Ray T. Ko

APPLICATION OF A GIANT MAGNETORESISTIVE (GMR) SENSOR FOR CHARACTERIZATION OF CORROSION IN A LABORATORY SPECIMEN

This paper describes the use of a Giant Magnetoresistive (GMR) sensor to detect thickness variation in a multi‐layered specimen. A series of frequency‐response and modes‐of‐operation tests has been carried out to characterize a GMR sensor. Both the first and second harmonics of the GMR signal for corrosion detection were collected and analyzed. The experimental data showed that phase approach seemed to have better thickness discrimination over that of the magnitude approach.

Numerical and Experimental Study of Eddy Current Crack Detection around Fasteners in Multi‐Layer Structures

Previous work on the detection of cracks around fastener holes in multi‐layer structures using eddy current NDE has met with some success; however, challenges remain in detecting smaller corner cracks while minimizing false call rates. To address this issue, a 3D model was developed for this problem and successfully validated with experimental data. Using this model, a series of numerical studies were performed to identify potential features for defect classification.

A Novel Multi‐Frequency Eddy Current Measurement Technique for Materials Characterization

In an effort to meet the needs for high frequency eddy current measurements and be able to distinguish small conductivity variations in different materials, a new eddy current module capable of measuring magnitude, phase, and frequency shift was developed and integrated into a general‐purpose scanning system. Comparisons of three different parameter images are presented. The potential application of the multi‐frequency, multi‐parameter eddy current measurement technique for materials characterization to discriminate small conductivity changes is discussed.

Application of Resonant Frequency Eddy Current Technique on a Shot‐Peened Nickel‐Based Engine‐Grade Material

The shot peening conditions of a nickel‐based engine‐grade material were evaluated using a novel eddy current measurement technique. With this technique, the shift of a resonant frequency was found to be dependent on variables which also affect conventional eddy current testing. The cable effect is another important variable, which is often neglected in a routine eddy current testing, is also discussed. Experimental results showed that at high frequencies, the shot peening conditions were easily distinguishable using this frequency shift technique.

Electrical Circuit Model of an Eddy Current System for Computing Multiple Parameters

An electrical circuit based model for eddy current system has been developed using commercial electrical engineering software. The model allows incorporation of individual characteristics of the signal generator, the cable, the eddy current sensor and the sample under test. Computational results of the characteristics of the system, obtained by sweeping the frequency, under normal and varying test conditions are presented. The sensitivity of the eddy current system response due to changes in different parameters during test conditions is discussed.

EDDY CURRENT ASSESSMENT OF ENGINEERED COMPONENTS CONTAINING NANOFIBERS

The eddy current approach has been used to assess engineered components containing nanofibers. Five specimens with different programmed defects were fabricated. A 4‐point collinear probe was used to verify the electrical resistivity of each specimen. The liftoff component of the eddy current signal was used to test two extreme cases with different nano contents. Additional eddy current measurements were also used in detecting a missing nano layer simulating a manufacturing process error. The results of this assessment suggest that eddy current liftoff measurement can be a useful tool in evaluating the electrical properties of materials containing nanofibers.

Characterization of Retrogression and Re‐Aging Heat Treatment of AA7075‐T6 Using Nonlinear Acoustics and Eddy Current

Nonlinear acoustic parameter and eddy current methods have been utilized to characterize the heat treatment process of retrogression and re‐aging of aluminum 7075‐T6. The results of nonlinear acoustic parameter measurements show two distinct peaks at 30 minutes and 45 minutes of retrogression time. The phase of the through‐thickness eddy current signal shows a minimum at 42 minutes of retrogression time. Application of combined methods for identifying the optimized properties in the material is discussed.

Ultrasonic characterization of polymer infiltration into foam materials

The development of advanced multi-material systems has become increasingly complex with regard to performance requirements, tailorable material options, and recent innovations in manufacturing technologies. The increased development and use of engineering foam material systems, for example, has recently led to significant improvements in the weight, cost, and performance of materials used in insulating, thermal protection, sound deadening, and lightweight structural materials. An important aspect of such complex multi-material systems includes the ability to effectively join two or more materials and characterize the processing results. In the present effort, the use of ultrasound for characterizing the infiltration of a polymer material into a foam is investigated using computational models and experimental validation studies. Characteristic pulse-echo signal responses utilizing time-of-flight reflection and attenuation were used to provide an effective means for estimating the fill-factor of the polymer into a foam material, where optimization of the thermal and structural performance of the material system is desired.

EDDY CURRENT TESTING AND FOUR‐POINT PROBE MEASUREMENT OF NANO‐ AND MACROMATERIALS ENHANCED POLYMERIC COMPOSITES

Nano‐ and macromaterials are being engineered into polymeric composites to enhance the electrical conductivity for lightning strike protection purposes. Electrical conductivity can be measured using a four‐point probe method. Eddy current methods can provide a more localized measure of conductivity on the surface of a component. In this study, engineered composites containing both macro‐ and nanomaterials have been fabricated. Eddy current measurements were made on these specimens to quantify the sensitivity of this method to the relevant variables. Eddy current results in the sub‐MHz frequency range agreed well with the electrical conductivity data from four‐point probe measurements.

EDDY CURRENT MONITORING OF SURFACE ROUGHNESS

The surface conditions of a steel plate specimen have been examined with an eddy current technique. Previous examinations have shown promise in using white light interferometer to measure the surface roughness and thus the corrosion of the specimen. Additional nondestructive evaluation (NDE) ultrasonic testing was utilized. The goal of this study is to examine the feasibility of using eddy current to monitor the surface roughness. The apparent surface roughness based on the eddy current data is in close agreement with a white light interferometer.