Fatigue Life Prediction using Abaqus.pptx

Fatigue Life Prediction using Abaqus

1. Corrosion Fatigue Life P rediction U sing Abaqus DINESH SHIVAKOTI (R01449875)

2. Table of Contents Introduction Workflow in Abaqus/Fe-Safe Creation of Model for FEA Creation of Material in Abaqus Meshing of the geometry in Abaqus Application of the Load Fatigue Analysis using Fe-Safe Fatigue Analysis using CDM

3. INTRODUCTION 1

4. WORKFLOW IN ABAQUS/FE-SAFE 2

5. Abaqus teaching license has a limitation on number of mesh elements. With teaching license, we can model up to 100,000 nodes which is around 20,000 elements for hexahedral(cube/box) mesh. While it sounds a lot, when it comes to modeling a notch(or a corrosion pit) of 10-50 μ m(0.01-0.05mm), It is very limited and modeling multiple notches or complex small features is almost impossible with the available number of mesh elements. d) e) f) Corrosion pit modeled with a half sphere shape or oval shape Zoomed view of the notch Cross section of the notch showing the depth of the notch(pit) 0.01-0.05 ( mm) Fatigue test specimen(E466) Fatigue test specimen with clamp shape reduced Fatigue test specimen used for the modeling of the small notch(pit) Fig: a) b) c) 5-6mm Creation of the model for FEA 5-6mm 3

6. Fig: Engineering Stress vs Engineering Strain Fig: True Stress vs True Strain Fig: Creating an elastic-plastic material in Abaqus Creating a material (NAB) in Abaqus Stress(mm/mm) Stress( Mpa ) True Stress/Strain Engineering Stress/Strain Engineering Stress-Strain data from the Tensile test is converted to True Stress/Strain to create an elastic-plastic material in Abaqus. 4

7. To correctly model the notch with depth of 0.05mm in FEM model, mesh size of at least 0.0025 mm must be used. Mesh size must be kept small in the region where crack is likely to propagate to correctly predict the damage behavior . Smaller mesh results in better accuracy of the result at the cost of computation time. Optimum mesh size is when the result doesn't change with further decrease in mesh size. Number of Mesh and Mesh type(Tetrahedron, Hexahedral etc.) greatly affect the stress/strain fields. C3D10, a 10-node quadratic tetrahedron mesh element is used for the FEM analysis in Abaqus. Fig: Meshing of the model Fig: Meshing of the notch with very small elements Fig: Selection of Mesh type in Abaqus Meshing the geometry in Abaqus Mesh size changes from very fine to fine to coarse as we move away from the notch. 5

8. FIX FORCE STRESS/STRAIN result One end is fixed, and another end is subjected to the stress at different levels(80% to 120% of YS) Meshing of the model to capture the notch shape(corrosion pit) Visualization of the result in Abaqus Fig: Application of the Load in Abaqus Apply uniaxial Tension Load Cyclic Load (Fully reversed, R= -1) Cyclic Load (Fully reversed, R= -1) Fig: Different load application based on the approach For Stress/Strain Life approach, ramp tensile load is applied. For CDM/LEFM approach, Cyclic load is applied. 6

9. Best Fit line Fig: S-N data of WAAM NAB Fig: S-N curve of WAAM NAB Creating a material (NAB) in FE-SAFE Material constant A and B are computed Material is created in FE-SAFE from experimental data(SN data). 7

10. Stress Life(Von Mises Stress) > Finite Life (Goodman/Gerber et) Fig: Creating a Material in FE-SAFE FE-SAFE is a fatigue analysis software for finite element model that comes with Abaqus. Fig: result from Abaqus(. odb file) Fig: Fully reversed load(R= -1) is applied Fig: Analyzing the FEM model with Stress Life(Goodman) criteria Fig: visualizing the result in Abaqus to find the Fatigue Life Fatigue Analysis using FE-SAFE 8

11. Fig: More accurate representation of the geometric morphology of corrosion pit Table: SN data of WAAM NAB (2hrs corrosion) & Prediction of the corrosion fatigue life by FE-SAFE Reference: Table: SN data of WAAM NAB Fatigue Analysis using FE-SAFE Fig: simplest representation of corrosion pit FE-SAFE results differs significantly from the experimental data . Following can be considered to improve the result. Calculation in FE-SAFE solely based on stress/strain field which fluctuates largely with different mesh parameters. Mesh parameters tuning may be required. Since only one notch(pit) is modeled, it acts as the only stress concentrator so Fatigue life is much less. More accurate representation of corrosion pit can be considered to improve the result. 9

12. (D=1) Continuum Damage Mechanics (CDM) is a theoretical framework used to model the gradual degradation of materials under mechanical loading . Damage variable(D) is used to degrade the material and when D reaches 1, material is declared failure. Abaqus supports Damage modeling for the uniaxial/biaxial tensile loading but doesn't support damage modeling in cyclic loading, Damage doesn't accumulate in each cycle. To incorporate Damage Evolution Model in cyclic loading in Abaqus, a special program written in Fortran program called ‘subroutine’ is required. Subroutine USDFLD or UMAT can be used, USDFLD being less difficult to program. Fatigue Analysis using CDM 10

13. Damage Evolution Model in Fatigue Parameters to calibrate Fatigue Life Estimation in CDM Fatigue Analysis using CDM Table: SN data of WAAM NAB Material parameters( α,m) must be calibrated from experimental data(S-N data) Calibration of material parameters( α,m) Using log-log linear regression method (equivalent to Paris Law in Fracture Mechanics) Damage Evolution Law 11

14. Abaqus Subroutine USDFLD written in Fortran How Abaqus Subroutine works? Fatigue Analysis using CDM The abaqus teaching license cannot be used to run analyses that include any user subroutine(USDFLD, UMAT etc.) Creating a Ductile Damage material in Abaqus 12

15. THANK YOU!!