Handbook of Measurement in Science and Engineering, Volume 1

PART I

CIVIL AND ENVIRONMENTAL ENGINEERING

CHAPTER 1

NEW AND EMERGING TECHNOLOGIES IN STRUCTURAL HEALTH MONITORING

MERIT ENCKELL, JACOB EGEDE ANDERSEN, BRANKO GLISIC, AND JOHAN SILFWERBRAND

1.1 Introduction

1.2 Background

1.3 New and emerging technologies

1.3.1 General

1.3.2 Fiber-optic sensors (FOS)

1.3.3 The global positioning system (GPS)

1.3.4 Microelectromechanical Systems

1.3.5 Corrosion monitoring

1.3.6 B-WIM, WIM

1.3.7 Nondestructive testing (NDT)

1.3.8 Interferometric radar

1.3.9 Photogrammetry

1.3.10 Smart technical textiles

1.3.11 Specific issues around usage of new technologies

1.3.12 Chosen technologies and motivation

1.4 Fiber-optic technology

1.4.1 General

1.4.2 Sensors based on Sagnac, Michelson, and Mach-Zehnder interferometers

1.4.3 Sensor based on the Fiber Bragg Gratings

1.4.4 Sensors based on Fabry-Perot interferometry

1.4.5 Best performances of discrete FOS

1.4.6 Distributed sensors

1.5 Acoustic emission

1.5.1 Theory of acoustic emission

1.5.2 Sources of acoustic emission

1.5.3 The development of acoustic emission in industry and civil engineering

1.5.4 Acoustic emission systems

1.5.5 Codes, standards, and recommended practice in acoustic emission

1.6 Radar technology

1.6.1 General

1.6.2 Ground-penetrating radar

1.6.3 Interferometric radar

1.7 Global Positioning System

1.8 Corrosion monitoring systems

1.9 Weigh-in-motion (WIM) systems

1.9.1 Weigh-in-motion

1.9.2 Railway weigh-in-Motion

1.9.3 Bridge weigh-in-Motion

1.10 Components of structural health monitoring system

1.10.1 Sensory system

1.10.2 Data acquisition system

1.10.3 Data processing and control system

1.10.4 User interface

1.10.5 Maintenance tools

1.11 Structural health monitoring system design

1.11.1 Structural analysis for new structure

1.11.2 Structural analysis for existing structure

1.11.3 Sensor selection

1.11.4 Data acquisition issues

1.11.5 Responsibilities and installation planning

1.12 System procurement and installation

1.12.1 System procurement

1.12.2 Commissioning

1.12.3 Installation

1.12.4 Lifetime support

1.12.5 System efficiency and redundancy

1.12.6 Dismantling environmental issues

1.13 Application of structural health monitoring systems

1.13.1 High-rise building, Singapore—2001

1.13.2 The New Årsta Railway Bridge, Sweden—2005

1.13.3 Stonecutters Bridge, Hong Kong—2010

1.13.4 Severn River Crossing, UK—2010

1.13.5 A4 Hammersmith Flyover, UK—2010

1.13.6 Streicker Bridge, United States—2010

1.13.7 Messina Bridge, Italy—2018

1.14 Discussion

1.14.1 Development of new and emerging technologies

1.14.2 Obstacles

1.14.3 Need for education and collaboration

1.14.4 Future use and development

1.15 Conclusion

Acknowledgments

References

1.1 INTRODUCTION

Structural Health Monitoring (SHM) and modern sensory technology together with advanced data acquisition are currently available for various applications. The area of monitoring is also very wide and incorporates several disciplines. Civil engineers working with monitoring have to cooperate very closely with various kinds of specialists to assure that the chosen monitoring system provides the information that they are looking for.

Organized SHM became a well-known concept during the last decades. Health Monitoring according to Aktan et al. (2000), may be defined as

the measurement of the operating and loading environment and the critical responses of a structure to track and evaluate the symptoms of operational incidents, anomalies, and/or deterioration or damage indicators that may affect operation, serviceability, or safety reliability

SHM helps to control and verify structural behavior: the condition or changes in the condition of a structure. SHM gives more improved and precise information than visual inspection about the real condition of the structure at real time. Decision making concerning the maintenance, economy, and the safety of the structures is easier with an appropriate Structural Health Monitoring System (SHMS).

Factors such as shortened construction periods, increased traffic loads, new high speed trains causing new dynamic and fatigue problems, new materials, new construction solutions, slender constructions, limited economy, and need for timesaving demanded for better control as well as verification and benefited for SHM.

Old deteriorated structures, especially, the ones that do suffer about fatigue effects may have malfunction and could collapse. But also structures that are not at the end of their lifetime do fail. Some serious collapses have taken place in recent years, for example, Sport Arena Bad Reichenhall in south Germany and Arena in Katowice, south Poland in 2006; I-35W Mississippi River bridge in Minnesota, United States in 2007. Many people were killed and injured caused by these mentioned collapses. The newly built bridges called Gröndal Bridge and Alvik Bridge in Stockholm revealed extensive cracking in the webs of their concrete hollow box girder sections just after a few years of operation (Sundquist and James, 2004), and two tension rods and a crossbeam from a recently installed repair collapsed in Oakland Bay Bridge, San Francisco in 2009, causing the bridge to be closed temporarily. Structures also do collapse during the construction period, and workers may be killed or injured as recently for the cable-stayed bridge across the Chambal River in India. SHM can start with a sensor installation and monitoring already during the construction period and therefore provide information throughout the whole life span of the structure: construction, testing, operation and also demolition. It may capture the behavior that visual inspection does not accomplish and therefore there is possibility that it may save human life.

The rapid development of technology in the fields of sensors, data acquisition and communication, signal analysis and data processing provide SHM with great profit. Buildings, bridges, wind farms, nuclear power plants, geotechnical structures, historical buildings and monuments, dams, offshore platforms, pipelines, ocean structures, airplanes, wind plants, turbine blades, and so on, may be objects for monitoring activities. The monitoring can be periodic or continuous, short term or long term, local or global, and the monitoring system can consist of a few sensors up to hundreds or even thousands of them depending on the demands of the monitoring object.

Structural Health Monitoring System for a structure consists of sensors, data acquisition systems, data transfer and storage systems, data management that normally includes data analysis as well as presentation, and data interpretation. The number of sensors used in monitoring is endless. Different applications with various techniques such as electrical, optical, acoustical, and geodetical are available. Various parameters such as strain, displacement, inclination, stress, pressure, humidity, temperature, different chemical quantities, and environmental parameters such as wind speed and direction can be monitored.

