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Eddy current techniques using absolute probes find limited application to weld testing. Signals from the defects are masked in noise produced by lift-off, probe wobble and local changes in the permeability of the weld. Also, variations in the coil impedance caused by defects in the test specimen are often minuscule in comparison to the quiescent value of the coil impedance. These reasons led previous researchers to conclude that reliable detection or sizing of weld defects was difficult using Eddy current methods. Differential probes overcome these shortcomings. They are being used increasingly in flaw detection and are ideal for testing tube geometries. In this thesis, an approach using differential probes for inspection of weld defects in small diameter tubing is described. A finite element code is used to model the weld region and defects. The validity of this code is assumed, as proven by previous research, and parameters in the 'weld region moder are changed to obtain nwnerical results that match experimental results. Simple defects are simulated in the 'weld region model' developed and compared with experimental results to cross check the validity of the code. The comparison of numerical and experimental results is also used to study weld region conductivity. An approach to overcome the problem of permeability variations in the case of ferromagnetic tube inspection is described.

Eddy current inspection of weld defects in tubing