The upper fuselage skin of the F-16 (Figure 1) has exhibited fatigue cracks growing from the bolt holes that are used to attach removable access covers. These fatigue cracks typically initiate in the bolt hole, grow through the access cutout ligament, then back through the remaining web area. These fatigue cracks have been discovered in the USAF F-16 fleet on aircraft by approximately 2,500 flight hours. The design of the F-16 is such that the access doors on the upper fuselage skin are low or non-load carrying. The fuselage skin is expected to carry most of the axial, bending and shear loads with low load transfer into the door.

The method of attachment of the access doors onto the fuselage skin is with clearance bolts into riveted nut plates. The aircraft skin, if found cracked beyond repair limits, must be replaced.

To reduce the likelihood of this fuselage skin cracking, FTI investigated a process that provides residual compressive stresses into the aluminum skin surrounding the bolt hole and existing nutplate rivet holes and then test its effectiveness. The FTI ForceTec (FtCx) process is a rivetless nut plate with enhanced fatigue life performance. As a part of this modification effort, reducing the stress in the skin by increasing the effectively of the door was assessed to see if it would improve the fatigue life. To increase the door effectivity, the door material was changed from aluminum to a stiffer metal matrix composite door. Also, the rubber door gasket was changed to a metallic gasket.

Figure 1