| Ferromagnetic metals are widely used in the fields of aerospace,railroad and transportation,and their complex working conditions make the crack expansion of ferromagnetic components eventually lead to damage.When the crack is in steady-state expansion,the expansion rate is slow or even stops;while when the crack evolves from steadystate expansion to unstable expansion,penetrating fracture will occur soon,resulting in complete loss of institutional working ability and possibly causing significant economic loss or casualties.To address the problems of difficulty in obtaining traditional fracture mechanics parameters of crack fatigue extension and difficulty in characterizing destabilization extension in engineering practice,this paper introduces metal magnetic memory nondestructive testing technology,combines microscopic in situ observation and DIC digital image correlation techniques to obtain mechanical parameters of crack extension under cyclic loading,establishes a crack extension rate characterization model with magnetic property parameters,and establishes an elasto-plastic multi-axial stress magnetization model under complex stress states based on J-A theory to complete a force-magnetic coupling simulation study of the crack extension process.In order to calculate the force-magnetic coupling simulation in the elasto-plastic stage,and also to solve the problem that the J-A model is only applicable to uniaxial elastic stress and cannot calculate the complex stress state of elasto-plasticity under the action of multi-axial stress,while the change of magnetic properties under the action of multi-axial stress in engineering practice is significantly different from that of uniaxial stress.In this paper,based on the J-A model,a modified magnetization model is established to reflect the effect of complex stress and plastic deformation on the magnetization of ferromagnetic materials in the elastoplastic stage by introducing the equivalent force based on the magneto-elastic energy and considering the effect of stress and strain on the model parameters.The simulation results are compared with the experimental data,and the feasibility of the model is verified.In order to study the change law of magnetic characteristic parameters and mechanical parameters during crack instability extension,this paper uses Q235 B steel three-point bending specimen as the test material,and adopts TSC-5M-32 metal magnetic memory detector with metallographic microscope and DIC digital image correlation technology to observe and record in situ the crack extension evolution process,the change law of strain distribution and the change law of magnetic characteristic parameters during the crack extension evolution process.This provides a new idea for the effective characterization of the crack instability expansion process by obtaining simple magnetic characteristics instead of fracture mechanics parameters in the actual inspection process.To perform finite element simulation of stress magnetization phenomenon during the evolution of crack instability extension.Based on the elasto-plastic multi-axial stress magnetization model proposed in this paper,an improved fatigue force-magnetic coupling model is obtained by introducing the effect of cycle cycle cycle on material parameters.The crack instability expansion process of the three-point bending specimen is studied numerically by using the finite element method,and the simulation results of the crack instability expansion process are obtained,and the force-magnetic coupling simulation is completed by secondary programming to obtain the surface leakage signal during the crack expansion process and analyze the magnetic signal change law during the crack expansion process. |
