Investigation of Flux Superposition in Steel using Magnetic Barkhausen Noise Tetrapole Probes

Transcription

1 Investigation of Flux Superposition in Steel using Magnetic Barkhausen Noise Tetrapole Probes P. McNairnay,, T. W. Krause,L. Clapham Department of Physics, Engineering Physics and Astronomy, Queen s University, Kingston, ON, Canada, K7L 3N6 Department of Physics, Royal Military College of Canada, Kingston, ON, Canada K7K 7B

2 Magnetic BarkhausenNoise Magnetic Barkhausen noise (MBN) refers to changes in magnetization, occurring in ferromagnetic materials, that result from the abrupt motion of domain walls as they overcome pinning sites Surface MBN is typically generated using a dipole electromagnet and detected using a surface mounted pickup coil MBN has been correlated with among other properties: Residual stress Microstructure Anisotropic properties require angular-dependent measurements

3 Magnetic Barkhausen Noise and BN E 3 Barkhausen Noise Magnetic Flux Density 3 Pickup Coil Voltage (mv) Magnetic Flux Density (mt) Magnetic Barkhausen Noise Energy (BN E ) θ α cos β Phase ( ) Typical MBN Pickup Voltage Signal MBN E angular dependence fitting equation for dipoles Dipole Probe Design Tetrapole Probe Design 3

4 Flux Controlled MBN System Flux/Voltage Control Circuit LabVIEW Measurement Software Tetrapole Probe Dipole Probe

5 System DesignBenefits Tetrapole allows for rapid electronic rotation of the excitation field reducing measurement time (<6 s) Flux/Voltage control improves repeatability of measurements by counteracting probe lift-off effects System Design Issues Validity of flux superposition in non-linear, hysteretic materials is uncertain Flux control at probe poles does not ensure controlled flux in sample under pickup coil 5

6 Flux Superposition H y H Flux superposition is assumed to be achieved with a linear vector addition: θ H x /μ cos /μ sin Magnetic flux in controlled at the pole tips but µ is not isotropic and non-linear FEM modelling has demonstrated flux superposition is achieved with the tetrapole but such models do not account for material anisotropy or non-linearity 6

7 Flux SuperpositionInvestigation Dipole Measurements Tetrapole Measurements 3 3 7

8 Initial Results on Mild Steel Dipole Tetrapole at 9 * 9 6 Dipole MBNe Dipole Fit MBN Energy (mv s) αcos (θ φ) + β α =.86 ±. β =.67 ±.6 φ =. ±.3 R =.9 Dipole Fit Equation BN = α ( θ φ ) E cos E + β - φ E is the magnetic easy axis direction MBN Energy (mv s) *One tetrapole pair is aligned with the easy axis 7 3 Tetrapole Orientation

9 Initial Results on Mild Steel Tetrapole at 5 Tetrapole at * MBN Energy (mv s) MBN Energy (mv s) Tetrapole Orientation *One tetrapole pair is aligned with the easy axis Tetrapole Orientation 9

10 Initial Observations Tetrapole results at 9 appear similar to dipole but with poorer fit to angular dependent equation: BN E ( θ φ ) β = E α cos + Tetrapoleresults at other angles (5,3,5...) indicate that MBN energy generated by the tetrapole, arises very differently than for a dipole. Generating a four lobe pattern with lobes located in the probe pole directions Similar results have been observed for measurements on carbon steel, oriented and non-oriented laminates and HY-8. The presumption that flux superposition in a tetrapole can simulate a dipole is incorrect A model for the generation of Barkhausen noise under flux superposition is required

11 Empirical Fitting Equation!cos " # sin " # $ cos % Independent variables: # -Tetrapole angle Superposition angle Dependent variables:!,, $,% Constants -Easy axis angle Fitting Results: = -.3! =.9 =.9 $ =.7 % = -. & =.83

12 Development of Tetrapole Theory MBN energy is a function of field/flux amplitude 75 5 MBN Energy, mv s Flux, mt At a superposition angle of 5 the orthogonal fields are.77 of the maximum field The dependence on field amplitude and the observed minimum at 5 suggest that the MBN energy observed by the tetrapole may be generated independently by each pole pair.

13 Experimental Evaluation of Theory MBN energies were measured for each pole pair independently by setting the field amplitude at the value required for a given superposition angle and switching off the other pole pair H y H H x -Field amplitude of - pole pair H y -Field amplitude of -3 pole pair H-Field amplitude of superposition θ H x If pole pairs are acting independently under flux superposition, the sum of MBN energies generated by H x andh y should equal the MBN energy generated by H 3

14 Results Tetrapole at 8 TF Ind TF Sup MBN Energy, mv s Angle, degrees

15 Results Tetrapole at 5 6 MBN Energy, mv s Angle, degrees T5F Ind T5F Sup

16 Discussion Comparing the dipole and tetrapole results clearly indicates that the MBN generated by the tetrapole arises very differently than by a dipole Tetrapoleresults were, however, affected by anisotropy, as evidenced by the stretching of the four lobe pattern in the direction of the easy axis The cause of the pattern has not yet conclusively been determined, however, the proposed model of independent and orthogonal dipoles appears to describe the results well.

17 Discussion Potential explanations for these results include: ) The angle and magnitude of the superposition field is only valid in a small region at the centre of the probe, leading to a non-uniform magnetization in the surrounding region, whose Barkhausen events are detected in the pickup coil ) The orthogonal geometry of probe poles permits independent growth of magnetic domains in the direction of the pole pairs influencing the domain structure or superposition field in the region around the pickup coil

18 Conclusions The MBN energy generated under superposition appears similar to the MBN energy generated by two perpendicular dipoles acting independently of each other Attempts to develop a theoretical Tetrapole MBN energy fitting equation in the same manner as the Dipole fitting equation, have not been able to explain the results Empirical fitting equations have been developed, and have been shown to identify the magnetic easy axis direction Continuing work lies in developing a theoretical model for domain wall motion under the condition of two superimposed magnetic fields.