Effect of overloading and band overloading on fatigue crack growth of aluminum alloy 7075 -T651

Present investigation involves the study of fatigue crack growth behavior of thermo-mechanically treated aluminum alloy 7075-T651. The investigation is focused to study the effect of single and multiple loading cycles on crack growth behavior. The test were carried out at stress ratio, R=0.3. The fatigue cracked specimens were subjected to overloading spike or band at a/w=0.35 followed by constant amplitude loading cycles. Significant crack growth rate retardation was noticed for multiple overloads in case of OLRs 1.5 and 1.75. However, there were insignificant retardation in case of overload spikes of 1.25 and 1.5. No sign of retardation were noticed even on application of 10 overload cycles of OLR 1.25. Insignificant retardation in case of OLR=1.25 may be due to low value of maximum stress at the notch tip and corresponding stress intensity factors ƒ¢K(6.14 MPaãm) and Kmax(6.822 MPaãm). This has resulted monotonic plastic zone size not large enough to retard growing crack. Overload of higher magnitude induce large monotonic plastic zone and therefore the significant retardation is observed. The application of band overload, on the other hand, developed a series of monotonic plastic zone, identical to plastic wake and enhanced the effect of retardation for given value of overload