Eddy current testing to detect welded zones in drawn wires

This thesis discusses how to detect the defects of welded steel wires by eddy current testing. The theoretical bases of the method are initially addressed. The reference parameter for eddy current testing is the impedance of the probe; the interaction between different kind of steel and the probe is modeled using lumped parameter schemes. The effects of permeability, excitation frequency and fill-factor on the complex impedance are analyzed and discussed. The necessity of magnetic saturation of ferromagnetic steels to reduce the noise has been exploited. An automatic procedure to identify the coil parameters for common steel testing has been implemented in EXCEL. Finally, 3 types of wire, a 3mm stainless steel and 2 common steel wires in 1mm and 3mm diameter, have been tested by different frequencies to find the optimum frequency to detect the defects. There is a wide range of excitation frequency for stainless steels however for each ferromagnetic steel there is an optimum frequency which has highest signal to noise ratio. For 3mm common steel wire by fill-factor of 1 optimum frequencies are the low pass filtered 30 kHz signal and low pass and high pass filtered 40 kHz signal which have the highest signal to noise ratio of around 7 and in our tests for 1mm common steel wire of 1/3 fill-factor optimum frequencies are 30, 40 and 50 kHz of low pass filtered signals and low pass and high pass filtered 30 kHz signal by signal to noise ratio of 3 that are the most accurate and clean signals to detect the weld defects.