Southwest Research Institute® (SwRI®) News

Barkhausen Noise Stress Measurement System

San Antonio, TX -- Sept. 24, 1981 -- A system developed at Southwest Research Institute® (SwRI®) for measuring residual and applied stress in ferromagnetic materials has been honored as one of the 100 most significant technological advances of the year.

Known as the Barkhausen Noise Stress Measurement System, Series 200, the development was selected as a winner in the 1981 I-R 100 competition conducted by Industrial Research and Development, officials of the publication announced Thursday in Chicago.

John R. Barton, vice president in charge of the Institute Instrumentation Research Division, is principal developer and leader of the group involved in development of the system.

A magnetic phenomenon at times viewed as an annoyance is put into service by the new system to provide industry with what is seen as a significant improvement in ability to determine a body's state of stress.

Residual stresses are important in the design, fabrication, and service life of many structures and components. In some cases means such as shot peening (working a metal surface by bombarding it with a stream of shot) are used deliberately to induce residual compression stress with the intent of prolonging fatigue life. On the other hand, some service-induced stresses are considered harmful predecessors of fatigue cracking.

Developers of the method foresee its use in measuring stresses from these sources as well as from grinding and other steps in processing of a wide variety of industrial components -- such as bearing races, steel helicopter spars, steam turbine blades, crankshafts, hydraulic pistons and cylinders, trunnion pins, railroad rails and wheels, and steel bridge structures.

To detect and measure stresses such as these, the Barkhausen noise method makes use of the distinctive behavior of magnetic domains, small regions of local magnetization oriented in various directions within a ferromagnetic material (such as iron, steel, nickel, cobalt, or alloys of like magnetic character).

Even in a specimen of material considered to be unmagnetized, theory holds that all of these tiny domains actually are fully magnetized. Their directions of magnetization are oriented at random, however, so they cancel one another out.

When the material is magnetized by the application of an external field, the walls between domains are sent into motion through the material, as regions with orientation favored by the external field grow larger at the expense of their unfavored neighbors.

This movement of domain walls is the key to the new system's operation, the source of the stress-revealing signals it relies upon.

Domain walls do not move smoothly. Instead, they advance in abrupt, discontinuous steps, known as Barkhausen jumps (after German scientist Heinrich Barkhausen who noted the effect in 1917). Electromagnetic changes accompanying these jumps are sensed by some instruments as electrical "noise," which previously has often been viewed as more troublesome than useful to those trying to measure material properties.

Institute investigators made the noise useful by emphasizing the fact that stress also affects these signals -- that the way domain walls move is strongly influenced by the stresses of mechanical forces as well as by magnetic forces acting on the material. Barkhausen signals therefore should contain useful information, for those who can extract it.

A measurement is made by applying a controlled, changing pattern of magnetization, and sensing resulting electromagnetic effects in the form of voltage pulses induced in a small inductive coil probe. Amplified and electronically processed, this signal can be presented on an oscilloscope or as a meter reading.

Precise quantitative results can be achieved when material and processing are known and suitable calibration data are available, Barton said. Even without that kind of support, valuable qualitative assessments can be made. The amplitude of Barkhausen signals, for instance, is known to increase with increasing tensile stress, and to decrease with growing compressive stress.

Despite the value of residual stresses and the need for such measurements, residual stress measurement is not currently in wide use in industry. he most widely accepted method now is X-ray diffraction, which although improved in speed and versatility in recent years still is not readily applicable in complex configurations, and requires removal of covering paint and plating.

The Barkhausen noise method is unusual in that it requires no surface preparation, is rapid (less than 10 seconds per measurement point), is applicable to complex configurations with use of special probes, senses below the surface (to depths of 0.01 to 0.02 inch), senses very local regions (approximately 0.05 inch square), and can readily be automated for high speed digital measurement.

For more information about the Barkhausen Noise Stress Measurement System, contact Joe Fohn, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas, 78228-0510, Phone (210) 522-4630, Fax (210) 522-3547.

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