Gary Steffes

EC-GMR Data Analysis for Inspection of Multilayer Airframe Structures

Eddy-current testing (ECT) is widely used in inspection of multilayer aircraft skin structures for the detection of cracks under fasteners (CUF). Detection of deep hidden CUF poses a major challenge in traditional ECT techniques largely because the weak eddy-current signal due to a subsurface crack is dominated by the strong signal from the aluminum or steel fastener. Giant magnetoresistive (GMR) sensors are finding increasing applications in directly measuring weak magnetic fields associated with induced eddy currents. The measured flux image at a fastener site is in general symmetric and an asymmetry is introduced by the presence of a subsurface crack, which is used for defect detection. This paper presents novel methods that employ the resident phase information, for improving detection probability of GMR signal analysis. Using computational model, the effectiveness of the proposed methods for enhancing detection of CUF is investigated. Results demonstrating the potential of these techniques for detection of second layer CUF are presented.

Aging Aircraft NDE: Capabilities, Challenges, and Opportunities

US Air Force aircraft are managed by Damage Tolerance Assessment (DTA) and Nondestructive Evaluation (NDE) plays a critical role in insuring their flight safety. This paper provides an overview of current NDE capabilities for aircraft structures. The potential migration to Condition‐Based Maintenance (CBM) will require NDE to evolve from detection of defects to characterization of their location and size. This represents multiple significant challenges as aircraft have many material and geometry factors that complicate this analysis. This paper provides a strategy to resolve these factors.

MILLIMETER WAVE HOLOGRAPHICAL INSPECTION OF HONEYCOMB COMPOSITES

Multi‐layered composite structures manufactured with honeycomb, foam, or balsa wood cores are finding increasing utility in a variety of aerospace, transportation, and infrastructure applications. Due to the low conductivity and inhomogeneity associated with these composites, standard nondestructive testing (NDT) methods are not always capable of inspecting their interior for various defects caused during the manufacturing process or as a result of in‐service loading. On the contrary, microwave and millimeter wave NDT methods are well‐suited for inspecting these structures since signals at these frequencies readily penetrate through these structures and reflect from different interior boundaries revealing the presence of a wide range of defects such as isband, delamination, moisture and oil intrusion, impact damage, etc. Millimeter wave frequency spectrum spans 30 GHz–300 GHz with corresponding wavelengths of 10−1 mm. Due to the inherent short wavelengths at these frequencies, one can produce high spatial...

MODEL‐ASSISTED PROBABILISTIC RELIABILITY ASSESSMENT FOR STRUCTURAL HEALTH MONITORING SYSTEMS

This paper describes a model‐assisted probabilistic methodology to ensure the reliability of SHM systems for damage detection, localization, and sizing. A hierarchical approach is presented that attempts to minimize the number of samples, the length of time, and degree of full‐scale testing required for statistically meaningful characterization results. The feasibility of applying this approach to typical sensing methods found in SHM systems is investigated, and additional challenges concerning model reliability and uncertainty propagation are addressed.

Scattering of obliquely incident shear waves from a cylindrical cavity

Prior work has proposed the use of ultrasonic angle-beam shear wave techniques to detect cracks of varying angular location around fastener sites by generating and detecting creeping waves. To better understand the nature of the scattering problem and quantify the role of creeping waves in fastener site inspections, a 3D analytical model was developed for the propagation and scattering of an obliquely incident plane shear wave from a cylindrical cavity with arbitrary shear wave polarization. The generation and decay of the spiral creeping waves was found to be dependent on both the angle of incidence and polarization of the plane shear wave. A difference between the angle of displacement in 3D and the direction of propagation for the spiral creeping wave was observed and attributed to differences in the curvature of the cavity surface for the tangential and vertical (z) directions. Using the model, practical insight was presented on measuring the displacement response in the far-field from the hole. Both analytical and experimental results highlighted the value of the diffracted and leaky spiral creeping wave signals for nondestructive evaluation of a crack located on the cavity. Last, array and signal processing methods are discussed to improve the resolution of the weaker creeping wave signals in the presence of noise.

