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.
Longitudinal and Circumferential Cracks in a Ferromagnetic Bar Detected Simultaneously
In ferromagnetic materials it is possible to detect surface cracks using two principles. One is the magnetic leakage test that is very often applied with the testing by D.C magnetic fields. Magnetic particle methods are based on this principle. The second principle is testing by eddy currents that can be applied for ferromagnetic and for non-ferromagnetic materials. The aim of our investigation is to construct a universal arrangement of coils that could be used for ferromagnetic bars for detecting surface cracks of any orientation. 1. Magnetic particle methods It is very often explained that magnetic leakage fields can be used for detecting surface cracks in ferromagnetic material. The part must be magnetized in the direction being perpendicular to the length of the expected surface crack. Usually a strong D.C. magnetic field is used and magnetic particles (in the form of powder or suspension) are spread over the surface being investigated. From the Maxwell equations it is evident that when crossing the boundary between two materials the normal component of the magnetic field density (B) must be conserved and the tangential component of the magnetic field strength (H) must be conserved too. Since the magnetic permeability of a ferromagnetic material is considerably bigger than 1 the flow of the magnetic field is different from the flow in a non-ferromagnetic material. In the defective places a part of strong magnetic flux comes out of the material and can be registered by a suitable instrument. The principle of the magnetic particle method is in concentration of the field in the vicinity of the crack that influences the magnetic particles in the suspension so that they get concentrated in the spots of emerging field of both sides of the crack. Sometimes it is possible to observe the change of the magnetic particle distribution on the surface by naked eye sometimes it is better to use a fluorescent suspension and to make a good photo of the part. The surface cracks being oriented along the field cannot be detected in this way. The method is very practical and there are several procedures available for changing the orientation of the field accordingly. It has however some drawbacks. It does not tell much about the crack depth. When the magnetic leakage fields are measured it is possible to connect the measurements with the depth of the crack [1]. ECNDT 2006 Poster 102
Finite Difference Modeling on the Temperature Field of Substrate in Friction Surfacing
Friction surfacing is an advanced process of great potential, especially in the field of repair and reclamation of worn and damaged components. As a kind of thermal processing technology, the temperature profile and changes of the substrate in friction surfacing are important factors to get coatings of high quality. In this paper, the temperature field of the substrate during the process is simulated by finite difference method. The results are consistent with experiments, reappearing the temperature change of the substrate during friction surfacing, allowing further study on figuration and quality of coatings.