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Eddy Current
A down side to performing eddy current is that if the crack is not visible to the human eye it can be difficult to correlate to a customer or untrained person that the squiggly mark on the screen equals a crack or flaw. A nickname that has emerged from this dilemma is ET black magic
Eddy current is dependent on a calibration block or standard: a piece of material of the same composition as the part being inspected with known indications or crack examples. Sometimes the metals have no flaws and are used to verify the conductivity of a material. With other methods, such as liquid penetrant and magnetic particle, these blocks are not required. This can represent a problem when trying to perform an inspection. If you do not have an appropriate calibration block you cannot perform the test in conformance to a governing code. This becomes of the utmost importance when inspecting tubing or pipe. The diameter of the probe and the standard need to be near optimum dimensions in order to attain acceptable inspection data.
Equipment is available in special use, single and multi-frequency capacities. Conductivity and metal sorting testers with a digital readout are special use which require less training to operate. Single frequency is usually used for basic surface and subsurface crack detection. Multi-frequency is used for pipes and tubing inspection requiring sensitivity on the surface and through the material.
High frequency equals less penetration and higher sensitivity. Low frequency equals more penetration and less sensitivity. This principle adds value to multi-frequency instrumentation that allows you to provide sensitivity and penetration at the same time. The selection of frequency is usually specified in the procedure being used to perform the inspection.
A common problem to eddy current inspection is ferromagnetic materials which create erratic data. This is the main purpose for utilizing RFT testing. Remote field testing utilizes the properties of the magnetic material to its advantage. A major draw back is the loss of data in curved sections which usually need to be followed with another method such as UTT (ultrasonic thickness) if access is available.
