Cracking on the front spar of CF-188 (F/A-18) model A/B wings both in-service and during fatigue testing, resulted in cold expansion repair being incorporated on the full scale fatigue test of the CF-188 wing. Monitoring of strain gauge measurements during the modification recorded an increase of approximately 1,200 microstrain in the flange area adjacent to the holes. At a significant time later, the unmodified, port inner wing was found cracked at the same location and repaired in an alternate manner. This prompted the Institute for Aerospace to further investigate the impact of the repairs using a variety of experimental stress analysis measures, including strain gauges and photoelasticity. Two coupons were manufactured to simulate the front spar region in the vicinity of the critical area. The residual stress state of the manufactured coupons was characterized using X-ray diffraction. Then both coupons were instrumented with photoelastic coating and strain gauges, in order to measure strain response during each step of the repair procedure. Coupon 1 was the experimental coupon simulating the condition of the port side wing repair and coupon 2 was the control coupon simulating the original starboard wing repair. The port side wing coupon simulated repairs to the port wing on the test. This repair was intended to restore the same interferences as the original equipment manufacture (OEM) Hi-Loks. Coupon 2 imitated the starboard side repair to the test specimen that utilized reaming of holes and Fatigue Technologies Inc (FTI) hole cold expansion on four holes adjacent to the critical area. This was the same technique used by the repair and overhaul contractor. The results showed that cold expansion (CX) of holes in coupon 2 created compressive strains in excess of 10,000 myepsilon in the low edge region adjacent to the fasteners. In contrast to this the Radial-Lok repair, coupon 1 generated compressive strains in the order of 2,500 myepsilon around the fasteners on the low edge distance side. The full field photoelastic results showed that the Jo-Bolts installed in the cold expanded coupons did not modify the strain field significantly, except to shift the location of the Isotropic points without significantly changing the maximum measured strain. The photoelastic results also showed that the low edge distance, on one side of the fastener, was the likely cause of this increase in strain as a result of the need to accommodate the balanced compression/tension field. The large edge distance side of the coupon showed significantly lower strains, suggesting that future repairs using cold expansion of holes should take hole diameter, hole spacing and spar geometry into account before implementing repair procedures on actual aircraft structures. This investigation also showed the value of using experimental stress analysis techniques and coupon level tests to baseline the structural changes of repair modifications.
Experimental measurement of strain changes around fastener holes during cold expansion
Experimentelle Messung der Dehnungsänderungen um Befestigungslöcher herum bei der Kalterweiterung
2006
20 Seiten, 4 Quellen
Aufsatz (Konferenz)
Englisch
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