Ultrasound is a valuable tool for the detection of damage in structures and the characterisation of material properties. Its
detection is conventionally done by piezoelectric transducers, however fibre optic sensors can operate over a greater
range of frequencies and also yield information on the direction of wave propagation. The interaction between fibre
sensors and ultrasound both demonstrates the integrating features of intrinsic fibre optic sensors and presents new
opportunities in ultrasonic detection, offering enormous diversity in polar and frequency response. This paper
summarises the interaction mechanisms between ultrasound and fibre sensors and confirms their functional flexibility.
We use these results to demonstrate the practical use of these sensors to detect and locate damage in a sample.
The detection and location of holes in an isotropic aluminium plate using fibre Bragg grating rosettes to detect
ultrasound Lamb waves is described. This is followed by a description of the anisotropic properties of a carbon fibre
plate and their effect on hole detection. Finally, the issues involved in attempting to locate holes in an anisotropic
samples are discussed and the possibility of achieving this assessed
The possibility of combining load history with ultrasound signal information offers, potentially, an extremely powerful
method for determining the structural health of a structure. Here we describe how suitably configured fibre Bragg
grating arrays can be used to both monitor strain fields and to detect the magnitude and direction of ultrasound waves.
The directional responses of fibre Bragg gratings to both static and ultrasonic strain fields have been investigated and
the results used to enable the FBGs to be configured into rosettes. These can be interrogated to give both the magnitude
and direction of either the principle static strain or an incident ultrasound Lamb wave.
Measurement of the static strains enables us to perform strain mapping and hence determine load history, whilst
ultrasound detection allows us to monitor a structure for defects such as cracks. If Lamb waves are generated by, for
instance, PZTs bonded to the structure, comparing signals from the rosette with data previously obtained for the healthy
structure allows possible defects to be identified and located. Location is carried out by calculating the intersection point
of ultrasound direction from 2 or more rosettes. The geometry of the rosette configuration is described, together with the
parameters affecting efficient strain transfer and the optimisation of FBG interrogation techniques.
We describe and compare two novel methods of detecting ultrasonic Lamb waves used for damage detection and location, and then go on to compare their characteristics with those of more conventional PZT transducers. The two methods are measurements of the change in polarization state of the light in an optical fibre and the changes in reflected power from a fibre Bragg grating. Since different transducers measure different properties of Lamb waves by different methods, their relative sensitivities to the S0 and A0 modes can also vary. This can be of interest because, for instance, the A0 mode is more sensitive to the presence of delaminations in a sheet due to the larger shear strain component that this mode contains. We also describe the directional properties of the sensors and demonstrate the ways in which these can be used to advantage in the detection and location of damage.
In this paper we describe and compare two methods for detecting Lamb waves used for damage detection. One is a polarimetric sensor that integrates the pressure effects of the acoustic wave along its length, whilst the other is the Bragg grating that measures linear strain and is in effect a point sensor. Both sensors are highly directional, which gives benefits in damage signal detection and source location. We demonstrate both the detection and location of damage and discuss the advantages and disadvantages of the two sensors.
We describe a novel method of damage location using ultrasonic Lamb waves detected by fiber Bragg gratings. The gratings are configured in a rosette configuration in order that the direction of an incoming wave may be determined. Location of holes was determined by finding the intersect point of the reflected ultrasound signals obtained from two rosettes. Accurate results are obtained when optimum geometries source transducers an fiber gratings are used.
We describe a novel but simple method of detecting Lamb waves through the measurement of the changes in the polarimetric state of light propagating through an optical fibre which has been either embedded into or bonded onto the plate to be tested. The directional properties of the sensor are described, as is its ability to detect a hole produced in the plate. We also show how the relative sizes of the detected signal amplitudes from the source PZT and from the hole vary according to the alignment of the source with the sensor axis.
We describe a novel fiber-optic system that is able to detect both ultrasonic Lamb waves and the location of their source. The aim of the system is to detect damage in structures such as those found in aerospace applications. Our system involves the use of fiber Bragg gratings, which may be either bonded to the surface of the material or embedded within it in order to detect the linear strain component produced by the acoustic waves. Interrogation of the Bragg gratings is carried out using a laser, which is tuned to the wavelength that gives the maximum sensitivity on the grating response curve. An amplitude modulated signal is produced by the interaction of the Lamb wave with the grating. The well defined directional properties of the Bragg grating (compared to the isotropic response of the more commonly used piezoceramic disc transducers) are used to determine the direction of propagation of the acoustic waves by mounting three of the gratings in a rosette configuration. Two suitably spaced rosettes are used to locate the source of the ultrasound by taking the intersection of the directions given by each rosette. This will become important when we extend the technique to include the study of the use of changes in the propagation properties of Lamb waves as a method of damage detection. We will present both theoretical and practical results on the interaction of the Lamb waves with the grating and the extraction of directional information from the response of the rosettes.
In future aircraft, integrated sensors for monitoring structural performance and structural integrity will become increasingly important. For integration into carbon-fiber- reinforced composite structures, fiber-optic Bragg grating sensors represent a very promising approach. In order to characterize the behavior of integrated sensors for different mechanical load cases theoretical and experimental investigations were conducted.
This paper describes a novel, patented wavelength- and time-division multiplexing sensor system based on a tunable laser source with an additional optical modulator geared for intenogating a large number of low-reflective Bragg grating sensors with high accuracy.
We developed a surface mounting technique where fiber-optic Bragg grating (FBG) sensors are glued to the surface of structures and tested the technique on the surface of a CFRP- wing at the DASA Airbus test center Hamburg for over one year. The FBG sensors were interrogated with a measurement system capable of determining the Bragg wavelength in a few seconds over a spectral range of 60 nm (around 1.53 μm) with an absolute accuracy better than 1 pm. A polarization scrambler was used to account for polarization effects. Excellent consistence between the values of electrical strain gauges and the FBG sensors were found during all measurements. However because this method shows drawbacks in a harsher environment such as a flight test, we are currently investigating the possibilities of integrating FBG sensors into the varnish of the structures. For reasons of their better mechanical performance we use FBG sensors produced on the fiber draw-tower with a special UV-curable coating. The sensors are integrated into an original Airbus varnish build- up. We observed linear strain sensitivities in a temperature range between -50 and +100 °C. Furthermore, at negative temperatures we found a vanish- induced polarization dependence which could be used to differentiate between strain and temperature effects.
We present results on optical and mechanical stability of single-layered acrylate coated fiber Bragg gratings produced on a draw tower, and exposed to high temperature annealing and to cyclic loading. Optical stability was assessed in terms of strain response and reflectivity changes with annealing temperatures up to 400 degree(s)C. Cyclic loading of Bragg gratings with mean stresses between 2.4 GPa and 3.0 GPa was compared with predictions made by using power-law based crack growth theory with parameters obtained by dynamic tensile tests. Comparison with theory confirms predicted strength decrease and lifetime reduction.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.