Recent advances and trends in structural health monitoring

Recent advances and trends in structural health monitoring

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Shukla Alokita 1 , Verma Rahul 1 , Kandasamy Jayakrishna 1 , V.R. Kar 3 , M. Rajesh 1 , S. Thirumalini 2 , M. Manikandan 4 1 School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India; 2School of Civil and Chemical Engineering, Vellore Institute of Technology, Vellore, India; 3Department of Mechanical Engineering, NIT Jamshedpur, Jharkhand, India; 4 Department of Mechanical Engineering, Amrita College of Engineering and Technology, Nagercoil, Tamil Nadu, India

4.1

Introduction

In recent years, researchers from academic institutes, government agencies, and industries have been concentrating more on structural health monitoring (SHM) in different fields such as civil, marine, mechanical, military, aerospace, power generation, and offshore oil and gas. In engineering, the age, material type, service condition, and layout of the structures influences their performance. Other than performance, safety, reliability, and serviceability of any engineering structure are crucial aspects [1]. Hence, it is important to use technology to monitor the engineering structure by evaluation and assessment [2]. SHM technology is employed for various applications worldwide. For example, long-term monitoring systems have been implemented to monitor large structures in various countries such as Europe [3e5], the United States [6,7], Canada [8,9], Japan [10,11], Korea [12,13], China [14e16], and other countries [17e19]. Development of SHM helps in detecting damage and analyzing strategies, which further helps to increase the service life of engineering structures or components by avoiding their failure [20]. In general, engineering structures fail due to damage in the material and due to certain geometric properties including some boundary conditions of that system, which negatively affect their performance. The main aim of SHM is to alert the system in initial stages of initiation of damage and avoid further propagation of failure with the help of continuous monitoring by structurally integrated sensors. In general, SHM is used to monitor the structure by measuring strain, load, displacement, impact, pH level, moisture, crack width, vibration signatures, and presence of cracks. In SHM process, dynamic responses, extraction of damage-sensitive features, and statistical analysis are used to monitor the structure [21]. Wang et al. [22] analyzed merits and limitations of different gear damage monitoring techniques using vibration measurement. David et al. [23] present a critical review for diagnostics of rotating machinery using acoustic emission technology. Loutas et al. [24] used vibration and acoustic emission recordings method to monitor the condition of gears. They used Structural Health Monitoring of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102291-7.00004-6 Copyright © 2019 Elsevier Ltd. All rights reserved.

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an oil debris monitoring (ODM) system along with vibration and acoustic emission recordings method to increase the diagnostic capacity. Tandon and Choudhury [25] analyzed the vibration and acoustic measurement method to find the damage in the rolling element of bearings. Fadden and Smith [26] also reviewed vibration-based monitoring of rolling element of bearings by high-frequency response method. Staszewski et al. [27] used active and passive approaches to monitor the aerospace composite structure. They estimated the severity in composite plates using 3-D laser vibrometry to locate delamination. Strain waves transmitted from an impact that are applied to the aircraft composite structure were detected using piezoceramic sensors. Damage detection is important to ensure the service life and performance of the structure. In recent years advances in SHM have been taking place at a faster pace. Recent developments in the material field influence the performance of structures. Especially, low-weight materials such as composite materials and alloys are used to replace conventional materials for different engineering applications. Main advantages associated with light-weight composite materials are high strength-to-weight ratio and easy manufacturing. These properties increase the usage of composite materials. Hence, it is important to monitor the damage occurring in the composite material. In the composite structures, strength is based on fiber-matrix interaction, fiber pullout, debonding of fiber-matrix, and crack formation. Similarly, geometry monitoring is also important. Henceforth, it is important to monitor the performance of composite structures continuously for safety. Complying with the current trends, researchers used various nondestructive approaches in this area such as ultrasonic testing, X-rays, vibration/modal analysis, and numerous optical methods such as shearography or holography [28]. Todd et al. [29] analyzed the geometry changes using eight-degree-of-freedom spring-massdamper vibration-based damage detection in a system. Montalvao et al. [30] reviewed vibration-based SHM for composite material for damage detection, localization, and assessment for certain kinds of structures. Kang et al. [31] used carbon nanotube material to form a piezoresistive strain sensor for SHM. Higher van der Waals attraction forces associated with carbon nanotube produces strain measurement. The reason for using strain measurement is that, higher van der Waals attraction force allows axial slipping of the smooth surfaces of the nanotubes. FabryePerot interferometer (EFPI) and fiber Bragg grating (FBG) sensors were employed by Leng and Asundi [32] to monitor the curing process of carbon fiber composite laminates. Results revealed that both embedded EFPI and FBG sensors detect the damage occurring in the composite material due to curing process. Murukeshan et al. [33] used FBG sensors in the curing process of composite material. It was also used to analyze the mechanical changes for 3- and 4-point bending. Kalamkarov et al. [34] measured the strain produced in pultruded carbon fiberereinforced composite rods using optical sensors. Cracks and corrosion occurring in aircrafts have been monitored using piezoelectric wafer active sensors. Lau et al. [35] used optical fiber sensors for SHM of civil infrastructure elements. They carried out experimental investigations to measure the strain of composite-strengthened concrete structures by fixing single- and multiple-point strain measuring techniques. Ciang et al. [36] improved the safety of wind turbines by monitoring the wind turbine system to avoid downtime and to lower the frequency

