During the last decade, high strength aluminum alloys have been increasingly applied for the design and construction of high speed vessels; and over the same time the size of high speed vessels has grown and their operation has moved into increasingly harsher environments. Subsequently, the design and building process to ensure the structural safety of aluminum high speed vessels has become more complex in terms of strength or reliability analysis and fabrication quality control. The aim of the present paper is to obtain the relevant design database in terms of the statistics of initial imperfections that occur during welding fabrication of aluminum stiffened plate structures for marine applications, while it has been cognized that weld induced initial imperfections significantly affect (reduce) the load-carrying capacity. In the present study, a total of 78 single and multi-bay stiffened plate prototype aluminum structures which are full scale equivalent to sub-structures of an 80m long all aluminum high speed vessel are constructed by metal inert gas (MIG) welding. The material of plating and stiffeners is varied, using 5083-H116 (rolled), 5383-H116 (rolled), 5383-H112 (extruded) and 6082-T6 (extruded) aluminum alloys which are today the most popular types of aluminum material for marine applications. The types and dimensions of the structures are varied with different types of stiffeners (flat, built-up T-bar, extruded T-bar), stiffener web height, thickness of plating and stiffener, and principal panel dimensions, in addition to the number of frame bays. The statistics for six primary forms of weld induced initial imperfections, namely initial distortion of plating (between stiffeners), column type initial distortion of stiffeners, sideways initial distortion of stiffeners, residual stresses of plating, residual stresses of stiffener web and softening of the welding heat affected zones (HAZ) are obtained by direct measurements of the prototype structures. A statistical analysis of the measured database is performed to determine mean and coefficient of variation (COV) of each of the six above noted imperfection parameters based on presumed Weibull probability density functions. Three (slight, average and severe) levels for each of the six imperfection parameters are then determined and proposed for applications to limit state strength assessment for purposes of design of welded aluminum marine structures. The various pertinent details of the 78 test structures and the related measured imperfections for each structure are also documented at some length since these could constitute a good source of data for use by other investigators. The insights developed from the present study will also be very useful for reliability analyses and code calibrations of ultimate limit strength and fabrication quality for welded aluminum plate structures.
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