Health Monitoring of Aerospace Structures: Smart Sensor Technologies and Signal Processing / Edition 1

by Wieslaw Staszewski, C. Boller, G. R. Tomlinson

Overview

Providing quality research for the reader, this title encompasses all the recent developments in smart sensor technology for health monitoring in aerospace structures, providing a valuable introduction to damage detection techniques. Focussing on engineering applications, all chapters are written by smart structures and materials experts from aerospace manufacturers and research/academic institutions.

This key reference:

• Discusses the most important aspects related to smart technologies for damage detection; this includes not only monitoring techniques but also aspects related to specifications, design parameters, assessment and qualification routes.
• Presents real case studies and applications; this includes in-flight tests; the work presented goes far beyond academic research applications.
• Displays a balance between theoretical developments and engineering applications

Product Details

ISBN-13:	9780470843406
Publisher:	Wiley
Publication date:	02/13/2004
Pages:	288
Product dimensions:	6.93(w) x 9.88(h) x 0.81(d)

About the Author

Wieslaw Staszewski, The Faculty of Mechanical Engineering and Robotics, University of Science and Technology, Poland
Wieslaw Staszewski is a Professor at AGH University of Science and Technology in Poland. He has authored circa 280 publications, predominantly in the areas of damage detection and advanced signal processing. He has written and edited a book, authored circa 90 journal papers and is also an editor and associate editor of five journals. He was jointly awarded the "2004 Person of the Year" title by Structural Health Monitoring journal for outstanding contribution in the field of SHM.

C. Boller is the editor of Health Monitoring of Aerospace Structures: Smart Sensor Technologies and Signal Processing, published by Wiley.

G. R. Tomlinson is the editor of Health Monitoring of Aerospace Structures: Smart Sensor Technologies and Signal Processing, published by Wiley.

Read an Excerpt

Health Monitoring of Aerospace Structures

Smart Sensor Technologies and Signal Processing

---

John Wiley & Sons

ISBN: 0-470-84340-3

---

Chapter One

Introduction

G. Bartelds, J.H. Heida, J. McFeat and C. Boller

1.1 HEALTH AND USAGE MONITORING IN AIRCRAFT STRUCTURES - WHY AND HOW?

To ensure structural integrity and hence maintain safety, in-service health and usage monitoring techniques are employed in many engineering areas. Structural health is directly related to structural performance and in this respect it is one of the major parameters with regard to safety of operation. This aspect of structural health is particularly relevant to transportation systems including various elements of transportation infrastructure. In this context structural health monitoring is a safety issue.

At the same time a change in structural health may affect structural performance to a degree that remedial maintenance actions become necessary. Structural repairs increase the cost of transportation in at least two ways. First, the design and implementation of repairs implies direct costs. Second, the execution of repairs generally requires the transportation system to be temporarily taken out of service and this induces indirect costs due to the loss of production volume or as a result of leasing a substitute system. To reduce repair and maintenance cost an attempt to repair can be undertaken at a very early stage of damage development to limit direct costs. Alternatively, it might bedecided to postpone repair until the transportation system has to be taken out of service for scheduled major overhauls to reduce indirect costs. In this context structural health monitoring becomes an issue of cost savings. In excess, structural health monitoring may be considerable with regard to monitoring advanced repair methodologies, which have so far not received approval due to lack of knowledge in their performance and where this lack could be overcome by the application of structural health monitoring.

In case of the option relying on the delay measure it may be necessary to adapt operational usage to limit or even stop damage growth. If sufficient knowledge exists to relate damage rates to mission types this can be achieved by usage monitoring. In general usage monitoring can be viewed as a valuable addition to structural health monitoring. Prescribed maintenance schedules are based on an estimated usage pattern. Knowledge of the actual utilisation can be translated into a severity parameter that can be compared to the value corresponding to the estimated loading spectrum. In this manner prescribed inspection intervals and times between overhauls can be tuned to actual needs.

It is worthy to note that there are substantial differences in damage development and as a consequence in the manner structural health will deteriorate with time between metal and composite structures. Whereas in metallic components cracking is a gradual and predictable process with a high probability of occurrence, the wear-out of a composite component as a result after loading environment is much less pronounced but composites may suffer from discrete traumas due to accidental damage of a nonpredictable random nature. The situation suggests that different health monitoring philosophies should be applied to the two families of structural components.

