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  Journal of Space technology Vol-4, No-1, July 2014 History, Current Status and Challenges to Structural Health Monitoring System Aviation Field Abid Ali Khan a , SuhaibZafar   b , NadeemShafi Khan c , ZahidMehmood d a Institute of Space Technology, Islamabad, Pakistan  b,c,d  National University of Sciences and Technology, Pakistan  Abstract-- The paper provides an overview of the Structural Health Monitoring System (SHMS) that has evolved from research on smart/intelligent structures. The authors have elaborated the principle, advantages and benefits of using such systems on modern day aircraft structures. In addition the challenges being faced by the designers for implementation of available techniques and theuseof available sensors have been compared for their suitability to health monitoring. The importance of SHM in the time-dimension through monitoring that makes the full history database analysis possible and also provides a prognosis has been emphasized. Furthermore, therequirement of a modified inspection planfor going beyond the design life span in the context of operational life has been explained. Keywords: Preventive Maintenance, Aircraft Structures, Design Life Span, Health Monitoring Sensors I. INTRODUCTION Large, complex and costly engineering structures such as aircraft are made to last long. Aircraft are used intensively and their long endurance combined with usage leads to deterioration. Aircraft are designed to withstand deterioration for a certain anticipated lifetime. Such engineering structuresin normal practice are designed for maintainability. In reality the facts have been different andupon completion of their initial design life they are often considered for an extended life span. Aircraft structures in the past have been traditionally made of metals and designed for a specific number of flight hours. Design life was defined on the basis of an average usage in accordance with a predefined load spectrum. Application of the load spectrum to a structure leads to fatigue. Experienced  professionals in fatigue tests are aware of the fact that fatigue life for even a pre-defined component geometry, load spectrum, and material scatters significantly.Aircraft structures,other than designed fail-safe, are designed towards low probability of fracture;it is unlikely that the component fractures on reaching its design life[1, 2].Due to scatter it may even endure for time that it has already lasted or maybe even more. II. HISTORICAL BACKGROUD In the early days, the only means of monitoring usage and managing the fleet was based on flight hours or landing cycles. Therefore, aircraft would retire on reaching a pre-defined number of hours. Later, with the advancement in the science of fatigue and load-cycle counting methods evolved, relatedthe load cycles with stresses and structural damage. Subsequently, fatigue meters were developed to compile a count of divergence from preset positive and negative g-levels during service. Further down the time line, usage of strain gauges and mechanical strain recorders were introduced for direct derivation of stress. Most recent advancement is measuring and recording of hundreds of parameters with exceptional frequency through computer based multi-channel recorder systems such as Flight Data Recorder (FDR) or Electro-Mechanical Mission Computer (EMMC).Historically, monitoring of the structure has been based on certain assumptions, however, advancements in technology has  proved these assumptions to be in correct:   (a)   The effect of different missions and configurations on aircraft is not too pronounced : This assumption has  been proved to be wrong and in fact, aircraft configuration makes a significant difference on structural loads to an extent which cannot be neglected. (b)   Usage of all aircraft in a large fleet averages out with time : This is not true since some aircraft accrue fatigue damage at a higher rate than others. Therefore, it depends on the usage of each aircraft. (c)   Design load spectrum is an adequate basis for maintenance management : This is not true either, since for many operators, the average user spectrum is more severe than the design spectrum which is the basis for the OEM maintenance management. It is important to examine those conditions under which we are able to take advantage of a structure beyond its initial design life.Flight hours, traditionally used as a measure of aircraft usage and lifetime are not the only criterion for this  purpose. In real life, theaircraft is not flown in the conditions for which it was initially designed, this is specially observed in the military aircraft sector. Hereone might ask whether the aircraft can be safely used till reaching its design life (in 67  HISTORY, CURRENT STATUS AND CHALLENGES TOSTRUCTURAL HEALTH MONITORING SYSTEMIN AVIATION FIELD terms of flight hours).The instantaneous answer to this question is a straight ‘NO’ as the environment and style in which aircraft is flown plays an