Influence of Yield Strength on Overload Affected Fatigue Crack Growth Behavior in 4340 Steel
Final rept. Feb 1973-1974
AIR FORCE FLIGHT DYNAMICS LAB WRIGHT-PATTERSON AFB OH
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Single tensile overloads were applied to 4340 steel specimens which were treated to give 120 and 220 ksi yield strength levels. The influence of yield strength level on the number of nonsteady state crack growth cycles subsequent to the application of a 100 percent overload was noted to be substantial. The number of nonsteady state cycles for the 120 ksi strength steel was approximately an order of magnitude greater than that of the higher strength steel. A retardation model was developed using a residual stress intensity factor concept similar to that proposed by Willenborg et al. The model was found to predict to within 10 percent the number of nonsteady state crack growth cycles required to move a crack from the pre-overload position to a subsequent position, one overload induced plane stress plastic zone radius ahead of the pre-overload position. The model indicates that the reason for substantial increase in nonsteady state crack growth cycles observed for the low strength steel is due to a corresponding increase in the overload affected zone size.
- Metallurgy and Metallography