Ali Sophian

Nondestructive Testing and Transient Electromagnetic-Thermal NDT

Nondestructive testing (NDT) is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component, or system without causing damage. NDT technologies have a crucial role in various industrial applications in the aerospace, automotive, manufacturing, petrochemical, transportation, civil, marine, and defense industries. There are currently hundreds of NDT methods being investigated or applied in the science and technique fields, which can be categorized as optical, acoustic, electromagnetic, infrared/thermal, or radiography NDT. Transient electromagnetic-thermal NDT techniques with a lot of advantages, such as high speed, great depth, high sensitivity, width spectrum, low cost, and easy quantification, are widely investigated in various industrial fields.

Unsupervised Sparse Pattern Diagnostic of Defects with ECPT

This chapter proposes an unsupervised method for defect diagnosis using eddy current pulsed thermography (ECPT). The proposed method is fully automated and does not require manual selection of the specific thermal frame images for defect diagnosis. The core of the method is the hybrid of a physics-based inductive thermal mechanism and a signal processing-based pattern extraction algorithm using sparse greedy-based principal component analysis (SGPCA). An internal functionality is built into the proposed algorithm to control the sparsity of SGPCA and to render better accuracy in sizing the defects. The proposed method is demonstrated for automatically diagnosing defects on metals and accurately sizing the defects. Experimental tests and comparisons with other methods have been conducted to verify the efficacy of the proposed method. Very promising results have been obtained where the performance of the proposed method is very near to human perception.

Volume or Inside Heating Eddy Current Thermography

This chapter presents volume heating thermography (VHT) and inside heating thermography (IHT) for analyzing advanced composite materials through these electromagnetic excitations. The physical principles of VHT and IHT for defect quantification have been investigated. Several specific VHT and IHT methods have been built in the style of (square) pulse and step analysis in the time domain, and phase analysis in the frequency domain (i.e., volume lock-in thermography). 1D solutions, simulation involving 3D finite element modeling, and experimental studies demonstrate that polytetrafluoroethylene (PTFE) inserts, impact, and delamination in carbon fiber reinforced polymer (CFRP) can be qualitatively detected and characterized using these proposed methods. This is especially true in phasegram after eliminating nonuniform heating effects and periodic structures. Through electromagnetic induction, microwave, and terahertz wave, VHT and IHT have the potential to be used for inspection and characterization of composites, polymers, and biomaterials.

Volume Heating ECT and Phase Analysis for CFRP Evaluation

This chapter presents eddy current volume heating thermography (ECVHT) and phase analysis for delamination and impact inspection in carbon fiber reinforced plastics (CFRP). The proposed method has been verified through experimental studies under both transmission and reflection modes. After discrete Fourier transform (DFT) of temperature responses, the phasegram and phase spectra can be used to image and characterize interface delamination and impacts in CFRP due to elimination of the nonuniform heating effect and carbon fiber structures. With the whole temperature response processed by DFT, carbon fiber structures and delamination can be differentiated due to periodic oscillation of phase spectra. With temperature response in the cooling phase processed by DFT, some characteristic features can be extracted to construct new phase images according to the shape of the phase spectra. In summary, using ECVHT and phase analysis greatly enhances image characterization for delamination and impact compared to conventional visible optical inspection system and eddy current pulsed thermography (ECPT).

Heat Conduction Based Eddy Current Pulsed Thermography (ECPT) for Defect Evaluation

This chapter is focused on defect evaluation using longitudinal and lateral heat conduction of eddy current pulsed thermography (ECPT). In the first section, characterization of wall thinning defects and inner defects in steel is investigated in the time domain. Characterization was performed under transmission and reflection modes through 1D analytical analysis, 3D numerical studies, and experimental studies. In the second section, logarithmic analysis to quantify the depth of subsurface defects is proposed and verified through numerical and experimental studies. Results show that temperature-time curves in the logarithm domain can be used to detect subsurface defects, and separation time can measure defect depth. The third section introduces lateral heat conduction (LHC) for the detection of cracks parallel to the inductive coil (parallel cracks) and natural oblique cracks. Due to the significant temperature gradient, the spatial derivative and gradient were proposed to improve detectability of natural oblique cracks in rail.