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of sudden breakdowns. The failure of structures is common due to damage occurring in the materials and geometry. It is very important to analyze the structural response of engineering structures with respect to various factors such as loading conditions, environment, materials, boundary conditions, etc. Hence, it is important to monitor the structure from time to time to avoid failure. In this chapter, recent developments in SHM are discussed for various engineering applications.

4.2

State of the practice in bridge monitoring systems

Long-term structural health monitoring systems are successfully employed in the monitoring of bridges [37]. Each monitoring technique is useful in various fields based on their advantages such as in the monitoring of underground tunnels, aerospace composites, submarines, and bridges. Among all the advantages of SHM systems, one major advantage is the monitoring of megastructures like bridges [37]. Ni YQ et al. [38] reviewed that about 40 bridges with spans of 100 m or longer in the world are being evaluated with SHM systems. Famous examples of such bridges are the Great Belt Bridge in Denmark [3], the Tsing Ma Bridge in Hong Kong [39], the Akashi Kaikyo Bridge in Japan [40], the Commodore Barry Bridge in United States [41], the Seohae Bridge in Korea [42], and the Confederation Bridge in Canada [43]. Long-term monitoring techniques have been implemented at the time of construction on some recently constructed bridges, such as the Shenzhen Western Corridor, the 4th Qianjiang Bridge, the Sutong Bridge, and the Stonecutters Bridge [44]. In the research of KY Wong et al. [21], there was major improvement in the monitoring system of the Stonecutters Bridge when compared to the Tsing Ma Bridge. The improvement includes the addition of advanced sensors such as corrosion sensors, hygrometers, barometers, and pluviometers [37]. There are some specific types of sensors like corrosion sensors, fiber-optic sensors, and strain gauge sensors that can be employed only at the time of manufacturing of the bridge [37]. In recent years, the main focus of monitoring systems was on the monitoring of bridges’ durability, integrity, and reliability [37]. This was clearly visible in the Sutong Bridge. The majority of embedded sensors in the Sutong Bridge are a foundation stability and safety monitoring system, with the intent of bringing longevity to the life span of the bridge and making the bridge more durable [37].

4.3

Factors affecting measurement data

SHM is an effective way to enhance the life span of a structure. In recent years, there have been huge advancements in the technology of SHM systems such as plantation of embedded sensors at the time of manufacturing and the statistical analysis of the structure to determine the present health of the structure [45]. There are numerous benefits of implementation of SHM, such as improving public safety, improving life span of the structures, and reducing effective costs of construction [2]. However, to meet the

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recent needs for proper monitoring of the structures, a number of technical factors and environmental factors need to be considered while conducting normalization and fusion of data [46]. The measurement data given by most of the monitoring techniques is very precise, but there are some factors such as environmental factors, on-site construction defects, and mixing of monitoring techniques that affect the measured data. These factors are discussed in the following subsections.

