Best Practices for Fatigue Calculations on FE Models


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Best Practices for Fatigue Calculations on FE Models

  1. 1. Best Practices for Fatigue Calculations on FE ModelsPresented by:Dr.-Ing. Stephan VervoortSenior Application EngineerHottinger Baldwin Messtechnik GmbH, nCode Products 1
  2. 2. Agenda• HBM nCode Products• Fatigue Analysis Process• Modeling Recommendations for FE Models• Defining the Loading Environment• Defining Material Properties• Conclusion 2
  3. 3. HBM-nCode ProductsData Processing System for Streamlining the Virtual Fatigue Web-based Processing forEngineers Engineering Process Engineering Data• Comprehensive analysis to reporting • Powerful fatigue analysis technology • Search, query and reporting data• Graphical, interactive & powerful • Integrated reporting and processing through secure web access• World leading fatigue analysis • Fast, expandable, and scalable • Analyze, trend and understand using capabilities configurable processing 3
  4. 4. A Simple Fatigue Process 4
  5. 5. Fatigue Analysis Process Inputs •Stress Life •Strain Life •Crack Growth Geometry •Etc Loading Fatigue Analysis Fatigue Results Environment Material Properties 5
  6. 6. The Inputs• Geometry Accuracy of surface stresses are important ±10% stress @ ±100% life Current structural FE modeling is generally sufficient• Loading Environment Large effect on fatigue life Must be correctly characterized• Material Properties Material fatigue properties are relatively inexpensive to obtain Materials Assurance Service available from nCode laboratory 6
  7. 7. Modeling Recommendations for FE Models 7
  8. 8. Modeling Recommendations for FE Models• Fatigue cracks usually initiate at free surfaces• Fatigue damage increases exponentially with stress ±10% stress @ ±100% life OK for load path & natural modes Required for Fatigue• Recommend using node on element or averaged node on element Check for convergence 8
  9. 9. Shell Model• Stresses calculated at Gauss points and extrapolated to node• Node has separate stress result from each element• Most FEA uses average nodal stress 9
  10. 10. Solid Model• Stresses calculated at Gauss points – but Gauss points are not on surface z’• Option 1 – skim surface with membrane or x’ thin shells Resolves stresses to surface plane y’ Uses element stresses from the shells as before• Option 2 – use surface node results Resolves stresses to the surface Uses node on element or averaged node on element on surface nodes only 10
  11. 11. Defining the Loading Environment 11
  12. 12. Different Types of Loading Environments• Linear static superposition Linear static superposition is an efficient FE analysis technique This process can also be used for modal superposition• Time step Time step is computationally intensive but allows for non-linear analysis and dynamic analysis• Harmonic response Harmonic analysis is very efficient for steady state random loading 12 12
  13. 13. Linear Static Superposition C1 C1 x L1=1 Real Load L1 + C2 C2 x Real Load L2 L2=1 = sA C = FE Stress tensor for Unit Load Cases Stress time signal at element 13
  14. 14. Modal Superposition Modal Stresses Geometry Loading Modal Loading Modal SN or EN Fatigue Fatigue Histories Transient Environment Coordinates Analysis Results SN or EN Material Properties Curve L1 L1 L2L2 Mode 1 Mode 2 s1A* f1(t) + s2A* f2(t) + ... = sA(t) f1 sA f2 14
  15. 15. Time StepLoad time signals processed by FE L1 sA L1 Stress time signal at element L2 L2 Stress for combined loads calculated by FE point by point For long time histories, issues with solution time and disk space requirements 15
  16. 16. Harmonic Response Transfer Function C f  C f   2L f  1 Real Load L1 = sA C(f) = Complex transfer tensor for unit Load Cases L(f) Stress PSD at element 16
  17. 17. Non-linear Contact Real load L1 L1 t L2 Real load L2 t• Non-linear contact is modeled as 2 linear static load cases• Load cases scaled by real loads Combined load case• +ve loads applied to load case 1• -ve loads applied to load case 2 t 17
  18. 18. Defining Material Properties 18
  19. 19. Defining Material Properties SN Fatigue T est Analysis• The fatigue life is often described with a single regression curve through data points at which 50% of samples have failed Stress Range S• More data points result in better confidence, but range of life is also Supplier A Test Data important Supplier A Regress ion fit 2 s igma range 2 s igma range Supplier A Des ign curve Supplier B Tes t D ata• The distribution of points allows for Number of Cycles to failure N different Certainty of Survival levels Supplier A• The distribution of points also allows for Supplier B the comparison of different batches or supplier 4 2 0 2 4 6 8 19 19
  20. 20. Conclusion• Accuracy of damage results depends on accurate surface stresses• There are three basic types of loading environments Linearstatic superposition Time step Harmonic response• Determine if the loading is dynamic or quasi-static If max frequency in PSD < 1/3 f1 then use static analysis• The fatigue life curve, certainty of survival, and range of life are all important when characterizing material properties 20
  21. 21. HBM GmbH nCode Produkte Carl-Zeiss-Ring 11-13 85737 IsmaningThanks You Tel: +49 (0)89 960537218 Fax: +49 (0)89 960537221 Email: 21