Trainguy:
Actually, creep (not your creeps) in the glue line would be one of the things you would want to look into. Theoretically it should be possible, and in fact be an improvement over taking the loads at discrete locations (rivets) or at just one line (welds) on thin material, since the shear flow is distributed over several sq. inches instead of an individual rivet.
The problem has always been the durability/reliability of the glue w.r.t. vibration, cycling stresses, weathering, expansion & contraction, changing glue line mechanical properties over time and the like. Then, there is the materials prep. for proper bonding, clamping pressure and its uniformity, etc. as relates to fabrication. I would talk to the glue suppliers, aircraft designers and builders, maybe truck trailer builders, and the like, makers of large stressed skin panels, Hexcel comes to mind. The people who manufacture the glue products should have some testing on things like vibration, freeze/thaw, water and UV damage, etc. The question is longevity and durability of the glue material, not initial strength which seems to be quite good with minimal special effort. I’ll bet that in the deep dark files of outfits like Pullman Standard, ACF, Bombardier, Valdunes, Hawker Siddeley, Budd, etc. there would be some very interesting reports and testing on this very subject.
If you can control the glue degradation it works well because the shear stress is only a few psi, whereas the stresses around the rivet, in the sheets, and in the rivet are quite high and susceptible to vibration, fatigue, etc. I’ve epoxied teflon sheets to steel blocks and plates to make slide bearings and am not aware that there were any problems with those designs. I also used up some of the left-over epoxy to glue some samples of .5" x 2" steel bar stock together, in lap joints of varying lengths, and these joints were tougher than hell. I never did do any serious testing to get meaningful forces or stresses. You might be able to use many fewer rivets since the glue really carries the forces/stresses, but the rivets prevent a peeling type failure. You are keeping your detail, loadings, etc. top secret, so it’s difficult to comment further. But, you want that joint to act in shear, a shear flow in lbs./sq.in., in the plane of the glue joint and sheets being joined, not in prying or peeling, or in a tension perpendicular to the plane of the joint. If you were cutting internal stiffeners in your application I probably wouldn’t do it, because of the way they concentrate loads. But, if you were cutting two skins in an area btwn. two stiffeners, I could imagine fab’ing. a strengthening/stiffening frame for that opening and maybe glueing it btwn. the skins. Then after the glue had set up, adding a few rivets at corners, etc. just to minimize out of plane flexure due to vibration, etc. Then the window could be set in this opening. It would be a mighty interesting experiment if there wasn’t too much at stake. But, I wouldn’t quote 500 cars with this detail without more to hang my hat on.