4.3.1

Environmental factors

The major difference between structural health monitoring and conventional measurement systems is the damage diagnostic and prognostic methods [46]. In the past decades, numerous studies by Ko JM et al. [46] on the methods of structural damage identification have been conducted and stated in detailed reviews [19,47]. The most extensively studied methods are vibration-based damage detection methods [46]. Vibration-based damage detection methods work on the principle of using measured changes in dynamic features to analyze the alterations in physical properties that can lead to structural degradation [46]. There are various environmental conditions such as humidity, solar radiation, wind, and temperature that play a major role in changing the modal parameters, which can further cause irreparable damage to the structure. The assessment results of numerous vibration-based damage detection methods practiced on bridges reflected that the environmental effects were the main reasons for reducing the on-field applicability of modal-based methods [6,48]. For better performance of damage detection methods, it is mandatory to differentiate the anomaly changes in dynamic features caused by structural damage from simple changes due to the variations in environment and operational fluctuations [46]. Numerous investigations concluded that temperature is the major source causing modal variability; the temperature may reach 5%e10% for highway bridges due to the variations in modal frequencies [46]. Ko JM et al. [49], with a one year of data measurement on Ting Kau Bridge, give the effectiveness of different statistical learning algorithms [50] for modeling the effect of temperature on modal frequency. Due to variation, the modal frequency can exceed the tolerance frequency value of a structure, which leads to the failure of structure. With in-depth knowledge of effects of environment on modal properties and including well-defined corrections in the suitable detection method, it is feasible to be able to find the smallest structural damage in the future [51e54].

4.3.2

On-site construction defects

The manufacturing of aerospace and automotive structures is done in a very precise way, whereas civil infrastructures are not prepared with such accuracy. Therefore, there is increased possibility of error in design in civil structures. Exact dimensions are not achieved at the actual construction site as described in the blueprints of projects. These complications create problems for applying monitoring methods on the structures for the analysis of data. Initially, complete analysis of the structure is done with respect to the blueprint of the structure and construction is done accordingly,

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but after the completion of a structure the actual analysis performed by monitoring techniques may incur errors due to the presence of defects like dimensional errors, different composition of the material with respect to the blueprint, and improper installation of embedded sensors. These errors are the causes that produce different measurement data on the monitoring of structures [55,56].

4.3.3

Misinterpretations due to mixing of data by different monitoring techniques

The construction of megastructures is not feasible by a single company. In order to manufacture a megastructure, numerous companies are appointed to handle various departments like the construction management, management for the production of raw material, financial management to control the overall expenditures, and also for the monitoring of structural health. When it comes to the monitoring of huge structures, various specialists are appointed to obtain the most precise results. Different specialists employ different types of monitoring techniques on the same structure. The complexity of structures does not favor each monitoring technique and causes errors in the analyzed data. When different monitoring techniques are applied on a single structure, such as optical-fiber sensors, eddy current sensors, thermography, and others, the final collected data is difficult to interpret and thus leads to improper conclusions. This misinterpretation alters the resultant analysis to a large extent. The final results can seldom help to reach a common conclusion, which can cause overlooking of actual damage, thus making catastrophic damage inevitable [57].

4.4

Benefits of structural health monitoring

Modern human society is marked by a number of structures like bridges, roads, railways, skyscrapers, etc. Any country or state will develop and prosper only if they carefully maintain and monitor these key structures. Maintaining and monitoring the structures will not only prevent economic losses but will also ensure public health and safety. It is evident that if the important structures are not properly monitored and the prescribed guidelines for their maintenance are not met, this leads to catastrophic results. These accidents are a setback to the national as well as world economies and claim huge numbers of lives. In recent times, improved technological support is used to effectively analyze this structural health. With the increasing realization of the importance of structural health monitoring, various automated tools are being developed for the benefit of society. The government is also implementing stringent rules regarding procedures and the duration of the maintenance cycles of massive structures. This decreases capital investments and also reduces the risk due to accidents. These benefits of structural health monitoring are described in the following subsections.

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4.4.1

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Enhanced public safety

One of the major benefits of SHM is enhancement of public safety. The advanced SHM methods use sensors to collect data and then carry out an appropriate analysis. The government ensures public safety by mandating municipalities to employ the monitoring of all crucial structures. When the structures start aging, they start developing many cracks and weak points. The SHM process can help in detecting the cracks and faults in old structures. When the faults are known, the management can either rectify them or isolate the structure for public safety. In the case of new structures, faulty manufacturing techniques can render a number of weak points in the structures. Very often during the design phase of a structure, the operational environmental conditions are not well considered. SHM ensures that the structures are adapted to the operational environment, therefore reducing the risk associated with its usage [58,59].

