Wayne R. Philp

Broadband Lamb wave measurements in aluminium and carbon/glass fibre reinforced composite materials using non-contacting laser generation and detection

Abstract Broadband measurements of Lamb wave dispersion characteristics have been performed in aluminium, carbon and carbon/glass fibre hybrid composite materials using a non-contacting optical technique. A high powered Q-switched Nd:YAG laser was used to initiate the Lamb wave spectrum in the materials, and an optical fibre Michelson interferometer was used to monitor the outof-plane displacements associated with the propagating modes. Comparison of experimental results with theoretical modelling in both the aluminium and composite samples indicated good agreement for the excited modes. The application of this technique to the non-destructive testing and elastic property measurement of composite materials is discussed.

Wavelet signal processing for enhanced Lamb-wave defect detection in composite plates using optical fiber detection

A wavelet-transform-based technique to enhance defect de- tection in a carbon fiber composite plate interrogated using ultrasonic Lamb waves and incorporating an optical fiber receiver is described. Fundamental symmetric (S0) Lamb waves were introduced into the sample plates using a conventional piezoelectric transducer operating at a frequency of around 250 kHz. Coupling into the plates was achieved using a perspex phase-matching wedge. The propagating acoustic pulses were monitored using a simple embedded or surface-mounted singlemode optical fiber forming the signal arm of an optical fiber Mach- Zehnder interferometer. The direct Lamb wave reflections from delami- nations in the sample plates were of low amplitude, although a degree of defect visibility enhancement was achieved by correlating the received signals with the outgoing ultrasonic pulse. A considerable improvement in the defect visibility over the latter technique was found by using a wavelet-transform-based novelty technique to identify the defective plate zones. Using an orthogonal wavelet transform to compress the data, important structurally related features were extracted by setting appro- priate threshold levels on the wavelet coefficients. The reconstructed (uncompressed) data from defect-free portions of the plate were used to construct a template representing a normal condition. Defect location was achieved by analysis of the departure of signals arising from defec- tive plate regions from the no-fault condition template.

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.

Detection of defects in composite plates using Lamb waves and novelty detection

The problem of detecting damage in composite plates is addressed here using L amb waves and novelty detection. Damage can be inferred from the scattering and modification of the Lamb wavefield as it passes through a defect. In order to produce an automatic diagnostic tool which can operate on measured time data, the method of novelty detection is used. This depends on establishing a description of normality which then allows subsequent signals to be flagged as anomalous if they deviate from normal condition. Three methods of novelty detection are illustrated: two statistical methods and one neural. The methods are demonstrated on experimental data captured from two composite plates.

Interferometric signals in fiber optic methane sensors with wavelength modulation of the DFB laser source

We examine the performance limitations of a fiber optic methane sensor using microoptic GRIN lens cells in either transmission or reflective mode. We derive the worst case values of sensitivity due to interference effects caused by reflections within the cell as a function of the cell parameters. We also show both theoretically and experimentally how the interference signal may be minimized by suitable choice of the amplitude of the wavelength modulation. Although, theoretically, reflective cells could match the performance of transmission cells, in practice, transmission cells are superior in terms of interferometric noise levels. With reflective cells, two secondary reflections from the cell and secondary cavity effects in the system enhance the interference so that in practice their performance is inferior.

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.

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.

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.

Ultrasonic inspection of CFRP plates using surface-bonded optical fiber sensors

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.