Structural health, or equivalently, the state of damage can be established either directly or indirectly. The first approach checks for the damage type (e.g. cracks, corrosion or delaminations) by applying an appropriate inspection technique. These techniques, based on physical phenomena, in fact sometimes also amount to response measurements but in this case they have a very local and direct character. The established inspection techniques vary from visual inspection by the naked eye to passing the structure through a fully automated inspection gantry. In the indirect approach structural performance or rather structural behaviour is measured and compared with the supposedly known global response characteristics of the undamaged structure. If the effect of certain damages on structural response characteristics is known, this approach provides an indirect measure of damage and of structural health. Obviously in both the direct and indirect approaches the sensitivity and the reliability of inspection are important quantitative performance measures. They are determined on the one hand by the laws of physics but on the other in practice also by the hardware and software quality of the inspection equipment and, last but not least, by the equipment operator: the inspector. In this connection human factors such as the loss of alertness in case of rare occurrences of damage and inspector fatigue in case of long and tedious inspections are important reasons to consider a smarter solution to inspection as an element of structural health monitoring.

Safety, costs and performance issues of the structural health and usage monitoring are particularly important in the aircraft industry. At present monitoring techniques are primarily based on pessimistic prediction and periodic inspection. Flight parameters and a range of independent, nondestructive techniques are employed in practice.

1.2 SMART SOLUTION IN AIRCRAFT MONITORING

Structures which are able to sense and respond/adapt to changes in their environment are often referred to as smart. The design philosophy of smart structures is associated with the integration of sensors, actuators, controllers and signal processors. Smart solutions to structural health and usage monitoring relates to systems including sensors for damage detection combined with advanced signal processing and presentation. The sensitivity to damage and the reliability of performance are the major requirements with regard to smart technologies. In comparison to conventional solutions, smart sensors have to provide greater sensitivity, provided they are properly installed. This option is clearly related to a monitoring strategy being related to specific inspections at precisely known generally poorly accessible critical locations. On the other hand, smart sensor systems with advanced data processing may also be relevant for inspecting larger areas for a variety of defects, specifically in the sense of widespread and multi-site fatigue damage. If such smart systems virtually function continuously, the time between inspections effectively tends towards zero and then a moderate sensitivity might suffice, when compared to conventional inspection intervals.

Section 1.1 has identified a safety issue of structural health monitoring. Certainly in high performance transportation such as aerospace, high-speed trains and also automobiles, where structural failures may lead to fatal accidents, safety of operation is a prime consideration. Continuous research in the areas of fatigue and corrosion of metallic aircraft structures including inspection techniques (sometimes spurred and accelerated by dramatic accidents or incidents) has helped to achieve a very high level of structural reliability. Design for damage tolerance is now widely applied. It relies on a very profound understanding of material behaviour, on a very accurate description of the loading environment (both external and internal) all of this in combination with advanced manufacturing techniques and, of course, proven and reliable inspection and maintenance procedures. And in situations where brittle material behaviour or poor accessibility with regard to inspection are in the way of a damage tolerant design approach, detailed numerical analysis supported by advanced testing has produced the understanding of slow crack growth and allowed for the design of safe life structures. Interest for automated integrated inspection systems could thus result from a need for greater reliability of inspection. The damage tolerance chain is only as strong as its weakest link, which probably is inspection. Only in special situations an integrated sensor system may provide greater reliability than current methods. However, if in view of the rapidly growing air transport volume, expressed in billions of passenger miles flown, a significant reduction in structural failure rates is needed, smart solutions may become more relevant as a safety issue. The reality is that operators as well as technology providers have not sufficiently assessed the business case.

