Study On Material Properties Of Steel With Marine Pitting And Ultimate Strength Of Damaged Structures

The marine environment is a kind of complex corrosive environment.Steel structures in such a corrosive environment for a long time are prone to corrosion damage due to chemical or electrochemical reactions of materials.Pitting corrosion is one of most harmful corrosion type whose randomness makes it difficult to predict the ultimate strength reduction of a damaged structure.The current research indicates that there is a great randomness in the depth,diameter and ratio of diameter to depth of the pits,but in regard to the pit shape it can be simplified to be cylindrical,semi-ellipsoidal and conical.As for ultimate strength reduction caused by pitting damage,most of the current studies only consider the influence of regularly or irregularly distributed pitting corrosion damage,without considering the comprehensive randomness of pitting corrosion.Especially,the pit shape is considered to have no effect on the ultimate strength reduction.In this thesis,the influence of pit shape on the properties of corroded steel was studied based on regular pitting corrosion,and then on random pitting corrosion.Material constitutive models were established respectively for the steels subject to the regular and random pitting corrosion,which were used to evaluate the ultimate strength of pitted steel structures,taking the place of modelling the random pitting corrosion.The main work is as follows:(1)The Q345 steel tensile specimens were designed,which were introduced with pits of different shapes in regular distributions using mechanical drilling method,and tensile tests were carried out.Meanwhile,the numerical models of the tensile specimens were established through a validation on their accuracy.Results showed that the pit shape has a critical effect on the material properties of the specimen,and its influence can be represented by the ratio of pit diameter to pit depth.The larger the ratio of diameter to depth,the lower the yield strength and tensile strength of a pitted specimen.The material degradation of pitted steel was determined by the ratio of volume loss(DOV)of corroded material and the ratio of diameter to depth(d/h).Finally,a two-parameter equivalent material constitutive model of pitted steel was established for the regular pitting corrosion.(2)The work similar to that done on the Q345 steel tensile specimens was carried out on the Q235 steel tensile specimens.The results showed that the degradation law of tensile properties of different types of steel is almost the same in view of the effect of pit shape.The equivalent material constitutive model correlated with DOV and d/h as two key parameters was also established for the Q235 steel.The comparison of the equivalent material models of the two types of steels shows that there is no obvious difference in their adverse effects on the tensile properties,manifesting a unity between different steels.(3)A modeling method was proposed considering the randomness of the pit depth,size,shape and pitting distribution simultaneously.A large number of numerical models were then established to study the effect of the distance and distribution location(denoted by the angle along axial load direction)of adjacent pits on material degradation in tensile specimens.The results showed that the larger the angle between pits and the smaller distance between pits,the greater the degradation degree of tensile properties of the specimen.In addition,multiple intersecting pits could be modeled as a single pit whose diameter was the projection length of multiple pit openings on the cross section of the sample,and whose depth was the average depth of those intersecting pits.The results of a large number of numerical simulations showed that the material property degradation of randomly pitted steel is also determined by the DOV and the equivalent d/h,and the latter is determined by the mean of the diameter and depth of all the random pits.Finally,the equivalent material model of random pitted steel was proposed.(4)The ultimate strength of pitted steel structure was predicted using the equivalent material model of regular or random pitted steel in lieu of modelling complex pitting corrosion.The comparision between the prediction and existing tests showed that the equivalent material model of regular pitting corrosion has a good accuracy in predicting the ultimate strength of pitted structure with regular corrosion pits.However,the equivalent material model of random pitted steel seems to not present a satisfied prediction on the ultimate strength of pitted plates subject to random corrosion pits,unless further treatments was performed on such a material model.This was achieved by introducing a reduction factor in the material model which is closely realted to the DOV.The modified material model was then used for randomly pitted plates with different aspect ratios,slenderness ratios and initial geometric defects,and stiffened panels,showing good accuracies to replace modelling of random pitting corrosion.