Conventional sensors used for civil engineering such as strain gauges, traditional accelerometers, inclinometers, load cells, vibrating wires, linear variable differential transformers (LVDT) are able to measure many parameters and have a long experience in use. On the other hand, the evolution of emerging technologies together with computer-based data acquisition, advanced signal and data communication have made the evaluation of new techniques and sensors for civil engineering purposes possible.

Fiber-optic sensors, microelectromechanical systems (MEMS), optical distance measurement techniques, acoustic emission, and different type of lasers and radars have been under great development in recent years and are now available on the market. They are characterized by high accuracy, straightforward usage, and data-collecting concept. These techniques often allow very delicate measuring in harsh conditions and in various applications. The automatic collection of the data saves time, and it has advantages with respect to manual measurements. The reliability and durability of the sensors become significant when choosing the appropriate instrumentation. These new high-tech sensors also allow for not only high accuracy but also high precision, high and constant sensitivity, stability over time, no drift, and they are often temperature compensated.

The market with fiber-optic sensors and their applications is massive. There are several different techniques and various kinds of sensors that also can be modified for unique monitoring needs for a particular structure. Fiber-optic sensors allow for measurements that have been unpractical or too costly with the traditional sensor technology. Hundreds measuring points along the same fiber, as well as distributed sensing, versatility, insensitivity for electromagnetic fields, operability under extreme climate conditions and also the fact that there is no need for protection against lightning are some of the advantages over the electrical-based counterparts (Ross and Matthews, 1995).

Numerous different monitoring projects have taken place in recent years, and SHM has become a standard when designing large or complicated structures all over the world. It is now an essential implement in managing structures for civil engineers as well as owners. The modern sensory technology is also more commonly used as well as accepted by the civil engineering society.

As the subject is large and the number of emerging technologies and SHM applications is numerous, the chapter principally brings up and discusses the subjects mainly from the civil engineering point of view. The most common techniques that are suitable for civil engineering applications are presented and discussed and examples are given.

1.2 BACKGROUND

Testing and measuring of certain desired parameters have taken place in the field of civil engineering in the latest century (Mufti, 2001). Steel strains, rock stresses, concrete curing temperature, shrinkage and stresses, pressure of the concrete in formworks, vibrations, and many other phenomena that engineers felt uncertain about have been measured and recorded. Lack of knowledge or experience was the driving force. The earliest reference to dynamic testing in the late nineteenth century can be seen in Salawu and Williams (1995). But as the Tacoma Narrows Bridge collapsed in 1940, it forced the engineers to face the problem with long-span bridge aerodynamics (Miyata et al., 2002; Miyata, 2003). As a result, the dynamic measuring techniques developed and increased significantly in the following decades. These activities were small scale and not really organized structural monitoring. The technology was not yet well developed in term of automation and data handling. The amount of data was held in small portions in order to be handled and used in a decent way.

Damage identification in aerospace and mechanical engineering started the organized SHM but were followed by civil engineering society. Organized SHM activities were acknowledged in the last decades. The subject also emerged various engineering disciplines as the new sensor technology and information technology entered the field. At present, many civil engineering structures are monitored continuously and true real-time information of these structures is provided.

Furthermore, a lot of discussion is going on between scientists and other related disciplinary in order to create standards and international guidelines. The concept Health Monitoring was defined by Aktan et al. (2000). They also published the report Development of a Model health Monitoring Guide for major Bridges (Aktan et al., 2001). ISIS Canada Research Network was established in 1995. ISIS published a report Guidelines for Structural health Monitoring in 2001 (Mufti, 2001) and several others design manuals and reports are published up today. International Society for Structural Health Monitoring of Intelligent Infrastructure (ISHMII), a nonprofit organization was founded in 2003. The aim of the Society is to advance the understanding and the application of SHM in the civil engineering infrastructure, in the service of the engineering profession and society. ISHMII also publish a paper called The Journal of Civil Structural Health Monitoring (JCSHM). JCSHM publishes articles to advance the understanding and the application of health monitoring methodologies for the condition assessment and management of the civil infrastructure systems.

The European Union project, sustainable bridges also started to work with functional requirements for railway bridges in 2003. The goal was to achieve increased capacities required to meet future demands for increased traffic levels and heavier axle loads. The activities would also deal with efficient condition monitoring systems for railway bridges, and numerous reports were published and can be seen in www.sustainablebridges.net/.¹

Farrar and Worden (2007) gives an introduction to SHM. A very comprehensible and extensive paper about SHM of civil infrastructure that illustrates the different topics of SHM of dams, bridges, offshore, buildings, towers, nuclear installations, tunnels, and excavations can be seen in Brownjohn (2007). Problems, challenges, and limitation of development of SHM are brought up in these papers, the subject that is very important but still left out completely by many other authors. Some standards are also already created and can be seen in BSI (2004).

Recently, the SmartEN research project for research into smart use of wireless sensor networks and the integration of SHM monitoring results with Maintenance Management Systems for the optimization of maintenance have been formed funded by the European Commission seventh framework for research. These developments open up a completely novel area of multidisciplinary research toward the smart management of sustainable environment. Even though there are top research institutions working in the field of wireless sensors and others in the civil infrastructure reliability and management (Stochastic Optimization Methods for Infrastructure Management with Incomplete Monitoring Data Le Thanh Nam, 2009, Kyoto University), most of the activity is fragmented and there is no significant activity in performing multidisciplinary structured research for developing integrated smart and dynamic systems for effective management of the built and natural environment. The aim of SmartEN is to fill this gap and push innovation through the development of an ITN network that will focus on the development and effective integration of emerging technologies targeting key application areas of current interest to the European Commission and internationally.

Major construction projects such as large bridges, dams, high-rise buildings, nuclear power plants, offshore structures, tunnels, and harbor structures demand a lot of investment and resources. Their malfunction or collapse may cause severe damage to the society, its inhabitants or to the environment. Therefore, need to control these structures have emerged needs for monitoring that in turn has been adapted to smaller scale projects.

1.3 NEW AND EMERGING TECHNOLOGIES

1.3.1 General

Established technologies are well known and have a proper long-term experience. Emerging technologies, on the other hand, are science-based innovations that have the potential to create a new industry or transform an existing one (Day et al., 2000). Emerging technologies demand for new kind of thinking in order to prevail and copy with them. They are also characterized with certain ambiguity and complexity as they are in accelerating change.

Authors of this chapter have been working with established as well as new and emerging technologies: both in theory and practice. Several both small-scale and large-scale installations, testing and measuring campaigns are completed and Structural Health Monitoring Systems are designed, mostly for international major bridges but also for other applications such as high-rise buildings, tunnels, machinery, and heritage structures. The authors’ intention is to provide a comprehensive presentation about the subjects as well as about related tasks. This will help the reader to understand the new challenges in order to accomplish sustainable SHM with new and emerging technologies.

