Xiaokang Yin

Detecting surface features on conducting specimens through an insulation layer using a capacitive imaging technique

This paper describes the application of the capacitive imaging (CI) technique to the detection of surface features on conducting specimens. The fundamental theory of the CI technique was briefly described. Comparison was made between the CI technique and the eddy current approach. A two-dimensional finite-element method was employed to model the electric field distribution from the CI probe, and how it interacts with conducting specimens through an insulation layer. Experiments using prototype CI probes were carried out. Effects of electrical conditions of the conducting specimen, lift-off distances and thicknesses of insulation between the CI probe and the conducting specimen were studied. The proof-of-concept results indicated that the CI technique is sensitive to surface features on conducting specimens through a significant air gap and/or insulation layer, thus could be used to detect Corrosion Under Insulation (CUI).

Air-coupled ultrasonic spectroscopy of highly damping materials using pulse compression

Air-coupled ultrasonic spectroscopy is described, whereby the output from a pulse compression system is used. It is demonstrated that the cross-correlation operation used within a pulse-compression system preserves amplitude and phase information. This approach allows the signal-to-noise ratio and, hence, signal-detection capability to be improved by the cross-correlation, while allowing noncontact spectral information for solid samples to be obtained. Results are presented for chocolate samples, where measurements of interest to the food industry have been obtained.

Preliminary studies on the design principles of capacitive imaging probes for non-destructive evaluation

As a novel electromagnetic non-destructive evaluation (NDE) technique, capacitive imaging (CI) has been successfully used on a various types of materials, including concrete, glass/carbon fibre composite, steel, etc. In this work, studies on the design principles for the CI probes were presented. Firstly, measures for the evaluation of the CI probe performance are presented. The concept of volume of influence is then raised, to evaluate the imaging ability of a given CI probe. Examples of CI probe designs are then provided, namely symmetric and concentric geometries. Design factors, such as separation between electrodes, guard electrodes, and electrode shapes are also discussed and examined experimentally. CI scans were performed on two typical specimens, namely Perspex and aluminium, to evaluation the imaging performance of CI probes with different geometries.

Negative measurement sensitivity values of planar capacitive imaging probes

The measurement sensitivity distribution of planar capacitive imaging (CI) probes describes how effectively each region in the sensing area is contributing to the measured charge signal on the sensing electrode. It can be used to determine the imaging ability of a CI probe. It is found in previous work that, there are regions in the sensing area where the change of the charge output and the change of targeting physical parameter are of opposite trends. This opposite correlation implies that the measurement sensitivity values in such regions are negative. In this work, the cause of negative sensitivity is discussed. Experiments are also designed and performed so as to verify the existence of negative sensitivity and study the factors that may affect the negative sensitivity distributions.

Developing and testing of a novel portable ACFM intelligent instrument

Considering the industry demand for enhancing the mobility and portability the nondestructive testing instrument, a novel portable Alternating Current Field Measurement (ACFM) intelligent instrumentis developed and tested in this paper based on the ACFM principle. The CPU and DAQ (data acquisition) module, core hardware of the portable ACFM intelligent instrument, are designed miniature and modular to make it possible to realize both high speed data acquisition and processing in function and portability in structure. The ACFM defect intelligent recognition software is developed to process data for crack sizing. Finally, the real crack detection experiments are designed and conducted to test the efficiency of the portable ACFM instrument. The results show that the portable instrument can recognize cracks intelligently and accurately and it is easy to use and convenient to carry.

Modeling and simulation of crack detection for underwater structures using an ACFM method

Considering the influence of seawater environment, numerical model of the alternating current field measurement (ACFM) method was built for underwater structure surface defect detection in this article. Based on ACFM principle and ANSYS simulation software, finite element simulation was performed to investigate rules and characteristics of the electromagnetic signal distributions in regions with defect, which are verified by the underwater artificial crack detection experiment. The experiment results show that the distributions of electromagnetic signals picked up in the artificial crack experiment are accord with the simulation results and the numerical model is validated.

