Multi-layer Aircraft Structures Research Articles

Detecting and Estimating Local Corrosion Damages in Long-Service Aircraft Structures by the Eddy Current Method with Double-Differential Probes

ABSTRACT Monitoring corrosion in aircraft structures through nondestructive testing is crucial for maintaining long-term aircraft serviceability. Corrosion monitoring is particularly challenging when corrosion damage is situated on internal surfaces of multilayer aircraft structures. The eddy current method is one of the most promising techniques for detecting and measuring such subsurface corrosion damage without direct contact or disassembly. However, due to their low sensitivity traditional eddy current probes with coaxial coils are not well suited for detecting corrosion damages of the local type, such as pitting or corrosion pits, in multilayer aircraft structures. This study tested the use of low-frequency eddy current probes of the double-differential type, characterized by 8 and 10 mm operational diameters, in detecting and measuring hidden corrosion damages of this local type. Such corrosion damages were simulated by means of flat-bottomed drilled holes of differing diameters and depths (or different diameters and residual thicknesses of the inspected sheet in the damaged area). The signals from the eddy current probes were evaluated in the complex plane using a universal eddy current flaw detector. The correlations between the amplitude and phase of the eddy current signal and depth of location of the local corrosion damages were analyzed. Results indicate that it is possible to estimate the residual thickness of the skin in locally corroded areas by measuring the eddy current signal phase, independently of the local corrosion damage diameter (size), providing useful information for residual service life determination.

Eddy Current Techniques for Detecting Hidden Subsurface Defects in Multilayer Aircraft Structures

Abstract In-service non-destructive inspection (NDI) is a very important part of the aircraft maintenance program that minimizes aircraft breakdowns due to the fracture of critical components. The eddy current (EC) NDI method is one of the most applicable methods for this purpose, due to its high sensitivity to fatigue cracks and corrosion damage in the main structural materials. In this paper, selective double differential type EC probes characterized by the enhanced possibility of detecting subsurface cracks initiated by fatigue or stress corrosion phenomena are presented. For different applications, a family of double differential type EC probes was developed with different sizes (from 5 to 33 mm) and different spatial resolutions. These types of probes are characterized by different operational frequencies in a wide frequency range (from 0.2 kHz to 1.0 MHz), high penetration depth and unique sensitivity to subsurface defects of different types (like elongated fatigue cracks or local corrosive pitting), and a high level of specific noise suppression concerned with the scanning inspection procedures. The EC probes proposed were investigated as effective tools for characteristic aircraft applications concerned with subsurface defect detection in multilayer structures, such as the detection of cracks in the second layer of a riveted two-layer structure or cracks initiated on the side surface of a multilayer structure with the suppression of the reinforcing hoop influence; the detection of subsurface defects in arc welding with a rough surface; the detection of cracks through repair patches fabricated from aluminum alloy or carbon fiber reinforced plastic, etc. These techniques create remarkable possibilities for the well-timed detection of dangerous damage without disassembling the aircraft structure or removing protective coating.

Detection of the Fatigue Cracks Initiated near the Rivet Holes by Eddy Current Inspection Techniques