4.4.2

Early risk detection

The application of SHM helps the engineers to recognize poor structural health and risk associated with the structure in the early stages. This early detection helps in prevention of events like floods, which are caused by damaged dams, pipelines, and dykes. The detection of faults is done by the sensors, which are installed in the system itself. The sensors monitor the change in water level and therefore identify the minor leaks. The engineers can rectify the minor leaks and thus further prevent the collapse of the entire structure. SHM is an effective method to collect geotechnical information about various structures like roads, bridges, buildings, and other such civil structures. If the movement of the ground is detected in time, the failures of structures due to landslides and earthquakes can be prevented to a great extent [2,60].

4.4.3

Improved life spans

Regular structural health monitoring helps in rectification of cracks and failures in their initial stage. This not only improves their efficiency but also enhances their life span. The traditional methods of SHM like the visual inspection technique do not provide such accuracy. This makes optical inspection method undependable for enhancement of the longevity of a structure. The smallest failure can easily be monitored by advanced techniques being used in SHM like the optical method, transient thermographic method, and eddy current method. The smallest cracks that are detected can be easily rectified. The propagation of cracks that will further lead to structural failure is thus postponed [61].

4.4.4

Cost effectiveness

Apart from having various benefits like increased life span of the structures and ensured public safety, implementation of proper SHM also reduces the short-term and long-term expenditures associated with structures. The business industry is

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particularly in favor of proper SHM to increase their overall profits. Detection of small failures and immediate rectification can help in lengthening the maintenance schedules of the entire structure. The fixing of failures in the initial stages helps to avoid major damage to structures and thus saves on expenditures involved in demolition and rebuilding of the entire structure [6,62].

4.5

Challenges for structural health monitoring

SHM is an efficient way to safeguard national property. Although SHM has various advantages, like maintaining only when required, which reduces capital expenditures and increases the life span and improves public safety, there are certain limitations of structural health monitoring. Limitations of SHM are that when neglected this can cause great damage to the structures as well as to public around it. Aktan et al. [63] state that the failure of civil infrastructure systems to perform at their expected level might decrease the national gross domestic product by almost 1%. However, improved structural health monitoring of civil infrastructures can help in improving the performance ratio. Differences in the shapes and sizes of the structures and also the age of the structures influence the SHM technique involved. The differences make it difficult for establishing a standard method for all the structures, which could further save time and efforts. The type of SHM system applied on any structure is based on several factors like shape and size of the structure. The structural health monitoring of a bridge like Tsing Ma Bridge in Hong Kong and a building like Shanghai Tower in the same city will be different. The Tsing Ma Bridge is the longest suspension bridge in the city with the main span of 1377 m, and the Shanghai Tower is 632 m tall. The structural differences in these two buildings thus demand different types of SHM techniques to be used [39]. The Tsing Ma is equipped with a wind and structural health monitoring system. The entire SHM system is comprised of 6 anemometers, 110 strain gauges, 115 temperature sensors, 3 data acquisition outstations, 2 displacement transducers, 19 accelerometers, 10 level sensing stations, 7 weigh-in-motion stations, and 14 GPS rover stations. The Shanghai Tower is monitored by a system comprised of 400 sensors of 11 types like the strain sensors, together with 11 substations [64]. Similarly, an old structure like Steccata church in Parma, Italy, and a new building constructed in the same city with similar environmental conditions will have different types of health monitoring systems involved. The church is monitored with a laser Doppler vibrometer technique, a noncontact detection technique providing data with great reliability and accuracy; in contrast, a recently constructed building will be installed with embedded sensors to measure the data [65,66]. The variations in the structures demand unique monitoring techniques to be employed with each structure. It becomes difficult when there is a lot of construction. Analyzing each structure individually is an enormous task and is prone to defects. Thus standard policies for employment of monitoring methods can save time and capital expenditures as well as reduce the errors involved in collection of measurement data.

