Exploring Pulsed Eddy Current (PEC) Responses in Ferromagnetic Cylindrical Structures

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Authors
Eddy, Ian
Keyword
pulsed eddy current , PEC , non-destructive testing , non-destructive examination , pipes , rebar , insulation , concrete , corrosion detection
Abstract
Corrosion of carbon steel pipes under insulation has a direct negative impact on the structural integrity of the pipe. Insulation prevents visual inspection of external wall corrosion. Long-time decay, up to 0.1 s, in the transient response of Pulsed Eddy Current (PEC), was examined. The transient response of a coaxial solenoidal drive-receive coil pair, oriented parallel to the longitudinal axis, was analyzed over a range of distances from the pipe (liftoff) and wall thicknesses. The single exponential long-time decay constant was dependent on liftoff, simulating a seemingly thicker pipe wall as liftoff increased. At constant liftoff, an inverse-square power law relationship of the long-time decay slope to wall thickness was observed. Pipe with non-concentric wall thickness variation provided an example of partial-circumference corrosion, which the probe was sensitive to. The decay constant increased with liftoff, except at the thickest part of a non-concentric pipe. The most significant conclusion is derived from comparing long-time decay slope with averaged neighbouring wall thicknesses. Averaging of wall thicknesses ±45o (selected window size) from probe location brings all data sets closer to an expected inverse-square relationship, when compared to the purely beneath-probe thickness. This window increases with liftoff, describing the changes in decay constant as interaction with more and more material, unless at the thickest location of a non-concentric pipe. Characterization of corroded rebar has important implications for evaluating the integrity of concrete structures. PEC measurements on rebar of varying diameter demonstrated that the long-time decay constant exhibited a power law relationship with rebar radius, in agreement with theory. Intercept of the linearly fitted long-time decay underwent an exponential decay as liftoff increased but showed a separable dependence on rebar diameter. In actual concrete structures, rebar is overlaid orthogonally to provide tensile strength in two mutually orthogonal directions. Altering probe angle with respect to the axis of a single rebar presented an intercept that decreased linearly as sine of the probe angle reached 1 (90). Response to two orthogonal rebar demonstrated a superposition of independent rebar signals at later times in the transient decay. These observed trends indicate use of PEC to characterize rebar.
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