Spring 2010, MET 415 - FEA Applications I

Prof. Dave Johnson, psuprofdj@psu.edu

Penn State - Erie, The Behrend College

Homework Assignment 3B

Concepts:

• 2D Axisymmetric Element Types
• Features for load application
• Density Material Property Units
• Inertia/Body Loads
• Solution: Multiple Load Steps
  • Multiple SOLVES vs. LS Files
  • Load Step Files
• Postprocessing: Multiple Load Steps



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A tractor trailer sized brake drum is shown (with a 90 degree sector cutaway) in the figure above. The dimensions are given in the cross- section figure below. This drum is made of a ductile cast iron with an elastic modulus of 24.0e6 psi, Poisson's ratio of 0.3, and a density given as 0.246 lbm/cu. in.  To permit axisymmetric modeling, the circle of 12 discrete 3/4" diameter bolt holes is treated as a continuous feature about the "revolved" geometry. 

A 2D IGES file is provided - it contains ONLY an area, lines, and points on the XY plane. 

• Right-click on this link:  IGES File or go the the Angel page for this class, under Lessons > HW files

• "Save Target As...", then "Save as type: All Files", and use a file name like: "brakedrum.iges"  

• After Import, check model scale by listing keypoint coordinates (or KLIST command) and comparing to the dimensions below.

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Define the appropriate element type and a linear, elastic material model for the cast iron. 
What are the proper units for density in a model with BIN dimensions ?  (HINT: NOT lbm)

Geometry Features Defined for Loading (Load Patch)

An UN-meshed solid model may be modified to permit the desired loading.  A "load patch" is not a geometry feature that would be needed to manufacture the part, but is needed to split a face or an edge to allow the applied loading to be placed and distributed correctly.

For this problem, it is convenient to use the ANSYS Working Plane as a cutting tool.
For example, position the WP 1.0" down from the highest edge, rotate the WP so you view it as a line from a front view of the model, and use the Boolean Operation: Divide > Line by WrkPlane.  This can break the inside wall line a the right location to facilitate the brake pad pressure loading.  It may be useful to view the ANSYS Work Plane as a "Grid & Triad" when positioning it for such operations.

The bolt circle constraint can be approximated by constraining the axial (UY) direction at the center of the bolt on the face of the brake drum where it mounts to the wheel hub. 

MULTIPLE LOAD STEPS

Several loads are to be solved and evaluated, individually:

• Inside pressure over a 5" wide band (simulating the brake pad contact) of 1000 psi.
• The drum spins about its axis at 120 rad/sec which corresponds to a truck speed of nearly 80 mph.
• Combined loading of both the pad pressure and the angular rotation.

PROCEDURE (A or B method)

A. The multiple solve method:

1. Specify analysis settings, loads and options for the first load step.
2. Solve - Current LS (and postprocess if desired).
3. Change the title and loading as required for the next solution.  Loads and constraints which are not changed or removed, remain in the next solution step. 
4. If you have left the solution processor (to do postprocessing for example) since the last solve, then specify a RESTART to avoid the new analysis overwriting the results file.
5. Solve, and postprocess as desired.
6. Repeat steps 3, 4, and 5 until all load steps are completed.

B. To use the LOAD STEP FILES method:

1. Specify analysis controls and specify loads and options for the first load step, change the title.
2. Write load step file. [Solution - Load Step Opts - Write LS File (start with number 1)]
3. Change loads and options as required for the next load step, change the title.
4. Write next load step file. [Solution - Load Step Opts - Write LS File]
5. Repeat steps 3 and 4 for remaining load steps.
6. Solve from load step files. [Solve - From LS Files - specify starting and ending LS File numbers]
7. Postprocess

LS Files can be edited (ANSYS commands) to correct mistakes or change input.  LS Files can also be useful if you discover model errors that require new solutions.

Once you solve for these load steps, in General Postprocessor, you can switch between sets using Read Results > First Set ... or Next Set ... or Last Set.  You do not have to re-solve the model.

Turn in:

• A plot showing the mesh
• A plot showing constraints and ALL loading - include a  statement explaining why the UY constraint alone is adequate constraint for this simulation.
• A simple* hand calculation of the weight of this part compared to the ANSYS solution output of the total mass of your model.  (During solution, ANSYS writes the "TOTAL MASS" in the Output Window).
      * simple: use only geometric formulae for disks and cylinders to estimate volume of the part
• Plots showing the deformed shape (including the mesh error) for each of the load cases.  (SEPC must be less than 5%)
• Plots showing the equivalent stress for each of the load cases.
• Compare the appropriate brake drum FEA hoop stress to a simple hand calculation based on strength of materials, pressure vessel formula.  Include your reference for the pressure vessel formula.  This calculation is only for the brake pad pressure load, treated as a simple cylinder with a uniform internal pressure.  Include the FEA results plot used for this comparison.
• The modeling session log file (with comments added).