design considerations for aluminum hull structures - Ship Structure ...

-41-(f) Weld and plate cracking in way of impact d~age on bottom orside shell.(g)cra~kfig in the welds attachfig highly loaded fittings, - . i.e.,bitts, chocks, etc. to the hull. -These instances of structural cracking ondeckhouses lead to the following conclusions:existing aluminum ships or(a)(b)(c)(d)(e)Aluminum is more susceptible to such moblerns than steel. sincethe hamspots, discont~nuities and po~r welding details firoducingthe stress concentrations invariably occur in either the weldof lower strength heat affected zone.Proper attention to the design of aluminum structural detailsis of paramount importance, particularly if the structure isto be highly stressed. Details should be designed so thataccess for welding is adequate all around. Continuous weldingshould be used in way of all connections of framing members,stanchions, etc. to avoid, or at least minimize, undercuttingand radial shrinkage stresses at the end of the bead.Conventional merchant ship details for frame and beam connectionsshould be avoided in aluminum construction since lapped bracketsor overlapped framing members are hard ‘Goweld properly, andhigh stress concentrations can result at the discontinuity ofthese members. Navy-type details, though more expensive initially,should be less expensive in the long run, since failures are lesslikely to occur.Careful attention must be paid to the stress flow in aluminumdeckhouses. lliscontinuitiessuch as large openings and inadequateattachment to the steel hull will result in stressconcentrations which will produce structural failure even whenthe theoretical stress levels sre relatively low. The transferof loads between deckhouse and hull must receive special attention,particularly at the fore and aft ends of long deckhouses.All longitudinally effective structure in am aluminw hull ordeckhouse must be continuous through transverse structure.REPAIRSSeveral owners or operators have noted the dfificulty in obtaininghigh quality repairs of aluminum structures. In many cases, the repairwork does more harm than good, since bad welding applied to a crackedor highly stressed structure seldom remains sourd. The basic problemsseem to relate to the following factors:(a) Unqualified welders working in a bad environment.“(b) Lack of proper surface or edge preparation in way of welds.(c)Insufficient access for the welding gun.