Near-field ultrasonic response (50-2000 kHz) of an underwater 1-3 piezocomposite transducer is experimentally
investigated using a polarimetric polarization maintaining fiber sensor. Measured outputs from our sensor and a
reference hydrophone are observed to be comparable.
This paper describes work conducted into mobile, wireless, semi-autonomous NDE inspection robots developed at The
University of Strathclyde as part of the UK Research Centre for Non Destructive Evaluation (RCNDE). The inspection
vehicles can incorporate a number of different NDE payloads including ultrasonic, eddy current, visual and magnetic
based payloads, and have been developed to try and improve NDE inspection techniques in challenging inspection areas
(for example oil, gas, and nuclear structures). A significant research challenge remains in the accurate positioning and
guidance of such vehicles for real inspection tasks. Employing both relative and absolute position measurements, we
discuss a number of approaches to position estimation including Kalman and particle filtering. Using probabilistic
approaches enables a common mathematical framework to be employed for both positioning and data fusion from different NDE sensors. In this fashion the uncertainties in both position and defect identification and classification can be dealt with using a consistent approach. A number of practical constraints and considerations to different precision positioning techniques are discussed, along with NDE applications and the potential for improved inspection capabilities by utilising the inherent reconfigurable capabilities of the inspection vehicles.
The interaction of fibre Bragg grating sensors with ultrasonic acoustic waves in water is investigated in the near field
region of a 1-3 piezocomposite transducer. Longitudinal and transverse strain coupling is studied using two different
grating designs and high fidelity measurement of the emitted field was demonstrated.
Mechanical properties of materials may be obtained from the inversion of ultrasonic Lamb wave dispersion curves. In
order to do this broadband excitation and detection of ultrasound is required. As sample size and, in particular, thickness,
are reduced to those of microstructures, ultrasound frequencies in the range of the gigahertz region will be required. We
look at two possible cw laser excitation techniques which, having far lower peak powers than the more frequently used
Q-switched lasers, therefore give a negligible risk of damaging the sample through ablation. In the first method the
modulation frequency of a sinusoidally modulated laser is swept over the required range. In the second, the laser is
modulated with a series of square pulses whose timing is given by a PRBS (pseudo random binary sequence) in the form
of a modified m-sequence.
In this work we present a successful non-contact ultrasound laser generation and detection system for the extraction of the structural properties on mechanical structures. The system uses a Q-switch Nd:YAG short pulse high power laser to generate a broadband source of Lamb waves that propagate along the plate, interacting with the structure’s entire thickness. A modified Michelson surface displacement optical fibre interferometer is used for the detection of the stress and strain waves. In order to extract the structural information stored in the generated and detected waves we present two completely different signal processing tools; the reassigned spectrogram as a time-frequency analysis and the two dimensional Fourier transform. We compare these two techniques and extract interesting conclusions of their properties.
Finally we apply these two techniques and the developed system to temperature change sensitivity and damage detection applications.
In this work we present a signal processing method applied to totally non-contact laser generated and detected acoustic waves for the estimation of the main elastic properties (Poisson ratio and Young's modulus) of structural materials.
This paper presents the basic principles of the laser generation of ultrasound (LGU) through thermal conversion and illustrates the approaches to its use in material evaluation using the broadband features of the source. Traditional LGU involves high energy optical pulse irradiation which often induces surface damage, especially in carbon or glass fiber composites. We therefore expand the concept into low power excitations using laser diode sources. This enables excitation without damage but requires coded temporal signals. Arrays of semiconductor laser sources can also produce very broadband acoustic signals, both temporally and spatially. Piezoelectric sources are usually the opposite constrained in both space and time. This basic observation opens new avenues of material investigation, some of which feature in this paper.
Low power laser diode sources were used in conjunction with electronically controllable time delays and fiber optic delivery arranges as a phased array to produce an electronically steerable ultrasonic source. Delays between the array elements of up to 520nS allowed steering of the fundamental longitudinal mode to be achieved at angles of up to 18 degree(s), whilst low pulse powers of 3.6(mu) J ensured that the testing mechanism was non-destructive. Beamsteering has been demonstrated for aluminium plates demonstrating great potential as a tool for structural assessment.
The DAMASCOS (DAMage Assessment in Smart COmposite Structures) project is a European Union funded program of work bringing together a number of academic and industrial partners throughout Europe. The aim of Damascos is to apply new ultrasonic detection and generation techniques integrated within the structure, together with advanced signal processing to realize damage assessment and ageing characterization in composite structures. This paper describes the background, experimental findings and future applications of the technology as the project moves into its final phase.
