Eddy current method for fatigue testing

US5698977A · kind A · utility

Assignee

• The United States of America as represented by the Administrator of the National Aeronautics and Space Administration · US

Inventors

• John W. Simpson · Madison, US
• James P. Fulton · Hampton, US
• Russell A. Wincheski · Williamsburg, US
• Ronald G. Todhunter · Grafton, US
• Min Namkung · Yorktown, US
• Shridhar Champaknath Nath · Gaithersburg, US

Key dates

• Filing date: Jun 5, 1995
• Grant date: Dec 16, 1997
• Expiry date: Jun 5, 2015

Classification

• Technology area (CPC G): Physics
• CPC primary: G01N27/902
• WIPO field: Measurement
• WIPO sector: Instruments

Abstract

Flux-focusing electromagnetic sensor using a ferromagnetic flux-focusing lens simplifies inspections and increases detectability of fatigue cracks and material loss in high conductivity material. A ferrous shield isolates a high-turn pick-up coil from an excitation coil. Use of the magnetic shield produces a null voltage output across the receiving coil in presence of an unflawed sample. Redistribution of the current flow in the sample caused by the presence of flaws, eliminates the shielding condition and a large output voltage is produced, yielding a clear unambiguous flaw signal. Maximum sensor output is obtained when positioned symmetrically above the crack. By obtaining position of maximum sensor output, it is possible to track the fault and locate the area surrounding its tip. Accuracy of tip location is enhanced by two unique features of the sensor; a very high signal-to-noise ratio of the probe's output resulting in an extremely smooth signal peak across the fault, and a rapidly decaying sensor output outside a small area surrounding the crack tip enabling the search region to be clearly defined. Under low frequency operation, material thinning due to corrosion causes incom…