Detection of Localized Plastic Deformation in Pipelines Using the Nonlinear Harmonics Method

    Published: Jan 2001

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    Pipelines can experience localized plastic deformation from third-party damage, rock impingement, and differential settlement that results in bending or wrinkling of the pipe wall. Cold working and stresses associated with these regions can result in cracking, which can lead to leaks or catastrophic failure. The risk of significant property damage or personal injury from such failures makes detection of localized deformation very desirable. The nonlinear harmonics (NLH) method is a magnetic approach for detecting stresses and plastic deformation. The NLH method is based on applying a sinusoidal magnetic field at a given frequency and then detecting the third harmonic frequency of the magnetic induction. The harmonic is generated because of the nonlinear permeability of the pipeline steel. Because of the magnetoelastic effect, stress and plastic deformation affect the magnetic properties and thus the harmonic signals.

    The NLH method was investigated for application to pipelines. Data were obtained from pipe specimens having plastically deformed regions consisting of dents and gouges of varying severity. Results show that plastically deformed regions can be detected by the NLH method. Because the stress field extends significantly outward from the deformed region, NLH can also detect the damage without scanning directly over the deformed region.


    plastic deformation, nonlinear harmonics (NLH), rock impingement, differential settlement, magnetic properties

    Author Information:

    Burkhardt, Gary L.
    Principal scientist and staff engineer, South-west Research Institute, San Antonio, TX

    Crouch, Alfred E.
    Principal scientist and staff engineer, South-west Research Institute, San Antonio, TX

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP42143S

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