important role. The answer for many real life issues rests with Structural Health Monitoring (SHM) thataims in scrutinizing a much required health status of the constituent materials of different  parts and aircraft assembly[3]. The state of the structure thus remains in the design domain, even being altered by aging due to usage and actions of the environment and accidental events[4]. III. STRUCTURAL HEALTH MONITORING Structural Health Monitoring (SHM) is an emerging technology. It is being applied in every field, examples of SHM applications include Aerospace [5-7], Buildings[8-10], Bridges [11, 12], off shore structures[13], wind turbines [14], Medical [15], Underground Structures [16], Ships [17] and many more industries. SHMdeals with development and implementation of techniques and systems such that monitoring, inspection and damage detection becomes an integral part of structures[18, 19]. Thus it is an issue of automation that merges with a variety of techniques related to diagnostics and prognostics.Many researchers have developed different techniques for SHM such as structural health monitoring using fuzzy pattern recognition[20], application of neural networks for the structural health monitoring [21, 22], diffused ultrasonic waves technique to detect structural damage in the presence of unmeasured temperature changes[23], vibration response detection based technique [24, 25], SHM using an intelligent parameter varying (IPV) technique [26], development of metastable ferrous alloy insert for composite materials damage detection which has an austenitic crystal structure at room temperature, but upon application of strain, this transforms to a thermodynamically stable marten site[27] and a novel approach for optimal sensor and/or actuator placement for structural health monitoring (SHM) applications [28].   SHM has its roots in the research of smart and intelligent structures. It isa multidisciplinary field that laterally encompasses specialties such as structural dynamics; materials and structures; fatigue and fracture; non-destructive testing and evaluation; sensors and actuators;micro-electronics and signal processing[29].In order to be effective in the development of such systems, a multidisciplinary approach is fundamental,without which engineers find it difficult to holistically manage the operation of an engineering structure through its life cycle in future. Furthermore, new breakthroughs in structural engineering would also be an uphill task. To ensure structural integrity and hence maintain safety in-service, health and usage monitoring techniques are being employed in various engineering disciplines. Structural health is directly related to its performance, thus in this respect, it is one of the key parameters with regard to safety of operation. As this aspect of structural health is predominantly relevant to transportation systems therefore, in this context it  becomes a safety issue. Most SHM systems today are based on the traditional damage tolerance philosophy, maintenance philosophy of Eurofighter is one such example [30],which generateda requirement of  periodic maintenance so that damage is detected before it  becomes critical. This has resulted in the development of  Non-Destructive Inspection (NDI) techniques, some of which are not only advanced but accurate as well. However, these are time-consuming, require trained inspectors and might also require disassembly of components. In short, the improvement of safety is a strong motivation [31].Maintenance requires grounding of aircraft thus resulting inloss of essential revenue in the case of airlines. Whereas, for defense organizations, it is a loss of time, that could be utilized for training or other purposeful missions.Although the economic motivation is important, but itsimpact is often difficult to evaluateas it depends on the usage conditions. SHM couldbe defined as thecontinuous monitoring of structures or components using integrated or applied sensors. SHM systems can be analogous to the human nervous system, with sensors concentrated in key areas where loads are highest.Itis aimed at assuring structural integrity of the aircraft through on-event and periodic inspections that detect damages resulting from fatigue, corrosion, excessive loads, impact, etc. during its usage. IV. PRINCIPLE OF SHM   Monitoring of structures does not necessarily mean knowing the status of the structure in real-time. Structures are designed with acceptable margins such that, after normal or exceptional events, maintenance tasks can be planned for the next scheduled inspection.Systems are available for aircraft condition monitoring, mostly for loads (accelerations, flight  parameters, etc.) and enable decisions to be made based on actual flight load levels. Indirect surveillance of the structure is not comprehensive or reliable enough to avoid interval inspections. Currentlynon-destructive testing is applied starting with visual inspections, for more subtle or hidden flaws, procedures are defined based on eddy current, ultrasonic, X-ray etc. Inspection intervals are usually based on knowledge of the structure residual strength, operating environment, applied loads, damage growth rate and failure consequences.Inspections result in downtime, resources and efforts in inaccessible areas of structure requiring significant efforts to remove equipment or strip protective coatings for access thatmust be restored after the inspection. Monitoring activity comes at a considerable cost and accounts for all on-aircraft maintenance man-hours that can be saved [32-35].   