Magnetic Leakage Fields as Indicators of Eddy Current Testing
With eddy current testing in a homogenous field it is often spoken about hindrances that are represented by cracks to eddy currents that cannot flow so as in non-defective material. Such homogenous condition can be met with the magnetizing in a very long coil. If however the coil is short the field is not homogenous any more and especially with testing of ferromagnetic materials the magnetic leakage field may not be neglected. The demagnetizing effect of eddy currents can be seen also in magnetic leakage field. By proper arrangement of primary and secondary coil it is possible to get more information. It is given in the article how the longitudinal and the transverse cracks can be detected in the proposed arrangement of primary and secondary coils. If the arrangement of coils with the eddy currents testing is not a standard one it is very important to understand the physics of the new arrangement to be able to interpret the results properly. A very useful tool is the mathematical treatment of field equations. The numerical methods are often applied, since Maxwell equations are rather complex partial differential equations and there are usually real and imaginary components of the field to be taken into account. There are main ideas given how to solve more complicated problems. The actual concrete results will be compared with practical experiments.
NUMERICAL METHOD FOR THE MAGNETIC FIELD CALCULATION WITH EDDY CURRENT TESTING
With eddy current testing it is sometimes difficult to describe the a.c. magnetic field distribution in the material analytically. For some ideal cases as for the case of homogenous field and for pieces of simple form and homogenous physical properties it is often shown also in text books. When we, however, have to deal with the real situation the things become more complicated, since for calculation of the magnetic field distribution the Maxwell equations in three dimensions are to be solved. There are normally six partial differential equations to be solved: three real components and three imaginary components of the magnetic field. For the complete description of the magnetic field six data are needed in any point of the space. The partial differential field equations can be written as a system of linear difference equations using the method of finite differences (FD) or the method of finite elements (FE). By means of the available software and hardware it is possible to solve such problem to certain precision even using the simple relaxation method. In the paper the calculated distribution of the magnetic field in the cross-section of a sound ferromagnetic bar is given together with the case of a surface and subsurface thin radial crack. From the corresponding graphical and dynamical presentation of the magnetic field it is possible to get better information about eddy currents.
Photoreflectance characterization of vanadium-doped GaAs layers grown by metalorganic vapor phase epitaxy
Abstract We characterized the vanadium-doped (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy. The conductivity of the samples is checked by the Hall effect measurements, and it is n type with electrons concentration ranging from 2×10 13 to 2×10 17  cm −3 . We used the photoreflectance technique (PR) to study the effect of vanadium doping on the optical properties of GaAs. Our experimental results show that the built-in electric field F and the broadening parameter Γ 0 increase with V-doping concentration. Furthermore, the temperature dependence of the built-in electric field and the broadening parameter of GaAs:V samples have been investigated by PR. It has been observed that these parameters increase with increasing temperature. The decrease of the built-in electric field with decreasing temperature is due to the photovoltage effect.We characterized the vanadium-doped GaAs (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy.