Stress Spectrum Statistic Analysis And Fatigue Life-Span Evaluation Of Grab Ship Unloader Steel Structure

Grab ship unloader is important equipment for unloading in port, play an important role in the shipping of the world. The use of grab unloader will be more and more especially after the quantity and throughput of port increase with the development of the domestic infrastructure construction. The failure of grab unloader mostly results by the fatigue failure of its structure under repeated stress, with the corrosion of seawater and salt wet air will certainly weaken the bearing capacity of the structure. Therefore, it is important to evaluate the life-span of the grab unloader scientifically and systematically and evaluating its life-span according to the real work condition is of great significance for its maintenance and even its design.At present, most evaluation inland take little consider of the effects of ship types, the grab points and quantity to stress spectrum of grab unloder. In view of this situation, the stress spectrum of ship unloader's danger point is worked out by numerical simulation, experimental test and statistical analysis of data, then evaluating the life-span of danger points. And this method could be referenced in some scale. The main content of the paper is as follows:1,Summarized the theory of fatigue life, and confirmed the reliability of safe life design method for life-span evaluation after analyzing some common fatigue analyzing methods in consider of the fatigue type of the grab. Considering the structural characteristics of tension link and girder confirmed the calculation method of P-S-N curve by nominal stress method. Selected miner linear accumulative damage model as the life span evaluation method according to the stress types of the grab. 2,The dynamic stress test points and their static stress value of these points has been determined after analyzing of the grab's static stress under some important work condition has been completed by ANSYS. Then tested the dynamic stress of those points online in consider of the effects of different grab points. Finally added the static stress to the dynamic stress to get the whole stress-time data.3,Discussed the influencing factor of the stress spectrum of the grab. after comparing some count models of Rain Flow Count Method, three peak and valley rain flow count model has been chose to statistically analyze the stress data in MATLAB to get the stress spectrum of each point in one circle, which can be used to extrapolate the whole year stress spectrum.4,Determined and corrected the P-S-N curves for both tension link and girder. Miner principle has been applied to evaluate the life span of the danger structure. At last advices for maintenance of the grab has been given according to the evaluation results and the real work condition.