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.
Buckling and postcritical analysis of prismatic metal bars, including complex cycles of loading are considered in the paper. The postbuckling path analysis of bars loaded in the form of increasing displacements is discussed in detail. The results of experimental research are reported as a dependence of the force P versus lateral deflection f as well as total shortening of the rod u. It is stated that only the simultaneous analysis of both methods of presentation of results comprises a complete tool for developing the column response. Particularly, the response that appears in the first, linear elastic phase of loading can be observed on P-u diagrams. The increase of the compressing force causes visible shortening of the bar without its buckling. This important phenomenon is not usually included in stability considerations of prismatic rods.
METODY OCHRONY MOSTÓW RUROCIĄGOWYCH PRZED SKUTKAMI UDERZENIA HYDRAULICZNEGOW artykule omówiono wybrane metody zabezpieczenia mostów rurociągowych przed skutkami uderzenia hydraulicznego. Wskazano na przyczyny i możliwe zniszczenia spowodowane uderzeniem hydraulicznym. Zamieszczono przegląd aktualnie stosowanych metod ochrony przed uderzeniem hydraulicznym, wykonano porównanie efektywności wybranych systemów ochrony przed uderzeniem hydraulicznym. Zwrócono uwagę na fakt, że częstości drgań własnych mostów rurociągowych mogą odpowiadać częstościom zmian ciśnienia fali uderzenia hydraulicznego i być powodem rezonansu.
Experimental research on mechanical properties of hardened polyethylene HDPE, is developed in the paper. The conditions of hydraulic impact simulation, caused by sudden opening or closing of the valve or by working pomp were adopted in the model. The created in such conditions shock wave moves at a high speed causing additional dangerous dynamic loadings, which lead to faster pipe wear process. The aim of this study is to determine Young's modulus of the pipe material in the cyclic load conditions. The assumed amplitude and frequency of the applied load relates to variation of the impact wave pressure also the speed of the disturbances propagation are taken from experimental measurement of the real water hammer. The measured Young's modulus is higher than that obtained from a static tensile test. The presented study arises from the need to verify the actual value of pipe material mechanical properties, i.e. longitudinal stiffness, for designing of hydraulic pipes under conditions of water hammer.
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