Abstract Eddy current (EC) method is considered as most applicable for in-service detection of fatigue subsurface cracks initiated in aircraft multilayer structures near the rivet holes. At the same time, the successful solution of this problem is obstructed by additional noise created by defect-free rivets. All EC inspection techniques for the detection of subsurface cracks around the rivets can be classified into three main groups: 1) static mode – carried out by placing the EC probe concentrically on the rivet head; 2) rotational mode – when the EC probe is rotated around the rivet axle and 2) sliding mode – performed by the movement of EC probe along the rivet line or near it. All these approaches have some advantages and limitations. In this study, known EC techniques for the detection of cracks in multilayer aircraft structures are analyzed. New advanced EC techniques for the detection of fatigue cracks in internal layers of the riveted structures based on different types (ring, sliding, and rotational) probes are presented. The static EC method with developed low-height ring-type probe creates the possibility to detect cracks in the difficult of access areas. The possibility to estimate the length of detected cracks by a ring-type probe is shown. The proposed rotational remote field EC probe can detect as small as 1.0 mm long cracks under the button-head rivet and 2 mm thick upper skin with a high signal-to-noise ratio. Therefore, in many aircraft structures, fatigue cracks will be detected before a critical threshold achieved. New EC sliding techniques based on remote field and double differential probes were proposed for the rapid detection of cracks in internal layers of riveted aircraft structures. Remote-field EC probe for reliable detection of fatigue cracks in third and fourth layers of five-layer units was proposed. Another sliding technique based on a double differential EC probe gives the possibility to detect transverse cracks in the second layer without the rivet row area access. The main advantage of developed techniques is high inspection reliability due to the possibility to discriminate the signals created by cracks and defect-free rivets. Presented inspection procedures include the selective signal analysis in the complex plane diagram. Proposed EC inspection techniques were successfully implemented into the aircraft maintenance practice.

PEC Frequency Band Selection for Locating Defects in Two-Layer Aircraft Structures With Air Gap Variations

Multilayer structures are widely used in aircraft fuselage. Because of the interlayer air gap caused by deformation or disbonding, conventional single-frequency eddy current cannot discriminate between second-layer defect signals and gap signals. In this paper, several defects at varied locations (i.e., first-layer surface, first-layer subsurface, second-layer surface, and second-layer subsurface) are manufactured into two-layer Al-Mn 3003 alloy specimen with various air gaps. Pulsed eddy current (PEC) is investigated in combination with principal component analysis (PCA) to classify and locate defects in the specimen. The new feature named differential frequency to zero is proposed, and the frequency responses of selected frequency band are processed through PCA. The principal components are used for locating defects. The experimental results show that first-layer surface defects, first-layer subsurface defects, second-layer surface defects, and second-layer subsurface defects can be classified when air gap is varied from 0 to 1.4 mm through the proposed methods. In conclusion, PEC testing with the help of PCA can eliminate the interlayer air gap and liftoff effect, which has potential for defect characterization in multilayer aircraft structures.

Magneto-Optic Imaging for Aircraft Skins Inspection: A Probability of Detection Study of Simulated and Experimental Image Data

The increasing fleet of aging aircrafts has resulted in an increasing demand for cost effective nondestructive evaluation (NDE) techniques that are accurate, reliable, and easy to use. Magneto-Optic Imaging (MOI) is such a technique, which has gained wide acceptance for detection of both surface and subsurface defects in multi-layer aircraft structures. The main advantage of MOI is rapid inspection and ease of interpreting image data in contrast to complex impedance signals from conventional eddy current instruments. One missing piece of the puzzle for advanced MOI systems is how to quantitatively analyse the MO images, and understand the detectability limits when image data are acquired under varying operational conditions. This paper presents a probability of detection (POD) study that is conducted using both simulation model-predicted and experimental MO image data. Simulated panels from a 3-D FEM model and experimental panels with machined defects are used to generate data for interpretation by human inspectors or automated systems, and subsequently for POD studies. The POD curves demonstrate the merits in optimizing inspection parameters that maximized the performance of current MOI systems. Parameters quantifying the detectability of MO image data using skewness functions are also presented and discussed.

PEC defect automated classification in aircraft multi-ply structures with interlayer gaps and lift-offs

Current pulsed eddy current (PEC) defect classification method requires highly trained personnel and the results are usually influenced by subjectivity of human perception. Lift-off effect and interlayer air gaps are the main obstacles in eddy current defect classification in multi-layer structures. Therefore, automated defect classification in multi-layer structures is very desirable and stringent. In this work, principal component analysis (PCA) and support vector machine (SVM) are investigated for defect automated classification under different interlayer gaps and lift-off effects. The experimental classification and predicting results show that 1st-layer surface defects, 1st-layer sub-surface defects, 2nd-layer surface defects, and 2nd-layer sub-surface defects can be classified satisfactorily when air gap and lift-off vary from 0 to 1.4mm through the proposed methods, which has the potential for automatic defect in-situ classification for multi-layer aircraft structures.

