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Fiber Optic Sensors Application for Structural Health Monitoring

Final Report Summary - FOSAS (Fiber Optic Sensors Application for Structural Health Monitoring)

Executive Summary:
The project is addressed to the exploitation of fiber optic based sensor for structural health monitoring of aeronautic structures and materials. The activity plan is organized in three tasks, according to the call description. The first activity is focused on preliminary research on the use of Fiber Optic Bragg Grating (FOBG) sensors for the strain evaluation during a fatigue test on metallic samples in corrosive environment, with the aim to define and characterize a new experimental methodology capable to provide an innovative tool for the prediction of corrosion initiation. The second activity is addressed to the development of a new approach for the realization and test of an apparatus for FOBG sensors interrogation. The task is motivated by the need to identify a robust and reliable instrumentation for flight test campaign. Starting from results of previous experimental campaign carried out together with Alenia, the actions to develop are devoted to the identification of a new class of commercial devices and/or systems for the integration of a novel instrumentation which should be qualified for flight test campaign and, at the same time, must provide higher performances than conventional equipments. The third research activity is aimed to the development of a distributed strain and temperature sensing system based on standard optical fibers employment. The scattering effects (Raman, Brillouin, Rayleigh…) on the basis of this new technique have been analyzed and prototypal and commercial solution, already certified for laboratory application, has been evaluated. The distributed technique has been tested during ground test campaign, with the aim to reach performances comparable to these typical of FOBG based discrete sensing systems. In conclusion, the project was aimed to provide theoretical and experimental support for the exploitation of new technologies for the realization of smart materials and structures.

Project Context and Objectives:
Activity 1: Fatigue Corrosion (FC) phenomenon is a mechanical degradation due to occurrence of both a mechanical cyclic stress and a corrosive environment. FC can be an occasional problem in commercial and military aircrafts, due to the presence of an effective protective surface layer (anodic oxidation, primer, enamel and so on). Although each single effect of both fatigue and corrosion have been extensively documented for metals, their synergic action is not thoroughly understood and it continues to be an area of considerable scientific and industrial interest. In this research project, a novel and effective approach has been proposed and applied with the aim to monitor, contemporary, the electrochemical behavior and the true strain of a metallic coupon, while it is subjected to a fatigue test. The main objective is to understand and quantify, the reciprocal influence of the fatigue and the corrosion by monitoring both electrochemical and mechanical properties of a manufact subjected to a cyclic load in presence of an aggressive environment. Further, the aim is to verify if and how the cyclic load, combined with environment, influences the pit formation and growth; sub sequentially it has been tried to appreciate this influence via the monitoring of an indirect electrochemical characteristic in order to discover a relationship between the electrochemical properties and the crack initiation and propagation. In a second stage the objective was to understand the transition from pit to crack, again by monitoring the chosen electrochemical characteristic and the strain. Fatigue experiments have been carried out on specimens in presence of an aggressive environment consisting, for example, of the well-known water solution with 3.5 % of NaCl in weight, simulating sea water. The mechanical stress was applied in such a way, preferably the simplest one (cantilever beam, four point bending, etc.), in order to easily know, i.e. calculate analytically, and measure the strain in high-cycle fatigue. At the same time the electrochemical behavior of the specimen was monitored by continuously measuring some relevant characteristic, such as the Open Circuit Potential (OCP). The actual true strain measurement in the monitored area of the specimen has been performed by means of a Fiber Optic Bragg Grating (FOBG) sensor. The above mentioned entities were recorded as a function of number of cycles: the results should provide information needing for a correct interpretation of the numerous and complex phenomena occurring on the material as time goes on. This activity has been carried out in the framework of cooperation between IMM-CNR and the Dept. of Materials and Production Engineering, University of Naples Federico II (DIMP-UNINA).
