| The river-sea direct transportation has unique advantages such as fewer transfer links,lower cargo damage and shortage,which is an indispensable part of the Yangtze River's "golden waterway" construction and "One Belt and One Road" strategy.As the carrier of river-sea coordinated transport,River-Sea-Going ship has two different routes between river section and sea section during its service period.As the sea route gradually extends to Japan,Korea and Taiwan area in the future,the fatigue problem of the River-Sea-Going ship will need more attention.While the fatigue assessment can not directly refer to sea ship specifications due to the special characteristics of two different route loads.Therefore,it is imperative to study the fatigue performance and propose a set of methods for evaluating the fatigue strength of River-Sea-Going ship.In this paper,by analyzing the load characteristics of river-sea route,the fatigue damage law of River-Sea-Going ship under two-step alternating load was discussed based on the model test and cumulative damage theory,and the reasonable suggestions for fatigue assessment of typical joints were given.The main research contents and conclusions are as follows:(1)Based on the analysis of joint form and route load characteristics of River-SeaGoing ship,a series of notched specimens fatigue tests under two-step alternating load were carried out considering the influence of river-sea load effect.The Miner criterion and S-N curve correction method were used to analyze the damage effect of small loads below fatigue limit.A new non-linear cumulative damage theory was proposed to evaluate the effect of load interaction.The fatigue damage evolution law of notched specimens under two-step alternating load was explained from the material science based on fracture and XRD analysis techniques.(2)Taking the 930 TEU River-Sea-Going ship as the research object,fatigue tests of longitudinal penetrating joint under constant and two-step alternating loads were carried out.Based on the static tests and finite element simulation results,the stress distribution at the end of weld toe was analyzed and the fatigue hot spot location was determined.The hot spot stress values under different load conditions were obtained by linear interpolation method.The fatigue failure mode of joint and the stages of crack propagation were observed,and the differences of crack growth rates under different loading conditions were compared and analyzed.The effect of small load on the fatigue performance of ship joints was discussed based on fatigue life,cumulative damage theory and fracture analysis methods.(3)The comparative analysis of fatigue performance between notched specimens and typical joints under two-step alternating load shows that the fatigue strength of River-Sea-Going ship should comprehensively consider the fatigue damage caused by river load,sea load and the load interaction effect.Based on the S-N curve method and referring to the relevant specifications of CCS and IACS,the detailed suggestions on the selection of P-S-N curve and the calculation of load and stress range have been gave.The cumulative damage degree of joints under different routes and load conditions is calculated by introducing the route distribution history coefficient and load interaction factor,which points out the direction for formulating fatigue strength criteria of River-Sea-Going ship's typical joints under different routes. |
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