Magnetizing power optimization
Magnetizing power optimization optimizes the alternating magnetic field strength that is applied by the Barkhausen noise sensor to the measured sample. Optimizing the magnetizing power is important to achieve maximum sensitivity to residual stress state and microstructural differences. If the magnetic field is too strong, the sample or the sensor will be saturated. If the magnetic field is too weak the signal to noise ratio will be poor. Both cases result in poor sensitivity to residual stress state and microstructural differences. Optimizing the magnetizing power is done by adjusting the magnetizing voltage.
1. Select procedure
There are two alternative procedures to determine magnetizing power parameters.
Procedure A is a one-point method in which the optimal magnetizing power is determined from magnetizing saturation point. This is the method for situations where residual stress state and microstructure are unknown. This method is simpler and not as accurate as the Procedure B.
Procedure B (recommended) is a two-point method in which the magnetizing power is the highest ratio calculated from high and low Barkhausen noise value. This procedure requires
- L-sample/spot with hardness in optimal level, and compressive residual stress in optimal level = low Barkhausen noise value
- H-sample/spot with hardness less than optimal, and low compressive or tensile residual stress state = high Barkhausen noise value.
NOTE: With procedure B, it is possible to determine an optimal parameter for the maximum sensitivity.
2. Prepare the sample(s)
Select sample(s) or scanning spots and mark them with a marker.
NOTE: Before measuring, check that the sample and the sensor are free of any debris and damage. Wipe the surface of the sample and sensor gently with a lint-free cloth. Mild solvent or soaps may be used on the contact surfaces of the sensor. The sample should be covered with a light oil film to improve contact and and sliding of the sensor.
NOTE: If the sample(s) have residual magnetism, it needs to be demagnetized.
3. Prepare the system (check the details in user manual)
- Installation: connect the cables
- Power up the system
- Communication: check the communication and software settings, and connect the data plotter software if needed
- Set the needed measurement settings, for example magnetizing frequency (initially 125 Hz)
4. Collect the data
- Place the sensor on the sample and make sure that the contact between the sensor and the sample is good
- Make automated magnetizing voltage sweep with data plotter software or Rollscan (this can also be done manually by step by step increasing the magnetizing voltage)
- With procedure 2, repeat with the other sample/spot
5. Analyze data and select the optimal setting
Procedure A: Initially the mp (magnetoelastic parameter) level increases as voltage is increased. At some point the mp level begins to flatten out. This is characteristic of magnetic saturation and results in a loss of sensitivity of the system. The proper setting for voltage is usually between the steepest slope and the “knee” of the curve before the saturation.
NOTE: In rare cases it’s possible that you can’t find the saturation point. In that case, please contact Stresstech support.
Procedure B: Calculate the following ratio for each voltage setting: ratio = mp H : mp L. The optimum magnetizing voltage level is the one that gives
the highest ratio.
NOTE: In rare cases with the L sample it’s possible that you can’t get a decent Barkhausen noise signal with the highest ratio voltage. In that case, please contact Stresstech support.
Magnetizing frequency optimization
The most commonly used magnetizing frequency is 125 Hz, but in some cases optimizing the frequency gains a significant increase to the sensitivity to residual stress state and microstructural differences. Magnetizing frequency affects the magnetizing voltage at which the highest ratio is found and for that reason magnetizing voltage optimization needs to be done for each frequency. For optimizing the magnetization frequency follow the instructions below:
1. Select the magnetizing frequencies
Select the frequencies, for example 50, 125, 250, and 500 Hz.
2. Optimize the magnetizing power
Perform magnetizing power optimization using Procedure B with all selected frequencies individually (see magnetizing power optimization instructions).
3. Analyze the result
Calculate the following ratio for each voltage and frequency combination: ratio = mp H : mp L.
The optimum magnetizing voltage and frequency combination is the one that gives the highest ratio.
NOTE: You can test different frequencies and smaller steps. If for example 250 Hz shows better results than 125 and 500 Hz, test frequencies nearby 250 Hz.