Activity 2: This activity has provided to the final user support and assistance during the development and the qualification process of a new technology for structural health monitoring for avionic materials and structures. The innovation with respect to the past approach is related to the employment of fiber optic sensors instead on traditional strain gauges. Fiber-optic sensor technology is the most attractive device currently under investigation in the aerospace and aircraft industry for in situ monitoring of large-scale metallic and Carbon-Fiber Reinforced Plastic (CFRP) structures. It is embedded into a structure to form a novel self-strain-monitoring system. In order to improve the reliability of the structures, an SHM system must be designed and realized for ground and flight test applications. The development of smart composite structures involves the integration of sensors (such as optical fiber sensors) into these advanced laminated materials allowing in situ process monitoring and continuous structural health check. At present, optical fiber sensors (OFSs) are most promising among all. This is because optical fibers have enough flexibility, strength, and heat resistance to be embedded easily into composite laminates. Furthermore, OFSs have some advantages when compared with previous sensors, such as immunity to electromagnetic interference and multiplexing capability. OFSs suitable for health monitoring of composite structures are mainly classified into the following types: intensity-based OFS, interferometric OFS, fiber Bragg grating (FBG) sensors, and distributed OFS. Our project deals with the exploitation of both FBG and distributed OFS, even if with a different degree of development. FBG sensors (typical gauge length: 5–10 mm) consist of a series of parallel gratings (typical grating period: 0.5 m) printed into the core of an optical fiber, and a narrow wavelength range of light is reflected from the sensors when a broadband light is illuminated, as discussed in detail later. As the wavelength at the peak of the reflected signal is proportional to the grating period, the axial strain can be measured through the peak shift. The FBG sensor can be easily multiplexed and the measurement by the FBG is really robust and has high sensitivity for monitoring of strain and internal damages. FBG sensors were applied to detect buckling of CFRP aircraft skin/stringer panels. FBG sensors were also used to monitor the integrity of composite repair patches. Sensitivity of embedded FBG sensors to dis-bonds in bonded composite joints was also investigated. Both quasi-static strain and high-frequency damage signals of a graphite/epoxy cross-ply laminate have been monitor simultaneously with a single FBG sensor by combining two different demodulators. Furthermore, Blue Road Research has developed an FBG sensor that can measure tri-axial strain and temperature simultaneously by overwriting two different gratings in a birefringent optical fiber. Then, multi-axis FBG strain sensor was applied to damage detection in a composite pressure vessel.
Activity 3: Although the above-mentioned Fabry–Perot sensors and FBG sensors are used for localized or multipoint measurements, distributed OFS can measure distributions of strain and temperature using conventional optical fibers without processed sensor parts. The simplest method of distributed OFS is the optical time-domain reflectometer, where pulsed signal is transmitted into one end of the fiber and returning Rayleigh-scattered signals are recorded from the same fiber end. Then, the disturbed position can be detected from the relation between the received light intensity and the return time. In recent years, in order to improve the quantitative response to strain and temperature, the frequency shift of the Brillouin scattering has been utilized (named Brillouin optical time-domain reflectometer (BOTDR)). More recently, the time and strain resolution has been improved using a pulsed pre-pumped Brillouin optical time-domain analysis (ppp-BOTDA). Optical fibers bonded onto structure surfaces are also found sensitive to dynamic disturbance induced in structures. Fibre-optic Mach-Zender or Michelson interferometer and laser Doppler velocimeter have been used for the detection of Lamb waves or acoustic emissions. The project deals with the study and the evaluation of this technique in the aeronautic field. In particular, different aspects were addressed, such as the preparation of the specimen, in particular with respect to the optical fiber path definition, as well as the elaboration and interpretation of the data provided by the instrumentation itself. The underlying objective is to assess and verify the feasibility of the integration of innovative fiber-optic based sensor technology with novel technologies related to the use of advanced structures and materials in the aerospace industry. Moreover, a number of issues related to this new sensing technology have been investigated, such as the problem of temperature/strain discrimination, as well as the limited sensing length allowed by swept-wavelength interferometry. Finally, it was also investigated the possibility to perform dynamic strain measurements by increasing data acquisition and data transfer rates.

