Best Practices for Fatigue Calculations on FE Models

Transcription

1 Best Practices for Fatigue Calculations on FE Models Presented by: Dr.-Ing. Stephan Vervoort Senior Application Engineer Hottinger Baldwin Messtechnik GmbH, ncode Products 1

2 Agenda HBM ncode Products Fatigue Analysis Process Modeling Recommendations for FE Models Defining the Loading Environment Defining Material Properties Conclusion 2

3 HBM-nCode Products Data Processing System for Engineers Streamlining the Virtual Fatigue Engineering Process Web-based Processing for Engineering Data Comprehensive analysis to reporting Graphical, interactive & powerful World leading fatigue analysis capabilities Powerful fatigue analysis technology Integrated reporting and processing Fast, expandable, and scalable Search, query and reporting data through secure web access Analyze, trend and understand using configurable processing 3

4 A Simple Fatigue Process 4

5 Fatigue Analysis Process Inputs Geometry Stress Life Strain Life Crack Growth Etc Loading Environment Fatigue Analysis Fatigue Results Material Properties 5

6 The Inputs Geometry Accuracy of surface stresses are important ±10% ±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 Modeling Recommendations for FE Models 7

8 Modeling Recommendations for FE Models Fatigue cracks usually initiate at free surfaces Fatigue damage increases exponentially with stress ±10% ±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 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 Solid Model Stresses calculated at Gauss points but Gauss points are not on surface Option 1 skim surface with membrane or thin shells Resolves stresses to surface plane Uses element stresses from the shells as before x y z 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 Defining the Loading Environment 11

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 Linear Static Superposition L 1 =1 C 1 C 1 x Real Load L 1 + C 2 C 2 x L 2 =1 Real Load L 2 = s A C = FE Stress tensor for Unit Load Cases Stress time signal at element 13

14 Modal Superposition Modal Geometry Stresses Loading Histories Modal Transient Loading Modal Environment Coordinates SN Fatigue or EN Analysis Fatigue Results SN Material or EN Properties Curve L 1 L 1 L 2 L 2 Mode 1 Mode 2 s 1A * f 1 (t) + s 2A * f 2 (t) +... = s A (t) f 1 f2 s A 14

15 Time Step Load time signals processed by FE L 1 s A L 1 Stress time signal at element L 2 L 2 Stress for combined loads calculated by FE point by point For long time histories, issues with solution time and disk space requirements 15

16 Harmonic Response Transfer Function L f 1 C f C f 2 Real Load L 1 = s A C(f) = Complex transfer tensor for unit Load Cases L(f) Stress PSD at element 16

17 Non-linear Contact Combined load case L 2 L 1 Real load L 1 t Non-linear contact is modeled as 2 linear static load cases Load cases scaled by real loads +ve loads applied to load case 1 -ve loads applied to load case 2 Real load L 2 t t 17

18 Defining Material Properties 18

19 Defining Material Properties Stress Range S The fatigue life is often described with a single regression curve through data points at which 50% of samples have failed SN Fatigue T est Analysis More data points result in better confidence, but range of life is also important The distribution of points allows for different Certainty of Survival levels Supplier A Test Data Supplier A Regression fit 2 sigma range 2 sigma range Supplier A Design curve Supplier B Test Data Number of Cycles to failure N The distribution of points also allows for the comparison of different batches or supplier Supplier A Supplier B

20 Conclusion Accuracy of damage results depends on accurate surface stresses There are three basic types of loading environments Linear static superposition Time step Harmonic response Determine if the loading is dynamic or quasi-static If max frequency in PSD < 1/3 f 1 then use static analysis The fatigue life curve, certainty of survival, and range of life are all important when characterizing material properties 20

21 Thanks You HBM GmbH ncode Produkte Carl-Zeiss-Ring Ismaning Tel: +49 (0) Fax: +49 (0)