Study On Surface Crack Propagation Behavior Of The Deep-sea Manned Spherical Hull

The spherical pressure hull of a deep-sea manned submersible is a multi-hole welding structure.In the process of submergence,the welded toes bear the combined action of alternating load of sea water and welding residual stress,which is easy to produce fatigue cracks and lead to structural failure finally.For fatigue life prediction and fracture endurance of spherical hulls,it is necessary to evaluate stress intensity factor distribution along the surface cracks fronts.It is of great significance to ensure the normal operation of equipment and the safety of personnel.The main contents and conclusions of this study are as follows:?1?In this work,the deep-sea manned spherical pressure hull is taken as the research object,and the ultimate submergence depth is 7000m.The finite element model of manned spherical hull is established.Through static analysis by ABAQUS,it is determined that the weld toes of hatch,main viewport and auxiliary viewport are dangerous.By inserting surface cracks at the weld toes,the finite element models of the spherical hull with cracks are established by FRANC3D,and the combined effects of seawater external pressure and welding residual stress were taken into account.The stress intensity factors K_I,K_II and K_III at each weld toe were computed based on M-integral.The results show that the cracks at each welding toe are mainly mode I cracks,and the value and distribution of K_I at different welding toes are basically the same.Compared with K_I,K_II and K_III have little effect on cracks.?2?The factors affecting the stress intensity factor of surface crack are analyzed.The influence of welding residual stress,crack depth a and crack depth to half-length ratio a/c on the K_I of surface crack is analyzed.The results show that the welding residual stress is the main factor affecting the K_I value,and the maximum value of K_I increases with the increase of residual stress.Therefore,reducing the tensile residual stress at the welding toe is an effective way to slow down the crack propagation and increase the fatigue life of spherical hulls.With the increase of crack depth a,the K_Ic of surface point also increases,and the K_Ia of deepest point first increases and then decreases.When a/c<0.8,the crack propagates preferentially in the depth direction,and when a/c>0.8,the crack propagates preferentially in the length direction.With the increase of a/c and constant depth of cracks,the K_Ic varies slightly,which indicates that K_Ic is insensitive to the change of crack length.?3?The interaction and influence law of multiple cracks are analyzed.The main crack and disturbing crack are inserted on the surface of the spherical hull by defining the location parameters???????.It is found that the disturbing crack at different positions strengthens or shields the stress intensity factor of the main crack.And the disturing coefficient?is defined to evaluate the influence of disturbing crack on the main crack.The stress intensity factors at the surface and deepest points of the main crack are analyzed through?to investigate the interaction of multiple cracks.?4?The propagation of surface crack on a spherical hull is simulated and analyzed.Fracture toughness test and crack growth rate test of titanium alloy are carried out to determine the parameters of crack growth model.The quasi-static crack propagation simulation of surface crack is carried out based on FRANC3D.The crack propagation path along length and depth and the change of stress intensity factor are analyzed.Based on the crack growth rate model,the service fatigue life of a manned spherical hull under extreme working condition is predicted,and the predicted results are 7302 times.