Due to tremendous change in information technologies and sensor development, it is possible today to measure nearly any asked parameter and also perform automatic data processing and analysis in real time and with remote access. Sophisticated systems are present: over icing, corrosion, vibration, deflection, chemical concentration, humidity, strain, stresses and combined as well as distributed parameters can be measured. Some new technologies are better established like fiber-optic sensors but there are still a lot of challenges as research is ongoing and new sensors enter the field constantly.

A lot of work needs also be done in order to guarantee severity and long-term function of these innovative systems; one of the main tasks is to provide for defect localization as well as prediction of the future condition of the structure by automatic data analysis. New and emerging technologies have many advantages compared with old technologies such as adaptability, reliability, possibility for sophisticated measurements in real-time, and in harsh conditions. There are also disadvantages as some uncertainly is present. People working with new and emerging technologies need to be open for new ideas, ways of thinking and able to have a idea about the future development in order to find flexible, adaptable solutions that will meet the requirement not only now but also in the future. The following sections present some new and emerging technologies and areas that the authors’ find interesting and relevant for civil engineering structures. A few technologies are then presented in more detail.

1.3.2 Fiber-Optic Sensors (FOS)

The development that made possible to optical communication was first, the invention of the laser in 1960; and second, the invention of optical fiber. Anyhow, the first optic fibers had a lot of losses and it took some years before the discovery of low-loss silica-glass fiber led to the technology of fiber-optic communications. Telecommunication systems made fiber optics familiar to everybody. The use of fiber-optic applications in different kinds of engineering fields made also a huge expansion in the last decades, especially in communications. Fiber-optic sensory technology provides accurate, wide bandwidth measurements of various parameters in diverse harsh environments where the traditional techniques might fail. Many sensors do not need calibration, have no drift, allow either parallel or serial multiplexing, perform static and/or dynamic measurements and are insensitive to electromagnetic interference.

1.3.3 The Global Positioning System (GPS)

U.S. Department of Defense started the project for Global Positioning System in early 1970s to overcome the limitations of previous navigation systems. GPS system was created and realized, and it is a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth. In order to work properly, GPS needs an unobstructed line of sight to four or more GPS satellites. The system that was developed as a military system became fully operational in 1993, and it is maintained by the U.S. Government (El-Rabbany, 2002). Nowadays, it is also available for civil use and civil engineering needs. Dynamic deformation monitoring of structures, such as long bridges, towers, and tall buildings, is now possible using GPS technology. GPS can also monitor a wide variety of other structures and geologic features: dams, landslides, platforms, slopes, pipelines, volcanoes, retaining walls, constructions zones and railways, just to mention some.

1.3.4 Microelectromechanical Systems (MEMS)

Microelectromechanical Systems is the technology of the very small devices or systems that combine electrical and mechanical components (EMPA, 2004; Leondes, 2006). These devices usually range in size from a micrometer to a millimeter. They are fabricated using modified silicon fabrication technology, molding and plating, wet etching and dry etching, electro discharge machining and other technologies capable of manufacturing very small devices. A particular system can accommodate from a few to millions devices. These systems can sense, control and activate mechanical processes on the micro scale. On the macro scale they are able to function individually or in arrays.

Accelerometers, inertial sensors, optical scanners, fluid pumps, chemical, flow, humidity, temperature, and pressure sensors are a few application examples. Accelerometers based on MEMS technologies are fairly new but perhaps the simplest MEMS device possible, consisting of little more than a suspended cantilever beam or seismic mass with some type of deflection sensing and circuitry. MEMS Accelerometers are available in single axis, dual axis, and three axis models and in wide variety of ranges. Use of these accelerometers in the automotive industry has pushed their cost down radically and they are now available on the market for civil engineering monitoring purposes. A SHM project with MEMS Accelerometers can be seen in Wiberg (2006).

MEMS industry is still a young industry. MEMS will certainly invade more and more area as new products are invented, and frequency response and sense range are getting wider. Sensors based on MEMS technology are also getting more reliable, and their sensitivity is better and even the price is dropping down.

1.3.5 Corrosion Monitoring

Reinforced concrete (RC) structures and steel structures in aggressive environments are exposed to chemical attacks. For that reason, SHM techniques for evaluating the condition of these structures are essential. Maintenance and repair may be very expensive: if errors can be identified at an early stage of their occurrence, a lot of capital can be saved. Corrosion monitoring has many applications in the construction and maintenance of civil structures. High-rise buildings, bridges, dams, spillways, flood control channels, tunnels, piers, pylons, and harbor constructions are continuously monitored.

Parameters to be measured in RC structures are chloride concentration, resistivity, and temperature. The market provides with new innovative devices and methods, and a lot of research in ongoing as corrosion cost a lot of money for governments. The movement of aggressive substances in concrete occurs due to differentials in humidity, ionic concentration, and pressure and temperature within the microstructure of concrete cover (Srinivasan et al., 2009). The moisture content and temperature of the concrete can be monitored during the curing process to ensure maximum strength of the concrete (Dunn et al., 2010). Once construction is complete, the instrumentation can be used to conduct long-term monitoring of corrosion conditions over time.

1.3.6 B-WIM, WIM

Weigh-in-motion or weighing in motion (WIM) devices capture detailed data for each individual vehicle as vehicles drive over a measurement site. The following parameters can be measured dynamic weights of all axles, gross vehicle weights, axle spacing, vehicles distance and speed, vehicle classification according to various schemes, and statistic representations for all types of traffic parameters.

Modern WIM systems are efficient as they are capable of measuring at normal traffic speeds. Bridge weigh-in-motion (B-WIM) is the process by which axle and gross vehicle weights of trucks traveling at highway speeds can be determined from instrumented bridges. B-WIM systems are performed by attaching strain transducers to the soffit of a bridge and placing sensors for detecting axles in order to provide information on vehicle velocity, axle spacing and position of each vehicle. This is called nothing-on-the-road (NOR) or free-of-axle detector (FAD). B-WIM system and a wide range of field trials have been completed in recent years. These systems are becoming increasingly accurate, and they are exceptionally durable as no contact with tires is required (Obrien et al., 2008).

1.3.7 Nondestructive Testing (NDT)

Nondestructive testing is a commonly used tool in several engineering fields as well as in medicine and arts. McCann and Forde (2001) give an overview to NDT methods as applied to the civil engineering. The basic principles of NDT methods are described, and the main NDT methods used in engineering investigations are discussed. In addition, brief case histories from the literature are illustrated.

