Jinhee Park (M.S. Candidate) Date of joining Masters’ program : Fall 2002

1. Influence of Overload Induced Residual Stress Field on Fatigue Crack Growth in Aluminum Alloy Jinhee Park (M.S. Candidate) Date of joining Masters’ program : Fall 2002 Thesis advisor : Dr. M. A. Wahab Dr. S. S. Pang

2. The experimental facility (MTS 810)

3. Instrumented Specimen in the Corrosion Cell

4. < Overview > • Introduction • Theoretical Review • Numerical Modeling (FEM) • Design of Experimental Methodology • Numerical Result • Comparison of Numerical Result with Theory • Conclusion & Comment • Further Work

5. < Introduction > • Concept of Fatigue Crack Growth loading ∆a unloading ∆a Time loading unloading

6. • 2D - Modeling of Center Crack Specimen ASTM Standard E 1823 M(T)  Crack Tip a W 

7. • Material Behavior Modeling (elastic-perfect plastic) Bilinear Inelastic Isotropic Hardening Loading Unloading Dissipated Energy   Permanent Set Linear elastic material Elastic plastic material stress-strain loop

8. : Irwin’s plastic zone radius (1960) • Plastic Zone Size for Plane Stress At cyclic loading (+) Crack Distance from crack tip (+) At cyclic unloading (--)

9. • Compressive Residual Stresses ahead of Crack Tip after Overload Tensile Stress Overload Plastic Zone (+) Compressive Stress Distance from crack tip (--) Cyclic loading plastic zone

10. • Fatigue-life Extension due to Periodic Overloads Crack Length(a) Constant amplitude cycles Overload Periodic overload retardation Number of cycles(N) The slower crack growth continues until the crack grows beyond the overload plastic region(Thewheeler model of crack growth inside an overload plastic zone). This beneficial residual stress effect of overload is called crack growth retardation.

11. • The wheeler model of crack growth inside an overload plastic zone (+) • Before overload (-) a • After first overload a • After some crack growth a’ • After second overload a’

12. < Theoretical Review > • Consequences of Overload for Crack Tip Plastic Strain Loop A A E E   Cyclic elastic strain range F E F E A After overload Before overload F F

13. Elastic Modulus (E) Poisson Ratio () Yield Stress ( ) Room Temp. (T) Tangent Modulus (H) 70 0.33 200 20c 0 < Numerical Modeling > • Mechanical Properties of Aluminum Alloy (2024-T3, 7050-T7451) - Chemical Composition of Aluminum Alloy

14. 40mm Node just front crack tip 20mm Crack Tip 10mm 40mm • Finite Element Model for Center-Crack Plate  Symmetric Boundary Condition Element Size around crack tip : 0.5mm Element type : 8 node PLANE 82

15. • Cyclic loading condition with two overloads history Overload ratio = 80 / 30 80MPa 30MPa

16. < Design of Experimental Methodology > • Influence of low cycle fatigue(LCF) damage on high cycle fatigue(HCF) crack growth. • Modeling of overloading effects on crack growth, and considering various load amplitudes. • Formulating equations for lifetime prediction. • Providing recommendations for service life extension and developing improved fatigue life assessment tools.

17. < Numerical Results > After overload (80MPa) Before overload After one overload, cyclic strain range was considerably reduced and the crack growth rate will decrease accordingly(Retardation).

18. • Stress-total strain curve with two overloads (80MPa, 60MPa) After overload (60MPa) After overload (80MPa) Before overload

19. • Stress-total strain curve with two overloads (80MPa, 100MPa) After overload (80MPa) After overload (100MPa) Before overload

20. • Stress-strain curve with two overloads (80MPa, 80MPa) After overload (80MPa) Before overload This curve didn’t show any more decreased strain after the second overload.

21. • Substep time-total strain curve with two overloads 0.015 0.01 0.005 80MPa - 60MPa Total strain 0 80MPa - 80MPa -0.005 80MPa - 100MPa -0.01 -0.015 0 5 10 15 20 25 Substep Time

22. • Von-mises stress distribution along the crack plane 200 180 Before overload 160 140 120 Von-mises stress (MPa) 100 80 After first overload (80MPa) 60 After second overload (100MPa) 40 20 0 5 10 15 20 25 30 35 40 Distance from the crack tip (mm)

23. • Two overloads (80MPa - 100MPa) The plastic zone disappeared after each overload.

24. < Comparison of Numerical Results with Theory > After overload (80MPa)  Before overload F After overload F After overload (80MPa), strain range moves to the left. It becomes negative. The strain hardening should be considered.

25. < Conclusion and Comments > • After one overload, strain was decreased. (Crack growth retardation) • After the second overload, the second reduced strain was recorded in 80MPa & 100MPa. • Von-mises stress redistribution along the crack plane after first overload (80MPa) was reduced. After the second overload (100MPa), the stress redistribution was smaller than the first one. • Two overloads effects on the crack plane and crack growth rate was not well checked in this work. • This paper was accepted by ICCE 10, July 2003.

26. < Further Work > • Cyclic strain hardening and path dependent plasticity should be considered later on. • Various loading conditions like overload ratio, stress ratio, and stress range should be conducted later on. • To advance crack, the crack tip advance scheme (involving node release immediately after maximum load on each cycle) needs to be carried out.