Another more important factor stimulating the development of smart systems, however, is the cost of inspection. There are very little published data on the potential for cost reductions but the inspection efforts applied in current aircraft maintenance procedures are very considerable and moreover inspector training and motivation require continuous attention. It must be mentioned here that significant improvements have been achieved in traditional inspection equipment with regard to inspector friendliness and quantitative data presentation. A recent study on inspection requirements for a modern fighter aircraft (featuring both metal and composite structure) revealed that an estimated 40% plus could be saved on inspection time by utilising smart monitoring systems. The situation at hand is illustrated in Table 1.1. Another estimate derived for a fully automated impact sensing system for a composite structure, based on the use of integrated distributed piezoceramic sensors in combination with advanced signal processing software arrives at a 50% saving on regular inspection time again for a fighter aircraft. Admittedly, these estimates are based on data derived from laboratory demonstrators. They provide a drive, however, for the development of full-scale demonstrators of smart structural health monitoring systems. As long as inspections can only be performed at discrete intervals, new sensing techniques will bring cost benefits. Continuous, in-service health and usage monitoring offers the potential to reduce the number of scheduled and unscheduled maintenance actions and the downtime. Monitoring of damage and loads will enable the assessment of the complete condition of structural components from cradle-to-grave. In fact major research programmes in this area assume that up to 20% of current maintenance/inspection cost can be saved in civil and military transportation by the use of integrated on-line damage monitoring systems. This clearly suggests that the case for smart solutions to aircraft structural health monitoring requirements derives from cost considerations.

The development of structural health monitoring systems relies on different research disciplines and in addition it affects design and manufacture as well as operation and maintenance. As primary flight systems such as the airframe, landing gear or engines are considered, the airworthiness authorities will have to be involved. Novelty of the structural health monitoring systems is considerable and thus a broad acceptance among all parties involved is necessary to achieve implementation.

These considerations have led to a number of research programmes aimed at setting up collaborative research and development projects. Although, no aircraft operators currently use health and usage monitoring systems of the type envisaged in this book, such systems have been identified as key elements within United States, Europe and Japan. Not only countries that have significant aerospace programmes but also smaller nations with advanced system component expertise are involved in projects that are important for both ageing and future generation aircraft.

1.3 END-USER REQUIREMENTS

The development of structural health and usage monitoring systems depends largely on the available technology. However, the benefits derived from actual research and application programmes must be recognised and reflected by the potential end-users. Structural health and usage monitoring systems must satisfy realistic performance requirements to meet end-user needs. The main driver for automated inspection is reliability and cost reduction. Aircraft operators and maintenance providers will not buy expensive equipment otherwise. This is specifically true within the context of ageing aircraft and true as well with regard to any other ageing infrastructure.

Aircraft operators require in general: safe aircraft structures, low life-cycle costs, airframe operational life extension, high rate of operational reliability and potential for enhanced aircraft performance. The last requirement is particularly relevant to military aircraft operators. This section describes various requirement aspects of damage detection and load history monitoring. This includes: inspection types, global vs local monitoring, dynamic vs static techniques and special aspects of system performance.

1.3.1 Damage Detection

There are two possible options for automated inspection systems. These are: (a) retrofit in existing structures; (b) integration in the structural design of new aircraft. It seems very sensible to integrate inspection devices into the basic structural design to ease inter-service effort. Also, automated health and usage monitoring systems need to cover various inspection types. This includes not only levels of inspection but also areas, locations, types of damage and/or inspection techniques. All these elements can be referred to operational availability, accessibility and improved reliability, as summarised in Table 1.2. The nature and type of the damage mechanisms being of interests are discussed in more detail in Chapter 2.

Potential health and usage monitoring systems can offer either global or local inspection capability. Global techniques may be used to inspect relatively large areas with the aim of locating suspect positions that may then be covered in detail by a special inspection technique. There is a considerable interest among aircraft operators in automated global inspection techniques for:

fatigue cracking, particularly in joints at countersunk hole edges,

corrosion, particularly inside joints and closed compartments,

paint damage as an impact event signal,

disbond, possibly due to corrosion in joints and full depth honeycomb slats and flaps,

impact damage in composites,

manufacturing damage in composites,

disbond in stiffened composite panels.

