Numerical characterization of the knock-down factor on unstiffened cylindrical shells with initial geometric imperfections

With the evolution of composite materials and moreover of the manufacturing process of large composite structures, a new window of possibilities is opened from the optimization point of view. Currently, one has great materials and reliable manufacturing processes than can be used for extremely optimized structures. The problem is that even nowadays some calculation processes make use of design guidelines based on data from 50 years ago, which limits the optimization process due to outdated allowables and process tolerances, increasing the final cost of the structure and putting on the edge the reliability of the entire design process. Currently, imperfection sensitive shell structures prone to buckling are designed according to the NASA SP-8007 guideline, dating from 1968, using its conservative lower bound curve. In this guideline the structural behaviour of composite materials is not appropriately considered, since the imperfection sensitivity and the buckling load of shells made of such materials depend among other things on the layup design as well. In this context, a numerical investigation about the different methodologies to characterize the behaviour of imperfection sensitive composite structures subjected to compressive loads up to buckling is presented. A benchmark test is developed using a 500 mm diameter unstiffened composite cylindrical shell. A series of non-linear analyses considering geometric and thickness imperfection, obtained from real measurements, are carried-out to characterize the knock-down factor of the benchmark test. The effect of each type of imperfection on the knock-down factor is compared against the experimental results.