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

The experimental facility (MTS 810)

Instrumented Specimen in the Corrosion Cell

Introduction Theoretical Review Numerical Modeling (FEM) Design of Experimental Methodology Numerical Result Comparison of Numerical Result with Theory Conclusion & Comment Further Work

Concept of Fatigue Crack Growth ∆a loading unloading loading unloading Time

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

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

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

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

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

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

Consequences of Overload for Crack Tip Plastic Strain Loop Before overload After overload F A E  A E F Cyclic elastic strain range  F E A E F

Elastic Modulus (E) Poisson Ratio ( ) Yield Stress ( ) Room Temp. (T) Tangent Modulus (H)  c 0 Mechanical Properties of Aluminum Alloy (2024-T3, 7050-T7451) CuMnMgZnCr 7075-T %2.5%5.6%0.23% 2024-T34.4%0.6%0.45% - Chemical Composition of Aluminum Alloy

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

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

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.

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

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

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

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

Substep time-total strain curve with two overloads Substep Time Total strain 80MPa - 100MPa 80MPa - 80MPa 80MPa - 60MPa

Distance from the crack tip (mm) Von-mises stress (MPa) Before overload After first overload (80MPa) After second overload (100MPa) Von-mises stress distribution along the crack plane

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

After overload  F After overload (80MPa), strain range moves to the left. It becomes negative. The strain hardening should be considered. After overload (80MPa) Before overload F

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.

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.