| Description |
In fatigue and fracture critical aerospace structure, fastener holes represent one of the most highly prone locations for the nucleation and propagation of fatigue cracks. The Split Sleeve Cold ExpansionTM (SSCx) process was developed to help reduce the probability of fatigue crack propagation at fastener locations. The SSCx process has been demonstrated to provide a dramatic improvement in the fatigue life at processed hole, however within the current fatigue life paradigms there is limited ability to take analytical advantage of this process when calculating an inspection interval. One of the concerns related to the implementation of this type of deep, engineered residual stress into a fatigue life calculation is whether or not a fatigue crack that propagates through the residual stress/strain field is changed due to the presence of the fatigue crack. In order quantify the effect a fatigue crack has on the residual stress field around a cold expanded (Cxed) hole within two common aluminum alloys (2024-T351 and 7075-T651), first a series of baseline uncracked coupons were developed. From these groups of replicate coupons, the two-dimensional residual stress fields were quantified using the contour method. In the second phase of this program, fatigue cracks were then developed and propagated within identical coupons to specific cold expansion (Cx) mandrel entrance surface lengths, ranging from 0.08 inch to 0.50 inch. The contour method was used to determine the residual stresses within these fatigue-cracked coupons. With a statistical understanding of the baseline, uncracked condition, the effect of the presence of the fatigue crack spatial throughout the coupon was able to be quantified. The effect of the fatigue crack was then integrate into a range of FEA simulation processed used to calculate SIFs and provide a prediction of a given test condition. Predictions were made using the legacy, static residual stress field, as determined by the baseline, uncracked coupons. We then utilized the information gained through the residual stressed determined in the fatigue-cracked coupons to make another prediction. Through the integration of the determined effect of the fatigue crack a more accurate prediction of the test condition was able to be developed. |
