Research On Fatigue Damage Condition Monitoring And Evaluation Method Based On Piezomagnetic Field For Deep Sea Pipeline Steel

The deep sea pipeline suffers from sophisticated loading conditions,and it is inevitably affected by alternating fatigue loadings during the long-term services,which easily induce the fatigue failure.As the offshore oil and gas develope towards the deep sea and ocean,the fatigue problem has been the major problem.The traditional fatigue damage assessment methods based on the theory of fatigue damage accumulation and fatigue crack growth have certain limitations in the damage evaluation of deep sea pipelines.The evolution of fatigue damage of deep sea pipeline steel,from crack initiation to crack growth,is an accumulative process.With the accumulation of fatigue damage,the dislocations inside the materials develop,which has a greate iMPact on the magnetic domain structure and leads to the continuous change of magnetization at the damage zones.Based on the variation of piezomagnetic field under cyclic loading at the damage zones,a new damage detection method based on piezomagnetic field,can not only detect macroscopic defects on the surface of materials,but also characterize the process of fatigue damage accumulation,and provide early warning of early fatigue damage of deep sea pipeline.It overcomes the insufficiency of traditional fatigue damage evaluation methods to evaluate early damage and has great potential.The focus of this paper is to systematically carry out the experimental study on the correlation between fatigue damage accumulation and piezomagnetic field of deep sea pipeline steel X70 and X80.The evolution regularities of piezomagnetic field and fatigue damage accumulation are investigated.A new model between crack growth and piezomagnetic field obtained.The fatigue damage monitoring and fatigue life evaluation of deep sea pipeline based on piezomagnetic field is carried out.This paper aims to applying the damage detection method based on piezomagnetic field to fatigue damage evalution and life prediction of deep sea pipelines.The contents of this paper are as follows.(1)Uniaxial and cyclic tests were carried out and the regularity of piezomagnetic field variation during loading process was studied.The effect stress cycle,loading rate,loading rate history,stress history and fatigue loading history on the evolution of piezomagnetic field are investigated.Based on the theory of the interaction between domain and dislocation,as well as the related J-A models,test results are analyzed and discussed.The sensitivity of the piezomagnetic field detection signals to the stress and microstructure changes is investigated.The feasibility of the piezomagnetic field testing technique in characterizing the early damages of ferromagnetic materials,such as the stress state,the early plastic deformation and loading history is explored.(2)Uniaxial tests are carried out on defect X70 steel specimens and the evolution regularities of piezomagnetic field and the stress distribution on the surface of steel specimens are investigated.The quantitative relationship between the evaluation parameters of the piezomagnetic field and the defect area,together with the stress concentration factor is explored,and the stress concentration evaluation method based on the piezomagnetic field and the stress concentration factor is established.(3)Stress-controlled cyclic tests are carried out on X80 and X70 steel smooth specimens and the regularity of piezomagnetic field variation during loading process was studied.The effect of ratchet plastic accumulation and the history of ratchet loading on the evolution of the piezomagnetic field is analyzed.The evolution regularities of the piezomagnetic field and the hysteresis loop during the ratchet failure process is studied.The relationship between the ratchet strain rate and the piezomagnetic field change rate under different stress amplitudes is investigated,and the corresponding relationship between the magnetic field change rate and the fatigue life is established.(4)Cyclic tests are carried out on X80?Q235?U71Mn and GCr 15 steel smooth specimens.The effect of loading times?stress amplitude and chemical component on the evolution of the piezomagnetic field is analyzed.The corresponding relationship between piezomagnetic characteristic parameters and fatigue accumulation is investigated.Finding suitable piezomagnetic characteristic parameters to characterize the fatigue damage accumulation process.(5)Asymmetrical stress-controlled cyclic tests are carried out on X70 steel defect specimens.The cumulative evolution trend of piezomagnetic field and fatigue damage at stress concentration position and non-stress concentration position under the cyclic loading is investigated.The evolution of piezomagnetic field from the appearance of macroscopic cracks to the fracture process of specimens is explored.(6)Asymmetrical stress-controlled cyclic tests are carried out on X70 steel coMPact tension specimens.The evolution regularities of the piezomagnetic field and crack length under cyclic stress is studied.The corresponding relationship between crack growth rate,?stress intensity factor range and piezomagnetic field characteristic parameters in crack propagation low velocity stage?crack propagation stable velocity stage and crack propagation high velocity stage is established.