1、1.IntroductionThe first objective of this paper is to look at eddy-current inspection in its most basic form in terms of the physics of the fields,and to establish a general framework into which all eddy-current methods will fit.Against this framework,we can then look at how practical inspection tec
2、hniques have developed and where they fit into the framework.Next,the influence of modern technology is discussed to show the potential that exists to develop new and improved methods of eddy-current inspection.Finally,the development of transient eddy-current inspection is discussed;it is shown how
3、 modern techniques can be used to improve the effectiveness of inspection for subsurface defects.2.Eddy-current inspection a generalised frameworkEddy currents are presented here as the response to an electric field propagating through a conductor.Generic types of measurements are discussed,together
4、 with the information that is available from them,and how to extract and interpret it in principle.Fields and propagationWithin a conductor,the solution to Maxwells equations leads to travelling-wave solutions.In other words,an electromagnetic disturbance will propagate through a conductor with a ch
5、aracteristic velocity and attenuation.It is similar in principle to very fuzzy short-range radar.It should be noted that electromagnetic fields in a conductor do not travel at the speed of light;they travel comparatively slowly-at a velocity determined by the conductivity,permeability and frequency(
6、for harmonic excitation).For example,fields with a frequency of 1 kHz travel at a velocity of approximately 20 m/s in aluminium alloy.The inspection processThis consists of establishing an electric field distribution at the surface of a conducting structure.(Note that an electric field is the source
7、 function for eddy currents rather magnetic field and that a static magnetic field will not induce eddy currents).In Figure 1 the source electric-field vector in air,E is a function of space x and time t.The eddy-current distribution is determined by the source electric field at the surface:Es.The e
8、lectric field then propagates into the structure generating eddy currents as it travels.If part of the field encounters a discontinuity or a change in conductivity or permeability,such as associated with specimen geometry and defects,scattering takes place(as this is a travelling wave process,the fo
9、rmalism of scattering applies)and part of the scattered field propagates back to the surface where it can be measured(see Figure 2).It should be noted that scattering also takes place as a consequence of the dissipation of energy due to Joule heating.This is effectively dispersion.Scattering takes p
10、lace from almost everything;the trick is to isolate and identify the part associated with defects.Given that measurements can only be made at the surface,the easiest component to measure is the magnetic field.In order to interpret this,either a calibration is needed,or a theoretical model to relate
11、the scattered field to a particular geometric parameter.Some defects,such as corrosion between layers,can be identified directly in terms of changes in geometric parameters,but defects such as localised cracks cannot be measured directly;their presence must be inferred from the unusual behaviour of
12、geometric parameters Field sourcesCoils are almost invariably used to generate electric fields in air for the purpose of generating eddy currents in metallic structures,THE 2004 ROY SHARPE PRIZE LECTURETransient eddy currents are they here to stay?D J HarrisonDetails of transient eddy-current inspec
13、tion were first published over ten years ago,however the ideas that led to this technique date back much further to a programme of basic research into eddy-current imaging.The spin-off from this programme has led to several developments in eddy-current technology underlining the essential role of fu
14、ndamental research as a means of gaining insight into the solution of practical inspection problems.The conceptual development of transient eddy-current inspection is reviewed and evaluated here in order to determine whether or not this technique has the potential to make a significant contribution
15、to NDE.Dr David J Harrison recently retired from the NDE Group,QinetiQ Ltd,Cody Technology Park,Farnborough GU14 0LX.Original presentation transcribed and edited by Robert A Smith.E-mail:RASmithQinetiQ.com Figure 1.Diagram illustrating the generic principle of electric field propagation into and thr
16、ough a conductor 144 Insight Vol 47 No 3 March 2005Insight Vol 47 No 3 March 2005 145 although a moving or vibrating permanent magnet could be used in principle.In air,a coil produces a magnetic field that is directly proportional to the coil current.If the current,and hence the magnetic field,changes then an electric field is created that is related to the time derivative of the magnetic field and hence of the coil current.Thus a harmonic current will produce a phase-shifted harmonic electric f
