| Since the 21 st century,the development of deep-sea manned submersibles,especially the development of full-sea deep(11,000-meter-class)manned submersibles,has triggered a new round of technological competition.In the working process of the manned submersible,it will inevitably go through the process of multiple dives-work-floats,so the pressure hull of the deep submersible will also go through the cycle of loading-holding-unloading.In addition,the depth of the deep-sea manned submersible’s diving depth and underwater operation time will vary with each mission.To ensure the safety of the submersible in service,it is necessary to reasonably calculate the fatigue life of the pressure hull structure under such complex loads.The method based on fracture mechanics has increasingly demonstrated its superiority in the fatigue life prediction of metal structures.The basis of its evaluation is a reasonable and reliable crack growth rate model.This paper is based on a small time scale crack growth model describing the cyclic loading-unloading process(crack growth rate is described by da/dt),and the model is improved based on the analysis of creep and overload plastic zone changes and some experimental phenomena.The latter model has good applicability under constant-amplitude load,variable-amplitude load and holding load.Considering the material properties as random variables,the reliability of crack growth rate under different lengths is analyzed.Finally,the life calculation process of the pressure hull of the large-depth manned submersible is given by the improved small-time scale load-holding fatigue crack growth rate model.The research content and main results of this paper are as follows:(1)The application of fatigue crack growth model,load fatigue crack growth model and reliability theory in fatigue life under constant amplitude load and variable amplitude load are reviewed and analyzed,in order to study the small time domain load fatigue crack growth model Provide ideas and directions;(2)The stress state of the crack tip is not always a plane stress or a plane strain state,but a state between the two.This paper considers the influence of the crack tip stress and strain state on the size of the crack tip plastic zone and the crack opening displacement,and introduces a constraint factor to make the crack opening displacement more in line with the actual situation;(3)The prediction ability of the crack growth model in the small time scale describing the cyclic loading-unloading process on the crack growth rate under constant amplitude loading conditions is analyzed,and the expression describing the crack opening angle proposed in the model is modified,and the crack is proposed.The expressions related to the opening angle and the stress ratio,fracture toughness,and the threshold value of the stress intensity factor improve the agreement between the model and the experimental data under constant load conditions;(4)In order to improve the prediction accuracy of crack growth rate under long-term holding load conditions,this paper proposes that long-term holding load and large stress intensity factor will cause greater damage to the material to accelerate the crack growth rate and cannot be ignored.The parameters related to the holding time and the stress intensity factor describe the effect of material damage on the crack growth rate,which is verified by comparison with experimental data;(5)On the basis of the improved small time domain load-holding fatigue crack growth model,considering the randomness of some material properties,using Monte Carlo method,first second moment method and second second moment method,the model is analyzed in different crack lengths reliability of crack growth rate;(6)Based on the modified small-time scale load-holding fatigue crack growth model,taking the calculation of the pressure hull life of a manned submersible at a large depth as an example,the determination of the initial crack,the determination of material parameters,the loading stage and the load-holding process in the calculation process are introduced.The method of determining the stage parameters and the method of determining the load spectrum.Finally,the calculation process was demonstrated by separately calculating the fatigue life of the pressure hull considering only the random load process of descending and floating and the holding fatigue life of the pressure hull considering the random load process of descending-working-ascending.It is necessary to maintain the load process to reasonably predict the fatigue life of the pressure shell. |
