Fatigue Based Optimization – MSC Nastran Embedded Fatigue Case Study – Shock Tower

5
May

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It is currently a great challenge performing fatigue analysis on FE models of moderate size, say 1-2 million DOF or more.  Utilizing the new MSC Nastran Embedded Fatigue (NEF), engineers will be able to unite fatigue life calculations and SOL200, Design Optimization and Sensistitivy Analysis, and may use MSC Nastran to optimize components and assemblies for weight, strength, stiffness and durability in a cyclic loading envinronment.  This is the first time a commercial stress solver has been coupled with an industry recognized fatigue solution process in this way.

Since NEF handles both the stress and fatigue life calculations in memory, performing fatigue life studies on large models is feasible.  In addition, because the solution resides entirely within MSC Nastran, the models can be much larger, and the amount of work required for an engineer is greatly reduced.

The following is a fatigue analysis case study with NEF.

Shock tower portion of a vehicle body

 

Problem Description
In this analysis, the shock tower portion of a vehicle body is analyzed to illustrate more complex loading in the form of a duty cycle. Constraints and loads are applied at certain locations in order to simulate gravity and road loads. The sheet metal stresses are recovered and used in the fatigue analysis.

  • Design variables : 15 Shell thickness
  • Objective : Minimize Weight
  • Constraint: Increase Log of fatigue life from 0.70 to 0.80
  • Files Required: shocktowerDCY_op.dat

Results

Maximum Constraint Tracked Versus Design Cycle

 

Individual Design Variables Tracked Versus Design Cycle

 

Weight Improvement Tracked Versus Design Cycle

 

Summary

With NEF, the shock tower was optimized for fatigue loadings, not just static ones. The optimized design resulted in a 24% weight reduction and an increase of life by 14%, while preservering its intended function and safety.

Initial Design Optimal Design
  • Weight: 2.514E-02
  • Log of fatigue life : 0.70
  • Weight: 1.919E-02 (down 24%)
  • Log of fatigue life :0.80 (up 14%)

 

 
Fatigue life improved by 14% and weight was reduced by 24%
 
 

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