NDT methods can be divided into following main categories: ultrasonic, magnetic-particle, liquid penetrant, radiographic, remote visual inspection (RVI), radar-based non-contact applications, eddy-current testing, and low-coherence Interferometry. Methods such as ultrasound, radiography, impact echo, rebound hammer, photogrammetry, and crack detection with help of very fine ferromagnetic particles are also widely adopted by industry and engineers. Steel industry, for instance, use these methods for testing on welding, homogeneity of material, and so on, and these testing methods are also described in the handbooks.

1.3.7.1 Noncontact 3D Laser Scanning

Noncontact three-dimensional laser scanners are a tool for capturing geometrical information of the objects (Feng, 2001). This technique that was initially developed for car manufacturing analyzes objects or environment to collect data on its shape and possibly its appearance (i.e., color). It is widely used in industrial applications as well as in rock and bridge engineering. Use of three-dimensional lasers has increased in the untraditional fields in recent decades, and modern three-dimensional laser scanners provide improved resolution, high-measurement accuracy, high scanning speed as well as lower cost. They can also accommodate integrated camera and laser plummet. Adapted software converts the stored data to useful information such as digital, three-dimensional models. Several companies worldwide provide this technique in various performances. These existing systems in the market are based on three scanning principles, for example, triangulation, pulse-based, and phase-based techniques.

A high speed phase-based three-dimensional laser scanner consists of two major components: the single point laser measuring system and the mechanical beam reflection system. A scan takes just few minutes as the system is developed for high-speed, high-performance, and eye-safe scanning tasks. Sampling rate up to hundreds of thousands of points per second can be achieved and the system works both in indoor and outdoor environments. Scanning ranges are from around 0.1 m to about 50 m in average lasers and up to few 100 m for the latest technology. The achieved measurement accuracy is in millimeter range and can also be performed in darkness. An interesting study of development of a prototype system that combines the noncontact measurement technologies of photogrammetric imaging and three-dimensional laser scanning to create dimensionally accurate and pictorially correct three-dimensional models and orthoimages of a rock slope can be seen in Kwong et al. (2007).

Optical technologies encounter many difficulties with shiny, mirroring, or transparent objects. The reflectivity of an object is based on the object’s color or reflecting power of a surface. Transparent objects such as glass will only refract the light and give false three-dimensional information. Shiny objects can be scanned by covering them with a thin layer of white powder. A white surface reflects a lot of light and more light photons will reflect back to the scanner.

1.3.7.2 Infrared Thermography

Infrared thermography or thermal imaging is example of infrared imaging science. Thermal images are visual displays of the amount of infrared energy emitted, transmitted, and reflected by an object. Thermography allows fast scanning of objects and produces immediate images in real time. It is a noncontact and remote measurement that can be used on both still and moving objects.

Thermography is used in many different areas as construction industry, condition monitoring and predictive maintenance industry. It is a safe and convenient method that allows large areas of structures to be surveyed quickly, remotely, and cost effectively. Improved camera and software technology have supported the use in many various applications within the construction industry. Detection of hidden structures, deterioration, moisture, and heat losses can also be performed.

A thermal imaging camera detects and measures invisible infrared energy being emitted from an object. Data are processed and images of that radiation, called thermograms with variations in temperature can be produced and plotted. It is very easy to produce these thermograms, but the interpretation of the information gathered takes both education and experience in order to perform proper analysis.

This noncontact method technique is very interesting and promising. Improved manufacturing efficiencies and product quality emerge new applications for Infrared Thermography. More information can be seen in the website of the Institute of Infrared Thermography (http://www.infraredinstitute.co.uk/index.html).²

1.3.7.3 Acoustic Emission

A structure starts to deform elastically when it is applied to a load; either by internal pressure or by external mechanical loading. Thereby, the stress distribution and storage of elastic strain energy in the structure changes. Acoustic emission (AE) (Jaffrey, 1982) is a naturally occurring phenomenon that takes place and generates elastic waves with these before mentioned loading conditions that relate to rapid release of energy. It detects ultrahigh frequency sound that stressed materials release. Acoustic emission monitoring is classified as a passive nondestructive testing method and AE tests can be used to evaluate the structural integrity of a component or a structure, structural damage diagnosis, lifetime assessment and SHM.

In AE monitoring, a transient elastic wave generated within a material by rapid release of energy is electronically monitored. Defects are detected and located in real time while the phenomena are taking place and instant action can be taken in order to save resources and provide safety.

A lot of development took place in the past decades. This technique is reliable and there is genuine long-term practice (Miller and McIntire, 1987) to monitor material flaws, corrosion, leakage in tanks, pressure vessels, piping systems, and steam generators. Anyhow, AE monitoring have entered civil engineering field; and bridges, offshore structures, heritage structures, dams, and many other structures are monitored continuously. Phenomenon that is studied nowadays with AE monitoring is corrosion, occurrence, and extension of fatigue cracks, fiber breakages in composite materials or fiber breakages in bridge main cables, stay cables or pre-stressd cables as well as cracking in concrete or reinforce concrete members. This is also a perfect method to verify post-tensioning in a structure.

1.3.7.4 Ground-Penetrating Radar

GPR is an accurate geophysical method. It uses electromagnetic waves to map the spatial extent of near-surface objects, interfaces, or changes in soil media and produce images of those features. Radar waves are propagated in distinct pulses from a surface antenna, reflected off buried objects, features or bedding contacts in the ground, and detected back at the source by a receiving antenna. As radar pulses are being transmitted through various materials on their way to the buried target feature, their velocity changes, depending on the physical and chemical properties of the material through which they are traveling. When the travel times of the energy pulses are measured and velocity through the ground is known, distance (or depth in the ground) can be accurately measured. A three-dimensional data set is then produced. In the GPR method, radar antennas are moved along the ground in transects, and two-dimensional profiles of a large number of periodic reflections are created. There are now a number of commercially available equipments for civil engineering purposes (Conyers, 2002).

1.3.7.5 Remote Sensing

Remote sensing is technology of obtaining reliable information on a given object or area either wireless, or not in physical or intimate contact with the object. Any form of noncontact observation can be regarded as remote sensing. Microwave Interferometry and Photogrammetry are good examples of remote sensing and presented latter.

1.3.8 Interferometric Radar

Interferometric radar is a pioneering revolutionary technology in the domain of geodetic measurements that is now spreading out to other areas such as civil engineering. The measurement device is coherent radar generating, transmitting, and receiving the electromagnetic signals to be processed in order to provide movement and deformation measurements (Gentile, 2010). Both static and dynamic measurements of structures can be performed. This noncontact method to measure objects distances up to kilometers is very convenient for many applications such as stay cables and main cables in bridges. No instrumentation is needed, traffic can continue and the method saves time, money, and resources.

