Study On Mechanical Properties Of Offshore Platform Steel Considering Pitting Damage

The special temperature,humidity and salinity in the marine environment can easily lead to pitting corrosion of the offshore platform,and the presence of pitting damage will lead to the deterioration of the mechanical properties of the steel.In order to study the effect of pitting damage on the mechanical properties of high strength steel.According to Faraday's laws of electrolysis,the electrochemical accelerated corrosion test was carried out,and the NV-D36 steel pitting specimens with different damage degree were prepared,and monotonic loading tests were carried out,then,the influence of pitting depth and section loss rate on the elongation,ultimate strength and yield platform of the steel was analyzed.The yield platform disappears gradually with the increase of the ultimate strength and elongation of the pitting depth and section loss rate.The quantitative degradation relation of nominal limit strength and elongation rate with section loss rate is established.Based on the study of basic mechanical properties,the finite element analysis of steel specimens with pitting damage was carried out,the stress distributions of steel specimens with different damage degree were compared under tensile state,and the variation of stress distribution around the pitting damage caused by pitting corrosion was obtained.In addition,the cyclic loading performance of NV-D36 high strength structural steels is studied in the light of various cyclic loading systems.The one-dimensional stress-strain curve of steel was obtained by monotonic loading and cyclic loading,and its monotonic properties,hysteretic properties and ductility properties were studied.Based on the Ramberg-osgood model,the cyclic skeleton curve is fitted,the one-dimensional stress-strain relationship skeleton curve of NV-D36 high strength steel under cyclic loading is obtained,and the symmetrical cyclic loading and monotonic loading curves are compared.It is concluded that the constitutive relationship of steel under cyclic loading is different from that under monotonic loading.The cyclic loading causes the material accumulative damage to make its ductility become worse,the steel structure relation under the monotonic load is used to calculate the response of components or structures under cyclic loading.NV-D36 steel has good energy dissipation ability,but its energy dissipation coefficient change process has a big difference.The energy dissipation capacity of steel is related to its initial loading state and cyclic loading amplitude,and in a certain cyclic loading strain range,the initial loading state is in the elastic phase of steel with good energy dissipation capacity,the larger the cyclic loading amplitude,the more fully utilized the energy dissipation ability of steel.By using the plastic constitutive model of chaboche steel,the combined hardening parameters of high strength steel are calibrated by the test data,and the numerical simulation results are in good fitness with the experimental results.In the same way,the influence of pitting damage on the mechanical properties of cyclic loading of steel is analyzed,and it is shown that the existence of pitting damage will not only lead to the degeneration of the mechanical properties of steel's monotonic loading,but also the degradation of mechanical properties of steel cyclic loading.