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Proceedings Paper

Magnetic measurement of creep damage: modeling and measurement
Author(s): Martin J. Sablik; David C. Jiles
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Paper Abstract

Results of inspection of creep damage by magnetic hysteresis measurements on Cr-Mo steel are presented. It is shown that structure-sensitive parameters such as coercivity, remanence and hysteresis loss are sensitive to creep damage. Previous metallurgical studies have shown that creep changes the microstructure of he material by introducing voids, dislocations, and grain boundary cavities. As cavities develop, dislocations and voids move out to grain boundaries; therefore, the total pinning sources for domain wall motion are reduced.This, together with the introduction of a demagnetizing field due to the cavities, results in the decrease of both coercivity, remanence and hence, concomitantly, hysteresis loss. Incorporating these structural effects into a magnetomechanical hysteresis model developed previously by us produces numerical variations of coercivity, remanence and hysteresis loss consistent with what is measured. The magnetic model has therefore been used to obtain appropriately modified magnetization curves for each element of creep-damaged material in a finite element (FE) calculation. The FE calculation has been used to simulate magnetic detection of non-uniform creep damage around a seam weld in a 2.25 Cr 1Mo steam pipe. In particular, in the simulation, a magnetic C-core with primary and secondary coils was placed with its pole pieces flush against the specimen in the vicinity of the weld. The secondary emf was shown to be reduced when creep damage was present inside the pipe wall at the cusp of the weld and in the vicinity of the cusp. The calculation showed that the C- core detected creep damage best if it spanned the weld seam width and if the current in the primary was such that the C- core was not magnetically saturated. Experimental measurements also exhibited the dip predicted in emf, but the measurements are not yet conclusive because the effects of magnetic property changes of weld materials, heat- affected material, and base material have not yet been sorted out experimentally form the effects of creep damage.

Paper Details

Date Published: 14 November 1996
PDF: 11 pages
Proc. SPIE 2947, Nondestructive Evaluation of Utilities and Pipelines, (14 November 1996); doi: 10.1117/12.259164
Show Author Affiliations
Martin J. Sablik, Southwest Research Institute (United States)
David C. Jiles, Iowa State Univ. (United States)

Published in SPIE Proceedings Vol. 2947:
Nondestructive Evaluation of Utilities and Pipelines
Martin Prager; Richard M. Tilley, Editor(s)

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