1.3.9 Photogrammetry

Geometric properties of object can be determined from photographic images; this practice is called photogrammetry (Mikhail et al., 2001). The American Society for Photogrammetry and Remote Sensing (ASPRS) founded in 1934 is a scientific association. Their mission is to advance knowledge and improve understanding of mapping sciences to promote the responsible applications of photogrammetry, remote sensing, geographic information systems (GIS), and supporting technologies. They do define photogrammetry as follows (http://www.asprs.org/)

Photogrammetry is the art, science, and technology of obtaining reliable information about physical objects and the environment, through processes of recording, measuring, and interpreting images and patterns of electromagnetic radiant energy and other phenomena.

Photogrammetry is used in different fields: topographic mapping, architecture, engineering, manufacturing, quality control, archeology, meteorology, and geology; and it enables producing plans of large or complex sites very effectively. It can be divided to aerial photogrammetry, close-range photogrammetry or Stereophotogrammetry.

In aerial photogrammetry, a camera is installed on an aircraft and multiple overlapping photos of the ground are taken in order to create two-dimensional or three-dimensional models from aerial photographs. Close-range photogrammetry cameras are used to model buildings, engineering structures, vehicles, forensic, and accident scenes. The camera is normally set close to the subject and is typically hand held or on a tripod and the produced output is three-dimensional model or a drawing. In Stereophotogrammetry three-dimensional coordinates of points on an object are estimated. Two or more photographic images taken from different positions are required. Common points are identified on each image and a line of sight can be constructed from the camera location to these points on the object. Triangulation is used to establish three-dimensional locations of the points (Ackermann, 1984). A lot of information such as publications, and so on, can also be found on the web of the International Society of Photogrammetry and Remote Sensing (ISPRS) (http://www.isprs.org/).³

Monitoring of crack origin and evolution with photogrammetry can be seen in Benning et al. (2004).

1.3.10 Smart Technical Textiles

Technical textiles are frequently used in civil engineering and geotechnical applications for reinforcing, repairing, or retrofitting purposes (Veldhuijzen Van Zanten, 1986). Typical applications in civil engineering domain include repair of damaged parts of structures (e.g., cracking of the bridge deck), retrofitting of seismically week structure (e.g., old masonry structure). Typical geotechnical applications include reinforcing bearing capacity of soils beneath foundations (e.g., dams, dikes, tunnels) and stabilization of landmasses prone to subsidence or sliding (e.g., to prevent land sliding, or creation of sinkholes).

Once the technical textile is applied to the host structure, there is a need to evaluate both its performance and the performance of the host structure in long term, in order to assess effectiveness of the technical textile application and the improvement made to the host structure. It is important to detect any deterioration in the performances, since it may lead to failure of the structure. Two parameters that are of particular interest for assessment the condition of civil structures are strain and temperature. In geotechnical applications, besides these two parameters, monitoring water pressure and leakage is important for assessment of condition of reinforced soils and foundations.

By embedding a monitoring system in the technical textile, the latter is transformed into an innovative intelligent multifunctional material—smart textile—that simultaneously provides both reinforcing and monitoring capabilities. An integration of sensors into the technical textile is functionally beneficial for both parties. The sensors provide with assessment of performances of the technical textile, whereas the latter provides for protection and an easy and practically inexpensive installation of the sensors, because they were integrated in the technical textile during the production. Thus, research is being performed in order to create a multifunctional technical textile with monitoring capabilities (e.g., Messervey et al., 2010).

Various sensing technologies can be embedded in the technical textile; however, the fiber-optic sensors seem to yield the most promising results. Sensors based fiber Bragg gratings (FBG) are particularly suitable for embedding in layers of technical textiles used to repair and retrofit civil structures such as bridges and buildings (Messervey et al., 2010), owing to their small size and both serial and parallel multiplexing capability. Distributed sensing technologies are particularly suitable for embedding in technical textiles used in geotechnical applications, since they provide with coverage of long lengths (up to several kilometers), which are characteristic for geotechnical structures (Belli et al., 2009).

Besides the developments in domain of traditional, glass fiber-based sensors, research is ongoing in domain of plastic optical fiber based sensors too (Liehr et al., 2009). Although the plastic fiber features significantly higher losses, and consequently can be applied to cover limited length (few hundreds of meters), they are highly deformable (few tens of percents of the original length), which makes them particularly suitable for monitoring large deformations such as found in geotechnical applications.

The first smart technical textile based on Brillouin scattering in glass optical fibers appeared on the market and they are commercially available. The research is ongoing, and various new products are expected emerge in the next decade.

1.3.11 Specific Issues Around Usage of New Technologies

Structures such as wind turbines are increasing, they are often located in remote areas with difficult or no access at all. They do require new kind of solutions in order to be monitored. Lot of research is ongoing in the field and solutions can be adapted from other areas like from offshore structures that also have no easy accessibility.

Other important structures and structural components difficult to access are the insides of main cables and stay cables in bridges, heritage structures in poor condition, residential areas of high-rise buildings, surfaces of tunnel structures, and so on. Sensors can be required implemented by manufacturers when producing components, as installation is difficult or impossible afterwards.

New technologies are merging the field and techniques from other engineering fields as well as monitoring methods are adapted to civil engineering purposes. These applications might though lack long-term experience; therefore, it is good to perform proper investigations when choosing the technology so that capital is not invested in nonproven technology. Small-scale test in the field is sometimes appropriate to prove, if the technique is feasible in the given circumstances. It is also very important that the installation is done in a proper way; otherwise, there is a risk to jeopardize the function of the whole system.

1.3.12 Chosen Technologies and Motivation

Many large construction projects were instrumented with thousands of sensors in the last decades. New technologies entered the field and a lot of heuristic knowledge was gained when working with new technologies, both in theory and in practice.

FOS appears to have a bright future, and a lot of research has been going on presently. Corrosion is present in reinforced concrete and steel structures; therefore, better methods for corrosion monitoring need to be found in order to provide long lasting structures and to save capital. New NDE techniques offer for straightforward monitoring and testing of structures. True information about old structures can be recorded in order to avoid uncertainties and malfunctions.

First, the chosen technologies and areas of interests are FOS, acoustic emission, radar technology, WIM applications, and GPS, as they have shown their adaptability to civil engineering structures in recent decades.

And second, corrosion monitoring as this issue is present in nearly every structure and huge amounts of capital can be saved if corrosion problems can be solved. New developments and the chosen technologies that have potential for SHM of civil structures are discussed below.

