Mengchun Pan

Reduction of Lift-Off Effects in Pulsed Eddy Current for Defect Classification

Pulsed eddy-current (PEC) testing is an electromagnetic nondestructive testing & evaluation (NDT&E) technique and defect classification is one of the most important steps in PEC defect characterization. With pulse excitation, the PEC response signals contain more features in time domain and rich information in frequency domain. This paper investigates feature extraction techniques for PEC defect classification including rising time, differential time to peak and differential time to zero, spectrum amplitude, and differential spectrum amplitude. Experimental study has been undertaken on Al-Mn 3003 alloy samples with artificial surface defects, sub-surface defects, and defects in two-layer structures under different lift-off. Experimental results show that methods are effective to classify the defects both in single-layer structures and two-layer structures. Comparing the results of different methods, it is found that differential process can eliminate the lift-off in defect classification in both time domain and frequency domain. The study can be extended to defect classification in complex structures, where lift-off effects are significant.

Eddy current pulsed phase thermography and feature extraction

This letter proposed an eddy current pulsed phase thermography technique combing eddy current excitation, infrared imaging, and phase analysis. One steel sample is selected as the material under test to avoid the influence of skin depth, which provides subsurface defects with different depths. The experimental results show that this proposed method can eliminate non-uniform heating and improve defect detectability. Several features are extracted from differential phase spectra and the preliminary linear relationships are built to measure these subsurface defects depth.

1/f noise suppression of giant magnetoresistive sensors with vertical motion flux modulation

The 1/f resistance noise is one of the main noise sources of giant magnetoresistive sensors, which will cause intrinsic detection limit at low frequency. To suppress this noise, a vertical motion flux modulation (VMFM) scheme with high efficiency and simple structures is proposed. And the electrical coupling effect is investigated with an equivalent circuit model. We found that the electrical coupling disturbance can be suppressed by improving the symmetry of VMFM sensors. The modulation efficiency of VMFM sensors has reached 18.8%, which is higher than most prototype sensors with other flux modulation schemes.

Eddy current pulsed phase thermography for subsurface defect quantitatively evaluation

This Letter verified eddy current pulse phase thermography through numerical and experimental studies. During the numerical studies, two characteristic features, blind frequency and min phase, were extracted from differential phase spectra, and their monotonic relationships with defects depth under different heating time were compared. According to the numerical studies, 100u2009ms was employed as heating time during the improved experimental studies. The experimental results agreed with the numerical results. Based on their linear relationship with defects depths, both features can be used to measure the defects depth.

Eddy current step heating thermography for quantitatively evaluation

This Letter proposed eddy current step heating thermography (ECSHT) combing eddy current excitation with SHT. It has been verified through numerical and experimental studies that the temperature-time1/2 curve can be used to detect the subsurface defects. Separation time was defined and extracted from temperature responses as characteristic feature. Experiment studies with mild steel sample were conducted, and the experimental results showed that two features representing separation time can be used to measure the defects depth based on their linear relationships.

Integrating magnetoresistive sensors with microelectromechanical systems for noise reduction

1/f noise is the dominant detection limit of magnetoresistive (MR) sensors at low frequency. The vertical motion flux modulation (VMFM) integrating with microelectromechanical systems (MEMS) can reduce 1/f noise by tens or hundreds of times, although thermal-mechanical noise possibly has strong impact on the detection ability of VMFM sensors like common MEMS sensors. Surprisingly, the voltage noise originated from thermal-mechanical noise is actually far less than the noise base of MR sensors, which indicates a great perspective for the integration of MEMS and MR sensors.

Remedying magnetic hysteresis and 1/f noise for magnetoresistive sensors

Thermal domain hoppings cause magnetic hysteresis and 1/f resistance noise in magnetoresistive sensors, which largely degrades their response linearity and low-frequency detection ability. In this Letter, the method of constant magnetic excitation integrated with vertical motion flux modulation was proposed to remedy magnetic hysteresis and 1/f resistance noise together. As demonstrated in experiments, the response linearity of the prototype sensor is promoted by about 10 times. Its noise level is reduced to near Johnson-Nyquist noise level, and, therefore, the low-frequency detection ability is approximately enhanced with a factor of 100.

Defect identification and classification of multi-frequency eddy current test based on spectrum method

Multi-frequency eddy current test (MFECT) is an important NDT technology, and it can be applied to realize multi-parameter detection and interference elimination effectively. Aiming at the problem that conventional single-frequency eddy current test can only identify defects but cant classify defects effectively, a new MFECT technology which adopts LFM signal exciting and spectrum analysis method was proposed. The probe-coil in an ac bridge was excited by a LFM signal, and output signals of the ac bridge were amplified and sampled to detect defects property by spectrum method. These defects could be detected according to spectrum total energy variation. With the purpose of defect classification, a new feature, termed as spectrum barycenter excursion was proposed. Compared with traditional MFECT, it has the merits of small peak factor and low system cost. The agreement between theory and experiment shows that the present method is correct.

Magnetoresistance based resonance monitoring with pulse-excited planar coils

Magnetoresistance sensing is an attractive resonance monitoring technique for micro/nano-electromechanical systems, due to its merits of simplicity, effectiveness, and independence of capacitance and stress. Nevertheless, the previous schemes suffer from the uncertain magnetic disturbances. In this letter, current pulse based magnetoresistance sensing is proposed to resist this uncertainty. By energizing a pair of planar coils with current pulses, the magnetic disturbances correlated in time can be identified and eliminated in pulse intervals. The detection sensitivity is tunable by varying with the intensity of the pulsed current. Presently, an amplitude detection limit of 0.1u2009nm/√Hz has been achieved.

Simulation Analysis of Multi-Frequency Eddy Current Sensor Impedance Property

Multi-frequency eddy current test technique is applied in the industrial non-destructive testing more and more widely. Every frequency component contains the useful information and the harmful noise when the eddy current sensor is excited by several sine signals. The impedance property of the sensor is a key element in the test system, so this paper analyzed it by finite element method simulation. The simulation results show the variation law of the impedance in the air and on the non- ferromagnetic metal plate in different frequencies, and the law is helpful for the signal processing of multi-frequency eddy current testing.