A Comprehensive Multi-Modal NDE Data Fusion Approach for Failure Assessment in Aircraft Lap-Joint Mimics

Multi-modal data fusion techniques are commonly used to enhance decision-making processes. In previous research, a comprehensive structural analysis process was developed for quantizing and evaluating characteristics of defects in aircraft lap-joint mimics using eddy current (EC) nondestructive evaluation (NDE) data collected for structural health monitoring. In this research, a comprehensive multi-modal structural analysis process is presented that includes intra- and inter-modal NDE data fusion based on EC, millimeter wave (MW), and ultrasonic (UT) data obtained from five lap-joint mimic test panels. The process includes defect detection, defect characterization, and finite-element modeling-based simulated fatigue loading for structural analysis. The multi-modal structural analysis process is evaluated using four test panels with corroded patches at different layers of the lap joints and one painted pristine panel used as a reference. The test panels are subjected to two rounds of mechanical loading, preceded by multi-modal NDE data obtained before each round. Different NDE modality combinations are examined for test panel modeling, including: 1) EC, 2) UT, 3) MW, 4) EC and UT, 5) EC and MW, and 6) EC, UT, and MW. Experiments are performed to compare the simulated fatigue loading, based on models determined from the different modality combinations, and the mechanical loading results to find susceptible-to-failure areas in the test panels. Experimental results showed that the EC and UT modality combination yielded a correct vulnerable (crack) location recognition rate of 98.8%, an improvement of 14.7% over any individual modality, demonstrating the potential for multi-modal data fusion for characterizing corrosion and defects.

AUTOMATED ANALYSIS OF EDDY CURRENT (EC) GIANT MAGNETO RESISTIVE (GMR) DATA

Eddy current testing (ECT) has been widely used in aircraft inspection. However ECT is limited by skin depth in detecting defects in multi‐layered complex structures. Advanced electromagnetic sensing methods, such as giant magnetoresistive (GMR) sensors offer much higher sensitivity that allow detection of defects located deep in the test object. Traditional magnitude based GMR signal interpretation methods using symmetry of images developed for detecting cracks under titanium fastener heads are ineffective when analyzing data from steel fastener heads which results in reduced probability of detection (POD). This paper presents a signal processing algorithm based on mixing the in‐phase and quadrature components of the GMR sensor signal via an optimum detection angle (ODA). Features derived from the mixed signal are clustered for automated crack detection using the MR sensor data. Performance of the algorithm is evaluated using signal to noise ratio (SNR) that measures efficacy of different features.

A Comprehensive Structural Analysis Process for Failure Assessment in Aircraft Lap-Joint Mimics using Intramodal Fusion of Eddy Current Data

In the aerospace industry, nondestructive evaluation (NDE) techniques are commonly used for structural health monitoring, including finding manufacturing and service-induced discontinuities and defects such as corrosion and cracks. A comprehensive structural analysis process is presented for quantizing and evaluating characteristics of aircraft lap-joint mimics. The process investigated here consists of NDE data acquisition, defect detection and characterization involving material loss estimation, three-dimensional structural model generation, finite-element modeling to simulate fatigue damage and comparison with actual tensile fatigue-induced structural analysis data (mechanical loading). The structural analysis process is examined using five test panels consisting of stacked and riveted aluminum plates, configured as lap-joints, four of them with different corroded patches at different layers of the lap-joints and one painted pristine panel used as a reference. The test panels were subjected to three rounds of incremental and cyclical tensile fatiguing with the final round resulting in complete fatigue failure. Eddy current data was obtained from the test panels prior to each round of mechanical loading. Comparing the simulated fatigue loading and the mechanical loading results for identifying susceptible-to-failure areas on the test panels, this comprehensive structural analysis process found the correct location of failure areas at rates as high as 88.9%.

A data fusion based approach for evaluation of material loss in corroded aluminum panels

Nondestructive Testing (NDT) is used to detect hidden corrosion in ageing aircraft structures. Use of any single NDT modality provides an incomplete picture of the corrosion environment. Data fusion techniques can be used for improved visualization and automated detection of hidden corrosion in multilayered structures. This work investigates a data fusion based technique using ultrasound measurements from corroded aircraft samples for estimating material loss. Experimental results are presented and suggestions for future work are made for evaluating residual structural integrity of the panels for a damage tolerance approach to corrosion mitigation.

Enhanced image processing methods for GMR array inspections of multilayer metallic structures

The objective of this paper is to present a comprehensive approach to image processing for Giant Magnetoresistive (GMR) array sensors with sheet current sources for the inspection of fatigue cracks in complex metallic structures. The approach incorporates optimal phase angle adjustment, array sensitivity compensation, fastener site identification, an adaptive edge evaluation and removal algorithm and a classification metric. Processing examples are presented that highlight the benefits of these algorithms for improving crack detection for challenging edge and fastener spacing conditions.