Low-Frequency Electromagnetic Detection Method For Large-Scale Damage Of Ferromagnetic Materials Based On Broadband Excitation

A low-frequency electromagnetic detection method for large-scale damage of ferromagnetic materials based on broadband excitation, comprising the steps of:

• 1) a piece of ferromagnetic steel plate is selected as a test piece;
• wherein the thickness of the ferromagnetic steel plate is between 12 mm-16 mm;
• wherein there are crack defects of different depths on the surface of the ferromagnetic steel plate;
• wherein each crack defect has the same dimensions except for the depth;
• and wherein the maximum depth is less than the thickness of the ferromagnetic steel plate;
• 2) adjust the signal pickup direction of a magnetic sensor inside a low-frequency electromagnetic sensor to make the detection direction parallel to a surface of the test piece;
• wherein the detection result is the tangential leakage magnetic field strength on the surface of the test piece;
• wherein magnetic field strength in this direction is sensitive to the depth of defect;
• 3) place the low-frequency electromagnetic sensor on the surface of the ferromagnetic steel plate so that the low-frequency electromagnetic sensor is in a defect-free reference area of the ferromagnetic steel plate, and adjust the lift-off distance between the low-frequency electromagnetic sensor and the ferromagnetic steel plate to be less than 1 mm;
• 4) adjust a function generator to generate a Chirp signal with fixed output voltage and bandwidth for excitation, and activate a power amplifier;
• using a digital oscilloscope, simultaneously display an excitation signal and an electromagnetic detection signal of the low-frequency electromagnetic sensor in the defect-free reference area of the ferromagnetic steel plate;
• assign detected signal as a reference signal when there is no defect and collect the no-defect reference signal;
• 5) place the low-frequency electromagnetic sensor at one side of a crack, and under the same excitation conditions, manually control the moving direction of the low-frequency electromagnetic sensor so that the moving direction is perpendicular to the longitude of the crack;
• activate a power amplifier;
• whenever the low-frequency electromagnetic sensor is placed at a detection point, simultaneously display on the digital oscilloscope the excitation signal and electromagnetic detection signal of the low-frequency electromagnetic sensor at the detection point, collect the broadband detection signals of all detection points as defect detection signals;
• 6) process the collected reference signals and defect detection signals via a computer, by the steps of:
• 1, perform frequency domain analysis on reference signals and each defect detection signal to obtain the reference signal spectrum and the defect detection signal spectrum at different positions;
• 2, Euclidean distance calculation formula is used to obtain the frequency domain Euclidean distance between detection the signal and the reference signal at each detection point;
• 3, using the frequency domain Euclidean distance as a defect characteristic parameter to draw a curve of Euclidean distance versus detection position;
• 7) flip the tested piece and repeat the steps 3) to 6) on other surface of the test piece;
• obtain a curve of Euclidean distance versus detection position when defect appears below the other surface of the tested piece;
• and
• 8) based on the Euclidean distance curves, the surface defects of ferromagnetic steel plates at different depths are quantitatively characterized.