Ultrasonic Lamb waves have been proved to be a potentially desirable method for quick interrogation of large plate structures. Integrating fiber optic sensors offer great potential to monitor large areas due to their geometric flexibility. To date this potential has not been widely exploited and there have been only a few reports of non-destructive testing with integrating fiber optic sensors. In this report we present our preliminary measurements for the interrogation of hole defects in plate-like materials with surface-bonded integrating fiber optic sensors. Due to the signal integrating characteristics of fiber sensors very complicated signals were obtained. Therefore signal processing is needed to interpret waveforms for flaw detection.
The simultaneous detection of in-plane and out-of-plane ultrasound displacements is crucial for the interpretation of Lamb wave mode pattern and their interaction with defects within the plate. In this report we present our preliminary experiments for the simultaneous measurement of these two components. A two-channel fibre optic interferometer system has been built which enable the measurements of in-plane and out-of-plane displacements separately and simultaneously. One channel is a Michelson and the other a modified Michelson interferometer or a Mach-Zehnder interferometer. The Michelson interferometer allowed direct measurement of the absolute out-of-plane displacement while the modified Michelson interferometer measured the in-plane displacement. The Mach-Zehnder interferometer measured both components. This two-channel fibre optic interferometer allows a directly calibrated measurement of the two components of displacement simultaneously and offers a great insight into ultrasonic flaw interrogation in plate-like materials.
As a technique of diagnosing failure in structures and systems, the method of novelty detection shows considerable merit. The basis of the approach is simple: given measured data from normal condition of the structure, the diagnostic system builds an internal representation of the system normal condition in such a way that subsequent departures from this condition can be identified with confidence in a robust manner. The success or failure of the method is contingent on the accuracy of the description of normal condition. In many cases, the normal condition data may have quite a complex structure: for example, an aircraft may experience a wide range of ambient temperatures in the course of a single flight. Also, the operational loads experienced by the craft as a result of flight manoeuvres may have wide-ranging effects on the measured states. The object of the current paper is to explore the normal condition space for a simple benchmark monitoring system. The said system uses Lamb-wave inspection to diagnose damage in a composite plate. Both short-term and long-term experiments are carried out in order to examine the variations in normal condition as a result of run-in of the instrumentation and variations in ambient temperature. The exercise is not purely academic as the fiber-optic monitoring system is a serious candidate for a practical diagnostic system.
Ultrasonic Lamb waves have been investigated extensively for damage detection in advanced composite materials. They are particularly suitable for proving thin plate structures of large area, where the Lamb wave approach offers a considerable saving in time over through-the-thickness inspection. However the potential complexity of the propagation can introduce significant difficulties to the technique. We present a review of work conducted at The University of Strathclyde in collaboration with several European partners into the feasibility of Lamb wave inspection. Specifically we will address issues of Lamb wave generation, propagation, defect interaction and detection.
The use of low frequency (sub 20 MHz) incoherent optical frequency domain reflectometry (IOFDR) provides a low cost alternative to the conventional OTDR techniques often used in the interrogation of optical fiber microbend sensors. We have modeled the operation of the IOFDR and experimentally characterized the operation of this technique for monitoring distributed water sensors based on water-swellable polymers (hydrogels). We demonstrate that the IOFDR is capable of detecting and locating sections of increased loss in a graded index multimode optical fiber and discuss the fundamental limits on spatial resolution and range.
A linear array of embedded optical fibers was utilized to produce an ultrasonic beam in carbon composite plates as a potential tool for structural assessment. The embedded fibers were used to deliver laser pulses from low power (1.5 W) modulated laser diode sources for the generation of the ultrasound. These were modulated with a pseudorandom binary sequence, which was used in conjunction with a correlation based detection technique using interferometric fiber optic sensors. Furthermore time delays could be used to adjust the excitation of the elements in the array, thus enabling the direction of the generated ultrasonic beam to be controlled.
A novel distributed fiber optic sensor that incorporates liquid swellable polymers to transduce the swelling into a microbend loss is presented. Interrogation of the sensor using standard optical time domain reflectometry (OTDR) instruments provides the possibility of detecting target chemicals and fluids at any location along the sensor length. The location of multiple events along a sensor, which may extend to 4 km is readily achievable. In this paper we present an overview of the work conducted on the characterization of a distributed optical fiber water sensor. Following a discussion of the basic principles of the water sensor and the underlying technology we present a review of the significant developments achieved. Tests incorporating the sensor in civil engineering applications, which range from monitoring of concrete curing to leak detection in highways, are described. In addition to this, more recent developments to utilize the sensor technology to detect other fluids are discussed, in particular for the monitoring of pH changes and liquid hydrocarbons. We discuss some of the significant advantages in using this type of sensor construction and areas in which it can be practically used.