Project Results:
In the first work package it has been conducted a preliminary study regarding test arrangement, with particular attention focused on the geometry of the specimen, the loading condition and the way to perform electrochemical measurements. First, it was necessary to define the loading condition and, as a consequence, the geometry of the specimen and the methodology for its preparation. Three different configurations have been studied and compared each other: three points bending, four points bending and the simple cantilever beam. The choice took in account many different factors (as, for example, simplicity of realization or analytically knowledge of stress and deformations induced in the specimen), and at the end has been taken the configuration that mediate best all the requirements. It was also evaluated the possibility to prepare notched specimens in order to encourage crack initiation in preferred sites, just where the electrochemical measurements will be carried out. At the same time, it was studied the technique of monitoring the electrochemical properties and the methodology to bond the FOBG. The measurement configuration for the electrochemical properties is a focal point of this experimentation: this choice can influence all the subsequent phases of this experimentation itself. In literature there are many examples of different configurations used for the measurement of electrochemical properties, but for the particularity of this experimentation, i.e. the area to be investigated is not free but loaded, a novel configuration is needed. Since the measurement we performed is confined within a small area, the microcell technique was the most suitable. Notwithstanding, since the specimen is continuously deformed due to cyclic load, different arrangements have been evaluated in order to find the solution able to provide best results, i.e. highest stability and repeatability of the measurement itself. The choice of the solution to be used as environmental condition was made according to the international standard test. The choice was made between two different standard solutions: the former, able to encourage quickly corrosion phenomena occurrence, a more severe aqueous solution simulating sea water (3.5 % NaCl in weight aqueous solution) and the latter, able to encourage slowly corrosion phenomena occurrence, a less severe one (4.2 % Na2SO4 in weight aqueous solution). At the end of this phase it was necessary to define the method of bonding of the FOBG on the specimen. As usual different solutions will be studied and the one that responds better at the experimentation requirements will be chosen. In particular the wish is to leave uncovered by the glue the part of the fiber containing the grating in order to measure the electrochemical properties and the true strain in the same area. Preliminary at this phase is the choice of glue itself and the type of coating and recoating for optic fiber. Regarding the coating, it was used a standard coating and recoating in acrylate. The choice of the glue was made depending on the capability that the glue itself has demonstrated to endure to the mechanical stress and to remain inert with respect to the aggressive aqueous solution. In a second phase it was carried out a preliminary experimental campaign in order to evaluate the capability of the developed system to measure and monitor the true strain and the electrochemical properties, in real time, on the selected area of the specimen in the chosen configuration. Then it was chosen the electrochemical property that will be monitored during the experimentation. This choice has been done taking into account both the electrochemistry and the precision and simplicity of the measurement system. This was a focal point in the experimentation, since a wrong choice couldn’t allow a correct interpretation of the phenomenon and relative experimental data. To this scope, it was taken in account two options: the OCP (Open Circuit Potential) and the EIS (Electrochemical Impedance Spectroscopy) measurements. The OCP provides information about the evolution, along the full duration of the test, of the system composed by surface oxide layer, bulk metal and aqueous solution in order to detect pits born and growth, cracks initiation and propagation and some other phenomena induced by corrosion. The EIS measurement is able to provide an estimation of the degradation of the surface oxide layer, and as a consequence, of its actual thickness and corrosion rate. A certain number of tests have been carried out in order to evaluate the repeatability and precision of the measuring system. In the third phase it has been assessed a Design of Experiments (DOE) in order to investigate correctly, and quantify, the influence of all the factors involved in this experimentation. In particular it has been studied: the loading frequency, the R-ratio between the minimum and the maximum stress acting on the specimen in the monitored area, the environment influence(aqueous solution).
At the end the experimental results have been analyzed and statistical treated by means of Analysis of Variance (ANOVA) in order to understand and quantify the influence of all investigated parameters on the phenomena involved in the experimentation. All these results allowed the interpretation of those complex phenomena governing the pit formation and growth and, that is more important, the transition from pit to crack.
The second WP was dedicated to the activity for the definition of a new FBG interrogation instrumentation and to the identification of critical parts and functionalities of the optical system which is the core layer of the existing instrumentation. The need to provide a reliable setup for flight test application pushed to reduce weight and volume of the instrumentation itself, and, at the same time, guarantee the compliance with the severe aeronautic standards, in term of mechanical, thermal and electromagnetic compatibility. This work package has been focused on the definition of the optical sub-system modification to operate, with the detailed description of new optical components and/or systems to acquire. The related market analysis has been carried out with particular attention to commercial solution and proposal already qualified for application in aerospace field. Once the definition of the modification to carry out on existing instrumentation is completed and the consequent market analysis is available, Alenia acquired the optical components and sub-systems needed to integrate the new instrumentation. Our proposal, at this stage, scheduled the modification of the instrumentation and its test and characterization in our laboratory. Once the performances of the updated instrumentation were compliant with the specifications defined by the final user, it was possible to plan and organized the final test campaigns. In particular, this work package provided also the support to the electronic sub-system and software updating, and to both ground and flight tests campaigns organized by Alenia.
