Structural Health Monitoring of Aerospace Composite Structures offers a comprehensive review of established and promising technologies under development in the emerging area of structural health monitoring (SHM) of aerospace composite structures.
Beginning with a description of the different types of composite damage, which differ fundamentally from the damage states encountered in metallic airframes, the book moves on to describe the SHM methods and sensors currently under consideration before considering application examples related to specific composites, SHM sensors, and detection methods. Expert author Victor Giurgiutiu closes with a valuable discussion of the advantages and limitations of various sensors and methods, helping you to make informed choices in your structure research and development.
The first comprehensive review of one of the most ardent research areas in aerospace structures, providing breadth and detail to bring engineers and researchers up to speed on this rapidly developing field
Covers the main classes of SHM sensors, including fiber optic sensors, piezoelectric wafer active sensors, electrical properties sensors and conventional resistance strain gauges, and considers their applications and limitation
Includes details of active approaches, including acousto-ultrasonics, vibration, frequency transfer function, guided-wave tomography, phased arrays, and electrochemical impedance spectroscopy (ECIS), among other emerging methods
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This chapter presents an overview of how the structural health monitoring (SHM) concept could be applied to aerospace composites. Composite materials have known an increasing acceptance into aerospace construction over an evolutionary period that spans more than four decades. At present, the new airframes are predominantly composites, such as Boeing 787 Dreamliner and Airbus A350 XWB in which composites have 80% participation by volume (i.e., 50% participation by weight). Because the damage and failure modes of composite structures are significantly more complicated and diverse than those of metallic airframes, this widespread penetration of composite materials into commercial and military aircraft construction opens new avenues for studying in-service performance, nondestructive evaluation (NDE), and SHM. Hence, the rest of the book is dedicated to understanding these intricate phenomena and identifying sensors and methods by which they can be monitored in service through the NDE and SHM processes.
Keywords
Aerospace composites; structural health monitoring, SHM; nondestructive evaluation, NDE; nondestructive testing, NDT; failure modes; commercial aircraft, military aircraft; airframes
1.1 Preamble
The concept of composites has attracted the interest of both the engineers and the business professionals. To engineers, composites are the opportunity to create designer materials with palettes of properties that cannot be found in existing mineral materials. To the business professional, composites offer unprecedented business growth especially in areas where unprecedented material properties are in high demand. Not surprisingly, the aerospace market is one of the largest and arguably the most important to the composites industry. Commercial aircraft, military aircraft, helicopters, business jets, general aviation aircraft, and spacecraft all make substantial use of high-performance composites. The aerospace usage of high-performance composites has experienced a continuously growing over several decades (Figure 1).
Figure 1 Increase of the weight content of composites in aircraft structures over a 30-year time span: (a) trends in military aircraft composite usage [1]; (b) trends in civil aircraft composite usage [2]; (c) breakdown of weight content by material types in Boeing 787 and Airbus A350 XWB [3].
Composites have good tensile strength and resistance to compression, making them suitable for use in aircraft manufacture. The tensile strength of the material comes from its fibrous nature. When a tensile force is applied, the fibers within the composite line up with the direction of the applied force, giving its tensile strength. The good resistance to compression can be attributed to the adhesive and stiffness properties of the matrix which must maintain the fibers as straight columns and prevent them from buckling.
1.2 Why Aerospace Composites?
The primary needs for all the advanced composites used in aerospace applications remain the same, i.e., lighter weight, higher operating temperatures, greater stiffness, higher reliability, and increased affordability. Some other special needs can be also achieved only with composites, like good radio-frequency compatibility of fiberglass radomes and low-observability airframes for stealth aircraft.
High-performance composites were developed because no single homogeneous structural material could be found that had all of the desired attributes for a given application. Fiber-reinforced composites were developed in response to demands of the aerospace community, which is under constant pressure for materials development in order to achieve improved performance. Aluminum alloys, which provide high strength and fairly high stiffness at low weight, have provided good performance and have been the main materials used in aircraft structures over many years. However, both corrosion and fatigue in aluminum alloys have produced problems that have been very costly to remedy. Fiber-reinforced composites have been developed and widely applied in aerospace applications to satisfy requirements for enhanced performance and reduced maintenance costs.
1.3 What are Aerospace Composites?
Aerospace composites are a class of engineered materials with a very demanding palette of properties. High strength combined with low weight and also high stiffness are common themes in the aerospace composites world. Nowadays, engineers and scientists thrive to augment these high-performance mechanical properties with other properties such as electric and thermal conductivity, shape change, self-repair capabilities, etc.
