Geometric Imperfection Effects in an Elastically Deployable Isogrid Column

Abstract

The experimental study and analysis of a novel gossamer structural component is described. The component is a 3-m-long thin-walled isogrid column prototype that may be elastically stowed and deployed. The column has a diameter of 0.318 m, with a linear density of 46 g/m. The static and dynamic mechanical responses of the deployed prototype are examined and compared to an idealized model. These comparisons indicate global stiffness ranging from 11 to 28% and buckling strengths from 7 to 37% of the theoretically ideal performance. Initial local curvatures approaching the thickness of the isogrid ribs are found to be the primary source of this performance reduction. A modeling approach based on the postbuckled response of individual isogrid ribs is proposed for predicting the global effects of local imperfections, and good agreement with the test results is found. In general, initial rib curvatures greater than 10% of the rib thickness are predicted to result in signi cant degradations of the global structural performance. NomenclatureA = cross-sectional area, m 2 b = isogrid rib width, m c = compliance, m/N E = Young's modulus, Pa f = vibration frequency, rad/s h = isogrid rib spacing, m I = minimum rectangular moment of inertia, m 4 J = rotational inertia, kg ¢ m 2 k = stiffness, N/m (or Nm/rad) L = length, m P = axial load, N q = normalized axial load r = column radius, m t = column/rib thickness, m°= knockdown factor ± = axial de ection, m " = imperfection amplitude, m ¾ = stress, N/m 2 Á = thin-walled cylinder exponent Subscripts axial = axial properties bending = bending properties cr = critical load eff = effective properties ideal = ideal properties rib = individual rib properties

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