Be extremely careful... this is not really a "simple procedure" as it seems. Materials, assembly and installation techniques are everything when sealant is introduced 1n a joint [wet assembly and wet fastener install]. Rivets are particularly susceptible to installation problems with sealant.
Early testing revealed poor static allowables with sealant in riveted joints. Fatigue allowables were worse. this is why early integral tanks DID NOT assemble structure wet with fayng surface sealant, nor install fastener wet. Everything was assembled "dry"... and the gaps were packed, all edges were fillet sealed and fastener heads/tails over coated. Fuselages relied on tight multi-fastener lap-joints to retain pressurization. Internal edges were also fillet sealed and interior/exterior gaps injected. The resultant airframe was heavier than desired and a maintenance nightmare: and "possibility" that fay-sealed assembly with wet-fasteners could save weight, reduce assembly costs, reduce joint corrosion [fay and hole] and reduce sealant maintenance headaches [fluid and air leaks]; and possibly reduce weight by eliminating bulky/globed sealant... was all seen as a "gold objective"... if achievable.
The DoD, OEMs, fastener vendors and sealant vendors worked toward the objective noted above.
DoD testing revealed that structural pull-down [clamp-up] and tight fastener fit before actually installing the fastener, and/or during the installation process, of lock-bolts, blind-bolts, rivets and blind-rivets also allowed for maximum sealant escape from the hole and around the head/tail resulting in tight/hard joints.
Sealant vendors evolved lower viscosity sealants for fay and wet-fastener assembly... and DoD/OEM structural testing evolved the techniques of HIGH pre-clamp-up of "wet-assembled" parts prior to fastening for MAXIMUM sealant squeeze-out and tightest possible joints.
Fastener vendors "tweaked" their fastener-systems to account for additional structural and fastener "settling" due to presence of sealant. For instance: pin-pull lock-bolts were developed providing MUCH higher pre-load prior to collar-hammering; and most Hi-Lok collars have collar-break torques about 10--15% higher than recommended for nut-torque installations (assumes pre-load will soften ~10% prior to sealant cure). Blind-rivets and blind bolts were also improved for "sheet pull-up" forces to minimize looseness after joint relaxes. Etc…
Test with wet installation of rivets revealed a “hydraulic lock” effect with trapped sealant. Trapped sealant has to have an escape path and “squeeze-out time” to insure hard contact of the rivet to the structure. If the rivet is bucked before hard metal contact, then the head may actually be “floating” after the cured sealant will crumbles away due to bearing and cyclic loading. The technique for installing rivets “wet” with sealant” involves pressing the head of the rivet hard against the structure [into a countersink] for a few seconds with the rivet gun hammer-head… then driving the bucked-tail (swelling tail will squeeze sealant sideways, forming a natural fillet around the bucked head). For protruding head rivets this is no brainer. HOWEVER… for flush-head rivets, countersink depth is critical: the head must NOT ride below flush in the countersink when driven (may have to deliberately set flush fasteners in shallow countersinks, then mill-flush). The NAS 1097 shear head rivet has a tiny dome element to help ensure that the head of the rivet will see positive contact with the rivet gun hammer-head. NOTE: Douglas developed 100-deg shear and tension head rives with a slight crown to ensure solid contact with driving tools… and allow a slightly shallower countersink “than normal”. Look at the wings and fuselage rivets of an MD-8x and these rivets are clearly visible… and yes the aerodynamicists are “OK” with the crown protruding slightly above flush… because of the smooth rounded-shape of the dome.
NOTEs.
The MS14218 and MS14219 [120-deg flush-interference, solid] rivets styles MUST NOT be installed wet with sealant due to the precise counter-bore/countersink nature of the head recess: the sealant (and even primer) will be trapped under the head and cannot escape (rivets MUST be installed dry, OK to apply alodine holes/countersinks before installing rivets).
A new generation of precision rivets and precision drilling, reaming, countersinking and squeezing (etc) equipment allows OEMs to install rivets so consistent and tightly, that little or NO sealant is required for a life-time integral tank using fully automated assembly. Eliminating sealant has allowed automated assembly to flourish, because wet-sealant assembly and fastener installation is a notoriously time consuming and “dirty” process. Sealant must be controlled, mixed, applied, etc… and then wet sealant sticks to everything like a brother… and must be constantly cleaned-up… otherwise the cured sealant must be aggressively removed by tools or chemicals.
Wet installation of rivets with primer (epoxy) is consider the same as “dry” rivet installation… assuming primer is still “as-thin-as-water” and has not become more viscous like in the latter stages of curing. This has been used effectively for decades.
Some design manuals force a ~ 0.90X strength and fatigue factor if primary structure is assembled “wet with sealant”. Also some torque-process manuals require increased nut-torque to account for inevitable pre-load relaxation as additional sealant squeezes-out of a joint.
Regards, Wil Taylor
