Effects of Stress on Magnetization Curves

Mechanical stress affects the magnetic properties of pipe steel. This change in magnetic properties affects both the applied field strength and the uniformity of the flux density around a defect. Stress in a pipeline arises due to gas pressure, residual stresses from the fabrication process, field bends, ground shifts, etc. In addition, metal-loss regions, dents, and attachments act as stress risers that increase the effects of stress even further.

Stress makes the analysis of an inspection record more difficult. The effects of stress are complicated because stress changes the overall (bulk) permeability of a steel and it changes the local permeability around a defect. A change in the bulk permeability of a steel changes the applied flux density in the pipe. These effects are a function of the magnitude and direction of stress.

A change in the local permeability at a metal-loss region produces changes in the amplitude and shape of the leakage field. These changes in signal are difficult to predict because the changes in permeability vary around the defect with variations in stress. In addition, strain affects permeability. So, permanent or plastic deformation make the signals even more difficult to analyze.

The effects of stress are greatest at low magnetization levels ^{[Crouch96, Nestleroth98a]} . This is one reason why corrosion inspections are generally performed with very high applied field levels. At high levels, the inspection signals are nearly independent of stress. Because stress effects are greatest at low magnetization levels, inspections for mechanical damage and related defects must be performed with weaker applied fields.