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NDT: Non-Destructive Testing

Dexter Pipeline Inspection Gauge

Non-destructive testing has gained significant importance in modern industrial processes with its primary aim at reducing down time and enhancing safety and productivity.  Magnetic methods for NDT include magnetic flux leakage (MFL), electromagnetic acoustic transducer (EMAT) and hysteresis testing.

Each of these methods is sensitive to different defect configurations; while some are sensitive to surface cracks, others are sensitive to volume defects and yet others to internal stresses.  In each of these techniques, a magnetic field (time varying or static) is induced in the part to be tested and the response is analyzed to determine and characterize defects.

The MFL method is sensitive to surface cracks that impede the 'flow' of magnetic flux in ferromagnetic material -e.g. steel.   In this test method, a static magnetic field is impressed on the part to be tested.  Surface cracks cause a change in magnetic reluctance in the closed magnetic circuit resulting in a change in the amount of flux 'leaked' into air immediately outside the surface of the part.  Hall effect sensors are typically used to measure the leakage field.  Analysis of the leakage field over the surface thus permits characterization of the defect.

The EMAT method launches a surface acoustic wave in the ferromagnetic part.  Acoustic waves reflected from the defect couples back into the EMAT transducer through the magneto-elastic properties of the material.

The hysteresis method is sensitive to bulk properties.  In this method, an appreciable volume of the ferromagnetic part is subjected to a full magnetization cycle.  Changes in the hysteresis loop parameters, such as coercivity and remanence, are used as indicators of change in the physical properties in the bulk material.

We have a wide expertise in the design of optimal magnetic circuits and can assist you in designing your next non-destructive testing system.  We have designed pipeline inspection gauges (PIGs) for a range of pipe sizes, varying from ~5" to 36" and for operation at room temperatures and at elevated temperature (up to 150 °C).

When working with our engineering group, you might be asked:

  1. What are the pipe dimensions?
  2. What is the minimum field desired in the pipe?
  3. How much room is there for the magnets?
  4. What is the maximum temperature in the operating environment?

Low temperature applications manage well with Neodymium Iron Boron magnets.  In the case of elevated temperature applications (>120 °C) we use Samarium Cobalt magnets.

  1. How much magnet material will I need to saturate the pipe wall?
  2. What magnet material should I use?
  3. Will the magnet lose strength over time?
  1. Amount of magnet material and type depends on each application.  Larger pipes require more magnets and or quadrature design.
  2. We use NdFeB magnet for applications up to ~100C.  We use SmCo for higher temperature applications
  3. Typically permanent magnets are “permanent”.  However a small amount of loss due to aging of the magnet material is possible.  This loss is less at lower temperatures and in closed magnetic circuits.

The typical design process begins with you supplying an performance requirement which includes magnetic performance, physical envelope dimensions and interface description.  It will take our engineering team an average of 2 days to 5 days to design a solution.  Once an order is placed, it will take our manufacturing department 6 - 12 weeks based on design complexity to build the device.