Z. J. Chen

Assessment of creep damage of ferromagnetic material using magnetic inspection

Results of inspection creep damage by magnetic hysteresis measurements on Cr-Mo steel are presented. It is shown that structure sensitive parameters such as coercivity, remanence and hysteresis loss are sensitive to the creep damage. Previous metallographic studies have shown that creep changes the microstructure of the material by introducing voids, dislocations, and grain boundary cavities. As cavities develop, dislocations and voids move out to the grain boundaries; therefore the total pinning sources for domain wall motion are reduced. This, together with the introduction of demagnetization field due to the cavities, results in the decrease of both coercivity and remanence. Numerical computations with a modified Jiles-Atherton model are presented which are consistent with the proposed mechanisms. >

Estimation of fatigue exposure from magnetic coercivity

An investigation of the effects of fatigue on A533B steel under constant load amplitude is reported in this paper. It was found that the plastic strain of the sample accumulated logarithmically with the number of stress cycles after initial fatigue softening. Based on the fact that plastic strain is often linearly related to the coercivity of material, at least for small changes of Hc, a phenomenological relationship has been developed and tested to correlate the number of stress cycles to this magnetic parameter. This result represents the first successful attempt to relate the fatigue exposure directly to a magnetic parameter.

Evaluation of fatigue damage in steel structural components by magnetoelastic Barkhausen signal analysis

This paper is concerned with using a magnetic technique for the evaluation of fatigue damage in steel structural components. It is shown that Barkhausen effect measurements can be used to indicate impending failure due to fatigue under certain conditions. The Barkhausen signal amplitude is known to be highly sensitive to changes in density and distribution of dislocations in materials. The sensitivity of Barkhausen signal amplitude to fatigue damage has been studied in the low‐cycle fatigue regime using smooth tensile specimens of a medium strength steel. The Barkhausen measurements were taken at depths of penetration of 0.02, 0.07, and 0.2 mm. It was found that changes in magnetic properties are sensitive to microstructural changes taking place at the surface of the material throughout the fatigue life. The changes in the Barkhausen signals have been attributed to distribution of dislocations in stage I and stage II of fatigue life and the formation of a macrocrack in the final stage of fatigue.

Variation of coercivity of ferromagnetic material during cyclic stressing

The relationship between coercivity and the number of stress cycles is reported in this paper. The linear relation between coercivity and the logarithm of the number of stress cycles was confirmed in load controlled fatigue tests, for both pre-strained and unprestrained specimens. It is believed that the dislocation activity is the main factor changing the magnetic properties of a ferromagnetic material under fatigue, rather than other microstructures and internal stress. As a result, a theoretical model was developed to explain the correlation between the coercivity and the number of stress cycles. The model is based on domain wall dislocation interaction and the strain-dislocation relation. We predict the relation between magnetic properties and the stage of fatigue life as Hc-Hc/sub 0/=b ln(N), where Hc-Hc/sub 0/ is the change in coercivity, N is the number of stress cycles and b is a constant. >

Modelling of reversible domain wall motion under the action of magnetic field and localized defects

A simple physical model of the action of point defects on a domain wall bowing in the low field region is presented. A relationship between initial permeability and defect density is derived. Experimental results on the relation between initial permeability and carbon content in spheroidized iron carbon specimens are presented and the results are discussed in connection with this model. >

Detection of Creep in CR-MO Steel by Magnetic Hysteresis Measurements

Creep damage is the slow plastic flow of metal under stress and at high temperature, typically about 50% of the absolute melting temperature. The result is a very slow viscous flow of the metal which ends in sudden failure. This problem occurs in alloy steels that have been used in steam generators, turbines, and pipelines in power plants operating at high temperatures, typically in the range 500°–600°C, and under stress for an extended period of time. Creep failure occurs by a process of cumulative damage which involves plastic deformation, nucleation and growth of cavities at the grain boundaries, subsequent linkage of these cavities to form microcracks, and the propagation of these microcracks until failure.

Analytic model calculation of magnetic field in a magnetic half‐space due to surface magnetic charge

By analogy with electrostatic field, a simple analytic model is presented on the distribution of magnetic field inside a test material as a result of surface inspection with a magnetic probe. According to this model, the penetration depth of the magnetic field is defined and the permeability of the material at different depths is calculated. Finally, the magnetic flux inside the inspection head when placed on samples of different thicknesses is calculated and compared with experimental results.

Measurements of magnetic circuit characteristics for comprehension of intrinsic magnetic properties of materials from surface inspection

A transfer function is presented for calculating magnetic field and flux density inside a test material as a result of surface measurement. By considering flux leakage, we introduce a parameter η, called the leakage coefficient, which can be experimentally determined. It is introduced into the equations to make the transfer function more practical. The distribution of field inside a test material is then discussed in accordance with a surface magnetic charge model.

Magnetic Property Evaluation of Creep Damaged Cr-Mo Steel Components used in Power Plants

Magnetic properties of steel are sensitive to the total volume of magnetic inclusions in the material [1]. These magnetic inclusions may be precipitates or microscopic voids. When a material is subjected to an elevated temperature under stress for extended periods, slow plastic flow of material occurs which eventually leads to microscopic voids, cavities and finally macroscopic cracking. This process is known as creep damaged. Depending on the strain rate within the material, the creep process can be divided into three categories. In the early stage of the creep process known as primary creep, the material deforms rapidly with time. But this strain rate gradually decreases and becomes independent of time. This stage of creep process is known as secondary, or steady state, creep. In the final stage which is known as tertiary creep, the strain rate again increases with time and deformation continues until the material fails.

Improvement of magnetic interface coupling through a magnetic coupling gel

Surface measurement of magnetic properties is potentially a very advantageous technique for nondestructive evaluation. The effect of a magnetic coupling gel on surface magnetic measurement, is reported in this paper. Test results under three interface conditions are presented and compared. It was found that the magnetic gel reduces the variability of the measurement by improving the magnetic interface coupling between the sensor and test specimen. A simple model of a magnetic circuit is analyzed to show the coupling function of magnetic gel. From these results, it is deduced that the magnetic coupling gel plays an important part in improving reliability of the magnetic measurements.