This chapter does not discuss wireless sensing technologies as wireless structural monitoring systems are still in their infancy. A detailed overview in designing wireless sensing units for the health monitoring of civil structures can be seen in Lynch (2007). The chapter has reviewed the design of a number of different wireless sensing unit prototypes that have been designed explicitly for structural monitoring applications. The firmware required to operate the units and to locally interrogate structural response data and a case study is presented, and a novel wireless active sensing unit design is proposed.

1.4 FIBER-OPTIC TECHNOLOGY

1.4.1 General

An optical fiber is a thin, transparent fiber, usually made of fused silica for transmitting light over large distances with very little loss. The diameter of the optic fiber is similar to that of a human hair and the core of it serves to guide the light along the length of the optical fiber. There are both single mode and multimode fibers: the core of the single mode fiber is very small, 5–10 μm, whereas core of the multimode fiber is around 50 μm. The core is surrounded by cladding with slightly lower index of refraction than the core. The purpose of the cladding is to minimize the losses and also physically to support the core region as the light propagates in the fiber. Optical fibers operate over a range of wavelengths; 1310 and 1550nm is common for single mode fibers with minimal losses and 850 and 1300 nm for multi mode fibers.

Development and research is ongoing; existing technology is improved and new products appear on the market continuously. Clear and detailed descriptions over the different fiber-optic technologies can be seen in Measures (2001). The book is highly relevant still today and a perfect introduction into fiber-optic sensors, interrogators, and related aspects.

Several companies working with fiber-optic sensors were founded in the 1990s in collaboration with universities. Several doctoral theses, as well as books were published about various kinds of fiber-optic systems and SHM related topics. Fiber-optic sensors, smart materials, and structural technology were developed, and examples can be seen in Udd (1991, 1995), Inaudi (1997), Vurpillot (1999), Glisic (2000), Clark et al. (2001), Glisic and Inaudi (2007), and Imai et al. (2009).

The high performances of the fiber-optic sensors (FOS) are intrinsically linked to the optical fiber itself. The optical fiber can be used for both sensing and signal transmission purposes. The silica of which the optical fiber is composed is an inert material, which is resistant to most chemicals in wide range of temperatures and is, therefore, suitable for applications in harsh chemical environments (Udd, 2006). Various packaging especially designed for field applications made fiber-optic sensors robust and safe to use even in very demanding environments (Udd, 2006; Glisic and Inaudi, 2007).

The light used for sensing purposes in the core of the optical fiber does not interact with any surrounding electromagnetic (EM) field. Consequently, the fiber-optic sensors are intrinsically immune to any EM interference (EMI), which contributes significantly to their long-term stability and reliability. The ability to measure over distances of several tens of kilometers without the need for any electrically active component is an important feature when monitoring large and remote structures, such as landmark bridges, dams, tunnels, and pipelines (Udd, 2006).

Fiber-optic sensors cover large spectrum of parameters that can be monitored (e.g., strain, inclination, temperature, humidity); thus, multiple parameters can be combined on the same network (e.g., Del Grosso et al., 2005). Compared with conventional electrical sensors, fiber-optic sensors offer two new and unique sensing tools: long-gauge strain sensors and truly distributed strain and/or temperature sensors. The former can be combined in topologies that allow for global structural monitoring while latter allows for one-dimensional strain field and integrity monitoring.

As the area of fiber-optic sensors is really wide, it is not possible to present every single technique. Sensors that are generally used in civil engineering applications and their division based on gauge length and the functional principle are presented in Figure 1.1.

FIGURE 1.1 Division of FOS based on gauge length and functional principle.

Recent significant developments in the optical telecommunications market helped reduce the cost of the FOS, which is still higher compared with conventional sensors, but, however, affordable and justified by superior long-term performance of the FOS. In this section, several FOS techniques are presented, along with their typical applications and summaries of the best performances. Many sensors like the ones based on fluorescence are mostly used for medical and chemical applications and are beyond the scope of this chapter.

1.4.2 Sensors Based on Sagnac, Michelson, and Mach-Zehnder Interferometers

Sagnac interferometer, also called as a fiber-optic gyroscope (FOG) is a gyroscope that uses the interference of light to detect mechanical rotation. This is very developed sensor and principally used for rotation rate measurements as well as in aerospace engineering. Two counter propagating light beams travel along the fiber inside an interferometer in opposite directions. As the path is closed and same for both beams, the beam traveling against the rotation experiences a slightly shorter path than the other beam. This is called Sagnac effect. This sensor can also be used in dynamic measurements but is not well known in commercial civil engineering applications.

Both Michelson and Mach-Zehnder interferometers are easy to understand and manufacture. They are pretty similar in appearance: two fiber beams are inserted with light and then a relative phase shift between these beams is measured in order to measure eventual length difference. In the Michelson interferometer, the end of the fiber paths includes chemical mirrors that reflect the light back to the photo detector. Mach-Zehnder interferometer has two outputs instead of the mirrors and the light passes the fiber paths and is then measured by a photo detector. Michelson interferometer-based decoder allows for very stable, long-term static measurements, whereas Mach-Zehnder interferometer allows for very sensitive short-term dynamic measurements. A detailed description of Michelson Interferometer that is commercially available is given in the following subchapter.

1.4.2.1 SOFO

Michelson interferometer that is proven in large number of projects (Glisic et al., 2010a) is called SOFO (French acronym for Structural Monitoring using FOS, Inaudi, 1997). The standard SOFO sensor comprised two zones: the active zone that measures the deformations, and the passive zone that serves as the carrier of information between the active zone and the reading unit. The sensor is schematically represented in Figure 1.2.

FIGURE 1.2 Schematic representation of standard SOFO sensor.

The active zone is limited by two anchor pieces and consists of two optical fibers placed in a protection tube. The anchor pieces have a double role: to attach the sensor to the monitored structure and to transmit the deformation from the structure to the sensing fiber. The measurement fiber is pretensioned between the anchor pieces in order to measure the shortening of the structure as well as its elongation. The reference fiber is independent of both the measurement fiber and the deformation of the structure, and its purpose is to compensate for temperature changes. Both fibers have mirrors silvered at their extremities.

The passive zone transmits the information from the active zone to the reading unit. It comprised one single-mode fiber, connector, and coupler, which are all protected by a plastic tube. The coupler is placed in the passive zone of the sensor, close to the anchor piece in order to increase the precision and to facilitate the manipulation during the measurement.

