Overload induced retardation in 2024-T3 aluminum sheets

Abstract

61 IV. SUMMARY AND CONCLUSIONS Periodic overload tests have been conducted on M(T) specimens made from five mm thick 2024-T3 AT clad aluminum alloy for a load ratio R=0.11, with the resulting baseline AK value typical of Region II of the Paris plot. OLR was kept constant, i.e. 1.65, whereas OCR varied from 1/3 to 1/15000 among 10 values. In addition a few single overload tests have been performed to clarify the effect of AK on retardation. Based on the experimental results and subsequent discussion, the follo wing conclusions may be drawn: (1) Periodic overloads, with all the OCR values, except 0CR=1/3, employed in this study, provide additional life or caused retardation in crack growth. However, steadily decrease in retardation as OCR approaches to `4 clearly shows that acceleration with respect to constant amplitude life should be expected when OCR <4. (2) The maximum life occurs at OCR=1/7500, i.e. ~22 times that of constant amplitude, for crack growth between 20 and 30 mm. For -i -1 OCR <7500and OCR >7500 corresponding fatigue lives are observed to decrease with different slopes on logarithmic plot. (3) Periodic overload growth rates obtained for only those values of OCR where smooth crack growth curves obtained are shown to be reduced by a factor of AN/ANca at the equivalent loading conditions, i.e. baseline AK. The greatest reduction occurs when AN/ANca is maxi mum which 1s evidently when OCR= 1/7500.62 (4) The experiments which are started close to the notch showed great differences from the experiments which are started far enough from the notch. (5) Turn-off during the night has no influence on fatigue crack propagation for the present test conditions. (6) As NCA decreases or the number of overloads Increases shear lips develop earlier, that is, earlier transition to plane stress takes place. Limited data show that periodic overload behaviour under plane stress and plane strain is the same. (7) The fact that band width of overload induced markings is less than the expected single overload band width of the same baseline AK suggests that AKeff has been reduced in periodic overload spectrum.