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.
How to distinguish surface and subsurface cracks using electromagnetic ndt methods
Abstract Electromagnetic ndt methods provide crack indications but tell one very little about the nature of a defect. This paper outlines qualitatively the physical principles of a method for distinguishing surface cracks from subsurface cracks. The method is based on determining changes in the amplitude and phase of the magnetic flux over a cross-section in a metal bar containing defects. It is shown that due to impedance effects, for surface or subsurface cracks the resultant magnetic fields have phases which lead and lag respectively the field from a bar with no defects.
Persistence and Change in Minkowski Spacetime
There are famously two main metaphysics of persistence, namely three and four-dimensionalism. Both yield a particular solution to the so called puzzle of change. I argue that typical three-dimensionalist solutions to the puzzle face insurmountable difficulties even in the simplest relativistic setting, that of Minkowski spacetime