68  HISTORY, CURRENT STATUS AND CHALLENGES TOSTRUCTURAL HEALTH MONITORING SYSTEMIN AVIATION FIELD V. BENEFITS AND MOTIVATION   As stated before, the primary motivation for SHM is safety. One might attribute this to the infamous Aloha Airlines incident in 1988. The economic motivation is obvious too and some might argue that it even exceeds the former in the case of end-users.Another benefit of SHM for aircraft manufacturers is weight saving, which is possible by reduction ofsafety margins, in some areas. This will lead to lower fuel consumption and increased aircraft range. SHM can provide the operator with information on structure areas that are remote and inaccessible. Invaluable time is thus saved which would have been spent dis-assembling,fault diagnosing and re-assembling the structure. With nophysical operation a safe inspection can even be performed in the hazardous areas. Scanners are not essentiallyrequired thus it also eliminates time consuming setup. SHM would also allow for the reductionin frequency of scheduled inspections;saving operator’s money, time and effort.Inspection intervals are calculated conservatively based on fatigue and corrosion growth models. SHM will allow optimizing these assumptions with actual aircraft flight data. SHM sensors can greatly simplify inspections since affixed  permanently. Therefore, SHM has several advantages over conventional non-destructive techniques: (a) Sensors in SHM are anintegral part of the structureeliminating repeated setups. (b) SHM is an automated process eliminating human influence on inspection. (c) Substantial time is saved due to simultaneous inspection of multiplelocations. (d) Time spent on structure inspection to assure continued airworthiness increases aircraft age. SHM allows an optimal use of the structure, a minimized downtime, and the avoidance of catastrophic failuresgiving themanufacturer a product improvement. Itdrastically changesthe work organization of maintenance services: (i)   By aiming to replace scheduled and periodic maintenance inspection with performance-based/ condition-based maintenance on long term. (ii) At least by reducing the present maintenance labor, in  particular by avoiding dismounting parts where there is no hidden defect on short term. (ii) By significantly minimizing the human involvement, consequently reducing labor, downtime and human errors thus improving safety and reliability. The economic motivation is strongerfor end-users. Structures with SHM systemshave envisaged benefits of constant maintenance costsand reliability over classical structures without SHM withincreasing maintenance costs and decreasing reliability.In order to reduce the inspection burden, some industries have   implemented automated on-line structural health monitoring systems and they only  perform,need based maintenance actions.However, Condition Based Monitoring (CBM) approach to maintenance,when applied to aerospace structures has the potential to reduce inspection time and improve airworthiness by earlier damage detection.In case,an SHM system is installed for safe life components, it would be possible to detect early failures and withdraw the components from service ahead of their expected life and continue to use healthy components beyond their design life. Figure 1 depicts the potential safety risk and life enhancement regions with respect to usage of aviation structures with and without SHM. Figure 1: Monitoring of Aerospace Structures VI. DESIRED ATTRIBUTES   The following are the desired attributesfrom an effective Structural Health Monitoring system as discussedby Jan D. Achenbach [36]and shown in figure 2. a)   Permanently installed sensors.  b)   On demand or continuous condition monitoring in real time with known Probability of detection (POD). c)   Wireless transmission to central station. d)   Instantaneous interpretation of sensor data. e)   Detection of unacceptable material damage at critical high-stress locations. f)   Monitoring of growth of material damage into critical size. g)   Growth prediction by a probabilistic procedure. h)   Adjustments to growth prediction for actual damage state at prescribed intervals.  