Multiple yoke eddy current technique for detection of surface defects on metal components covered with marine growth

A method of detecting a defect on a metal component coated with irregular marine growth, the method comprising the steps of:

• winding an excitation coil around at least two magnetic U-shaped yokes;
• placing a magnetic flux sensor underneath each U-shaped yoke, each magnetic flux sensor being differentially connected with respect to each other;
• placing each magnetizing U-shaped yoke on the marine growth over the component;
• applying an alternating current to the excitation coil for producing an alternating magnetic field through each yoke thereby inducing an eddy current in the component parallel to a plane formed by said magnetic flux sensor underneath each U-shaped yoke, both the alternating magnetic field and the eddy current having a magnitude, a phase and a direction;
• moving the at least two U-shaped yokes along the cover of the component for scanning for defects;
• and
• monitoring the alternating magnetic field and the eddy current by the sensor for changes in at least one of the magnitude, the phase and the direction.

A device for detecting a defect on a metal component having a cover, the device comprising:

• at least two magnetizing U-shaped yokes, each U-shaped yoke having a body supported by at least two legs, the legs for contacting the cover and supporting the body over the cover such that when the U-shaped yoke is placed on the cover of the component, a space is formed between the body, the legs and the cover, each U-shaped yoke being moveable about the cover;
• an excitation coil wound around the body of each U-shaped yoke between the legs for receiving an alternating current and producing an alternating magnetic field through each U-shaped yoke thereby inducing an eddy current in the component parallel to a plane formed by said magnetic flux sensor underneath each U-shaped probe, the alternating magnetic field and the eddy current having a magnitude, a phase and a direction;
• and
• at least one magnetic flux sensor located within the space of each U-shaped yoke, each of said magnetic flux sensors being differentially connected to another, said sensors detecting changes in the magnitude, the phase and the direction of the alternating magnetic field and the eddy current.