Fatigue Analysis For Spherical Shell Of Manned Submersible

Manned submersible is able to carry one or more scientists while diving into deep-sea and play a vital role in biology studying,geological prospecting,lost-article searching,etc.As a critical component of manned submersible,spherical shell provides a safe space for both working personnel and equipment,thus its structure failure is not allowed.In this paper,fatigue analysis,which contains crack initiation and crack propagation fatigue life calculation,is carried out for titanium spherical shell.Fatigue reliability of spherical shell is also calculated and the influence of some parameters is discussed.Some conclusions are drew for design reference.Fatigue load spectrum of spherical shell is obtained by using statistics method to dive data of manned submersible which has been in survey for several decades.Results show that it could be described as Gumbel distribution which only depends on the depth of sea area where the manned submersible work at frequently.Crack initiation fatigue life of spherical shell is calculated based on local stress-strain method.Fatigue life assessment positions are determined by using FE analysis and tensile residual stress is considered.The influence of residual stress and fatigue strength reduce factor is discussed.Results show that the spherical shell meets requirement in crack initiation fatigue life.Both residual stress and fatigue strength reduce factor have a significant effect on fatigue life,thus should be determined reasonably.Spherical shell’s crack propagation fatigue life is calculated using unique curve model which could take negative stress ratio into account.Material parameters of titanium alloy are obtained by using statistics method.SIF of surface crack is calculated based on crack opening displacement method.The influence of dive depth distribution,SIF threshold range,initial crack size,critical crack size and residual stress are discussed.Results show that the spherical shell meets requirement in crack propagation fatigue life.Fatigue failure is more likely to occur at vice-viewport welded toe due to high compress stress.Fatigue reliability is analyzed based on unique curve model for spherical shell and the influence of design life coefficient to fatigue reliability is discussed.Target reliability index of spherical shell is set as 3.5 refers to reliability standards from related industries.Analysis results show that reliability index of the spherical shell is higher than target reliability index.The conclusions come from the analysis of influence of design life coefficient provide a reference for determination of spherical shell’s design life.