Equivalent Imperfection Amplitude Concept for Probabilistic Design of Buckling Sensitive Composite Cylinders

Abstract

Traditional design of aerospace vehicles structures against buckling is based on the computation of bifurcation loads of perfect shells, opportunely scaled by safety factors and by experimentally based knock-down factors (KDF) which cover the effect of geometrical & loading imperfections as well as pre-buckling non-linearity. KDF for composite cylinders 1 date back to a long time ago, they don’t take into account composite manufacturing specificities and they aren’t based on extensive testing 2 . More important, the comparison with experimental results shows that they are over-conservative. In this paper an original methodology is set-up in order to obtain a new, less conservative, reliability based KDF for composite cylinders under axial compression. The methodology is based on the “equivalent imperfection amplitude” (EIA) concept and has been developed making an analogy with current damage tolerance practices used in aeronautics to compute the “equivalent initial flaw size” (EIFS). In practice, in a first step, a collection of all available literature data upon experiments of composite cylinders under axial compression is performed and in a second step the EIA leading to the collapse load registered during experiments is computed with the help of non linear analysis of imperfect shells. By this way, tests performed on cylinders with different sizes and different imperfection sensitivities are put together, leading to a large population representative of different manufacturing abilities. Finally the statistical distribution of EIA is used to obtain reliability based KDF with Montecarlo simulations and imperfection sensitivity curves for different stacking sequences of industrial interest.