This aim of the third WP was the study and the evaluation the feasibility of a novel technique for ultra-high spatial resolution distributed temperature measurements using standard single-mode fiber. In particular, a new technology based on the so-called swept wavelength interferometry (SWI) to measure the Rayleigh backscatter as a function of length in an optical fiber with high spatial resolution has been considered. The activity was organized in three subsequent tasks:
Task A: Market analysis and instrument choice. In this task an analysis of the different implementation of the swept wavelength interferometry (SWI) has been performed. This analysis was useful to define the most affordable technologies on the market already certified for the use in laboratory environment and potentially qualified for aeronautic application.
Task B: Specimen preparation and experimental test campaign. In this task an experimental campaign has been set-up in order to test the technique for ground application. The test case was defined with the aim to fully evaluate the performance of the system. A number of issues related to this new sensing technology were investigated, such as the problem of temperature/strain discrimination, as well as the limited sensing length allowed by swept-wavelength interferometry. The instrumentation management, operation and maintenance during the test case were also addressed. A particular attention was devoted to the specimen preparation, in particular with respect to the optical fiber path definition.
Task C: Data elaboration and interpretation. In this task the data resulting from the ground experiments were elaborated and interpreted in order to evaluate the performance of the system in terms of resolution, accuracy, temperature/strain discrimination, as well as the sensing length.

Potential Impact:
The objective of the project is to let the best and most advanced optical sensing technologies to become an integral part of the aircraft structure and to thus implement Structural Health Monitoring (SHM) into aircraft structural design with respect to maintenance cost reduction, increased aircraft availability and significant weight savings. The main project target is thus to develop and validate monitoring technologies which are able to deliver the expected cost savings for maintenance and enable innovative structural design for metals and composites. The main innovation through the proposal is to equip aero structures with an integrated sensing function by adaptation of advanced sensor technologies to SHM systems with the SHM systems being ready to be adapted to virtually any type of real aircraft structure and under a real In-Service environment. One of the most suitable sensor-based monitoring technologies, i.e. the fiber optic based sensor technology, will be accurately developed for three scenarios (corrosion monitoring, discrete and distributed strain monitoring), by first identifying the physics of phenomena under investigation and by the development of the adequate sensors and sensor systems, thus to be in position to realize the optimum monitoring solution for the respective case study. Today, aerospace is an area with one of the highest safety requirements. When all these requirements are met, it is extremely exceptional that an accident follows from structural integrity issues. However, fulfilling these requirements by conventional methods often takes place at a high price:
Low acceptance of anything new and innovative (e.g. design, materials, repairs, etc.)
High cost of inspection and maintenance
SHM contributes simply to make safety more affordable and significantly increase air-craft performance and economy of resources.
The socio-economic strategic impact of the SHM technologies that have been investigated in this project has two aspects for aerospace, not only in Europe:
SHM is an enabling technology that opens the door to high performance, environment-friendly and economic aircraft operation by better exploiting available weight reduction potentials of new structural materials and design philosophies without compromising the existing, high aero-space safety requirements.

SHM is also an enabling technology that significantly contributes to sustain, or in some limited, but known cases (e.g. repaired or sensitive parts) even improve safety of aircraft operation by means of more affordable and efficient integrated monitoring technologies instead of time-consuming and costly conventional procedures (disassembly, reassembly, checks, etc.).
