Characterisation of surfaces and surface breaking defects using magnetic NDE methods

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Choudhury, A.R., 2000. Characterisation of surfaces and surface breaking defects using magnetic NDE methods. MPhil, Nottingham Trent University.

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Abstract

The Barkhausen phenomenon is reviewed in relation to current knowledge, and the direction of modern research is discussed. Acoustic Barkhausen Emission (ABE) and Electromagnetic Barkhausen Emission (EBE) from ferromagnetic specimens has been studied. Test samples were magnetised using a silicon iron yoke electromagnet and a triangular magnetising frequency of O.lHz. A coil transducer of 2000 turns and a piezoelectric transducer were used as the sensors for EBE and ABE respectively.

Initially, finite element analysis of the magnetising field spread through a test sample was studied. A central region of uniform intensity of magnetisation was found, from where consistent EBE signals were detected. The relationship between the Barkhausen signals and sample volume was studied. The signals were analysed and distinct relationships were found for the ABE and EBE signals, for a range of sample volumes. Utilising the fundamental differences between the ABE and EBE, the effect of surface modifications on nickel, generated by machining, has been examined. The Barkhausen signals indicated a relationship between these modifications and time of EBE peak.

The effect of an idealised surface breaking defect on the Barkhausen emission (BE) is considered. Differing EBE sensors and two different magnetising yokes and their effect on the detected ABE and EBE signals from the slotted steel specimen have been studied. EBE envelopes were found to split due to the presence of the slot and a range of Barkhausen signal parameters (time of peak, signal energy and signal amplitude) were used to indicate the slot location. Optimum combination of experimental variables would allow the detection of surface breaking cracks. The BE from a tight surface breaking fatigue crack, was also studied. Barkhausen envelopes were found to be similar from points across the specimen surface. The analysed BE parameters were found to yield useful information about the crack location and surrounding material.

Barkhausen emission from EN-43A steel specimens, plain, welded and containing a low- cycle fatigue crack within the weld, were studied. The effect of these material changes on the BE envelopes was found to be significant. The effects of machining the surface of each specimen, using electrical and mechanical machining, were also studied. Different machining processes yielded significantly different EBE signals and as a result envelope parameters were found to change. Time of peak was found to occur earlier over the crack affected region, where an increase in the energy was also noted. Mechanisms for the generation of the differing ABE and EBE results are discussed.

The effect of crack depth reduction by machining away the specimen thickness has been examined. The different machining techniques, used to reduce the crack depth, had significant effects on the Barkhausen emission. Barkhausen response from mechanically machined surfaces failed to exhibited any noticeable trend with decreasing crack depth. Electrical machining generated a consistently reproducible surface texture and as a result, the effect of reducing crack depth was found to have a measurable influence on the EBE envelope parameters. Possible applications of the Barkhausen technique and the reported results are also considered.

Item Type:

Thesis

Creators:

Choudhury, A.R.

Date:

2000

ISBN:

9781369317152

Identifiers: