A. Fahr

Time-Frequency Analysis of Pulsed Eddy Current Signals

Detection and measurement of the material loss due to corrosion in aircraft fuselage lap splices are of great importance to life management of ageing aircraft. Pulsed eddy current (PEC) techniques have shown the potential to fully characterise these structures. However, variations in the probe lift-off, the interlayer gaps and material thinning produce similar PEC signals, making signal interpretation very difficult. This paper describes time-frequency analysis algorithms applied to pulsed-eddy current signals from lap-joint specimens containing simulated flaws. The effect of probe lift-off, interlayer gap, and material thinning on PEC signals is examined in three-dimensional space (amplitude-time-frequency). This study shows that the time-frequency analysis of PEC signals provides specific visual patterns that can be related to the interlayer gap, lift-off, and material loss.

Nondestructive measurement of porosity in thermal barrier coatings

Porosity is an integral part of thermal barrier coatings (TBC) and is required to provide thermal insulation and to accommodate operational thermal stresses. The effective use of TBC in hot-section components of aircraft engines requires nondestructive testing (NDT) methods to detect porosity variations and measure thickness changes to reduce the risk of damage to the coating due to such variations. The eddy current method has been used to measure the thickness of a plasma-sprayed TBC coating and either ultrasonic or capacitance techniques have been applied to assess porosity content based on thickness values obtained using the eddy current tests. The porosity values estimated by the NDT methods have been confirmed by destructive testing, which included metallography and vacuum volumetric measurement using nitrogen absorption.

A Data-Fusion Scheme for Quantitative Image Analysis by Using Locally Weighted Regression and Dempster–Shafer Theory

Dempster-Shafer (DS) theory provides a solution to fuse multisensor data, which are presented in a hypothesis space comprising mutually exclusive and exhaustive propositions and their unions. The fusion result is a description of the proposition with the values of support, plausibility, and uncertainty interval. However, in some applications, numerical values of a continuous function, instead of a Boolean value or a proposition, are expected. In this paper, a scheme based on DS reasoning and locally weighted regression is proposed to fuse the data obtained from the nondestructive inspections of aircraft lap joints for the estimation of the remaining thickness. The proposed approach uses a pairwise regression that is optimized by the DS method when multiple inputs are involved. The scheme is evaluated with the experiments on fusing conventional eddy current and pulsed eddy current data obtained from aircraft lap joint structures for the characterization of hidden corrosion.

Detection of thermally grown oxides in thermal barrier coatings by nondestructive evaluation

The thermal-barrier coatings (TBC) sprayed on hot-section components of aircraft turbine engines commonly consist of a partially stabilized zirconia top-coat and an intermediate bond-coat applied on the metallic substrate. The bond-coat is made of an aluminide alloy that at high engine temperatures forms thermally grown oxides (TGO). Although formation of a thin layer of aluminum oxide at the interface between the ceramic top-coat and the bond-coat has the beneficial effect of protecting the metallic substrate from hot gases, oxide formation at splat boundaries or pores within the bond-coat is a source of weakness. In this study, plasma-sprayed TBC specimens are manufactured from two types of bond-coat powders and exposed to elevated temperatures to form oxides at the ceramic-bond-coat boundary and within the bond-coat. The specimens are then tested using nondestructive evaluation (NDE) and destructive metallography and compared with the as-manufactured samples. The objective is to determine if NDE can identify the oxidation within the bond-coat and give indication of its severity. While ultrasonic testing can provide some indication of the degree of bond-coat oxidation, the eddy current (EC) technique clearly identifies severe oxide formation within the bond-coat. Imaging of the EC signals as the function of probe location provides information on the spatial variations in the degree of oxidation, and thereby identifies which components or areas are prone to premature damage.

Development of POD from in-service NDI data

The economic drive towards using aircraft beyond their initial design life has created a great interest in damage-tolerance (DT) based maintenance. The DT approach relies on routine nondestructive inspections (NDI), and requires that the NDI performance to be quantified in terms probability of detection (POD) to determine the safe inspection intervals. The most common approach for determining NDI POD is to perform inspections on representative components or specimens simulating the actual parts. This approach is practical but can be very expensive. A more economical approach may be to use actual field inspection data to obtain POD. This approach is particularly attractive for airframe inspection techniques, since most airframe structures cannot be easily simulated. There are a number of difficulties with this approach: Firstly, there is usually a very limited amount of field data. This may require special statistical treatment. Secondly, crack growth data must exist to allow the estimation of flaw sizes a...

On the independence of multiple inspections and the resulting probability of detection

Probability of detection (POD) is a critical measure of the performance, in terms of sensitivity and reliability, of a nondestructive testing system. For operation of safety-critical components, the interval of time allowable between inspections, the safe inspection interval (SII), is a function of flaw growth rates and POD. Traditionally, many people have assumed that repeating inspections provides significant benefit to POD, based on the assumption of partial or total independence of repeated inspections. The author demonstrates the errors in assuming independence of repeated inspections, and presents actual experimental POD data which further demonstrates the very small amount of independence between inspections. The effect on the calculation of safe inspection intervals is examined using real inspection data.

Application of Dempster-Shafer theory for fusion of lap joints inspection data

In this work the Dempster-Shafer (DS) theory has been used for fusing nondestructive inspection (NDI) data. The success of a DS-based method depends on how the basic probability assignment (BPA) or probability mass function is defined. In the case of nondestructive inspection of aircraft lap joints, which is of interest here, the inspection data is presented in raster-scanned images. These images are discriminated by iteratively trained classifiers. The BPA is defined based on the conditional probability of information classes and data classes, which are obtained from ground truth data and NDI measurements respectively. Then, the Dempster rule of combination is applied to fuse multiple NDI inputs. The maximum mass outputs determine the final classification results. In this work, conventional eddy current (ET) and pulsed eddy current (P-ET) techniques were employed to inspect the fuselage lap joints of a service-retired Boeing 727 aircraft in order to map corrosion sites. Estimation of the remaining thickness from the inspection data is the aim of this work. The ground truth data was obtained by teardown inspections followed by a digital X-ray thickness mapping technique, which provides accurate thickness values. The experimental results verify the efficiency of the proposed method.

Numerical modeling of frictional heating based vibrothermography

This paper investigates the use of finite element to model frictional heating based vibrothermography for the detection of fatigue cracks in steel specimens. First, a finite element modal analysis is carried out to predict the optimal excitation frequencies. Some thermographic experiments using an infrared camera are carried out to help updating a coupled thermo-mechanical model built to simulate the thermographic inspection process and to explain the heat generation and transfer related to it. Experimental investigations also confirmed that the technique is able to detect cracks as short as 0.1 mm. The developed model is able to simulate the thermographic inspection process with a maximum error of 2.13 % on the temperature distribution. The Fourier transform applied to numerical data reveals that the temperature evolution at the crack face changes according to the excitation frequency and is modulated due to the nonlinearity induced by the crack. The model also serves to confirm that the test is non-destructive since the calculated stress at the crack tip is less than the specimen materials yield stress.

Investigation of alternative excitation sources for vibrothermography

In this study, a mechanical shaker operating at low frequencies is demonstrated to be a viable excitation source for vibrothermography. Additionally, a low-cost transduction approach based on commercially available piezoelectric materials (PZT) is investigated. These PZT transducers are assessed for their excitation efficiency to allow crack detection in metallic structures. Cracks as small as 1 mm are detected using the mechanical shaker regardless of a beam structure orientation or crack location. Although the low-cost PZT-based transduction approach had sufficient excitation power to generate vibration, localized heat generation was not observed at crack locations.

Quantitative interpretation of multifrequency eddy current data by using data fusion approaches

Multi-frequency techniques are widely adopted for eddy current testing. One of the advantages of these techniques can be deduced from the skin depth formula (formula available in paper) where delta is the standard depth of penetration at excitation frequency f, with the other two parameters, mu and sigma, related to material properties. Thus, an inspection can be performed at several depths into the material with the simultaneous use of multiple frequencies. To investigate the potential of a multi-frequency eddy current technique (MFECT) for corrosion quantification, an experiment was carried out on a two-layered fuselage lap joint splice. Two data fusion approaches, namely Bayesian inference and multiresolution analysis, are investigated in this study to fuse eddy current images of different frequencies. The corrosion types are classified based on the percentage of material loss. The estimated thickness results, based on the fusion processes, are compared with accurate thickness maps obtained from teardown X-ray inspection data.