Fibre Optic Sensors Application for Structural Health Monitoring

Executive Summary:

In the first work package, 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 had been studied and compared to 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 were 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 fibre 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 fibre. 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 fibre. 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 fibre with high spatial resolution has been considered. The activity was organised in three subsequent tasks:

1. 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.
2. 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 fibre path definition.
3. 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.