Search

Reduced Activation Ferritic/Martensite (RAFM) steel is a new material considered to be one of the important candidate structural materials for Tokamak fusion reactors. Plastic deformation inevitably occurs during the operation of fusion reactors due to giant unexpected loads. To ensure the structural integrity, reliable non-destructive evaluation (NDE) for RAFM steel is of great importance. Magnetic Barkhousen Noise (MBN), Magnetic Incremental Permeability (MIP) and Magnetic Flux Leakage (MFL) methods are found being effective especially for NDE of large plastic deformation in RAFM steel by authors. Its feasibility for small plastic deformation under 1%, however, is not clarified. At the same time, as mechanical damage may be caused by several times of large load, i.e., the current deformation state is possibly of complicated loading history. To clarify the influence of these factors is also very important for application of these magnetic NDE methods in practice. In view of this background, the influences of plastic deformation history on NDE signals and the feasibility of the magnetic NDE methods for relative small deformation are experimentally studied in this paper. From experimental results, it was found that the MIP and MFL signals decrease with an enlarged plastic deformation and are not significantly influenced by the plastic deformation history, while the MBN signals show significantly different tendency for specimen of different loading history. To understand the mechanism, micro-structure analyses and magnetic domain observations are conducted by using the EBSD and the magnetic powder-pattern method. Observation results reveal that the pinning effect of the micro defects prevents the movement of magnetic domain walls and perturbs the signals of magnetic NDE methods, especially on the MBN signals. These results show that the MIP and MFL method are more suitable for evaluation of plastic deformation in RAFM steel from practical application point of view.

The irradiation induced variations in microstructure and mechanical properties of the Cu64Zr36 metallic glasses (MGs) are investigated via large-scale molecular dynamics simulations. The irradiation condition is modeled by sequential and prolonged collision cascades. The simulation results show a simultaneous decrease of the density and the fraction of Cu-centered full icosahedron (CCFI) clusters in the irradiated samples. This microstructural evolution induced by the prolonged irradiation is proposed to be responsible for the change of deformation mode of tested samples under uniaxial tensile loading. When increasing the irradiation dose, the ultimate tensile strength of MG samples decreases gradually, while their deformation mechanism switches from localized shear banding to ductile necking and finally to ideally plastic flow, enhancing the MG ductility. Careful atomic-scale examinations on the initiation and development of shear transformation zones (STZs) are carried out for a better understanding of the irradiation-induced ductility enhancement in MGs. Two related effects of the irradiation are unveiled. On the one hand, the irradiation releases the stored strain energy by promoting the STZ activation within the MG matrix. On the other hand, the irradiation damages restrain the propagation of shear band through the innovatively proposed two-unit “STZ-vortex” mechanism. Under the combined effect of these two mechanisms, the low-temperature ductility of irradiated MGs is improved and the brittle-like failure driven by single shear banding is prevented.

Nonlinear Magnetic Flux Leakage Testing (MFLT) method is a relative new non-destructive testing (NDT) technique for inspection defect or micro damage in structure of ferromagnetic material by measuring the leakage magnetic field during an alternative magnetization process. To enhance the nonlinear MFLT technique, a numerical tool for simulating the inspection signal is very important in views to clarify the detection mechanism and to improve the detection efficiency. However, most of the simulation methods on magnetic NDT problems solve problem without considering the hysteresis effects of ferromagnetic materials and is difficult to deal with the influence of mechanical damage. In this paper, a numerical scheme is proposed at first to simulate the nonlinear MFLT signals considering the nonlinearity and hysteresis characteristics of ferromagnetic materials based on the equivalent magnetic polarization approach and FEM-BEM hybrid method. The validities of the proposed method and the corresponding numerical code are proved by comparing the simulation results of a typical nonlinear MFLT problem with the experimental measured signals. Secondly, in order to cope with the influence of plastic deformation in material on the nonlinear MFLT signals, a magneto-mechanical coupling constitutive relation is proposed and applied to the developed numerical simulation code. Finally, the nonlinear MFLT signals of specimen with different plastic deformation are simulated with the developed numerical code and the influence mechanism of plastic deformation on nonlinear MFLT signals is discussed based on the numerical results. The results reveal that the distortion of nonlinear MFLT signals mainly due to the magnetic remanence coefficient of material which can be significantly influenced by micro damage such as the plastic deformation.