| As a kind of strategic emerging industry,remanufacturing can repair and improve the waste products by advanced technologies,which can increase the resource utilization rate and promote the resource saving and the environmental protection.The remanufacturing coatings prepared by advanced surface engineering technologies have great resistance of wear,corrosion and high temperature.Therefore,they have been widely applied to the high-end equipment fields such as aerospace,petrochemical,precision machine tool and metallurgical mine.The remanufacturing coating/substrate is a type of metastable multi-material system.The mechanical properties at the both sides of interface are very different,which can induce stress and crack at interface under the effect of complex service environment such as high temperature,high pressure,fatigue and heavy load.It is significant to conduct the interfacial damage detection and evaluation during equipment service process.As a kind of novel nondestructive testing method,spontaneous magnetic flux leakage(SMFL)testing can evaluate the early damage degree of components based on spontaneous magnetization phenomenon in ferromagnetic materials.However,the discontinuity and inhomogeneity of coating/substrate structure lead to the complicated stress field,the stress singularities and the large scale yield at crack tip.Therefore,it is difficult to describe the varations of SMFL at interface only based on classic magnetomechanical model.In this paper,we improve the Jiles stress-magnetization model based on the cohesive zone concept in elastic-plastic fracture mechanics to overcome this issue.Then,the cohesive zone-magnetomechanical coupling model is successfully established to facilitate the damage evaluation at coating interface.The physical mechanism of interfacial spontaneous magnetization is also revealed.And the SMFL evaluation of stress concentration and crack at interface is studied under three point bending loads and high temperature.Finally,the SMFL evaluation method for remanufacturing coating interface damage is established.The main research contents include the following five sections:(1)The cohesive zone model(CZM)from elastic-plastic fracture mechanics is used to describe the spontaneous magnetization phenomenon at damaged coating interface.The traction ? considered as intermediate variable is combined with Jiles stress-magnetization constitution relationship,and the fatigue damage evolution law in CZM is also introduced.Finally,the static and the fatigue cohesive zone-magnetomechanical coupling model is established,and its numerical calculation method of SMFL is provided.In order to verify this model,the crack initiation time of coating interface is predicted under fatigue tensile loads.The fatigue traction-separation relationship,the damage evolution and the magnetization accumulation process at interfacial pre-crack tip are obtained.And the theoretical magnetic field intensity threshold of interfacial crack initiation is consistent with the experimental result.Finally,the physical mechanism of spontaneous magnetization at interfacial cohesive zone is explained by microstructure observation.(2)Based on the beam theory in mechanical model,the general formula of coating interfacial stress is obtained.The beam theory-magnetomechanical coupling model is then established combined with the Jiles magentomechanical model.The distribution of stress and residual magnetization at coating interface is calculated under the static three point bending loads.And the magnetic field peak value at supporting seat is proposed to characterize the interfacial crack length.Based on the static cohesive zone-magnetomechanical coupling model established in this paper,the quantitative relationship between interfacial crack length and magnetic field characteristic value can also be obtained by ABAQUS finite element simulation.Finally,the interfacial magnetic signals are collected and analyzed by SMFL testing.It can be found that the experimental results are consistent with the theoretical calculations of this two models,and the cohesive zone-magnetomechanical coupling model can predict interfacial crack length more accurately.(3)Based on the fatigue cohesive zone-magnetomechanical coupling model,the interfacial stress distribution and initial separation under the fatigue three point bending loads can be obtained by ABAQUS finite element simulation.The fatigue traction-separation relationship,the damage evolution law and the magnetization accumulation process at interfacial pre-crack tip are calculated.Based on this,the interfacial fatigue crack initiation time and propagation behavior are predicted and a new magnetic characteristic value,namely magnetic field increasing rate dHax/dN,is proposed to describe the crack propagation rate.Finally,relevant experiments are conducted to verify the theoretical calculation results.It indicates that this model can provide guidances for SMFL evaluation of coating interfacial fatigue crack.(4)Considering the effect of high temperature on SMFL testing results at coating interface,the saturation magnetization Ms and mean field parameter a are modified in the classic J-A model.Then,the heat/force/magnetism coupling relationship for single homogeneous material is constructed,which can be also suitable for the coating interface.And the cohesive zone-magnetomechanical coupling model is modified for the high temperature environment.Based on the theory calculations and the tensile experiments conducted under the different temperatures,the variations of SMFL with stress and temperature are discussed.By observing microstructure at fracture,the effect mechanism of temperature on magnetic signals is explained from the perspective of magnetic domain movement blocked by inclusions.Finally,the interfacial SMFL signals under different high temperatures are modified based on the theoretical analyses and experimental results.It can help guarantee the evaluation accuracy of SMFL under high temperature environment.(5)In order to apply the cohesive zone-magnetomechanical model to the SMFL evaluation for remanufacturing coating interface,the application strategies of this model under various service environments are summarized.Then,the remanufacturing composite sliding plate is chosen as the application case of SMFL evaluation method.It is prepared by suitable process parameters of plasma arc surfacing technology,and its load form is simplified.The stress distrubtion and dangerous zone of composite sliding plate under three point bending loads are simulated by ABAQUS software.After that,the static analysis results are imported into FE-SAFE fatigue analysis soffware and the lifetime of composite sliding plate is obtained.Based on the simulation results,the interfacial fatigue crack initiation time and propagation length are predicted by the cohesive zone-magnetomechanical coupling model.The fixture of SMFL nondestructive testing is designed for remanufacturing cornpsite sliding plate accelerated fatigue test,and the experimental results are compared with theoretical predictions.Finally,the SMFL evaluation method is established for the interface damage of remanufacturing composite sliding plate. |
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