A. McNab

Surface-bonded and embedded optical fibers as ultrasonic sensors

The effectiveness of surface-bonded and embedded optical fibers for the detection of ultrasonic Lamb waves in 2-3-mm-thick steel, carbon-fiber-reinforced plastic (CFRP) and glass-reinforced plastic (GRP) plates are compared. A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach-Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture. An embedded fiber is shown to be about 20 times more sensitive to Lamb wave motions than a surface-bonded fiber. However, the latter may be more practical.

Surface-bonded optical fibre sensors for the inspection of CFRP plates using ultrasonic Lamb waves

Surface-bonded single-mode optical fibre sensors have been used to monitor the interaction of ultrasonic Lamb waves with defects in carbon fibre composite plates. Lamb waves were initiated using Perspex-coupled piezoelectric transducers. The defects investigated comprised holes, regions of impact damage and delaminations. Holes could be identified by analysing direct reflections and impact damage by back-wall echo amplitude. Large delaminations gave a poor direct reflection. Evidence was found for mode conversion at centre plane delaminations.

Generation and reception of ultrasonic guided waves in composite plates using conformable piezoelectric transmitters and optical-fiber detectors

A condition monitoring nondestructive evaluation (NDE) system, combining the generation of ultrasonic Lamb waves in thin composite plates and their subsequent detection using an embedded optical fiber system is described. The acoustic source is of low profile with respect to the composite plate thickness, surface conformable, and able to efficiently launch a known Lamb wave mode, at operating frequencies between 100 and 500 kHz, over typical propagation distances of 100 to 500 mm. It incorporates both piezocomposite technology and interdigital design techniques to generate the fundamental symmetrical Lamb wave mode in both metallic and carbon-fiber composite plates. Linear systems and finite element modeling techniques have been used to evaluate the operation of the transducer structure, and this is supplemented by experimental verification of the simulated data. An optical fiber, either bonded to the surface or embedded across the length of the composite plate samples, is used to detect the propagating ultrasonic Lamb waves. Single mode silica fiber has been used in conjunction with a portable 633 nm Mach-Zehnder interferometer for signal demodulation and subsequent data acquisition. This hybrid system is shown to generate and detect the fundamental symmetrical Lamb wave (s/sub 0/) in both carbon-fiber and glass-fiber reinforced composite plates. Importantly, the system signal-to-noise ratio (SNR) associated with the acoustic source compares favorably with s/sub 0/ Lamb wave generation using a conventional transducer and angled perspex wedge arrangement.

Monolithic phased array for the transmission of ultrasound in NDT ultrasonics

Abstract A monolithic phased array for the transmission of ultrasound is described. Formed on a lithium niobate substrate, it emits steered longitudinal waves and is intended for the contact testing of steel at 5 MHz. A simple model of the single element source is proposed which assures that the element acts as a localized normal force on the surface of the steel. Experimental measurements were made on the radiated waves from actual sources to determine the nature of this force. By extending the model to a group of such elements, predictions on the performance of a phased array can be made. These results show that a monolithic transmission array has useful properties for NDT.

Ultrasonic condition monitoring of composite structures using a low profile acoustic source and an embedded optical fibre sensor

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 , 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.

1–3 connectivity composite material made from lithium niobate and cement for ultrasonic condition monitoring at elevated temperatures

We have designed, manufactured and tested a piezoelectric composite material to operate at temperatures above 400 degrees C. The material is a 1-3 connectivity composite with pillars of Z-cut lithium niobate in a matrix of alumina cement. The composite material produced shorter pulses than a monolithic plate of lithium niobate and remained intact upon cooling. Results are presented from room temperature and high temperature testing. This material could be bonded permanently to a test object, making it possible to carry out condition monitoring over an extended period. A new excitation method was also developed to enable remote switching between array elements.

Towards automated interpretation of ultrasonic NDT data

Expert systems have demonstrated potential for automated interpretation of ultrasonic NDT data, but rules do not provide the best solutions for all interpretation problems. In this work, 3D images are reconstructed from raw A-Scans, using CAD models of test specimen geometry to correct positions and remove geometric echoes. A knowledge model for ultrasonic NDT has been implemented in a working expert system, with uncertainty managed using fuzzy logic. A suite of data processing tools is available both to the expert system, and also through an interactive 3D visualization system, which therefore provides a development environment for automated interpretation, as well as being powerful in its own right.

Detection of ultrasonic Lamb waves in composite plates using optical-fibres

This paper presents experimental results for a new condition monitoring system that uses a combination of piezoelectric and fibre-optic technologies as the active and passive elements respectively. Composite plate samples have been manufactured containing two groups of optical-fibres (typically four strands per group) embedded across the length of the plate. Polyimide coated 4/125 /spl mu/m single-mode glass fibre has been utilised and connected to a portable 633 nm Mach-Zehnder fibre interferometer for signal demodulation and subsequent data acquisition. For this present work, 1-3 connectivity piezocomposite transducers have been used to generate a known Lamb wave within carbon fibre reinforced composite plate (CFRP) structures. The S/sub 0/ mode was selected due to the relatively large in-plane motion and low dispersion characteristics. Results using both externally surface bonded fibres and embedded fibres are presented, for a range of defective plates. Defects ranging from 1 mm diameter holes through to significant delaminations have been detected at an ultrasonic operating frequency of 285 kHz.

The design of embedded transducers for structural health monitoring applications

This paper describes a theoretical and experimental investigation concerning embedded piezoelectric transducers employed principally for condition monitoring of engineering composites. Both interdigital transducers (IDTs) and plate transducers are investigated with the aim of assessing their efficiency as uni-modal Lamb wave transmitters. The IDT configuration comprises a piezocomposite layer sandwiched between two flexible printed circuit boards, where the interdigital electrode spacing corresponds to the wavelength of the desired Lamb wave mode. The alternative configuration comprises a thin piezoceramic plate for which the lateral dimensions are chosen to efficiently couple energy into the desired mode. For both types of transducer, finite element models have been successfully employed to establish the design requirements for generating the zero order symmetrical mode (So) without simultaneously generating the zero order anti-symmetrical mode (Ao), which exhibits strong velocity dispersion. In this investigation the Ao mode is regarded as coherent noise. Generation of a pure So mode is shown to require positioning of the transducer at a depth which is exactly half way between the top and bottom faces of the plate-like structure within which it is embedded. For structural monitoring, the plate-type transducer is shown to be more suitable than the IDT. A scanning laser vibrometer was used to verify many of the theoretical findings.

Construction and evaluation of a new generation of flexible ultrasonic transducers

This paper describes the construction of a new generation of flexible ultrasonic transducers. The device consists of an array of piezoceramic platelet devices of high aspect ratio embedded within a soft polymer matrix, supported by a copper/polyimide flexible printed circuit board. Theoretical modelling using both a linear systems approach and the ANSYS finite element package was used to predict transducer behaviour, and compared with experimental results and surface displacement profiles with good correlation. In addition, the linear systems technique was utilised to evaluate potential piezoceramic materials and the results compared to a conventional thin piezopolymer film transducer structure. The ceramic/polymer platelet transducer has been implemented in two application areas: a 6 MHz multi-element linear array for flaw detection in curved test specimens, allowing imaging of test piece construction; and efficient generation of the fundamental symmetrical Lamb wave at frequencies below 500 kHz, using interdigital design techniques, in both aluminium and carbon-fibre reinforced polymer composite plates. Experimental results will be presented which will demonstrate the application of this flexible transducer technology to practical NDE.