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.
In this paper we present a complete non-contact, all-optical inspection tool, for the extraction of the principal mechanical
characteristics of plate-like structural materials (effective stiffness, Poisson ratio and plate's thickness).
Broadband ultrasonic guided waves - Lamb waves - are optically generated and detected for incremental source detector
distances. A two dimensional Fourier transform is applied to these measured time signals in order to extract dispersion
information, which is highly affected by elastic properties as well as by sample geometry.
The elastic properties and thickness of the structure under test are obtained from this experimentally extracted dispersion
information, by an inversion technique which is presented and analysed in the paper. These values are verified with a
Monitoring changes in the estimated material properties could be used to indicate changes in the structural condition.
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.
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.
This paper describes an all optical non-contact technique to determine material properties in a mechanical structure. Using optical generation of ultrasound and interferometric optical detection to determine the dispersion curves for acoustic (Lamb) waves, readily available numerical algorithms have produced values of Young's Modulus and Poisson ratio with confidence limits of a few percent. The approach may be applied to inspection regions with dimension in the range microns to centimetres.
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.
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.
Ultrasound has been demonstrated to be a perfect tool for NDT. There are several detectors that can be applied in NDT, for example fibre Bragg grating, interferometry, etc. Here we concentrate in polarimetric optical fibre detection.
In this paper we develop a simple but realistic analysis of the ultrasonic wavefront integration technique along an optical fibre for acoustic detection. Our model considers the perturbation caused by the acoustic wave as an isotropic change in the effective refractive index of the sensing fibre used as the detection system and neglects the polarization modulation. Also we assume the stress homoegeneous through the section of the fibre.
The theoretical analysis has been simulated in MATLAB. In this program we have analyzed the relation between the length of the sensing fibre, its distance to the ultrasound source and its sensitivity to ultrasound detection, for different orientations of the source with respect to the sensing fibre. The results indicate that optimum ultrasonic detection may be achieved through careful positioning and orientation of the optical fibre. These results may be applied, for example in NDT, where scattered ultrasound from defects introduces new effective sources that may be characterized by arrays of these integrating sensors.