Study On The Fatigue Strength Of Ship Structure Based On Crack Propagation Theory

Fatigue damage is one of the main failure modes of the ship and offshore structures, the main method for the fatigue strength check of ship structures are the simplified procedures by the classification societies, which is based on S-N curves and Miner linear cumulative damage rules. Based on the crack propagation theory, the crack propagation in ship structures under random waves is studied in this paper, with the hope of providing basis to further fatigue check of ship structures. The contents completed in this paper are as follows:(1) The calculation of stress intensity factor. The calculation methods of stress intensity factor such as empirical formula method, weight function method and finite element method are compared and analyzed. The weight function method is chosen after the comparison.(2) The simulation of irregular waves. The stress history of the hull structure in random sea conditions is obtained by simulating irregular waves.(3) The solution of effective stress intensity factor. Considering the crack closure, Newman model and Willenberg model are adopted to calculate the crack effective stress intensity factor.(4) The solution of equivalent stress. The crack propagation is considered by equivalent the varying amplitude loads to constant amplitude loads when ships voyage in the sea.(5) The study of fracture criterion. The fracture criterion is determined by combining the actual type of the ship structure, location, materials, effect of force as well as the design parameters.(6) The comparison of crack propagation data. The fatigue crack growth data which are obtained by whether the crack closure is taken into account or not during the crack propagation and by utilizing the equivalent stress method are compared. And a particular process of calculation is presented.