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Nondestructive Evaluation


Magnetic Property Measurement of Selected Pipeline Steel Specimens


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  image of instrumentation schematic--click to enlarge

Instrumentation schematic. Click image to enlarge

  image of graph showing magnetization curve--click to enlarge

Magnetization curve data. Click image to enlarge

  image of graph showing relative permeability--click to enlarge

Relative permeability data. Click image to enlarge

Highly accurate measurements were made by engineers at Southwest Research Institute (SwRI) of the magnetic properties (primarily the initial magnetic permeability) of steel specimens taken from multiple pipe samples. All procedures followed ASTM (American Society for Testing and Materials) standards for magnetic property measurement.


Six 5-inch-diameter rings with 0.25-inch-square cross section specimens were cut from samples of line pipe. The rings were wound with excitation and sensor coils (primary and secondary) with the windings evenly distributed around the circumference, as dictated by the ASTM standard. The drawing (above) shows the instrumentation.


The pipe specimens comprised three samples, each of two nominal API grades. Multiple measurements were made on each ring, with complete demagnetization between each measurement. Data were processed to produce total B-H loops, total permeability curves, and initial permeability near zero magnetic field.


The determination of initial permeability is difficult because system noise becomes very significant at extremely low field strengths. In a practical sense, it was not possible to make the permeability calculation at zero field, but it is known that the permeability curve is a straight line as the curve approaches H=0.


The measurement was effective to define this straight line, allowing a determination of the initial permeability by extrapolation. Typical results are shown on the right.


The project concluded that there was no usable correlation between pipe grade and magnetic properties. If line pipe is being purchased for specific magnetic properties, the properties must be specified to the manufacturer or measurements must be made to verify the properties for each lot of pipe.


For more information about our nondestructive evaluation (NDE) and magnetic property measurement of selected pipeline steel specimen capabilities, or how you can contract with SwRI, please contact Jay Fisher, Ph.D., at or

(210) 522-2028.


Contact Information

Jay Fisher, Ph.D.


(210) 522-2028

Related Terminology

cladding thickness

computer modeling



eddy current

magnetic sensor

nondestructive evaluation

orientation sensing

pipe wall thinning

position sensing

pulsed eddy current

reliability studies

specialized NDE system development

specialized procedure development

steel pipe inspection

| Sensor Systems and Nondestructive Evaluation (NDE) Technology Department | Mechanical Engineering Division | SwRI Home |

Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 9 technical divisions.

August 07, 2014