Although MFL is the most commonly used technique for examination of pipelines for corrosion, methods are being developed to detect mechanical damage. The potential of the MFL method to detect mechanical defects was quantified for the Gas Research Institute ^[Davis96], and it is being further developed for the Department of Transportation ^{[Afzal99, Davis99, Hwang97, Ivanov97, Ivanov98, Ivanov99, Xie97]}. Mechanical damage defects are commonly described as dents, gouges, or a dents with a gouge.

From an MFL prospective, gouges should be divided into two types:

• where the steel is worked to a point where significant residual stresses remain and a potential change in microstructure exists,
• where steel is moved or removed

The magnetic response of these two types of gouges are different.

A gouge causes a reduction in flux leakage while removed metal causes an increase. Also, the flux leakage signals from gouges and dents tend to have small peaks at the beginning and end of a damaged region, while the signals from removed metal have small valleys. Most mechanical damage defects are a combination of gouges, dents, and metal loss. The amount of removed metal, a critical parameter in assessing damage, can be obscured by the residual stresses and geometry changes associated with denting and gouging.

The relative amount of denting and gouging depends on the internal pressure and the mechanical properties. Pipeline operators primarily use only geometric information (dent depth and gouge size) in their serviceability calculations. Additional information about the stress, strain, and microstructure would help define the initial damage extent and the current condition of the mechanically damaged region.