Research Of Non-destructive Indicating Of Structural Instability Load And Load Identification Method

In marine working condition, to ensure the seabed exploration, offshore oil production platforms and other production equipments to work normally has become an important research topic, which has led researchers at home and abroad on the great attention. The structures are working in the water, such as deep submergence vessels, marine working platform, whose structure is complex, whose volume is large and whose cost is so expensive. Because of the project needs, the depth of their work and the weight of the upper structure is increasing more and more, which resulted in the pressure load of the structure is increasing, too. Therefore, the possibility of structures whose buckling instability occurs will increase. In the early stage of structural design, how to know influence of pressure load on natural frequency and dynamic response of structures, how to forecast its instability load and after the completion of the structure, how to determine the structure of the instability load by non-destructive method for safety monitoring.From the basic equation of elastodynamics, the commonly theory of continuum mechanics is adopt. The precise solutions and the analytical solutions of the relationship ,which is between the square of horizontal natural frequency and pressure load of the beam by the axial pressure in different boundary conditions, have been discussed, including a simply supported beam at both ends by the axial pressure, a beam clamped at both ends by the axial pressure, a beam fixed on one end and simply supported on the other end by the axial pressure, a beam fixed at both ends with allowed angel by the axial pressure and a cantilever beam with lumped mass on free end by the axial pressure.In different boundary conditions, the approximate expressions between the square of horizontal natural frequency and pressure load of the beam by the axial pressure derive from the variational method. Analytical and numerical solutions, approximate solutions, the standardization finite element solutions and the standardization experimental results are analyzed and compared, then, the conclusion of linear relationship that is between the square of the horizontal natural frequency and axial force of a simply supported beam at both ends by the axial pressure is got. And the conclusions that approximate solution is linear relationship which is between the square of the horizontal natural frequency and axial force of a beam clamped at both ends by the axial pressure, a beam fixed on one end and simply supported on the other end by the axial pressure, a beam fixed at both ends with allowed angel by the axial pressure and a cantilever beam with concentrated mass on free end by the axial pressure, are obtained. By comparing, the difference between the approximate solutions and the analytical solutions is small, which can meet the requirements of accuracy on engineering. Therefore, the conclusion that approximative linear relationship can instead of precise non-linear relationship has been obtained.On this basis, before the beam has been damaged, as long as the natural frequencies of the structure at two load conditions are measured by experimental methods, elastic buckling load of the compressed beam can be identified. So the no damage test method to identify the elastic buckling load of the structures is given.