In the past decade, advanced composite materials have been widely used in a variety of load-bearing structures such as rotor blades, aircraft fuselage and wing structures. For example 25% of the new Airbus A380 is made from composites. The unique properties of composite materials such as their high strength-to-weight ratio, high creep resistance, high tensile strength at elevated temperatures and high toughness have been attracting increasing interest in numerous automotive, aerospace, and sports applications.
Composite structures are frequently subjected to external excitations over a variety of vibration frequency ranges. Such dynamic interference may cause the structures to suffer from fatigue damage and/or catastrophic failures if the excitation frequency approaches to the natural frequency of the structures. A typical composite fails in a sequence of transverse micro-cracking, delamination and fiber failure. Polymer matrix composites accumulate damage in a general rather than a localized fashion, and failure does not always occur by the propagation of a single macroscopic crack. The micro-structural mechanisms of damage accumulation, including fibre breakage and matrix cracking, debonding, transverse-ply cracking and delamination, occur sometimes independently and sometimes interactively, and the predominance of one or the other may be strongly affected by both materials variables and testing conditions .
Nondestructive evaluation (NDE) techniques have been developed to detect internal or invisible damage. Traditional NDE techniques are ultrasonic scan, an eddy current method, X radiography, an acoustic emission method, and passive thermography. These NDE techniques are effective in detecting damages in materials and structures, but it is difficult to use them in operation due to the size and weight of the devices. Therefore, there is strong interest in development of smart composite structures with integrated optical fibre sensors which would allow in-situ monitoring of both the manufacturing process and service life . Compared to traditional NDE techniques, fiber-optic sensors offer unique capabilities: monitoring the manufacturing process of composite parts, performing nondestructive testing once fabrication is complete, and enabling health monitoring and structural control. Because of their minimal weight, small size, high bandwidth, and immunity to electromagnetic interference, fiber-optic sensors have significant performance advantages over traditional sensors. Furthermore, optical fibers are steadily becoming more cost-effective due to advances in the telecommunication and optoelectronic industries. There are three major types of optical fibers in current use: multimode stepped index, multimode graded index, and single-mode stepped or graded index fibers [3, 4].
To ensure the integrity of composite structures, it is desirable to simultaneously monitor the strain, temperature and vibration frequency applied to them in real time. A multi-functional sensor that can measure multiple parameters would offer significant economic advantages and end-user appeal. Furthermore, the ability to monitor multiple parameters simultaneously would be of significant benefit to material and structural engineers. An important requirement for the sensor is that it should be possible to embed it into the host composite without modifying its properties and functions.
In the last 10 years, researchers have been working on the principles and techniques to measure strain (e.g., by using fiber Bragg gratings [5-7], or other types of sensors such as Fabry-Perot cavities ), temperature or other parameters. Moreover, the process of fiber embedding has been investigated to ensure reliability and precision of the measurement and to ensure structural integrity of the host composite structure after embedding the optical fibre sensors .
One of the leading varieties in current use, optical fiber Bragg gratings (FBGs), are widely used and considered the most popular technology for implementing health monitoring systems. Typically, an FBG consists of a single mode optical fibre with a short optical grating written within the core. If an FBG sensor is embedded into a composite structure, any change of the strain in the structure results in the grating being subjected to strain which can be detected by measuring the wavelength of light reflected from the grating. Measuring the strain in the fibre can allow one to measure to the strain of the structures if a perfect bond between the fibre and composite is achieved.
However, FBG sensors are limited by their simultaneous dependence on both strain and temperature. To overcome this cross sensitivity a number of techniques have been proposed, most of them relying on the deconvolution of two simultaneous measurements. These methods include the dual-wavelength superimposed gratings, the use of first- and second-order diffraction grating wavelengths, FBGs in optical fibers with different dopants, hybrid Bragg grating/long period gratings, dual-diameter FBGs, FBG/EFPI combined sensors , FBGs in high-birefringence optical fibers , and the employment of strain-free FBGs, etc. The use of a strain-free reference FBG turns is the most efficient way to discriminate strain and temperature. However, it is not easy to implement this technique when sensors must be embedded into a host composite, since it requires placing a grating into a small capillary tube or another envelop protecting it from strain.