Magnetic Sensor Based Pulsed Eddy Current for Defect Detection and Characterization

This chapter discusses one class of nondestructive testing (NDT) pulsed eddy current (PEC), where a magnetic sensor is used for sensing the magnetic field in order to detect and characterize the defects in the specimen. The chapter starts with an introduction to PEC, followed with an example of a PEC system. The usages of signal processing and feature extraction using principal component analysis (PCA) and wavelet analysis on PEC signals are then presented. Finally, its applications in the inspection of ferromagnetic and nonferromagnetic materials by using both conventional features and the PCA-based features are presented with relevant experimental results.

Physics-Based Modeling and Pattern Mining of ECPT

The electromagnetic mechanism of Joule heating and thermal conduction in conductive material characterization broadens their scope for implementation in real thermography-based nondestructive testing and evaluation (NDT&E) systems. They impart sensitivity, conformability, and allow fast imaging detection, which is necessary for efficiency. The issue of automatic material evaluation has not been fully addressed by researchers and it marks a crucial first step in analyzing the structural health of the material, which in turn sheds light on understanding the production of the defect mechanisms. In this study, we bridge the gap between the physics world and mathematical modeling world. We generate a physics–mathematical modeling and mining route in the spatial-, time-, frequency-, and sparse-pattern domains. This is a significant step toward realizing the deeper insight in eddy current pulsed thermography (ECPT) and automatic defect identification. This renders ECPT a promising candidate for the highly efficient and yet flexible NDT&E.

Hall-Device-Based PEC Features for Material Properties Evaluation and Defect Detection

Pulsed eddy current (PEC) response is a complex mix of many factors, including conductivity, permeability, lift-off, and material thickness variation, which should all be taken into account in PEC testing. The influences of material properties on PEC responses in the time domain are investigated and normalization technique is used to reduce the lift-off effect. After this, two time-domain features, representing conductivity and permeability (magnetic field intensity) are extracted. These features are utilized to measure stress in an aluminum alloy, to detect defects in a honeycomb sandwich structure, to evaluate low-energy impact defects in CFRP materials, and to characterize atmospheric corrosion on steel samples.

Pulsed Inductive Thermal Wave Radar (PITWR)

Major problems of eddy current thermography include the compromise between depth dynamic range and resolution, the influence of surface emissivity variation, and the low defect detectability. This chapter presents pulsed inductive thermal wave radar (PI-TWR) by introducing cross correlation (CC) matched filtering in eddy current pulsed thermography (ECPT). CC phase and CC peak phase delay are used as characteristic features. The proposed method was verified through numerical and experimental studies, where a steel sample and a carbon fiber reinforced polymer (CFRP) sample were tested under transmission and reflection modes. The results illustrate a significant improvement in the dynamic range, depth resolution, emissivity variation reduction, and detectability of subsurface defects and inside delamination for the nondestructive testing applications.

Eddy Current Step or Time-Resolved Thermography (ECST)

In this chapter, eddy current step thermography (ECST) or eddy current time-resolved thermography (ECTRT) is proposed. In conventional ST, heat is most commonly generated by powerful incandescent lamps. IR radiation outputted by these lamps will saturate IR camera signals during the time the lamps are on. The key advantage of using eddy current heating is that no IR radiation is generated by the heating source during the heating time. This makes ECST an experimentally feasible approach. Quantitative analysis using the temperature rise response during step heating has been investigated through numerical and experimental studies. Through numerical studies, two characteristic features representing separation time were extracted from temperature responses, and their monotonic relationships with defect depth were obtained. A steel sample with four subsurface defects was tested during experimental studies. Based on their linear relationships, both features were used to measure defect depth.