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4.6

Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites

Advantages of structural health monitoring

Earlier, nondestructive evaluation (NDE) was used to monitor structures. Disadvantages associated with NDEs are that it is not suitable for bigger structures and requires specialized equipment and skilled labor. In order to improve the monitoring process, researchers developed the SHM technique by integrating sensors and actuators inside of structures. Also, the implementation of SHM reduces the monitoring downtime when compared to NDE. SHM provides quality input and eliminates failures, which helps in unpredictable environments and which accrues a stable level of reliability over its service life. Main advantages associated with implementation of SHM in structures are to be able to avoid the failure and ensure safety of the structure and avoid the uncountable human loses due to accidents. Holmes et al. [67] describe an alternative approach for NDE. The full matrix of time domain signals from every transmitterereceiver pair is captured and postprocessed. Performance of algorithm was matched by measuring its ability to image a point-like reflector. Drinkwater and Wilcox [68] reviewed ultrasonic arrays for nondestructive evaluation. Ultrasonic array increases the inspection quality and reduces inspection time. Meola et al. [69] carried out experimental investigations for NDE of aerospace material with lock-in thermography to determine damage such as delamination, impact damage, and fatigue failure used in aerospace materials such as composites, hybrid composites, sandwiches, and metals. Kinra [70] developed ultrasonic NDE to resolve the problems associated with conventional ultrasonic measurement techniques for thin specimens. They converted time domain signals collected from thin specimens into frequency domain using the fast Fourier transform algorithm. Clark et al. [71] analyzed the damage that occurred in concrete and masonry bridges located in the United States using infrared thermography to the NDT. They experienced problems to deduct the debonding and concrete spalling utilizing solar heating when the temperature is extremely low. Common repair schemes in military and civilian aircrafts made of metallic or composite patches are resolved by adhesively bonding the patches instead of mechanically fastening them. However, it is difficult to identify an adhesively bonded repair because of bonding of batch in the structure, according to Genest et al. [72]. Similarly, during crime investigation detection, identification of body fluids and DNA analysis is a crucial step. The main disadvantage associated with most of the current methods is design to detect a specific body fluid. So, investigators should have emphasized more different types of tests and finalizing the tests for different blood products. The case can be failed in the court because of variation of small deviation in the biological evidence, according to Virkler and Lednev [73]. Rausche [74] summarized most common NDTs used for deep foundations to analyze the intimate contact between pile material and soil, causing dissipation. Pulse echo method, transient response method, vibration method, two accelerometer method, bending wave, cross hole sonic logging, case method, and single hole sonic logging method are commonly used methods to detect this type of failure. Main disadvantages associated with the above-mentioned tests are the limitation of length, requirement of experience for interpretation, and requirement of an exposed section

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of pile. A further drawback for the tests is that the results may be difficult to interpret if sufficient free distance between accelerometers is not available. However, as compared to conventional measuring technique, NDE offers advantages that are important to resolve the problem and enhance the structures stability and durability. Tremendous developments in the electronic, sensor, and nonconventional methods help in the development of SHM. Peairs et al. [75] carried out research on impedance-based structural health monitoring. Surface-bonded piezoelectric transducers used to acquire the excitation signal measure the impedance of structure. This indicates the damage that occurred in the system. FBG sensors were used to ensure the safety and integrity of the civil structure by Majumder et al. [76]. Advantages associated with FBG sensors are light weight, immunity to electromagnetic interference and harsh environment, and ability to be multiplied for distributive measurement. FBG is used to measure the strain measurement under static and dynamic environment. Lynch and Loh [77] analyzed the advantages of wireless sensor and sensor network for SHM. The main advantages associated with wireless sensor technology in SHM are that it is inexpensive to install and extensive wiring is not required between sensors, and there is no further need of a data acquisition system. Raghavan and Cesnik [78] reviewed the guided-wave SHM. They discussed different transducer technologies, including both conventional and nonconventional piezoelectric transducers. Lu and Michaels [79] analyzed the advantages of diffused ultrasonic waves for SHM. Diffused ultrasonic waves offer advantages of simplicity of signal generation and reception, sensitivity to damage, and large area coverage. Experimental results revealed that for a small aluminium plate specimen, a high probability of damage detection can be achieved (over 95%) with a probability of false alarm of approximately 5%. This high probability can be achieved even with temperature variations of more than 30 C. Generally the smallest detectable flaws have dimensions in the range of 1e2 mm. Park et al. [80] summarized software and hardware issues of impedance-based SHM and carried out experimental and theoretical studies of various structures using high-frequency structural excitations. Nagayama and Spencer [81] carried out experimental investigations on damage analysis of civil infrastructure using smart sensors. Results revealed that implementation of smart sensor technique for the monitoring of structures can detect damage effectively. Wang et al. [82] conducted experimental and theoretical investigations on the applicability of time-reversal concept to guided waves in plate-like structures. They achieved temporal and spatial focusing with the help of time reversal concept. Lynch [83] reviewed wireless SHM for civil structures and invented the key design future of wireless sensing. Wireless sensing unit is associated with various factors such as collocation of computational power and precision of integration through computing sensors with self-interrogation of measurement data. The Alamosa Canyon Bridge in New Mexico was employed with wireless sensing units to analyze its performance. Forced vibration test revealed the accuracy and reliability of wireless sensor system in the monitoring of structure. Crack formation and its random propagation reduce the life span of the structure, thus decrementing its performance. Researchers made a number of additional attempts to model the cracks in the beam structure. Local stiffness reduction, discrete spring