Support vector machine and optimised feature extraction in integrated eddy current instrument

Eddy current, especially pulsed eddy current (PEC), is known as an effective tool to detect defects in aircraft structures. Current PEC defect classification methods require highly trained personnel and the results are usually influenced by human subjectivity. Therefore, automated defect classification is desirable in a PEC instrument. In this work, five eddy current based methods are integrated into an instrument using a universal model and modular structure. Then, a Support Vector Machine (SVM) is used to build the classifier model and predict the type of defect. Principal component analysis (PCA) and independent component analysis (ICA) are investigated for feature extraction and compared for classification results using SVM. Two-layer Al–Mn alloy specimens with four kinds of defects are used for classification. The experimental results show that the proposed methods have great potential for in-situ defect inspection of multi-layer aircraft structures.

Classification of pulsed eddy current GMR data on aircraft structures

This paper presents a technique to automatically detect third-layer cracks at rivet sites in aircraft structures using the response signals collected by giant magneto-resistive (GMR) sensors. The inspection system uses pulsed waveform as the excitation source of a multi-line coil and captures the transient fields associated with the induced eddy currents via a GMR sensor, which was developed to detect cracking and corrosion in multi-layer aircraft structures. An automatic scan of the region around the rivet generates C-scan image data that can be processed to detect cracks under the rivet head. Using a 2-D image of each rivet head, feature extraction and classification schemes based on principal component analysis and the k-means algorithm have been successfully developed to detect cracks of varying size located in the third layers at a depth of up to 10 mm below the surface.

Defect depth estimation using pulsed eddy current with varied pulse width excitation

Current industry requirements demand defect quantification rather than simple defect detection. Besides the defect geometrical size, its depth location plays a critical role for the integrity of a structure and the safe life operation of the overall assembly. It has been long known that pulsed eddy current (PEC) non-destructive testing (NDT) for electrically conductive materials has the advantage of gathering different depth information in a single excitation process. This fact is due to the low frequencies found in the pulse spectrum that is used for the excitation of the driving coil; however, quantifying the defect depth still remains a challenge. This study considers different pulse widths and frequency components as a method of defect depth discrimination with direct applications in the evaluation of corrosion defects in multi-layer aircraft structures. Experimental testing and numerical modelling approaches are concomitantly discussed as ways of defect depth quantification.

Pulsed Eddy-Current Measurements of Corrosion and Cracking in Aging Aircraft

ABSTRACTPulsed, or transient eddy-current methods are an effective tool for quantitative characterization of hidden corrosion and cracking in multi-layer aircraft structures. Eddy currents are the method of choice for this task, since they penetrate multiple layers of metal, whether or not the layers are mechanically bonded. The pulsed eddy-current technique is an important advance over conventional eddy-current methods because it rapidly acquires data over a wide range of frequencies, thereby providing more information than a conventional, single-frequency eddy-current instrument. We have combined a pulsed eddy-current instrument with a portable two-axis scanner to produce an instrument capable of rapidly scanning aircraft lap splices in situ, producing pseudo-color images that reveal hidden corrosion or cracking. A unique feature of time-domain eddy-current data is the ability to selectively filter clutter from the image by time-gating the pulsed signal. Time-gating permits the user to select the inspection depth, thereby eliminating interference from upper layers, air gaps, lift-off variation and fasteners. By using a theoretical model of the pulsed eddy-current system, it is possible to interpret the data quantitatively, yielding quantitative maps of corrosion damage. Some of the same advantages of the pulsed eddycurrent technique apply to the characterization of hidden fatigue cracks as well, although the tieory for crack signals is less advanced.