Institute of Fundamental Technological Research

The buckling and post-buckling behaviour of prismatic aluminium columns from stocky to very slender shapes is investigated. The unconventional, in terms of buckling tests, displacement control of compressive load and a series of loadings provided an enhanced insight into the buckling process. A phenomenon of buckling load drop has been detected in columns of intermediate slenderness, reaching over 20% of the load early critical value. This newly observed occurrence resembles finite disturbance instability, which until recently was commonly believed to only appear in cases of thin walled cylindrical shells, but not columns. The observation is in contradiction to predicted results from the elasto-plastic buckling models of Engesser or Shanley, with constant or growing values of load during the post-buckling process. Further tests on columns of intermediate slenderness, with strain gauges glued at node and anti-node locations of the buckled profiles, revealed that even minute buckling results in fields of highly non-symmetric residual microplastic strain. The results of the present study indicate that running column buckling tests under displacement control is worthy of being adopted as common practice. The envelope of column post-buckling states can be conveniently determined. This information will in turn allow for the quick and reliable estimation of the safety of a column, which has undergone accidental or deliberate damage in the form of limited buckling when under operational load.

Aluminium column instability, Finite disturbance buckling, Buckling load drop, Post-buckling, Micro plastic strains, Non-symmetric residual stresses, Column design criteria

The buckling phenomenon of prismatic column is experimentally investigated in this work, being major structural design concern. A number of tests were performed on aluminum made prismatic columns with slenderness index λ_ef remaining within range of 15 to 460 submitted to programs of multiple in time compressive loads – force and displacement controlled. The experimental evidence obtained here for stocky and slender columns confirm already well known information that column failure results from structural material plastic flow deformation or lateral buckling, respectively. A new, interesting observation has been made for intermediate slenderness columns that “drop” of buckling load takes place (reaching in some cases over 20% of early critical force value) after the very initiation of elasto-plastic buckling process. The observation is in contradiction to adopted in elasto-plastic buckling models assumption of constant (Engesser model) or growing (Shanley model) buckling load during buckling process. The obtained experimental evidence indicates that mysterious scatter in buckling loads for apparently identical columns with intermediate slenderness index can be attributed to minute residual stresses, which easily and in hardly controlled manner can be introduced during industrial production processes.

Maximization of the critical force of cracked columns, subjected to generalized follower force is discussed in the paper. The crack is assumed to be formed according to the opening and sliding modes and is modeled by a localized loss of stiffness. Influence of the crack stiffness and its localization on the value of the critical force is analyzed. The optimization process is based on the multimodal approach. The localization of crack with the critical force of the system equal to 137.17 EJ/L2 is found

STABILITY CONSTRAINTS, OPTIMIZATION OF CRACKED COLUMNS, FOLLOWER LOAD, localization of crack

Maximization of the critical force of cracked columns, subjected to generalized follower force is discussed in the paper. The crack is assumed to be formed according to the opening and sliding modes and is modeled by a localized loss of stiffness. Influence of the crack stiffness and its localization on the value of the critical force is analyzed. The optimization process is based on the multimodal approach.

localization of crackoptimization of cracked columns, stability constraints, follower load,