1.3.1 Definition of Aerospace Composites
From a pure lexical point of view, âcompositesâ seem to have a variety of definitions and there is no completely universal accepted one. One school prefers the word composite to include only those materials consisting of a strong structural reinforcement encapsulated in a binding matrix, while the purists believe that the word composite should include everything except homogeneous or single-phase materials. In a generic sense, a composite material can be defined as a macroscopic combination of two or more distinct materials, having a recognizable interface between them. One material acts as a supporting matrix, while another material builds on this base scaffolding and reinforces the entire material. Thus, the aerospace definition of composite materials can be restricted to include only those engineered materials that contain a reinforcement (such as fibers or particles) supported by a matrix material.
Fiber-reinforced composites, which dominate the aerospace applications, contain reinforcements having lengths much greater than their thickness or diameter. Most continuous-fiber (or continuous-filament) composites, in fact, contain fibers that are comparable in length to the overall dimensions of the composite part. Composite laminates are obtained through the superposition of several relatively thin layers having two of their dimensions much larger than their third.
High-performance composites are composites that have superior performance compared to conventional structural materials such as steel, aluminum, and titanium. Polymer matrix composites have gained the upper hand in airframe applications, whereas metal matrix composites, ceramic matrix composites, and carbon matrix composites are being considered for more demanding aerospace applications such as aero-engines, landing gear, reentry nose cones, etc. However, there are significant dissimilarities between polymer-matrix composites and those made with metal, ceramic, and carbon matrices. Our emphasis in this book will be on polymer matrix composites for airframe applications.
Polymer matrix composites provide a synergistic combination of high-performance fibers and moldable polymeric matrices. The fiber provides the high strength and modulus while the polymeric matrix spreads the load as well as offers resistance to weathering and corrosion. Composite tensile strength is almost directly proportional to the basic fiber strength, whereas other properties depend on the matrixâfiber interaction. Fiber-reinforced composites are ideally suited to anisotropic loading situations where weight is critical. The high strengths and moduli of these composites can be tailored to the high load direction(s), with little material wasted on needless reinforcement.
1.3.2 High-Performance Fibers for Aerospace Composites Applications
Fiber composites offer many superior properties. Almost all high-strength/high-stiffness materials fail because of the propagation of flaws. A fiber of such a material is inherently stronger than the bulk form because the size of a flaw is limited by the small diameter of the fiber. In addition, if equal volumes of fibrous and bulk material are compared, it is found that even if a flaw does produce failure in a fiber, it will not propagate to fail the entire assemblage of fibers, as would happen in the bulk material. Furthermore, preferred orientation may be used to increase the lengthwise modulus, and perhaps strength, well above isotropic values. When this material is also lightweight, there is a tremendous potential advantage in strength-to-weight and/or stiffness-to-weight ratios over conventional materials.
Glass fibers were the first to be considered for high-performance applications because of their high strength when drawn in very thin filaments. Considering that bulk glass is quite brittle, the surprising high strength of these ultra-thin glass fibers gave impetus to this line of research. Subsequently, a variety of other high-performance fibers have been developed: S-glass fibers (which are even stronger that ordinary E glass), aramid (Kevlar) fibers, boron fibers, Spectra fibers, etc.
The fiber that has eventually attained widespread usage in aerospace composites has been the carbon fiber (a.k.a. graphite fiber) that is used in carbon-fiber reinforced polymer (CFRP) composites. High-strength, high-modulus carbon fibers are about
in diameter and consist of small crystallites of âturbostraticâ graphite, one of the allotropic forms of carbon. Two major carbon-fiber fabrication processes have been developed, one based on polyacrylonitrile (PAN), the other based on pitch. Refinements in carbon-fiber fabrication technology have led to considerable improvements in tensile strength (~4.5 GPa) and in strain to fracture (more than 2%) for PAN-based fibers. These can now be supplied in three basic forms, high modulus (~380 GPa), intermediate modulus (~290 GPa), and high strength (with a modulus of ~230 GPa and tensile strength of 4.5 GPa). The tensile stressâstrain response is elastic up to failure, and a large amount of energy is released when the fibers break in a brittle manner. The selection of the appropriate fiber depends very much on the application. For military aircraft, both high modulus and high strength are desirable. Satellite applications, in contrast, benefit from the use of high-modulus fibers that improve stiffness and stability of reflector dishes, antennas, and their supporting structures.
1.3.3 High-Performance Matrices for ...
Table of contents
Cover image
Title page
Table of Contents
Copyright
Dedication
Chapter 1. Introduction
Chapter 2. Fundamentals of Aerospace Composite Materials
Chapter 3. Vibration of Composite Structures
Chapter 4. Guided Waves in Thin-Wall Composite Structures
Chapter 5. Damage and Failure of Aerospace Composites
Chapter 6. Piezoelectric Wafer Active Sensors
Chapter 7. Fiber-Optic Sensors
Chapter 8. Other Sensors for SHM of Aerospace Composites
Chapter 9. Impact and Acoustic Emission Monitoring for Aerospace Composites SHM
Chapter 10. SHM of Fatigue Degradation and Other In-Service Damage of Aerospace Composites