The light inserted in the passive zone is split into two fibers of active zone, travels to the extremities and reflects back off the mirrors. Since two fibers have in general different length, a shift in phase is created between the two reflected lights, and this shift in phase is proportional to the length difference in the optical fibers. The deformation of monitored structure will be transferred only to measurements fiber while the reference fiber remains unchanged. As a consequence, the shift in phase will change. The shift in phase is converted to length difference in the reading unit, which contains decoder in form of Michelson interferometer.

The SOFO sensor is true long-gauge sensor, because the light integrates the strain along its gauge length, which is typically limited between 20 cm and 10 m. The best performances of the SOFO system are given in Table 1.1, and applications can be seen in Enckell and Larsson (2005) and Enckell (2006).

TABLE 1.1 The Best Performances of Discrete FOS

1.4.3 Sensor Based on the Fiber Bragg Gratings

Fiber Bragg gratings (FBG) consist of periodical changes created in fiber core by appropriate exposing to ultraviolet light (Dadpay et al., 2008). If the light containing certain range of wavelengths is inserted into the fiber with an FBG, the latter will reflect back one specific wavelength and let pass through all the other wavelengths, as shown in Figure 1.3. The specific wavelength that is reflected depends on optical properties of the FBG that were preimposed by manufacturing process. The optical properties of the FBG depend linearly on strain and temperature. Increase in one or both of these parameters will practically change the wavelength that is reflected back to the light source. By determining the difference from initial wavelength it is possible to determine strain or temperature in the FBG. Since the change in wavelength depends on both strain and temperature simultaneously, strain sensors must be compensated for temperature using an appropriate procedure. The typical length of an FBG is several millimeters, thus, depending on packaging, they can be used either as short-gauge sensors (fiber-optic equivalent for classical strain-gauges) or as long-gauge sensors (e.g., if the fiber with FBG is pretensioned between two anchoring points). There is a large variety of both types of the sensors available on the market.

FIGURE 1.3 Principle of FBG sensors.

The source and decoder for FBG-based sensors are usually combined in the same device. The source is able to generate desired range of wavelength, while decoder measures intensity of reflected light and determines reflected wavelength using tunable laser with wavelength filter (e.g., Fabry-Perot cavity) or spectrometer. Both static and dynamic measurements are possible, depending on type of the reading unit.

An important advantage of FBG-based sensors is that several gratings with different specific wavelengths can be placed along a single fiber, allowing an easy multiplexing. Thus, several sensors can be read from a single channel on the reading unit. The total number of sensors that can be placed along single line depends on range of strain and temperature changes in the monitored structure. For typical civil engineering applications, involving steel or concrete structure, this number varies between 5 and 10. The best performances of the FBG sensors are given in Table 1.1.

1.4.4 Sensors Based on Fabry-Perot Interferometry

The previously presented sensors, SOFO and FBG, are called intrinsic sensors, because the optical fiber is used as the sensing body. Fabry-Perot interferometric sensors can be either intrinsic or extrinsic, but latter was proven to be more effective and reached market maturity, and that is why they are presented in this section. Schematic principle of functioning of an extrinsic Fabry-Perot interferometric (EFPI) sensor is shown in Figure 1.4.

FIGURE 1.4 Schematic principle of EFPI sensors.

The sensor consists of lead fiber and target fiber, both cleaved at 90° and both with partially reflective surfaces. There is a few millimeters air gap between the cleaved surfaces. The broadband light is sent from the source (multimode optical fiber is used as opposed to single-mode used in the case of SOFO and FBG sensors) through lead fiber. When the light reaches the cleaved end of the lead fiber a part of it is reflected back to the reading unit, and the other part passes through the air gap, reflects off the surface of the target fiber, and re-enters in the lead fiber. Two lights are then combined in the lead fiber into an optical signal (constructive or de-constructive interference of several wavelengths) that contains information about the size of the air gap (Krohn, 2000). This information is finally decoded in the reading unit.

The lead fiber is connected to target fiber using a packaging witch purpose is to couple the sensor to the structure. Any deformation of the structure will result in the change of the size of the air gap that is sensed by optical signal and determined in the reading unit.

The great advantage of the EPFI principle is its insensitivity to temperature changes. However, depending on the gauge length of the sensor and the position of the anchoring points between the packaging and the optical fibers, sensitivity to temperature is often present due to thermal expansion of the mechanical components of the sensor, and in general the sensors should be compensated for temperature.

Besides the strain, the EFPI sensors can be packaged so that they can sense temperature and pressure. They can be made extremely small (practically the size of optical fiber is the lower size limit) in and they found many applications in biomedical domain.

In civil applications, the EFPI sensors face some limitations regarding multiplexing and remote position of the reading unit; however, the complete system for smaller projects or for manual measurements is less expensive than SOFO and FBG systems. The best performances of EFPI sensors are shown in Table 1.1.

1.4.5 Best Performances of Discrete FOS

The best performances of discrete FOS are for comparison given in Table 1.1. These performances were extracted from commercial data sheets and web page of various companies.

1.4.6 Distributed Sensors

Distributed sensor (or sensing cable) can be represented by a single cable, which is sensitive at every point along its length. Hence, one distributed sensor can replace large number of discrete sensors. A distributed sensor requires single connection cable to transmit the information to the reading unit, instead of a large number of connecting cables required in case of wired discrete sensors. Finally, distributed sensors are less difficult and more economic to install and operate. An illustrative comparison between pipelines equipped with distributed and discrete sensors is shown in Figure 1.5 (this schematic drawing does not refer to real case, e.g., redundancy is not included).

FIGURE 1.5 Distributed versus discrete monitoring; schematic comparison (does not refer to real case).

Although a distributed sensor is sensitive to measured parameters (strain and/or temperature) at every point of its length, it delivers measurements at discrete points that are spaced by a constant value, called the sampling interval, and the measured parameter is actually an average strain measured over a certain length, called the spatial resolution (Lanticq et al., 2009). The spatial resolution can be considered as the gauge length over which the measurement is made. Sampling interval and spatial resolution are two parameters characteristic for the distributed systems. They can be set by the user depending on application. However, they are correlated with resolution and precision of measurement, and the time of measurement, and the trade-offs must be made. For example, a measurement that is very precise, and sensitive (small spatial resolution), are slow, while a fast measurement affects one or both other parameters.

There are three main principles for distributed sensing in the domain of FOS: Rayleigh scattering (e.g., Posey et al., 2000), Brillouin scattering (e.g., Karashima et al., 1990), and Raman scattering (e.g., Kikuchi et al., 1988). Each technique is based on the relation between the measured parameters, that is, strain and/or temperature, and encoding parameter, that is, changes in optical properties of the scattered light. This is schematically presented in Figure 1.6.

