A cantilever column carrying a compressive follower force and propped by a spring can become dynamically unstable by flutter or divergence depending upon spring stiffness. In this study, issues involved in the control of such a column by means of piezoelectric patches at the top and bottom surfaces of the column are studied. Attention is focused on negative feedback control with patch voltages proportional to the locally sensed bending strain rates. It is found such control is capable of significantly enhancing the critical load in the flutter range, such enhancement being limited often only by the limits on voltage developed across the patch of given thickness. In the range of spring stiffness corresponding to buckling failure, this control strategy is simply ineffective. A relatively light spring not only enhances the critical force significantly but also makes the control more effective. For prescribed limits on voltage across the patch, there is an optimal spring stiffness that results in the maximum of critical load and this lies in the flutter range of the spring stiffness. Softening nonlinearity reduces the critical loads in the range of transition from flutter to divergence and rounds off the sharpness of the drop in this range seen in the linear case. A partial patch spanning over half the length of the column from the fixed end is significantly more effective than a full patch in the flutter range. Nomenclature b = b P , width of the column section (width of piezoelectric patch) E = Young's modulus of the material of the host beam E P = Young's modulus of the piezoelectric material e P = piezoelectric constant H = depth of the column section G = control gain K 1 , K 3 = spring constants I = second moment of the area about the weaker axis L = length of the beam m = mass per unit length of the column N = number of harmonics in the Fourier series description P = axial (compressive) load P E = Euler buckling load of a column with pinned ends, 2 EI=L 2 P cr = critical load as determined from linear stability analysis P max = maximum load attained by the column for a give gain q o = uniformly distributed lateral load per unit length V = voltage across the piezoelectric patch V m = mth coefficient in the Fourier series representation of V v = lateral displacement of the column v m = mth coefficient in the Fourier series representation of v v m= v m =L 1 , 3 = nondimensional linear and nonlinear spring stiffnesses, respectively

Compressive behavior of two classes of “sandwich” structures is investigated. These structures have for their principal load bearing components two relatively stiff parallel horizontal sheets which are interconnected in one of the following ways: (i) by a highly compliant core material such as foam, or (ii) a set of discrete stiffeners connecting the parallel (top and bottom) sheets. In case (i), the structure can buckle in either a local mode in which the core and the facing bend together or a wrinkling mode in which the facing sheet undergoes severe bending with the core subjected to deformation in the transverse plane. It is found that these plates have neither post buckling stiffness nor do they exhibit any imperfection-sensitivity. In case (ii) the point of principal interest is the interaction of local and overall buckling. For the case of coincident local and overall buckling, it is found that 30% reduction in the maximum load can occur for modest levels of imperfections as a result of modal interaction.