them.
• Midsurface and its use is also an insight of this challenge. Exploring different options in the midsurface makes it easier to create a better one as per requirement.
• After practicing on this model and re-iterating it, helped to increase the efficiency and the reduced the time required for completion.
• Hyperbeam, an integral part of 1D element generation can also be understood with this task. Different types of cross-section in Hyperbeam make it easier to create various 1D elements.  

GDrive link for Parallel Brackets Model