Research Of Defect Detection Technology Based On Magnetic Memory

There are many safety risks with the pipelines in use,such as corrosion,perforation,buckling,cracking and so on.In order to prevent the terrible accidents,it is necessary to inspect the status of the pipeline regularly.At present,the main technology adopted in pipeline inspection is magnetic flux leakage testing(MFL).However,the MFL devices are usually too bulky as its need of magnetization devices,which means the pipelines are easily to be blocked by MFL devices.Furthermore,MFL devices are not able to evaluate the stress state of the pipelines.To solve these problems,it is necessary to develop new pipeline inspection technologies.Magnetic Memory Testing(MMT)provides a new approach for pipeline inspection due to its simplicity,non-contact and predictability.The research about MMT in this paper is carried out in the following three aspects:(1)The research of the principle of MMT based on magnetomechanical effect is conducted.First of all,basic ferromagnetism knowledge including spontaneous magnetization,magnetic domains,magnetic domain wall,magnetization mechanism,magnetic hysteresis loop,J-A magnetization model are studied.Secondly,the relationship between the magnetostriction coefficient and stress is explored.Based on all the above,the effective field model and the J-A stress-magnetic coupling model(J-A model)are established.Finally,the influence of the parameters in the models on the stress-magnetization curve is analyzed.(2)Three experiments including tensile test,compression test and cycling tensile test are designed to analyze the relation between stress and magnetic field qualitatively.And then the parameters of J-A model are determined based on the experimental results.(3)The J-A model is applied to the analysis of the bending of the plate and the internal defect detection of the pipelines.Firstly,the simulation of plate bending is performed in ANSYS.Then,theoretical and experimental analyses are both conducted.The comparison of simulation and experimental results shows that the magnetization of ferromagnetic material under complex stress can be well analyzed by using J-A model in three directions separately.Finally,the variation of the magnetic memory signals of pipeline defects is discussed under different internal pipeline pressure.The result shows that magnetic memory signals of internal defects have two characteristics— radial magnetic field in the pipeline mutating from minimum to maximum,while axial magnetic field has its maximum;the magnetic field inside the pipeline decreases as the pressure increases,defect's magnetic memory signals decrease and then increase.