Experimental and numerical study on the influence of imperfections on the buckling load of unstiffened CFRP shells

Due to a lack of statistical data, unstiffened cylindrical lightweight structures are designed following a knockdown factor (KDF) approach described in the NASA SP-8007 (Weingarten and Seide, 1965). This approach aims at a lower-bound, conservative design. The basis for the proposed KDFs are a number of tests with different, partially unknown boundary conditions and imperfection patterns for metallic cylinders that do not resemble the imperfection patterns achieved by state-of-the-art manufacturing methods for cylinders made of carbon fibre reinforced plastics (CFRP). Within this work, two CFRP tubes are manufactured using the filament winding method and are cut into a total of 12 cylinders from which optical measurements are taken to analyse characteristics of the corresponding imperfection patterns. 11 cylinders are tested in axial compression until buckling occurs and tests are accompanied by high speed displacement measurements, thermography and measurements of occurring load imperfections. Four different FE-models, which differ in the parameters considered, are set up for prediction of the buckling load of each cylinder. Model response showed the highest sensitivity to load imperfections. Negligence of this parameter lead to a 15% increase of the corresponding model bias while geometric imperfections made up for only 3%.