- Especially the first aspect of SHM steps beyond the improvement potential of new maintenance concepts, because it is driving innovative approaches in the design of aircraft structures: A self-sensing structure, which gives warnings about potential, existing or foreseeable breaches in structural integrity safety limits, does not need to be dimensioned to the current standards to overcome the limitations of conventional inspection and maintenance procedures. With SHM, the full structural performance of modern designs and materials is made available for exploitation at minimized risk. The second aspect is critical to the feasibility of future maintenance concepts that are currently developed. While the future maintenance processes yet have to be defined, it is clear that sensor technologies are playing a key role in monitoring aircraft. A self-sensing structure, which gives warnings about potential, existing or foreseeable breaches in structural integrity safety limits, does not need preventive (and statistically useless) inspection and maintenance, but allows optimized and reasonably plan-able operation and therefore reduced down-time. The development of a minimum set of reliable and more powerful, new integrated monitoring sensor technologies is mandatory to fill the superior scenario of increased, affordable and safe aircraft operation. SHM is an enabling technology that drives the primary and most obvious benefit: the opportunity for enhanced safety through on-condition maintenance and streamlining logistics support for advanced aerospace structures. In these scenarios, the periodicity of maintenance actions, including inspections and repairs, are determined by SHM-indicated breaches in structural integrity safety limits. In this way, more comprehensive vehicle health and life cycle management is afforded. This, in turn, enhances the operational availability and reliability of the aircraft and the fleet, and provides a direct opportunity for accident reduction and safety improvement. The purpose of introducing Structural Health Monitoring (SHM) into commercial transports is to enhance aviation safety by improving the effectiveness of the operator’s continued airworthiness programs. The primary consideration for assessing the effect of SHM systems on continued airworthiness is to determine their potential influence on scheduled maintenance programs and the potential to reduce unscheduled maintenance actions. SHM systems could be an important factor in improving the effectiveness of inspection and maintenance programs and enabling on-condition maintenance. Ultimately, these improvements would increase air carrier profitability by reducing maintenance program costs and increasing aircraft availability. These issues, along with recent technological advancements in sensor development, sensor data interpretation, prognostics, diagnostics and other key enabling technologies, provide the driving force for the development of advanced new integrated SHM systems that can be directly embedded into the material system or attached to a structure with limited increase in cost, weight, shape or size, such as the ones in the proposed project. If SHM technologies are effectively deployed, they can provide in-time (real-time or near real-time) indications of compromised structural integrity prior to life-limiting failure or fatigue. This can lead to earlier diagnosis and repair of affected components that might otherwise fail during flight. Obviously, this has cost savings implications in terms of the reduced extent of repair required, reduced necessity for component replacement (as opposed to repair), and reduced operational downtime. However, it also has the potential for enhancing overall user acceptance of the aircraft relative to its enhanced safety record and reliability. Hence, a success in the whole Clean Sky project will ensure a strong strategic impact and will have clear Socio-Economic benefits within the next five to ten years by contributing to:
- Enhance European aeronautic industry competitiveness
- Enhance European employment
- Meet societal needs for more environmental friendly, safer and efficient air transport
- Meet societal needs for more environmental friendly manufacturing
- In the strictest sense, relative to operational and logistical considerations, there are a number of additional perspectives from which these benefits can be analyzed, including:
- Reduced scheduled maintenance requirements,
- Operational Performance
- Environmental Considerations
- Resource Availability
- Validation of the design and engineering test models
- Actual response of the component under operational conditions
- Improvement in the design of future models
- Forensic analysis of structural anomalies
- Technology extension for in-process analysis of structural anomalies
- By including the major European airframe manufacturing companies in the Clean Sky Joint Undertaking, the rapid acceptance and approval of the new developments as well as the exploitation of the project results are automatically guaranteed and will contribute significantly to the growth and competitiveness of the European Aeronautic industry. Nevertheless, the final results obtained from the activity developed in the framework of the proposal have been transferred to the ITD leader to exploit the technology developed under this program. Each research group involved in this proposal documented his research activity and strategy, and results in the scheduled reports.
- Finally, the research partners together with the ITD leader benefit indirectly by their increased knowledge and understanding of the SHM technologies, which should enhance their reputation in this field generating greater opportunities for future collaborative programs. The project results can be exploited for clients of other related industries, such as automotive, rail, surface transport and marine engineering through consulting, seminars, website information and other activities. Thus at the scientific level the project acts as a driver for new research by material scientists and production engineers and gives impulse to new techniques for integrated SHM systems. This has initiated scientific investigations and led to a broad dissemination and to spin-offs in production tool improvements.

List of Websites:
Non Applicable.