It is clear that aircraft operators will request at least the same performance as currently available systems and possibly even better to not compromise the overall performance of the aircraft. As a guidance, new systems will be required to detect: 1-2mm cracks in aluminium sheet (at the base of a countersink), 5mm cracks in a metallic frame, 100mm cracks in large areas, 10% of sheet thickness in corrosion or 15 × 15mm debonds. Often the sensitivity of damage detection systems is motivated by the costs of such systems. The continuous automated monitoring will effectively reduce the inspection period and this should imply that considerably less strict requirements on sensitivity should be accepted.

Continues...

---

Excerpted from Health Monitoring of Aerospace Structures Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

---

Table of Contents

List of Contributors.

Preface.

1. Introduction (G. Bartelds, J.H. Heida, J. McFeat and C. Boller).

1.1 Health and Usage Monitoring in Aircraft Structures – Why and How?

1.2 Smart Solution in Aircraft Monitoring.

1.3 End-User Requirements.

1.3.1 Damage Detection.

1.3.2 Load History Monitoring.

1.4 Assessment of Monitoring Technologies.

1.5 Background of Technology Qualification Process.

1.6 Technology Qualification.

1.6.1 Philosophy.

1.6.2 Performance and Operating Requirements.

1.6.3 Qualification Evidence – Requirements and Provision.

1.6.4 Risks.

1.7 Flight Vehicle Certification.

1.8 Summary.

References.

2. Aircraft Structural Health and Usage Monitoring (C. Boller and W.J. Staszewski).

2.1 Introduction.

2.2 Aircraft Structural Damage.

2.3 Ageing Aircraft Problem.

2.4 LifeCycle Cost of Aerospace Structures.

2.4.1 Background.

2.4.2 Example.

2.5 Aircraft Structural Design.

2.5.1 Background.

2.5.2 Aircraft Design Process.

2.6 Damage Monitoring Systems in Aircraft.

2.6.1 Loads Monitoring.

2.6.2 Fatigue Monitoring.

2.6.3 Load Models.

2.6.4 Disadvantages of Current Loads Monitoring Systems.

2.6.5 Damage Monitoring and Inspections.

2.7 Non-Destructive Testing.

2.7.1 Visual Inspection.

2.7.2 Ultrasonic Inspection.

2.7.3 Eddy Current.

2.7.4 Acoustic Emission.

2.7.5 Radiography, Thermography and Shearography.

2.7.6 Summary.

2.8 Structural Health Monitoring.

2.8.1 Vibration and Modal Analysis.

2.8.2 Impact Damage Detection.

2.9 Emerging Monitoring Techniques and Sensor Technologies.

2.9.1 Smart Structures and Materials.

2.9.2 Damage Detection Techniques.

2.9.3 Sensor Technologies.

2.9.4 Intelligent Signal Processing.

2.10 Conclusions.

References.

3. Operational Load Monitoring Using Optical Fibre Sensors (P. Foote, M. Breidne, K. Levin, P. Papadopolous, I. Read, M. Signorazzi, L.K. Nilsson, R. Stubbe and A. Claesson).

3.1 Introduction.

3.2 Fibre Optics.

3.2.1 Optical Fibres.

3.2.2 Optical Fibre Sensors.

3.2.3 Fibre Bragg Grating Sensors.

3.3 Sensor Target Specifications.

3.4 Reliability of Fibre Bragg Grating Sensors.

3.4.1 Fibre Strength Degradation.

3.4.2 Grating Decay.

3.4.3 Summary.

3.5 Fibre Coating Technology.

3.5.1 Polyimide Chemistry and Processing.

3.5.2 Polyimide Adhesion to Silica.

3.5.3 Silane Adhesion Promoters.

3.5.4 Experimental Example.

3.5.5 Summary.

3.6 Example of Surface Mounted Operational Load Monitoring Sensor System.

3.6.1 Sensors.

3.6.2 Optical Signal Processor.

3.6.3 Optical Interconnections.

3.7 Optical Fibre Strain Rosette.

3.8 Example of Embedded Optical Impact Detection System.

3.9 Summary.

References.

4. Damage Detection Using Stress and Ultrasonic Waves (W.J. Staszewski, C. Boller, S. Grondel, C. Biemans, E. O’Brien, C. Delebarre and G.R. Tomlinson).

