Date of Award

2004

Document Type

Doctoral Thesis

Degree Name

Doctor of Philosophy

Department

Cork Institute of Technology

First Advisor

Mr. Matthew G. Cotterell

Second Advisor

Dr. Stephen Cassidy

Abstract

Magneto-acoustic emission (MAE) is a relatively new non-destructive detection technique. Its working principle is based on Barkhausen discontinuities and magnetostriction within a ferromagnetic material that is subjected to a varying magnetic field. Magneto-acoustic emission (MAE) and magnetic Barkhausen noise (MBN) sensing techniques were developed and employed to characterise plastically deformed and heat-treated AISI 430 ferritic stainless steel and AISI 304 austenitic stainless steel samples. A new waveform measurement parameter for magneto-acoustic emission was developed called MAE (Absolute “energy”).

Both MAE and MBN were found to decrease with increasing plastic deformation of AISI 430. MAE (Absolute “energy”) and MBN (RMS) were found to be inversely proportional to hardness. With increased deformation, the resulting change in dislocation density was found to effect material coercivity. It was found that both MAE (Absolute “energy”) and MBN have an inverse exponential relationship to material coercivity. These results are explained in terms of the different mechanisms that affect dislocation-domain wall interactions.

The work-hardening of austenitic stainless steels, such as AISI 304, during deformation is due to the appearance of a'-martensite and to an increase in dislocation density. MBN was found to be an effective means of studying the transformation mechanisms of austenite to a'-martensite during deformation. It was found that the dislocation density, rather than the a'-martensite phase, was the cause of material workhardening. Coercivity was found to be a useful quantitative and non-destructive method for characterising work-hardening of AISI 304. It is suggested that s-martensite plays an important role in the dislocation deformation mechanisms, particularly at higher strains (>45%) and in the heat treatment of AISI 304 below 500°C. MAE activity was not detected in the AISI 304 studied, due to single domain magnetic processes and the high dislocation density associated with martensite formation.

Access Level

info:eu-repo/semantics/openAccess

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