Ultrasonic Lamb waves have been extensively investigated for non-destructive testing of materials. Embedded or surface bonded optical fiber, acting as the signal arm of a Mach- Zehnder interferometer, is one method previously utilized to detect the Lamb waves. Optical fibers therefore have potential as permanent sensors for structural monitoring of damage and defects in materials. A greater understanding of the ultrasound interaction with the optical fiber sensor will bring application closer. In order to probe this interaction we built a two channel interferometer allowing ultrasound traveling through a material to be monitored simultaneously by a Mach-Zehnder interferometer and also a Michelson interferometer. The Michelson interferometer allows a non- constat measurement to be made of the absolute surface displacement associated with an ultrasonic Lamb wave. Comparison of the ultrasound signals detected by the two different interferometers provides a greater insight into the detection mechanism and sensitivity of the Mach-Zehnder interferometer. The work is then extended to look at embedded fibers in composite materials and damage detection.
We have interrogated a commercially available (SOFO) fiber optic displacement sensor in a dynamic mode using microwave sub-carrier interferometry. The detection threshold is in the region of 50 nm and the linear dynamic range extends to several centimeters using a sub-carrier frequency around 900 MHz.
This paper describes ongoing work into the use of acoustic Lamb waves for composite material characterization. Particular emphasis will be placed on the use of optical fiber based sensing techniques. Previous work described by the authors has concentrated on the utilization of optical fibers for the detection of acoustic waves in composite plates, and the subsequent evaluation of the acoustic signature to gauge damage in the material. In this paper we intend to explore materially integrated techniques for effective acoustic wave generation.
This paper describes ongoing research into the development of a fiber optic sensor for humidity sensing. Particular attention is paid to the compatibility of this fiber optic sensor with an existing system which is already in use for structural deformation monitoring. In order to achieve this, a special transducing coating induces a length variation of the optical fiber as a function of the surrounding humidity levels. An advantage of this setup is that the sensor can not only be read by the same reading unit but can also easily be multiplexed with other sensor types to form a multi functional sensor network. This is of a particular interest for monitoring materials such as reinforced concrete, where structural health assessment criteria include deformation, depassivation and humidity. Several sensor configurations have been tested using dry-wet cycles at room temperature. Through this testing a prototype humidity sensor has been developed which responds consistently to humidity. Using pH sensitive coatings, the same design could be used for a fiber optic pH sensor.
The authors describe the application of laser-ultrasonic sources in combination with interferometric detectors to monitor ultrasonic Lamb waves propagating in metal and composite structure plate samples. Using a high powered pulsed laser source, and temporally and spatially sampling the resultant acoustic field, it was possible to directly measure the Lamb wave phase velocities of several modes simultaneously. A further development has been the demonstration of a low power CW laser source capable of generating Lamb waves in a steel sample at a power density well below the ablation damage threshold.
Conventional ultrasonic Lamb wave testing of materials often relies on the efficient generation and detection of a single Lamb wave mode that propagates in a relatively non- dispersive region of its characteristic. Such an approach allows identification of defect sites within the material by looking for distinct pulse reflections in the time domain. The fundamental symmetric mode is often utilized due to its low dispersion and relatively high velocity. However, this mode is restricted to a low frequency-plate thickness product where its wavelength may be several tens of millimeters with obvious implications for defect sensitivity. An alternative approach to single mode excitation is to utilize broadband Lamb wave measurements. Since a wider portion of the Lamb wave spectrum is now excited it is anticipated that defect resolution could be enhanced. We have investigated the propagation characteristics of ultrasonic Lamb waves in thin plates of metal and composite structure. Ultrasonic generation and detection was accomplished using a combination of non- contacting optical techniques, material integrated optical fiber sensor and conventional piezoelectric sources. Utilizing the broad optical techniques, material integrated optical fiber sensor and conventional piezoelectric sources. Utilizing the broad temporal and spatial bandwidth of a pulsed laser source combined with a point detector, it was possible to simultaneously measure the dispersion characteristics of several propagating Lamb wave modes in both aluminium and carbon fiber composite plates. An alternative approach to signal interpretation was undertaken by developing a NARMAX model of composite plates. An alternative approach to signal interpretation was undertaken by developing a NARMAX model of Lamb wave propagation in a sample plate and correlating properties of the model to damage in the samples. A possible extension to this techniques using a modulated laser diode for acoustic generation is also described. It is anticipated that these related techniques could be used to identify the presence of defects in addition to material ageing effects in composites.