4.1 Introduction.

4.2 Acoustic Emission.

4.2.1 Background.

4.2.2 Transducers.

4.2.3 Signal Processing.

4.2.4 Testing and Calibration.

4.3 Ultrasonics.

4.3.1 Background.

4.3.2 Inspection Modes.

4.3.3 Transducers.

4.3.4 Display Modes.

4.4 Acousto-Ultrasonics.

4.5 Guided Wave Ultrasonics.

4.5.1 Background.

4.5.2 Guided Waves.

4.5.3 Lamb Waves.

4.5.4 Monitoring Strategy.

4.6 Piezoelectric Transducers.

4.6.1 Piezoelectricity and Piezoelectric Materials.

4.6.2 Constitutive Equations.

4.6.3 Properties.

4.7 Passive Damage Detection Examples.

4.7.1 Crack Monitoring Using Acoustic Emission.

4.7.2 Impact Damage Detection in Composite Materials.

4.8 Active Damage Detection Examples.

4.8.1 Crack Monitoring in Metallic Structures Using Broadband Acousto-Ultrasonics.

4.8.2 Impact Damage Detection in Composite Structures Using Lamb Waves.

4.9 Summary.

References.

5. Signal Processing for Damage Detection (W.J. Staszewski and K. Worden).

5.1 Introduction.

5.2 Data Pre-Processing.

5.2.1 Signal Smoothing.

5.2.2 Signal Smoothing Filters.

5.3 Signal Features for Damage Identification.

5.3.1 Feature Extraction.

5.3.2 Feature Selection.

5.4 Time–Domain Analysis.

5.5 Spectral Analysis.

5.6 Instantaneous Phase and Frequency.

5.7 Time–Frequency Analysis.

5.8 Wavelet Analysis.

5.8.1 Continuous Wavelet Transform.

5.8.2 Discrete Wavelet Transform.

5.9 Dimensionality Reduction Using Linear and Nonlinear Transformation.

5.9.1 Principal Component Analysis.

5.9.2 Sammon Mapping.

5.10 Data Compression Using Wavelets.

5.11 Wavelet-Based Denoising.

5.12 Pattern Recognition for Damage Identification.

5.13 Artificial Neural Networks.

5.13.1 Parallel Processing Paradigm.

5.13.2 The Artificial Neuron.

5.13.3 Multi-Layer Networks.

5.13.4 Multi-Layer Perceptron Neural Networks and Others.

5.13.5 Applications.

5.14 Impact Detection in Structures Using Pattern Recognition.

5.14.1 Detection of Impact Positions.

5.14.2 Detection of Impact Energy.

5.15 Data Fusion.

5.16 Optimised Sensor Distributions.

5.16.1 Informativeness of Sensors.

5.16.2 Optimal Sensor Location.

5.17 Sensor Validation.

5.18 Conclusions.

References.

6. Structural Health Monitoring Evaluation Tests (P.A. Lloyd, R. Pressland, J. McFeat, I. Read, P. Foote, J.P. Dupuis, E. O’Brien, L. Reithler, S. Grondel, C. Delebarre, K. Levin, C. Boller, C. Biemans and W.J. Staszewski).

6.1 Introduction.

6.2 Large-Scale Metallic Evaluator.

6.2.1 Lamb Wave Results from Riveted Metallic Specimens.

6.2.2 Acoustic Emission Results from a Full-Scale Fatigue Test.

6.3 Large-Scale Composite Evaluator.

6.3.1 Test Article.

6.3.2 Sensor and Specimen Integration.

6.3.3 Impact Tests.

6.3.4 Damage Detection Results – Distributed Optical Fibre Sensors.

6.3.5 Damage Detection Results – Bragg Grating Sensors.

6.3.6 Lamb Wave Damage Detection System.

6.4 Flight Tests.

6.4.1 Flying Test-Bed.

6.4.2 Acoustic Emission Optical Damage Detection System.

6.4.3 Bragg Grating Optical Load Measurement System.

6.4.4 Fibre Optic Load Measurement Rosette System.

6.5 Summary.

References.

Index.

Editorial Reviews

"...very relevant and timely...strongly recommend this multidisciplinary book...an integrated volume of real value..." (Measurement and Control, Vol 37(5), June 2004)

From the Publisher