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models, and complex models in two or three dimensions are the three major categories to design the cracks in the structure. Friswell and Penny [84] addressed effects of the excitation for breathing cracks and considered the nonlinear effect. Introduction of sensor and wireless technology in SHM improves the efficiency of the damage deduction.

4.7

Advance technology used for structural health monitoring

Construction of massive structures such as bridges, buildings, dams, pipelines, aircraft, ships, etc. is unavoidable. Introducing developments in new fields such as materials, design, manufacturing, and technology must ensure the safety of these structures. The inevitable natural disasters like climate changes, unexpected earthquakes of high magnitude, and severe environmental conditions are major setbacks to the safety of massive structures. Industrial disasters and natural disasters do not take place under similar circumstances. While natural disasters are unfortunate and inevitable events, industrial disasters are results of pure technological and human failure. This was clearly seen in 1984, in the city of Bhopal, India. In the middle of the night when the entire city was asleep, highly poisonous gases were released from the Union Carbide pesticide plant [85]. The accident killed 3000 people initially; updated figures indicate 8000 casualties at the time and a total of 12,000 since [86]. The final catastrophic event, also famous for being the “worst industrial disaster,” could have easily been avoided if the faults in the system were detected by proper structural health monitoring. The event was caused when an uncontrolled chemical reaction led to the release of methyl isocyanate (MIC) gas into the air. The accident was a direct result of a cheap engineering solution to a known maintenance problem. Not only were the prescribed standards overlooked and low-quality materials were used but also the monitoring schedules were lengthened. The lengthening of the monitoring schedule made it difficult to identify and rectify the cracks and dislocations in the plant. The management had also cut short the safety systems and the sensor alarms to save money, which further caused the accident [87,88]. The failure of a system is similar to that of the metal. It begins in the form of small cracks, which spread and grow unless rectified within a certain time. When any weak point is encountered by these cracks, the further propagation is magnified. In the Bhopal plant, the microscopic cracks were developing continuously since there was no proper structural health monitoring employed. Following this, when the reaction in the MIC tank became uncontrollable, this rendered a number of weak points, which magnified the existing propagation [89]. This resulted in the final accident, famously known as the “Bhopal gas tragedy.” One of the many accidents that also lie in the roster of accidents due to poor structural health monitoring is the crash of American Airlines flight 587. On November 12, 2001, the airbus A300 crashed shortly after takeoff. All 260 people died onboard the plane, including 250 passengers and 10 crew members [90]. Before the takeoff of an aircraft, a proper monitoring of the structural health of the aircraft has to be done. But