j)   Probabilistic forecast of damage state for near term and of lifetime. Figure 2: Typical Structural Health Monitoring System 69  HISTORY, CURRENT STATUS AND CHALLENGES TOSTRUCTURAL HEALTH MONITORING SYSTEMIN AVIATION FIELD VII. MULTIDISCIPLINARY TECHNIQUES Sensors are preferably recommended for installation during manufacturing of the airframe. This allows a reduction in the safety margins in some critical areas leading to weight reduction resulting in an increase in aircraft performance especially with respect to maximum aircraft range and lower fuel consumption. SHM could be passive or active. Passive SHM  means thatuser is observing the structure as it evolves, meaning its physical parameters and its state are evolving as a result of interaction with the environment[37]. This sort of situation occurs with acoustic emission (AE) techniques being utilized for SHM. Active SHM  means that structure is equipped with both sensors and actuators meaning the users are not just observing the structure; rather they are able to generate/actuate agitations in the structure with actuators and then through sensors, monitor the response. Sensors for SHM can be incorporated into the components during the manufacturing process of the composite. Aircraft incorporating composites into their structures such as the Boeing 787Dreamliner have been planned to be embedded with SHM systems.The composite structural anisotropy and the fact that they contain different phases of material (fibers and matrix) generally end up with various types of damage having different propagation characteristics. Flaw detection and determination of the remaining strength and life of a structure is a demanding and challenging task [38]. Boeing  plans to do itwith maturity in technology.A350 another wide  body from Airbus is likely touse structural health sensors in areas of interest like upper shell fuselage, lower wing skin and door structures where probability of damage is high during loading. Composites have different failure modes as compared to metals. Engineers are interested in impact damage and delaminationfor composite materials. SHM diagnostics development is critical for Composites, such as Carbon Reinforced Fiber Polymer (CFRP) whose use is on rise in aircraft.A number of techniques are being proposed by researchers for SHM in composite structures[39-43]. Challenges in the field comprisedetermination of critical areas that requiremonitoringfor operator's benefit andhighly reliable and durable sensors/systemsdevelopment. SHM system’s cost is another issue which is not to exceed the  benefits gained or become unaffordableso as to slow down the systems acceptance/adaption at mass level. One of motivating factor for use of SHM is the fact that maintenance costs increaseswith aging of structure, theproblem is thus arrested  by SHM as maintenance costs along with reliability remains constant.Dr.Roach[44] proposes there approaches to SHM that are: (a)   In-situ sensors permanently installed in the structure require periodic monitoring using a separate unit when the structure goes out of service. (b)   Sensors with in-situ data acquisition capability (data logging during operation) that needs periodic data downloads when structure is out-of-service. (c)   Sensorscapable of real-time data transmission to a remote site, allowing real-time structural monitoring when in operation VIII. HM Sensors A number of SHM sensing technologies are currently under development or in use[45-53]. The most common are the following:- (a) Fiber Bragg diffraction grating sensors .Fiber-optic cables embedded in the structure are laser marked with optical interference patterns. Any local strain causes a slight change in the sensor's light transmission wavelength. (b)   Acoustic emission . Transducers listen for acoustic signals generated by cracks, delamination or fiber  breakage. (c)   Acousto-ultrasonics . Low-frequency acoustic pulses are sent through a part and received by transducers. Damage causes a change in the reflected acoustic energy. (d)   Smart or sensitive coatings . Coatings or paints with integrated piezo- and ferro-electric elements or carbon nanotubes can function to detect strain. Some sort of spectroscopy is needed to detect changes in the coating. (e)   Microwave sensors . Small microwave sensors embedded in the structure send and receive signals that indicate moisture ingress. The method is good for monitoring composite sandwich structures. (f)   Imaging ultrasonic . A small ultrasonic wave transducer generates a signal that passes through the material. Changes in wave reflection indicate flaws or damage. (g)   Comparative Vacuum Monitoring .Monitoring of vacuum vs. atmospheric pressure in fine tubes within a simple manifold that is adhered to the surface of a structure and can detect crack propagation in that structure. Some concepts involve embedding the sensors within the composite laminate (Fiber Bragg diffraction sensors) or  placing the sensors on the surface of the structure (coatings or comparative vacuum monitoring sensor patches). IX. CHALLENGES OF SHM While there might be numerous benefits, there are some challenges that need to be dealt with.The challenges faced by SHM system has been discussed by many researchers [54-59], which can be summarized as follows:- 70
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