Yunlai Gao

Eddy Current Pulsed Thermography with Different Excitation Configurations for Metallic Material and Defect Characterization.

This paper reviews recent developments of eddy current pulsed thermography (ECPT) for material characterization and nondestructive evaluation (NDE). Due to the fact that line-coil-based ECPT, with the limitation of non-uniform heating and a restricted view, is not suitable for complex geometry structures evaluation, Helmholtz coils and ferrite-yoke-based excitation configurations of ECPT are proposed and compared. Simulations and experiments of new ECPT configurations considering the multi-physical-phenomenon of hysteresis losses, stray losses, and eddy current heating in conjunction with uniform induction magnetic field have been conducted and implemented for ferromagnetic and non-ferromagnetic materials. These configurations of ECPT for metallic material and defect characterization are discussed and compared with conventional line-coil configuration. The results indicate that the proposed ECPT excitation configurations can be applied for different shapes of samples such as turbine blade edges and rail tracks.

Electromagnetic pulsed thermography for natural cracks inspection

Emerging integrated sensing and monitoring of material degradation and cracks are increasingly required for characterizing the structural integrity and safety of infrastructure. However, most conventional nondestructive evaluation (NDE) methods are based on single modality sensing which is not adequate to evaluate structural integrity and natural cracks. This paper proposed electromagnetic pulsed thermography for fast and comprehensive defect characterization. It hybrids multiple physical phenomena i.e. magnetic flux leakage, induced eddy current and induction heating linking to physics as well as signal processing algorithms to provide abundant information of material properties and defects. New features are proposed using 1st derivation that reflects multiphysics spatial and temporal behaviors to enhance the detection of cracks with different orientations. Promising results that robust to lift-off changes and invariant features for artificial and natural cracks detection have been demonstrated that the proposed method significantly improves defect detectability. It opens up multiphysics sensing and integrated NDE with potential impact for natural understanding and better quantitative evaluation of natural cracks including stress corrosion crack (SCC) and rolling contact fatigue (RCF).

Motion-induced eddy current thermography for high-speed inspection

This letter proposes a novel motion-induced eddy current based thermography (MIECT) for high-speed inspection. In contrast to conventional eddy current thermography (ECT) based on a time-varying magnetic field created by an AC coil, the motion-induced eddy current is induced by the relative motion between magnetic field and inspected objects. A rotating magnetic field created by three-phase windings is used to investigate the heating principle and feasibility of the proposed method. Firstly, based on Faraday’s law the distribution of MIEC is investigated, which is then validated by numerical simulation. Further, experimental studies are conducted to validate the proposed method by creating rotating magnetic fields at different speeds from 600 rpm to 6000 rpm, and it is verified that rotating speed will increase MIEC intensity and thereafter improve the heating efficiency. The conclusion can be preliminarily drawn that the proposed MIECT is a platform suitable for high-speed inspection.This letter proposes a novel motion-induced eddy current based thermography (MIECT) for high-speed inspection. In contrast to conventional eddy current thermography (ECT) based on a time-varying magnetic field created by an AC coil, the motion-induced eddy current is induced by the relative motion between magnetic field and inspected objects. A rotating magnetic field created by three-phase windings is used to investigate the heating principle and feasibility of the proposed method. Firstly, based on Faraday’s law the distribution of MIEC is investigated, which is then validated by numerical simulation. Further, experimental studies are conducted to validate the proposed method by creating rotating magnetic fields at different speeds from 600 rpm to 6000 rpm, and it is verified that rotating speed will increase MIEC intensity and thereafter improve the heating efficiency. The conclusion can be preliminarily drawn that the proposed MIECT is a platform suitable for high-speed inspection.

Emissivity correction of eddy current pulsed thermography for rail inspection

Eddy current pulsed thermography (ECPT) is an emerging non-destructive testing and evaluation (NDT&E) technique for conductive material. However, surface emissivity variation of metallic object influences accurate temperature mapping in infrared thermography, which leads to false alarm of defect detection. This paper proposes a new emissivity correction method for ECPT based on spectral reflectance measurement in infrared and visible wavelengths. A graphical indicator was calculated based on normalized difference vegetation index (NDVI) algorithm and utilized for reducing the influence of emissivity variation. Experimental studies were conducted on steel plate and rail samples with different calibrated emissivity to verify its capability for artificial and natural rail cracks characterization. Results demonstrated this method can effectively improve defect detectability of ECPT for rail inspection.

The effect of stress on the domain wall behavior of high permeability grain-oriented electrical steel

This paper studies the effect of applied tensile stress on the hysteresis loop and magnetic microstructure characteristics with domain wall (DW) behavior based on the magneto optical imaging. Both B-H curve and magnetic DW behavior of high permeability grain oriented (HGO) electrical steel sample are measured and validated to be highly correlated with the applied tensile stresses by using a longitudinal Magneto-Optical Kerr Effect microscopy. Specially, the optical flow (OF) algorithm is used to track the DW motion, and the velocity of DW movement is verified to be decreased against the increase of tensile stress based on the OF velocity field. The variation of hysteresis loop is also closely related with change of speed of DW motion. This study provides detailed understanding on the relationship between macro and micro magnetic properties of HGO steel under tensile stress.

Ferrite-yoke based pulsed induction thermography for cracks quantitative evaluation

This paper proposes a new ferrite-yoke based pulsed induction thermography (PIT) for cracks visualization and quantitative evaluation. The proposed approach combines the advantages of both alternating current field measurement (ACFM) and eddy current pulsed thermography (ECPT) in physical level. It can produce relative uniform heating in surface and subsurface of ferromagnetic material and have an open-view for infrared imaging. The structure and physical mechanism of the ferrite-yoke based PIT are introduced. Numerical simulations are conducted to understand the general behavior and multi-physical interactions of the proposed method with stimulated magnetic flux, eddy current and induced heating in samples. Experiments are implemented to verify simulation results and identify the characteristics of the proposed method for characterization of the artificial and natural cracks. Results demonstrate the advantages of the proposed method on cracks quantitative evaluation in terms of combined magnetic flux and eddy current heating to defect orientations, uniform inductive stimulation and open-view for cracks visualization. The proposed method greatly enhances the capability for cracks detection and quantitative evaluation compared with previous induction coils.