Structural Health Monitoring Introduction

Structural Health Monitoring constitutes a relatively new scientific field. Its aim is to provide reliable data regarding the static and dynamic condition of a structure, or even data regarding its loading cases and loading combinations. The analysis based on the structural monitoring is useful to decide the continuation of the structure’s use, any necessary repairs or its withdrawal, changes in the construction phases’ program or detection of dangerous loading combinations. The monitoring is performed with the use of various types of sensors. They are attached properly to the critical regions of the structure (e.g. regions of concentrated stresses), providing data regarding its static or dynamic response during the application of various loads. Comparison of these data to the model behaviour of the construction offers the possibility of “continuous diagnosis” and the determination of the potential failure modes.

The technology of optical Fibre Bragg Grating sensors and the relevant networking of recording instruments and software constitute the technological peak in the domain of field instrumentation and structural health monitoring. This technology is widely used on technical works in many areas of the world, including Great Britain.

Fibre Optic Bragg Grating Sensors

An optical fibre consists of a glass core surrounded by a glass cladding which differs in index of refraction. The glass fibre is coated during the fibre drawing process with one of several polymers. The polymer coating protects the glass fibre surface from being flawed by abrasion, which weakens the fibre and accelerates crack growth, especially in the presence of moisture.

Fibre Bragg Gratings are made by laterally exposing the core of a single-mode fibre to a periodic pattern of intense ultraviolet light. The exposure produces a permanent change in the refraction index of the fibre's core, creating a fixed index modulation according to the exposure pattern.

This fixed index modulation is called a grating. At each periodic refraction change a small amount of light is reflected. Light propagates through the grating with negligible attenuation or signal variation.

Transformation of the optical spectrum through a Bragg Grating. The grating refracts a part of the optical signal back to the decoder:

Mechanical deformation of the grating alters accordingly the refracting index and hence the signal refracted back to the decoder:

Due to the temperature and strain dependence of the parameters n and Λ, the wavelength of the reflected component will also change as function of temperature and/or strain.

FBGs measure mechanical strain. They can be regarded as the evolution of the simple electrical strain gauge. By engineering the attachment of the sensor appropriately, the FBGs can be used to measure a variety of other physical parameters such as humidity, pressure, displacement and even water leakage.

It is possible to record the deformations and the temperature alterations by incorporating the sensor into the under study construction, via the decoding of the change of the wavelength: λb1 → λb2 → λb3 etc.

Either strain or temperature or both can be determined by a suitable fibre optic measurement system via the relationship: Δλ = Κε•εz + KT•Τ

The gauge factor Kε of the sensors is 1.18 μstrain per nm.

Furthermore, Bragg sensors are not affected by electromagnetic fields or humidity and have a long lifespan.

The characteristics desired for the ideal Fibre Optic Sensor for strain measurement in typical structures would include the following:

1. stable
2. localized
3. adequate sensitivity and dynamic range
4. linear response
5. sensitive to direction of field change
6. insensitive to thermal fluctuations
7. capable of absolute measurement
8. non perturbative to the structure
9. immune to power interruption
10. able to multiplex
11. easy to mass produce
12. durable for the lifetime of the structure

The above general specifications are usually met by combination of sensor arrangements that incorporate thermal sensors and appropriate cover for the sensor body and armour for the fibre optic cable.

The Fibre Optic Bragg grating sensor strain monitoring is a mature technology now with many applications all over the world. The FBG as a strain gauge has shown accuracy, repeatability and in general reliability with no parallel. The same time the Fibre optic that connects the FBGs of an array with the decoder is the most secure way to convey data from all aggressive environments. And as a matter of fact the cost per meter of monitoring drops with the distance, or length of the substrate (e.g. slopes, slope stability cases, embankments, reinforced earth monitoring).

Application steps

In the trivial case the steps followed include a structural “reconnaissance” and post measurement analysis.

• Choice of places for the growth of network of sensors:

This is achieved via the detailed calculation and the simulation of the expected (model) behaviour of the construction and the localization of its “sensitive regions” (e.g. regions of concentrated stresses). The type of the sensors to be used is then determined, depending on the type of the measured values, the required precision and the environment. The most usual methods of placing the sensors are stiff anchors and glue.

• Field Measurements:

Measurements are taken during the operation of the construction while imposing the representative and maximum loads.

• Comparison of the measurements by the sensors to the model behavior of the construction:

Through the analysis of the theoretical and field data the experimental results are evaluated and the construction’s possible failures are located.

• Continuous diagnosis of the construction’s structural integrity:

During regular time intervals the sensors’ data are recorded, creating in this way the "deformation history database" of the structure. These data analysis give us the ability to locate changes in the structure’s behaviour in time.

CRD Group's FBG sensors info

CRD Group’s intense research activity in structural health monitoring of technical works, gives our company the ability to manufacture its own optical sensor layouts and sensor mounts.

Our sensors follow the FBG (Fibre Bragg Grating) type and are designed to meet the requirements and specifications of the Greek construction industry. CRD Group’s sensors offer credibility, flexibility of solutions, complete adaptivity to each project’s individual requirements and low cost.

Our sensors have been subjected to a complete series of tests conducted by the research faculties of the National Technical University of Athens, which have tested their efficacy with strict criteria. The results of these tests were recorded in authoritative certificates.

Furthermore, our personnel and collaborators are excellently trained to undertake the instrumentation of any project on the whole: analysis and design, manufacture of the sensor layout, layout’s installation, measurement acquisition and data analysis.

The CRD Group offers the option of manufacturing custom sensor layouts that can adjust to every case of structural health monitoring.

Our experience on Civil Engineering works guarantee the extending of our support beyond structural health monitoring, in the prevention of structural failure and repairing or reinforcing works. CRD Group’s associates will always be by your side to support and redesign our sensor layouts.

CRD Group would like to remind you that the above mentioned sensor mounts are used as parts of the final sensor layouts, as derived from each project’s individual analysis. Custom sensor mounts and layouts are also available to cover the needs of every project’s structural health monitoring.