The combination of chemically sensitive, swellable polymer materials with novel optical fiber cable designs to transduce the swelling activity into microbend loss enables a simple yet powerful sensor to be produced. Interrogating such cables with standard optical time domain reflectoctrometry (OTDR) instruments allows particular chemicals of interest to be detected and located along a cable which may extend to several kilometers. We report here on a sensor cable which uses a water swellable material, a hydrogel, to detect positions of water ingress, relative humidity level or pH value. In direct water ingress tests, wet sensor lengths as small as 5 cm in several hundreds of meters have been detected using conventional OTDRs. Following a review of the sensor design, we present the results of an investigation of the mechanical interaction between the hydrogel polymer and the optical fiber within the sensor. The behavior of the sensor is then characterized within environments of different relative humidity levels from 70 percent to 100 percent at temperatures ranging from 0 to 60 degrees C. The sensor was initially designed for applications within civil engineering but can be applied to a much broader range of measurement requirements, for example soil moisture measurement. We will report details on experimental observations on concrete cure within reinforcing tendon ducts and soil humidity measurements within different soil types.
Numerous optical fibre sensing techniques have been evaluated to monitor composite material structures. Most of these sensor systems aim to measure responses to static loads or vibrational spectra. An alternative approach is to monitor the ultrasonic signature over structure and evaluate changes in this signature in terms of modifications to structural properties. This paper describes the principles and implementation of such a system. A simple line integrating interferometric optical fibre sensor is used to detect the propagation of Lamb waves at typically 250kHz within the composite material. The wavelength of these Lamb waves (typically 2cm) exceeds the dimensions of structural defects of interest to the optical fibre sensor is, in effect, detecting changes in the ultrasonic scattering signature of the test structure and relating these changes to the evolution of faults or damage within the structure. The paper will present preliminary results which explore the basic sensitivity mechanisms within the optical fibre sensor and examine the modifications to the ultrasonic signature with the introduction of predetermined damage and deterioration. The eventual aim is a fully integrated system which will enable “plug in” structural testing. These preliminary results indicate that such systems concepts are a medium term prospect.
The purpose of this paper is to provide a concise introduction to the developments and recent findings of a BRITE-EURAM program of work (BRE2.CT94-0990 <SISCO>, structurally integrated system for the comprehensive evaluation of composites). The aim of the program has been to develop an acoustic/ultrasonic based structural monitoring system for composite structures using material compatible sensors. Since plate-like structures have been investigated, it has been a requirement to utilize the propagation of ultrasonic Lamb waves through the sample materials. Preliminary investigations utilized conventional piezo-electric sources coupled to the sample via perspex wedges. The Lamb waves generated by these sources were monitored using either a fully embedded or surface mounted optical fiber sensors. The system was tested with a variety of different carbon and glass fiber reinforced panels, and the interaction of the lamb waves with different defects in these materials was monitored. Conventional signal processing allowed the location of defects such as impact damage sites, delaminations and holes. Subsequent investigations have endeavored to refine the system. This paper reports the development of advanced wavelet based signal processing techniques to enhance defect visibility, the optical connectorization of composite panels, and the development of flexible low profile acoustic sources for efficient Lamb wave generation.
This paper describes some initial results of an evaluation technique, applied to composite plates, which effectively measures the ultrasonic transfer function of these plates and interprets these transfer functions in terms of damage thresholds within the material samples. The samples are probed using Lamb waves at frequencies in the region of 250 kHz. The detectors are interferometric optical fiber systems embedded within or mounted on the surface of the sample. The system is sufficiently sensitive to detect damage and/or structural deterioration before its magnitude is sufficient to compromise structural integrity.
This paper describes a non-destructive inspection technique based on the interaction of ultrasonic SO Lamb waves with holes, regions of impact damage, and delaminations, in carbon fiber composite plates. The Lamb waves were detected using a surface bonded, single mode optical fiber sensor operating at a wavelength of 633 nm forming one arm of an optical fiber Mach Zehnder interferometer. Lamb generation was accomplished by use of perspex coupled piezo-electric transducers.
Glass reinforced plastic (GRP) and carbon fibre
reinforced plastic (CF'RP) laminates are durable,
versatile and light-weight materials which are
progressively replacing metals traditionally used in
aerospace, automobile, rail, gas-storage and many other
industries. If these composite structures were to debond,
fracture or seriously degrade whilst in-service, public
injury or catastrophic failure could result. Therefore,
in-service condition monitoring techniques for
composite structures are most significant and are of
immediate importance.
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.