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due to the negligence of the ground crew members, the importance of proper monitoring was overlooked for quite a long time for this airbus A300. The result of this negligence was seen as the reason of the crash, attributing the problem to the debonding and delamination of carbon fiber present in the composite. This was the final inevitable result of the micro-level damage, like fiber cracking, matrix cracking, fiber buckling, etc., which must have been taking place already for a long time. It was later found on further investigation that before the actual fiber delamination and debonding, small cracks had formed on the surface and it was corroded. The aircraft was made of composites and alloys of aluminum and titanium [91]. The component of the aircraft that undergoes high stress concentration was made of composite and was not supposed to be made of composite. As the flight took off, it directly entered into a high turbulence zone, which caused stress concentration beyond its design limit in the wings as well as in the vertical stabilizer. Investigations by the National Transportation Safety Board suggested that the vertical stabilizer was separated as soon as the airbus A300 took off, causing the aircraft to lose control and crash [92]. Avoidance of such an accident was easily possible with the help of monitoring operations using various sensors and could have helped prevent the world’s second most destructive aircraft crash. Researchers proposed numerous methods to ensure the safety of massive structures under dynamic environments [93]. They introduced various smart sensors such as optical-fiber Bragg grating (OFBG) and polyvinylidene fluoride (PVDF) sensors, equipped with self-sensing mortar and carbon fiberereinforced polymer (CFRP). It also includes a wireless sensor network called stochastic dynamic damage locating vector (SDDLV), which is one of the most promising algorithms to ensure the safety of structures by effective damage detection (Jang et al. [94]). Wireless sensor networking for structural health monitoring was studied in this article. In this work they used stochastic dynamic damage location method to validate the wireless sensor network for the structural health monitoring system. In an experimental setup they used three-dimensional truss structures and lmote2 sensors to acquire the signals for the structures. Additionally, the authors proposed the decentralized damage identification methods to reduce the data transition traffic. Marin et al. [95] used SDDLV approach to monitor the damage using vibration-based damage localization and compared the results with finite element model of a structure for both reference and damaged states. Sim and Spencer [96] proposed a multiscale approach to measure the acceleration and strain measurements of massive bridges and compared the experimental results with numerical methods. Hill [97] introduced OFBG sensors to monitor damage for civil infrastructures. Fig. 4.1 shows the encapsulated OFBG strain sensors developed by Ou and Zhou [98] and sense minimum strain 1e2 mm. Ou et al. [99] developed the FibreReinforced Polymer FRP-OFBG sensor as shown in Fig. 4.2 to measure the strain of concrete.

Figure 4.1 Encapsulated OFBG strain sensors.

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Figure 4.2 CFRP-OFBG bars.

Chan and Zhou [100] used FRPeOFeOFBG sensing unit to measure the interface strain. They found that OF-FBG sensing principle along a single fiber is effective to measure the temperature and strain. Zhou et al. [101] analyzed the sensing performance of OFBG for rough civil infrastructure. Results stated that the sensing performance under harsh environment and durability is significantly enhanced due to the FRP materials. They introduced OFBG to monitor the system of Aizhai Bridge located in China. Most of the civil structures fail due to formation of cracks and their random propagation. In order to monitor the formation of cracks and their propagation in civil structures, the sensing speed of sensors used in the civil structure must high. High sensitivity and toughness, compatibility with matrix, area sensing, adaptive to complex surface, high sensitivity coefficient, and fast response associated with PVDF have become popular in SHM to monitor the civil structures’ cracks and strains. Duan et al. [102] used fiber Bragg optic sensors for measurement of strains and PVDF sensors for monitoring the structural local response by measuring cracks and fatigue life gauges for accumulative fatigue damage. Rathod et al. [103] evaluated the quasi-static and high-frequency dynamic strain using large-area PVDF thin films with the help of linear strainevoltage relationship. Similarly, dynamic strain was sensed by comparing the measured signals from PVDF films with the continuous surface electrodes using piezoelectric wafer sensors as a reference. Ren and Lissenden [104] used PVDF multielement Lamb wave sensors to analyze the mode content. They bonded PVDF sensors directly to the wave surface and confirmed that the curved surfaces have low mass, low profile, low cost, and minimal influence on passing Lamb waves. Ke et al. [105] used PVDF nanocomposites to improve the conductivity and piezoresistive sensitivity. Loyola et al. [106] analyzed damage that occurred in fiberreinforced glass composite. They achieved damage detection by monitoring multiwalled thin film carbon nanotube, which altered the electrical conductivity. Hurlebaus and Gaul [107] analyzed the damage diagnostics using piezoelectric films. They used a thin self-sensing actuating layer of PDVF copolymer to identify the location and size of cracks and delamination in composite; see Fig. 4.5. Rosa and Sarasini [108]

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Figure 4.3 Fiber optic sensor.

used piezopolymeric-based in-situ damage detection technique, which is based on acoustic emission with PVDF sensors for different fields, such as automotive, aerospace, and complex civil structures. Wireless ultrasonic SHM system has been developed by Zhao et al. [109] to inspect the aircraft wing. Small and light-weight piezoelectric discs bonded to different parts of the aircraft wing were used to monitor the crack formations and corrosion. Bremer et al. [110] developed a fiber-optic crack sensor and two different fiberoptic moisture sensors to detect the moisture ingress in concrete-based building structures. They developed the fiber-optic crack sensor based on a textile net structure that transfers elongation due to the presence of cracks in the concrete structures. Gianti et al. [111] recorded vibrations in pendulum using vibration sensors coupled with optical fibers. Total reflection of fiber in fiber-optic sensor provides the intensity of vibration due to loss caused by the bending of the fibers. Microbending of fiber is shown in Fig. 4.4. Fiber-optic sensor (Fig. 4.3) can be used to determine the damage in tunnels due to excessive loading caused by corrosion, displacement, mechanical pressure, flooding, and landslides. Thomas and Khatibi [112] analyzed the reduction in the flexural stiffness of carbon/epoxy laminates using piezoelectric wafer active sensors (PWAS) under impact loading, and performance of PWAS was monitored using impedance analysis. Soh et al. [113] used piezoceramic transducer patches to monitor the damage that occurred when the destructive load is applied on a prototype reinforced concrete bridge. Raghavan and Cesnik [114] conducted guided wave testing for SHM by surface-bonded/embedded piezoelectric wafer transducers.

Signal “leaking out” from the fiber Macrobending

Figure 4.4 Microbending scheme.

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Structural Health Monitoring of Bio-, Fibre-Reinforced Composites and Hybrid Composites Exploded view of delaminated region

Clamped region

50 mm

25 mm

229 mm

Figure 4.5 Schematic of the delamination modeling procedure.

Reliable damage detection is critical for the use of composite resources in different applications. Zhou and Sim [115] reviewed development of in situ fibre-optic damage detection and assessment systems implanted in fiber-reinforced composite structures for detecting damage, its location and also assessing the nature of that damage. Kessler et al. [116] carried out experimental and analytical survey for in situ damage detection in composite materials. They used finite element approach for modal analysis and Lamb wave techniques on the quasi-isotropic graphite/epoxy test specimens containing specific damage to their composite structure. Further, experimental analysis has been carried out to verify the analytical results obtained from finite element model. They concluded that for small amounts of global damage in composite structures, passive modal analysis method was reliable, however, active Lamb method was sensitive for all types of local damage present between the sensor and actuator. Similarly, they used frequency response method to detect the damage occurring in composite material [117]. They compared the changes in the natural frequency and modes of the material using a laser vibrometer with 2-D finite element model. On comparing results, it was concluded that at low frequency models accurately predicted the response but generally fail at a higher frequency. Acousto-ultrasonic waves method has great potential to detect the damage that occurs in metallic and composite structures. Initially, the wave was introduced through the structures, and was used to propagate long distances. The main drawback associated with this method is that it requires a greater number of transducers for monitoring large structures. Mallet et al. [118] used a new scanning technique based on laser vibrometry to detect the damage occurring in aluminium plates and validated them with numerical simulations. Leong et al. [119] used a commercial laser vibrometer designed for vibration/modal analysis, which is used to detect the crack propagation in composite plate.

4.8

Conclusion

Monitoring of structures using recent advances and trends in structural health monitoring has been reviewed and emphasized. In this chapter, benefits of implementation of SHM, such as enhancement of public safety, early risk detection, improvement in the life span of the structure, and decrease in the capital expenditures involved, are discussed. The state of the practice in bridge monitoring systems for some famous bridges

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like the Tsing Ma Bridge, Commodore Barry Bridge, and Great Belt Bridge have been listed. Factors affecting measurement of data influence the overall analysis and thus the final monitoring results are not obtained accurately. Factors like environmental effects, on-site construction defects, and mixing of data by different monitoring techniques significantly alter the measurement data obtained. Further challenges in SHM due to unstandardized policies are also reviewed. Lastly, advantages of SHM have been explained with the help of some catastrophic disasters due to improper implementation of required SHM techniques.

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