A study of magnetic non-destructive evaluation for degradation detection in nuclear steels

Yue Cao

The goal of this project is to develop new and safe micro/meso-scale magnetic characterization methods to study the degradation of reactor structural materials. This project will focus on studying magnetic signatures caused by defects using various magnetic characterization methods. Despite the fact that the relationship between magnetic properties and structural defects has been studied experimentally by many groups, there has lacked a systematic study on application of all these measurements to practical magnetic monitoring scenarios. This project will provide more specific information about the magnetic signatures expected from nuclear reactor steel degradation. The primary magnetic characterization methods discussed in this proposal are major hysteresis loop, minor hysteresis loop, and first order reversal curve (FORC). Other methods, such as magnetic Barkhausen noise methods and isothermal remanence magnetization (IRM) curves are also applied on some samples. The relationship between the defects and the magnetic behavior shown by each of these measurements are discussed in this research work. In order to understand the experimental results, simulations of different hysteresis behaviors using the Preisach model are presented. Magnetic characterization is performed using a vibrating sample magnetometer (VSM). To sequentially investigate the effects of defects on magnetic properties, we first start investigating a series of single and polycrystalline Fe thin films, then go to Fe-1%wt. Cu samples, and finally discuss the irradiated single crystal Fe thin films. Characterization of these materials, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD) are used to analyze the density of grain boundaries and dislocations, which are the defects in samples. In this research, we aim to answer these questions: 1) How specifically do defects affect the magnetic properties? 2) What is the characteristic information that can be obtained by each magnetic measurement? 3) What are the merits and disadvantages of each magnetic measurement? 4) Can the magnetic signatures of different defects (grain boundaries, dislocations, precipitates) be distinguished in Fe-1%wt. Cu samples? 5) How can the hysteresis behavior be understood computationally?

A study of magnetic non-destructive evaluation for degradation detection in nuclear steels

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