FIGURE 1.6 Scattered light properties as encoding parameter for strain and/or temperature measurements (Glisic and Inaudi, 2007, courtesy of SMARTEC SA).

Rayleigh scattering can be used for both strain and temperature monitoring. It is based on the shifts in the local Rayleigh backscatter pattern, which is dependent on the strain and the temperature. Thus, the strain measurements must be compensated for temperature. The main characteristics of this system are high resolution of measured parameters and short spatial resolution, but the maximal length of sensor is limited to 70 m (Lanticq et al., 2009). Thus, this system is suitable for monitoring of localized strain changes over relatively short distances. Best performances achievable in strain monitoring using the Rayleigh scattering are given in Table 1.2.

TABLE 1.2 Comparison of Best Performances Achievable in Strain Monitoring Using Distributed Systems

Brillouin scattering can also be used for both strain and temperature monitoring. It is based on the change in frequency of Brillouin scattered light, which is dependent on the strain and the temperature. Thus, as in case of the Rayleigh scattering, the strain measurements must be compensated for temperature, and temperature measurements are to be performed with sensors containing a loose (strain-free) optical fiber. Both spontaneous (Wait and Hartog, 2001) and stimulated (Nikles et al., 1994, 1997) Brillouin scattering can be used for sensing purposes. Monitoring system based on stimulated Brillouin scattering is less sensitive to cumulated optical losses that may be generated in sensing cable due to manufacturing and installation, and allows for monitoring of exceptionally large lengths (Thevenaz et al., 1998), for example, in the case of strain monitoring, a single reading unit with two channels can operate measurement over lengths of 10 km, while in the case of temperature monitoring, the lengths of 50 km can be reached. Remote modules can be used to triple the monitoring lengths. The measurement specifications of Brillouin-based measurements are not as good as these of Rayleigh-based measurements; however, the great advantage of Brillouin-based systems is significantly longer length of the sensor (several kilometers). Thus, the Brillouin-based systems are better suited for monitoring global strain changes over large distances. Applications and different methods of Brillouin monitoring can be seen in Nikles et al. (2004) and Inaudi and Glisic (2005, 2006). The best performances in strain monitoring achievable with Brillouin-based systems are given in Table 1.2.

Raman scattering is the result of a nonlinear interaction of the light traveling in the silica fiber core and can only be used for temperature monitoring. It is based on the change in amplitude of Raman scattered light, which is dependent on temperature only. The insensitiveness of this parameter to strain is actually an advantage compared with Rayleigh and Brillouin-based temperature monitoring, since no particular packaging of the sensor must be made to make sensing fiber strain-free. Typical spatial resolution of Raman systems is 1 m, and typical resolution is better than 1°C. Since the leakage of pipelines, dykes, dams, and so on, often changes thermal properties of surrounding soil, besides the temperature monitoring; the Raman-based systems are used for leakage monitoring in large structures.

There is a sever tradition of monitoring dam constructions with Raman systems and some examples can be seen in Aufleger (1998).

1.4.6.1 Crack Detection with Distributed Techniques

Crack-related parameters: detection, identification, localization, and quantification, can be measured with distributed fiber-optics techniques.Brillouin-based technology for crack detection and crack width estimation is tested and discussed by Bao et al. (2005), Imai et al. (2009), and Nöther et al. (2009). Shi et al. (2005) and Zhang and Wu (2008) do report about distributed sensing system based on Brillouin scattering for monitoring geotechnical engineering structures and large Earth structures.

Enckell et al. (2011) do report about evaluation of a large-scale bridge strain, temperature and crack monitoring with distributed fiber-optic sensors (Ravet et al., 2009). The main goals of the system are to detect and localize new cracks, measure high strain, and unusual strain behavior. Also new methods and procedures in installing, testing, modifying, and improving a SHMS were developed, tested and proven, both in laboratory and on-site. The system sends warning messages to the bridge authorities with well-specified scenarios (Myrvoll et al., 2009; Enckell et al., 2011). The system is installed over five 1-km long girders of the Gota Bridge, Gothenburg, Sweden (Glisic et al., 2007, 2009) and monitoring is ongoing and intended to the last about 10 more years, to the end of the bridge’s lifetime. This is the first application of distributed systems on the bridges at such a large scale.

1.5 ACOUSTIC EMISSION

1.5.1 Theory of Acoustic Emission

When a material deforms and cracks, the subsequent atomic rearrangement produces transient elastic waves, which is known as acoustic emission (AE). Degrading mechanisms within materials, such as cracking, corrosion, and yielding, which produce AE signals, are called sources. AE signals contain information about the nature and the severity of the changes occurring within the material. Signals propagate through the material away from the source. AE technology is a passive technique that detects these signals using surface mounted transducers (sensors). Active defects are point sources, emitting nondirectional AE, which radiate out in a spherical wave front and detection is permitted by sensors placed anywhere on the structure. AE has the ability to locate the position of active defects, using two or more sensors. Location is achieved by identifying the time of arrival of an AE signal at different transducers. AE has progressed greatly over the past 30 years, with the increase in computing power allowing increased accuracy through data filtering and processing, which has seen the establishment of many commercial applications. Advantages of acoustic emission are high sensitivity; early and rapid detection of defects such as corrosion, flaws, wire break, and cracks; real-time monitoring of structures in-service; cost and time reduction; defect localization; global and local monitoring; monitoring of nonaccessible zones; defective area location; repeatable; and the evaluation is comprehensive and source detection; and it is not limited by defects geometry. Disadvantages are complicated signal analysis that requires highly trained and experience personal, low signal to noise separation and difficulty to separate external noise in some applications.

1.5.2 Sources of Acoustic Emission

Sources that emit detectable AE are numerous; in fact most materials emit AE when they are put under load. Common sources include the fracture of ice cubes in a drink or the crunch of an apple being eaten. Acoustic emission can be generated by many mechanisms, which include

Fracture of crystallites/inclusions

Crack nucleation and propagation

Micro-cracking including fracture of inclusions

Phase transformations in solids (e.g., marstensitic)

Boiling and electrical discharge (electrical transformers)

Matrix cracking (fiber-reinforced plastics)

Delamination (concrete and fiber-reinforced plastics)

The creation, annihilation of point, line or surface defects

Other forms of acoustic emission mechanisms commonly encountered include

Mechanical sources, for example, pumps, actuators

Joints and supports of structures, for example bridge expansion joints and bearings

Practitioners of AE try to differentiate between real damage sources and extraneous noise. Fundamental to the identification of signals of an individual application is the development of a database. A database contains examples of AE data from tests where the origin of the signals was confirmed either through visual inspection or other nondestructive testing. Using source confirmed AE examples, AE data taken in relatively