CAPACITIVE IMAGING TECHNIQUE FOR NDE

The Capacitive imaging (CI) technique is introduced in this paper as a non‐destructive evaluation (NDE) tool. The fundamentals of the CI technique are presented. Finite element simulations were carried out to demonstrate the physical principles of this technique and characterize the CI probes. The potential of this technique for making realistic predictions to the in‐service performance of practical structures is then shown in detecting corrosion under insulation (CUI) and indentifying cracks and voids in concrete.

Detection and evaluation of weld defects in stainless steel using alternating current field measurement

Stainless steel is widely used in nuclear power plants, such as various high-radioactive pool, tools storage and fuel transportation channel, and serves as an important barrier to stop the leakage of high-radioactive material. NonDestructive Evaluation (NDE) methods, eddy current testing (ET), ultrasonic examination (UT), penetration testing (PT) and hybrid detection method, etc., have been introduced into the inspection of a nuclear plant. In this paper, the Alternating Current Field Measurement (ACFM) was fully applied to detect and evaluate the defects in the welds of the stainless steel. Simulations were carried out on different defect types, crack lengths, and orientation to reveal the relationship between the signals and dimensions to determine whether methods could be validated by the experiment. A 3-axis ACFM probe was developed and three plates including 16 defects, which served in nuclear plant before, were examined by automatic detection equipment. The result shows that the minimum detectable crack length on the surface is 2mm and ACFM shows excellent inspection results for a weld in stainless steel and gives an encouraging prospect of broader application.Stainless steel is widely used in nuclear power plants, such as various high-radioactive pool, tools storage and fuel transportation channel, and serves as an important barrier to stop the leakage of high-radioactive material. NonDestructive Evaluation (NDE) methods, eddy current testing (ET), ultrasonic examination (UT), penetration testing (PT) and hybrid detection method, etc., have been introduced into the inspection of a nuclear plant. In this paper, the Alternating Current Field Measurement (ACFM) was fully applied to detect and evaluate the defects in the welds of the stainless steel. Simulations were carried out on different defect types, crack lengths, and orientation to reveal the relationship between the signals and dimensions to determine whether methods could be validated by the experiment. A 3-axis ACFM probe was developed and three plates including 16 defects, which served in nuclear plant before, were examined by automatic detection equipment. The result shows that the minimum detectable c...

Application of induced circumferential current for cracks inspection on pipe string

Pipe strings (such as drill pipe, tube, pipeline, riser) are critical facilities in oil & gas industry, which are highly susceptible to cracks caused by stress corrosion and fatigue damage. The most common defects are longitudinal and transverse surface cracks on pipe strings in oil & gas industry. Conventional nondestructive testing (NDT) methods are inadequate for these surface cracks on pipe strings. In this paper, a full 360° circumferential current induced by a coaxial excitation coil is present for inspection of longitudinal and transverse surface cracks on pipe strings. The finite element method (FEM) is employed to obtain characteristics signals by analyzing the distorted electromagnetic field above the cracks. The induced circumferential current test system is set up and crack inspection experiments are carried out. The results show that both longitudinal and transverse surface cracks can be detected effectively at one pass scanning on pipe string using the induced circumferential current.

Further capacitive imaging experiments using modified probes

In recent years, capacitive imaging (CI) is growing in popularity within the NDE communities, as it has the potential to test materials and structures for defects that are not easily tested by other techniques. In previous work, The CI technique has been successfully used on a various types of materials, including concrete, glass/carbon fibre composite, steel, etc. In such CI experiments, the probes are normally with symmetric or concentric electrodes etched onto PCBs. In addition to these conventional coplanar PCB probes, modified geometries can be made and they can lead to different applications. A brief overview of these modified probes, including high resolution surface imaging probe, combined CI/eddy current probe, and CI probe using an oscilloscope probe as the sensing electrode, is presented in this work. The potential applications brought by these probes are also discussed.