| The deep submersible is an important vehicle for humans to move into the deep sea,explore the deep sea,and develop and utilize deep-sea resources.The pressure hull is a key part of the deep submersible,and its comprehensive performance has a direct effect on the safety and reliability of deep submersible during operation.As the deep submersible has to frequently float and dive during its service,the pressure hull is subjected to the cycle loading of ultra-high hydrostatic pressure.Besides,the pressure hull is a complicated welded structure,which inevitably has welded defects and welding residual stress due to the lim itation of welding conditions.And the fatigue cracks are easily induced in the weld toe because of stress concentration.When the structure is subjected to the cyclic loading,t hese defect positions are prone to crack propagation and lead to damage.Thus,the low-cycle fatigue damage of the pressure hull for the deep submersible cannot be ignored.As some of the traditional low-cycle fatigue analysis methods have poor prediction accuracy or some methods cannot simultaneously predict the total life of fatigue crack initiation and crack propagation,this thesis aims to establish a low-cycle fatigue analysis method based on the continuum damage mechanics,which could consider the fatigue failure for the whole life of the deep submersible structure.According to the mechanical characteristic of deep submersible in high pressure,this method could consider the c rack closure effect under the compressive stress state,and also predict the crack initiation and propagation life of the structure.First,the cyclic stress-strain behavior of titanium alloy will be simulated based on the combined hardening model of Isotropic hardening model and Non-linear kinematic hardening model.Then,a new Lemaitre fatigue model considering the crack closure effect is established based on the continuum damage mechanics and the second law of thermodynamics.In this thesis,the room temperature tensile tests and low-cycle fatigue performance tests of Ti80 alloy material were carried out in order to obtain material parameters of fatigue damage accumulation model and research the low-cycle fatigue characteristics of Ti80 alloy.According to the experimental results,it can be found that the peak stress and valley stress decrease with the increasing number of cycles,which demonstrates that the Ti80 alloy is a cyclic softening material.The results also shows that the peak stress and valley stress decrease at different rates,which reveals that the damage of material is different under tensile or compressive stress states.Next,a numerical simulation method of the low-cycle fatigue full life prediction for Ti80 alloy is established by applying the secondary development of ABAQUS finite element software.Further,the fatigue crack initiation and propagation experiments of the notched CT specimens are carried out.The numerical simulation results were compared with the experimental values,whic h preliminarily verify the accuracy of the numerical method.This thesis also researches the fatigue crack growth characteristics of the notched CT specimen under tension-tension cyclic loading and compression-compression cyclic loading by using this numerical simulation method.The result shows that the fatigue crack growth rate of the notched CT specimen under compression is much lower than the tensile cyclic load.Finally,taking a single spherical pressure hull of a manned deep submersible as an example,the thesis establishes an analysis method for the low-cycle fatigue full life prediction of the pressure hull.This method can provide theoretical basis and technical support for the design of the pressure hull of deep submersible. |
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