1、APPLICATION OF THE ACFM INSPECTION METHOD TO RAIL AND RAIL VEHICLES D.Topp and M.Smith TSC Inspection Systems,Milton Keynes,England Abstract:The ACFM inspection technique is an electromagnetic inspection method,which is widely used in the Oil and Gas industry for the detection and sizing of surface
2、breaking cracks.It provides a number of advantages over conventional inspection methods,particularly in the ability to inspect without removing coatings or grease,and has recently been adapted for a number of rail specific applications.The technique has now been accredited in the UK for use on vehic
3、le wheelsets and for the inspection of plain rail.This paper describes the background to the ACFM technique and its application to in-service inspection.The development of a new Walking Stick for inspection of the rail head is described,together with some of the challenges of inspecting the complex
4、crack geometries associated with cracks generated by rolling contact fatigue(RCF).ACFM has also been applied to both plain and driven axles,examples of these applications together with its use on bogies are also given.This paper also considers some of the challenges faced when moving technology from
5、 one industry to another.Operator training has to be considered as well as the validation requirements for acceptance of a new technique.Validation often requires comparison with existing technologies in blind trials but these are not necessarily quantitative in terms of performance.Destructive sect
6、ioning provides an absolute measure of system capability and was also used during these developments.Results from both destructive and non-destructive trials are discussed.Introduction:The Alternating Current Field Measurement(ACFM)technique is an electromagnetic inspection method capable of surface
7、 crack detection and sizing.The technique does not require coating removal and was originally developed in the late 1980s for use in the Oil Industry,where there was a requirement for improving the reliability of subsea inspection.The first application of ACFM in the rail industry was for the inspec
8、tion of axles.This came about from a review of available crack detection methods conducted by the Engineering Link following a fatal accident involving the failure of an axle.This review,which looked beyond those methods traditionally used in the rail industry,was followed up by a series of blind tr
9、ials1 in which the ACFM technique was found to outperform conventional Magnetic Particle Inspection(MPI)and an advanced eddy current method.These results then generated further interest in the technique and in particular whether it could be used for the inspection of rails.At the time,the UK had exp
10、erienced a major accident involving a rail break,which identified a problem in the network infrastructure from Head Checking.More specifically,gauge corner cracking(GCC)was identified as a potential problem and significant effort was put into addressing this problem,both in terms of identifying the
11、causes and controlling the rail condition once cracks were known to exist.The ACFM method was developed for this particular application in an effort to identify the severity of the defects,this meant categorising the defects in terms of their depth.Although the crack morphology was complex,good perf
12、ormance was achieved from the system,which is now in use on the UK network.Another application,which has been developed recently,is the inspection of bogies.Like axles,these are traditionally cleaned to bright metal before inspection using MPI.The use of ACFM reduces the cleaning requirement and pro
13、vides auditable data recording of the results.This application was pioneered by Bombardier Transportation UK Ltd2,and has recently achieved accreditation in the UK.The ACFM Method:The Alternating Current Field Measurement(ACFM)technique is an electromagnetic technique capable of both detecting and s
14、izing(length and depth)surface breaking cracks in metals.The basis of the technique is that an alternating current can be induced to flow in a thin skin near the surface of any conductor.By introducing a remote uniform current into an area of the component under test,when there are no defects presen
15、t the electrical current will be undisturbed.If a crack is present the uniform current is disturbed and the current flows around the ends and down the faces of the crack.Because the current is an alternating current(AC)it flows in a thin skin close to the surface and is unaffected by the overall geo
16、metry of the component.Figure 1.ACFM currents flowing around a defect Associated with the current flowing in the surface is a magnetic field above the surface which,like the current in the surface,will be disturbed in the presence of a defect.An important factor of the ACFM technique is its capability to relate measurements of the magnetic field disturbance to the size of defect that caused that disturbance.The breakthrough came from a combination of studies at University College London,which pr
