Fatigue Strength Analysis Of Ship Hull Structures With Combined Bending Stress Under Nonlinear Wave Loads

With the outstanding of the question of ship hull structures' fatigue strength, the fatigue failure that is one of the main failure modes of ship hull structures has been one of the most vital research fields. The major purpose of this thesis is to do some research work on calculation of fatigue damage of ship hull structures with combined bending stress under nonlinear wave loads. For this reason, the following problems are emphatically investigated.Firstly, the slice theory is adopted to calculate linear wave loads (including bending moment of section and hydrodynamic pressure) that act on ship hull structures, short-term distribution of each wave induced total stress (including total bending stress and local bending stress) range got according to the spectrum analysis method, and S - N curve together with the theory of linear accumulated damage is applied to analysis the fatigue damage of ship hull structures with combined bending stress under linear wave loads. On the basis of above, two simplified methods, one is to introduce an equivalent regular wave for each work condition, the other is to introduce an overall equivalent regular wave of long-term in the life time, are proposed in order to simplify the calculation of fatigue damage of ship hull longitudinal structures with combined bending stress under linear wave loads.Secondly, nonlinear wave loads is calculated in time domain during overall longitudinal bending of ship hull, then stress time history with combined bending stress is obtained considering local bending of ship hull. The joint probability density of the stress range and mean stress is got by means of rain-flow cycle counting and mathematical statistics. On terms that damage is equivalent, long-term distribution mode of stress range is found. With the influence of mean stress being considered, the fatigue damage of ship hull structures with combined bending stress under nonlinear wave loads is evaluated.Furthermore, for simplifying the calculation of fatigue damage of ship hull longitudinal structures with combined bending stress under nonlinear wave loads, a simplified method based on the concept of equivalent regular wave is proposed in this thesis. On the premise of damage equivalence, the accumulated fatiguedamage is calculated in regular wave instead of irregular one. On the basis of the simplified method, a regression expression of nonlinear overall longitudinal bending stress range long-term Weibull distribution's shape parameter is presented, and a utility method is proposed.Finally, statistic value and reliability of fatigue life of ship hull structures with combined bending stress under nonlinear and linear wave loads is studied by using structure reliability theory. Maximum permissible stress range and maximum permissible accumulate damage of ship hull structures in service life is investigated also.