Eddy Current inspection

The Eddy Current inspection method is conducted by inducing electrical current into electrically conductive materials. Any changes in geometry, material, or discontinuities such as cracks, pits, thinning or other anomalies will disrupt the flow of the Eddy Current and produce a readable signal.

The Eddy Current that flows through the test object is affected, not only by the properties of the material, but also by the operating frequency. As the frequency increases, the penetration of the Eddy Current into the material decreases. The signal that is developed across a probe coil is a voltage, resulting from the current being driven through the coil by the Eddy Current instrument. The signal has both amplitude and angular components which are affected by the impedance change that develops across the coil, therefore, the amplitude and angular values are dependent on the inductance of the coil. Variations in the electrical characteristics; anomalies, defects or geometry changes within the test object, will show in the signal and can be read and evaluated by a trained operator.

Instrument

At Horizon Testing Inc., we use the advanced, multi frequency Zetec MIZ-28. This instrument delivers high technology and performance for a high speed and efficient inspection of heat exchange tubing. Combining multiplexed and simultaneous injection technologies into one box, it is capable of testing most tube materials, including magnetic alloys.

SIGNAL ANALYSIS

In accordance with ASME Section V, Article 8 guidelines, the technology displays the resultant signal in a vector (X-Y) format. This form of two-dimensional display allows the data analyst to use pattern recognition for characterizing signals. This is a distinct advantage over the single dimensional strip chart (amplitude only) analysis technique.

Typically, the analysis parameters are established as a phase or angular relationship of the signal, as measured in degrees, relative to the x-axis, equating to flaw depth. The amplitude in volts, or size of the signal, will relate proportionally to the volume loss of the material, however, the phase of the signal is volumetrically dependent. The ID and OD defect phase planes are distinguishable between the angular measurements.

RESOLUTION

Resolution is the result of phase separation between flaws of varying depths, and will vary with the excitation frequency.

SENSITIVITY

Sensitivity is dependent on the type of material, grade, dimensions, wall thickness, residual stresses, operating condition such as aging, ambient temperature, tube cleanliness, and location.

External materials such as tube support plates, tube sheets and weld attachments can shunt or distort the field and may mask the indication.

Defect orientation, axial versus circumferential, and location factors, such as the roll transition or the land area, will determine the type or style of probe that is used.

TRAVERSE SPEEDS

Probe traverse speeds are dependent upon the data sampling rate, which can never be more than the lowest frequency. 

LIMITATIONS

  • The ID defect flaw plane has half the resolution of the OD defect flaw plane

  • Bobbin probes have a low resolution for circumferential anomalies

  • Fill factor is critical

  • Cannot be used on ferrous material

     

For additional information please contact us.