The state-of-the-art of the HB fiber-based polarimetric sensors has been recently been reviewed in detail in [12,13], underlining the physical origin of the perturbation effects (such as those induced by pressure, strain, bend, twist, temperature) on the lowest-order mode propagation in HB PM fibers along with their impact on applications in optical fiber sensing and systems.
Highly birefringent (HB) polarization-maintaining (PM) fiber has created a new generation of fiber-optic sensors known as polarimetric fiber sensors which utilize polarization (phase) modulation within these fibers in response to various external perturbations describing the physical environment. Polarimetric optical sensors have created a powerful sensing technique, in which polarization of the guided optical field is the important issue.
It is a general feature of a HB fiber that the output signal of the polarimetric sensor based on HB fibers is a periodic function of the external strain and the period of this function (sensing range of the polarimetric sensor) can be conveniently adjusted by choosing an appropriate length of the sensing fiber. However, this periodicity requires zero strain calibration for the HB polarimetric sensors. Due to a variety of HB fibers and different responses to external perturbations there are a lot of possibilities to construct fiber-optic strain gauges precisely adjusted to particular needs and applications.
To compensate temperature disturbing effect in HB polarimetric sensors several techniques have been proposed. Temperature compensation has recently been realized by using specially-designed side-hole HB fibers that have two orders of magnitude higher pressure (strain) sensitivity coefficients in comparison to their thermal sensitivity .
FBG sensors are very capable of sensing absolute strain and temperature but distinguishing strain from temperature is difficult in a composite material. HB polarimetric sensors can be made temperature insensitive but to measure strain they require a means of setting a zero strain reference. Combined in a hybrid sensing approach FBG and HB polarimetric sensors work together to provide not only strain sensing but also temperature and vibration frequency sensing. For example the FBG sensors can provide a reference zero strain for HB polarimetric sensors, while the HB polarimetric sensors compensate for the temperature dependence of the FBG sensors. The proposed research will investigate the development of smart composite structures based on this hybrid sensing approach in a composite in order to provide simultaneous in-situ measurements of multiple parameters: strain, temperature and vibration frequency. Using both sensor types in a single composite structure should overcome the limitations of the individual sensor technologies, provide a capability of simultaneous measurements of multiple parameters and ensure high accuracy and reliability of the measurements. Another important advantage of the proposed approach is that having two sensing systems working together provides built-in redundancy in the event of failure of one of the sensing systems.
The fabrication of the smart structure will utilize typical methodologies for composites but in a manner that achieves the most reliable and accurate sensing capability. Furthermore a methodology will be developed for the location of sensors to best measure strain, vibration frequency or to detect the onset of damage/failure of the structure.
The objective of the research planned in this project is to develop smart composite structures based on the proposed hybrid optical sensing technique and to open the way to commercialisation of this technology. The main technical milestones of the project are 1) the development of reliable techniques for the sensor positioning and protection within the composite structure which would provide simultaneous measurements of strain, temperature and vibration frequencies without compromising structural integrity and properties of the composite. 2) The development of calibration and discrimination techniques for the polarimetric and FBG sensors that would gain maximum benefit from the complementary nature of the two sensor types, provide redundancy in the event of failure and ensure high reliability and accuracy of measurements; 3) The development of a common interrogation approach and 4) The investigation of the route to commercialisation.
The expected result of the project will be the development of methodologies for the reliable integration of the hybrid photonic sensor systems into composite structures, along with techniques for interrogating the sensors in a way that gains the maximum benefit from the complementary nature of the two sensor types used.
The consortium partners have accumulated a very significant level of expertise in a number of complementary areas: