Meandering Winding Magnetometer Research Articles

Breakthroughs in Nondestructive Measuring Techniques

Abstract The Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute (WPI) in Massachusetts is conducting a cutting-edge research project aimed at measuring the surface hardness and case depth on carburized steels for process verification and control. The results will enable companies to improve the quality of heat-treated products faster and more cost-effectively. Nondestructive techniques being evaluated initially include eddy current, meandering winding magnetometer (MWM), Barkhausen noise testing, and alternating current potential drop.

Sensors for monitoring early stage fatigue cracking

Three sensor systems were evaluated for their ability to detect early stage fatigue cracking in open holes. The sensor systems were (1) a Meandering Winding Magnetometer Array sensor system that induces eddy currents to monitor conductivity changes; (2) a through-transmission ultrasonic technique that monitors energy loss, and (3) the Electrochemical Fatigue Sensor which detects fatigue-induced changes in a metal surface through their effects on the electrochemical double layer. The sensors were mounted individually or in tandem in samples of 7075-T651 aluminum alloy which were then fatigued using a spectrum loading sequence. The samples were also examined at various stages during fatigue cycling using optical or scanning electron microscopy. Detection thresholds of approximately 100 μm were observed, and calibration curves for crack size in terms of the sensor outputs were obtained.

Early detection and monitoring of fatigue in high strength steels with MWM-Arrays

Fatigue monitoring of cyclically loaded shot peened high-strength steel components can be accomplished via magnetic permeability measurements during laboratory tests or in service. These measurements can be performed either continuously using permanently mounted Meandering Winding Magnetometer Arrays (MWM ®-Arrays) or intermittently with scanning MWM-Arrays. The results obtained to date suggest that MWM-Array permeability measurements can provide early detection of fatigue damage in steels before conventional methods can detect any changes. This has been demonstrated to be particularly significant in the presence of high compressive stresses introduced by shot peening. One of the fatigue tests was suspended when accelerating changes in local permeability were detected. Examination of the fatigue specimen in a scanning electron microscope detected only a few relatively small cracks, e.g. 50–200 μm long at the surface. Fractography, however, revealed significantly longer cracks. For the same specimen, conventional eddy current and ultrasonic testing failed to provide any indications of cracks, and fluorescent liquid penetrant detected only an inconclusive spot indication. This paper provides a comparison of the permeability changes and fractography data with a fatigue crack growth curve based on a FASTRAN analysis accounting for residual stresses from shot peening. A comparison of the experimental data and crack growth analysis results suggested that MWM-Array magnetic permeability measurements may detect cracks in the compressive stress field when they are about 50 μm deep.

MWM eddy-current arrays for crack initiation and growth monitoring

Permanently mountable and scanning MWM ® (Meandering Winding Magnetometer) eddy-current arrays provide an effective means of monitoring fatigue in laboratory tests of specimens and test articles as well as in service on structural components, e.g., aircraft high-strength components, heavy machinery components, and bridge members. The MWM-Arrays can detect and monitor early stage fatigue damage and cracks by measuring electrical conductivity or, in the case of ferromagnetic materials, magnetic permeability. Preliminary results of fatigue tests with linear MWM-Arrays, mounted on a shot peened surface or on a shot peened and cadmium plated surface in the semi-cylindrical gage region of custom-designed high-strength fatigue specimen, are presented and discussed. The results indicate that both permanently mounted MWM-Arrays and scanning MWM-Arrays can detect fatigue damage in steels well before formation of cracks as verified by post-test scanning electron microscopy.