Wenwei Ren

Transient-spatial pattern mining of eddy current pulsed thermography using wavelet transform

Eddy current pulsed thermography(ECPT) is an emerging Non-destructive testing and evaluation(NDT & E) technique, which uses hybrid eddy current and thermography NDT & E techniques that enhances the detectability from their compensation. Currently, this technique is limited by the manual selection of proper contrast frames and the issue of improving the efficiency of defect detection of complex structure samples remains a challenge. In order to select a specific frame from transient thermal image sequences to maximize the contrast of thermal variation and defect pattern from complex structure samples, an energy driven approach to compute the coefficient energy of wavelet transform is proposed which has the potential of automatically selecting both optimal transient frame and spatial scale for defect detection using ECPT. According to analysis of the variation of different frequency component and the comparison study of the detection performance of different scale and wavelets, the frame at the end of heating phase is automatically selected as an optimal transient frame for defect detection. In addition, the detection capabilities of the complex structure samples can be enhanced through proper spatial scale and wavelet selection. The proposed method has successfully been applied to low speed impact damage detection of carbon fibre reinforced polymer(CFRP) composite as well as providing the guidance to improve the detectability of ECPT technique.

Nondestructive Evaluation of Early Contact Fatigue Using Eddy Current Pulsed Thermography

Cyclic loading can lead to fatigue damage on the surface or subsurface of a gear tooth. In order to evaluate the contact fatigue damage, this paper applies eddy current pulsed thermography (ECPT) for fatigue damage characterization at different intervals of the loading cycle. The challenging task of fatigue evaluation is one of solving the qualitative microstructure state characterization before microcrack initiation. This paper proposes the thermooptical flow entropy tracking method to trace the heat flow and characterize the degree of fatigue damage while in this status no macrodefects appears using ECPT. In addition, the thermooptical flow is mathematically modeled to yield several desirable unique properties to evaluate minor variations in the microstructure of the material during the fatigue process. The nondestructive evaluation of fatigue damage with ECPT thermooptical flow is derived. The relationship between the entropy of thermooptical flow and the degree of contact fatigue at an early stage is established. The experimental study validates that the proposed method can detect and characterize the implicit damage and that the entropy of thermooptical flow is highly correlated with fatigue cycles which has the potential to evaluate the degree of fatigue damage.

Early contact fatigue evaluation of gear using eddy current pulsed thermography

One of the most common gear damages is contact fatigue damage, which is difficult to be detected at an early stage. As one of the integrative non-destructive testing and evaluation techniques (NDTE) of eddy current and thermography, eddy current pulsed thermography (ECPT) has been applied for fatigue damage characterization. However, quantitative material state characterization before microcrack initiation do not be solved completely in previous work of ECPT. In this paper, the heat flow can be tracked by using optical flow method and the transient thermal behaviour among areas where the gear flank is suffered contact fatigue and not suffered contact fatigue is evaluated. The degree of fatigue is quantified across a thermal image sequence. The relationship among optical flow, the transient thermal behavior and microstructure of gear fatigue at an early stage is established. The results show that the fatigue damage of gear can be well evaluated by using this method.

Repeatability study of magnetic domain wall behaviors for stress characterization

Magnetic domain behaviors directly characterize the macroscopic properties of ferromagnetic material, the understanding of domain wall dynamic is a way to explain the characteristics of magnetization process and performance of material. This paper presents the repeatability study of domain wall behavior for stress characterization. A high grain-oriented (HGO) electric steel is employed for domain textures observation using magneto-optical Kerr effect (MOKE) microscopy. Three conditions of different stress load, magnetic disturbance and time are considered for the domain wall repeatability analysis. The experimental results indicate that domain behavior can be applied for strain measurement.

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.