Daniel Betz

Acousto-ultrasonic sensing using fiber Bragg gratings

This paper describes a fiber-optic system which is able to detect ultrasound in structures. The aim of the sensing system is to monitor structures, in particular aircraft structures, by detecting ultrasonic Lamb waves. This type of monitoring technique has recently become a key topic in structural health monitoring. Most common approaches use piezoceramic devices to launch and receive the ultrasound. A new way of fiber-optic detection of Lamb waves is based on fiber Bragg grating sensors. In addition to the well known advantages of fiber-optic sensors, this new interrogation scheme allows the use of Bragg gratings for both high-resolution strain and high-speed ultrasound detection. The focus of the paper is on the ultrasonic part of the system. The theoretical approach and the implementation into a laboratory set-up are elaborated. Experiments have been carried out to calibrate the system and first results on simple structures show the feasibility of the system for sensing ultrasonic Lamb waves.

Structural Damage Location with Fiber Bragg Grating Rosettes and Lamb Waves

The aim of this study is to present the results of testing a damage detection and damage localization system based on fiber Bragg grating sensors. The objective of the system is to detect and locate damage in structures such as those found in aerospace applications. The damage identification system involves Bragg gratings for sensing ultrasound by detecting the linear strain component produced by Lamb waves. A tuneable laser is used for the interrogation of the Bragg gratings to achieve high sensitivity detection of ultrasound. The interaction of Lamb waves with damage, e.g., the reflection of the waves at defects, allows the detection of damage in structures by monitoring the Lamb wave propagation characteristics. As the reflected waves produce additional components within the original signal, most of the information about the damage can be found in the differential signal of the reference and the damage signal. Making use of the directional properties of the Bragg grating the direction of the reflected acoustic waves can be determined by mounting three of the gratings in a rosette configuration. Two suitably spaced rosettes are used to locate the source of the reflection, i.e., the damage, by taking the intersection of the directions given by each rosette. A genetic algorithm (GA) can be used to calculate that intersection and to account for any ambiguities from the Lamb wave measurements. The performance of the GA has been studied and optimized with respect to the localization task. Initial experiments are carried out on an aluminum structure, where holes were drilled to simulate the presence of damage. The results show very good agreement between the calculated and actual positions of the damage.

Advanced layout of a fiber Bragg grating strain gauge rosette

A temperature-compensated strain-sensing scheme based on fiber Bragg gratings (FBGs) that is suitable for strain mapping applications is described. FBGs are bonded to a backing patch, together with an extra grating that is used for temperature compensation/measurement. The patch provides a simpler and more robust way of attaching the FBGs to a structure than directly mounting a bare fiber, though it was necessary to design it in such a way that there was minimal reduction in the strain transferred from the structure to the sensing fibers. Finite element (FE) analysis was used to help design the patch, which was then constructed accordingly. The authors have demonstrated experimentally that the use of the backing patch produces a reduction in strain sensitivity of only around 4%, which is slightly better than theoretically predicted values. The temperature measuring FBG had to be bonded in such a way that it experienced the changes in temperature, but not the strain, to which the structure was subjected. A design for doing this was developed and proven. The use of a backing patch to develop a rosette configuration of Bragg gratings, each having a different peak reflective wavelength, is described. The rosette configuration is one that is frequently used with electrical strain gauges and allows here to determine both the magnitude and the direction of strain.

Identification of structural damage using multifunctional Bragg grating sensors: I. Theory and implementation

Structural health monitoring has become a respected and established discipline in engineering. Health monitoring involves the development of autonomous systems for continuous monitoring, inspection and damage detection of structures with minimum involvement of labour. The ultimate goal of structural health monitoring is to increase reliability, improve safety, enable light-weight design and reduce maintenance costs for all kinds of structures. The identification of structural damage is therefore a key issue in structural health monitoring. The scope of this paper is to present the results of testing a system for the identification of structural damage based on fibre Bragg grating sensors. The basic idea is to use fibre Bragg gratings as acoustic receivers of ultrasonic Lamb waves. The layout of such a damage identification system is introduced and its theoretical limits are studied numerically and experimentally. The set-up for damage identification experiments is described and the results of initial experiments introducing damage detection based on the analysis of Lamb wave signals are presented. The results for the Bragg grating sensors are then compared to the results of established technology for Lamb wave detection using piezoceramic transducers.

The Detection of Ultrasound Using Fiber-Optic Sensors

Ultrasound is a valuable tool for the detection of damage in structures and the characterization of material properties. Its detection is conventionally done by piezoelectric transducers, however fiber-optic sensors can operate over a greater range of frequencies and also yield information on the direction of wave propagation. The interaction between fiber sensors and ultrasound both demonstrates the integrating features of intrinsic fiber-optic sensors and presents new opportunities in ultrasonic detection, offering enormous diversity in polar and frequency response. This paper summarizes the interaction mechanisms between ultrasound and fiber 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.

Structural damage identification using multifunctional Bragg grating sensors: II. Damage detection results and analysis

Damage detection is an important issue in structural health monitoring. Lamb waves are the most widely used acousto-ultrasonic guided waves for damage detection. This paper gives the results of experiments carried out to study the identification of damage using Bragg grating sensors as ultrasonic receivers of Lamb waves. The experiments involve a rectangular aluminium plate. Damage was introduced into the plate by drilling a hole into the centre of the plate. In order to obtain different severity of damage, the hole diameter was increased step by step. Several signal processing tools are presented and then applied to the Lamb wave signals in order to find a parameter that corresponds to the severity of damage. The parameter that serves as the damage index has to have small cross-sensitivity to other physical parameters, e.g. temperature. Therefore, additional experiments have been carried out to study the temperature dependence of the Lamb wave signals. In order to determine the influence of the temperature on the damage detection results, the cross-sensitivity is studied within this paper.

Structural Monitoring Using Fiber-Optic Bragg Grating Sensors:

For many applications in the aerospace industry, monitoring of structural performance is becoming increasingly important in order to reduce maintenance and inspection costs and enhance efficiency. One of the most attractive sensors to build large area sensor networks has become the fiber-optic Bragg grating. Despite the fact that Bragg grating sensors have been of considerable interest to the structural health monitoring community for the last few years, many challenges associated with applying Bragg grating sensors in real-world applications have not yet been solved. One of the major issues remaining is the method of attaching the sensors to the structure to be monitored. It is essential to understand the optical signal of the sensor and its relation to the actual strain fields in or on the structure. DaimlerChrysler and EADS have studied several solutions for the fiber attachment over the past few years. This paper discusses three of these methods, which are currently under investigation in both lab and field tests.

Smart sensing of aviation structures with fiber optic Bragg grating sensors

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.

Test of a fiber Bragg grating sensor network for commercial aircraft structures

The field test reported on in this paper has shown the great potential of fiber-optic Bragg gating sensors. They are suitable for load monitoring of aviation structures under real-world conditions. The results obtained with the Bragg gratings show excellent consistency with the strain gage results. This indicates that both the sensor installation technique of gluing the sensors to the surface and the sensor interrogation technique using a tunable laser-based system, both of which were studied in the field test, are well-suited for these measurements. In addition, the fiber-optic sensor network has several advantages: it is immune to electromagnetic interference and has high multiplexing capability, for hundreds of these sensors can be aligned along a single optical fiber, if desired. This significantly reduces the effort needed for wiring and also leads to a considerable decrease in weight compared to standard electrical sensor networks.

Comparison of point and integrated fiber optic sensing techniques for ultrasound 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.