Q. Shan

Surface‐breaking fatigue crack detection using laser ultrasound

Surface‐breaking tight fatigue cracks in mild steel have been examined with laser‐generated ultrasonic pulses. Before the arrival of transmitted Rayleigh waves arriving at the detector, evidence is presented of a fast skimming longitudinal pulse which is also transmitted through the crack. Additionally, another ultrasonic feature is consistent with a longitudinal wave which is mode converted to a diffracted shear pulse by the tip of the fatigue crack. Such an interaction mechanism can form the basis of laser‐based fatigue crack depth measurements.

Modelling of confocal Fabry-Perot interferometers for the measurement of ultrasound

A mathematical and physical description of a confocal Fabry-Perot interferometer used for the measurement of ultrasound is presented. It analyses the interferometer performance in terms of ultrasonic transfer functions derived for rays of types 1-4. Instrument performance is modelled for two different schemes of operation, namely transmission and reflection. The frequency response is derived from a combination of ray types. In the case of the reflection scheme, a new beam separation method is suggested, which makes full use of the instrument etendue. Additionally, this latter scheme offers a significant increase in sensitivity over a wide frequency range extending towards the free spectral range of the interferometer, typically exceeding 100 MHz.

A conjugate optical confocal Fabry-Perot interferometer for enhanced ultrasound detection

To detect ultrasound remotely, an optical detection scheme is described which uses a confocal Fabry-Perot interferometer and conjugate detection. A conjugate signal is derived from the interferometer by making simultaneous use of signals derived from optical back reflection, and optical transmission, through the interferometer. Theoretical analysis and experimental results have demonstrated that the new scheme has a higher sensitivity than those described previously. Sensitivity enhancements between 2 and 17 times can be expected. Depending on mirror reflectivity, the frequency response is approximately flat in the range of 10 MHz and above, limited only by the free spectral range of the interferometer device.

New field formulas for the Fabry - Pérot interferometer and their application to ultrasound detection

Inaccurate field formulas for light transmitted through Fabry - P?rot interferometers have been identified in the open literature. A phase lag between the first emerging ray with respect to the incident ray is generally omitted. Such an error is not important when only one type of emerging ray is investigated. However, when more than one type of ray is monitored simultaneously, such as in the application of the confocal Fabry - P?rot interferometer to the detection of ultrasound, the error leads to an inconsistency between modelling and experimental measurement. This paper presents corrected field formulas for plane and confocal interferometers, and goes on to describe ultrasound transfer functions for a confocal interferometer. These functions are shown to give good agreement with independent experimental measurements. Finally, a variety of confocal inteferometer configurations for ultrasound detection are presented, together with an outline of their essential features including their detection limits.

Controlling the quality of thin films by surface acoustic waves

Abstract Techniques based on laser-induced surface acoustic waves have been developed with the potential for characterizing thin films. They enable variations of elastic parameters, density and film thickness to be detected. Previous work has demonstrated that from the measurement of the frequency dependence of the surface wave velocity, film parameters may be solved by using an inverse dispersion relation. In the present paper, a method is described that can be extended to the use of surface acoustic waves to the control of film quality in an on-line process. The method exploits the phenomenon that surface waves in coated materials show dispersion, giving rise to a characteristic deformation of wide-band surface wave impulses. Having traversed a fixed measuring distance on the test piece sample, an impulse signal is detected and compared with a reference signal from a standard sample. Signal processing yields two test functions: a cross-correlation of the test signal with a reference signal and the envelope of this cross-correlation function. If the film quality of the test sample agrees with that from the reference sample, the cross-correlation function is symmetric and its maximum coincides with the maximum of the envelope. Deviations from the required quality causes both test functions to shift against each other and gives rise to non-symmetric shape of the cross-correlation. The shifts and the deformation of the test functions are shown to contain information about the variations of film quality. As an example, results are presented for the case of TiN-coated steel samples having different film thickness. The sensitivity of the method is discussed and found to be dependent on signal-to-noise ratio (SNR) and bandwidth of the measurement. Tests were performed with two different types of laser-ultrasound instrumentation. They both used a pulse laser to generate the surface wave impulses. In the first type of instrumentation, ultrasonic impulses were detected with a piezoelectric transducer. It showed good signal-to-noise ratios (SNR) and high-frequency (60 MHz) bandwidth, but required mechanical coupling to the surface. The second type of instrumentation used a Fabry-Perot interferometer, which enabled a complete non-contact measurement to be performed up to a 15 MHz frequency bandwidth. The principle of the measurement method was again confirmed using the interferometer.

Characterisation of laser-ultrasound signals from an optical absorption layer within a transparent fluid

Characteristics of laser-ultrasound signals are presented from photoacoustic interaction with a layered, optically absorbing medium surrounded by a transparent fluid. A thermoelastic model is presented describing the interaction, with signal predictions in the fluid arising from polymer transducer detection. By taking the optical absorption coefficient and finite layer thickness into account, the amplitude and shape of transient elastic waves are calculated for both forward and backward travelling directions. Additionally, wave interaction with the PVDF transducer has been characterised using a discrete-time algorithm for the transducer response. With just three constants to define transducer response characteristics, the response function may be used to predict voltage signals. Good agreement with experimental waveforms is demonstrated, so that the response function may form the basis of system modelling when miniature laser-ultrasound probes are used in various applications.

POLYMER FILM THICKNESS MEASUREMENT USING LASER-ULTRASOUND TECHNIQUES

The thickness of thin polyester film has been measured using ultrasonic Lamb waves which were generated and detected using noncontacting laser techniques. The uncertainty analysis of such a measurement technique has been examined. Systematic errors in both time of flight and distance between laser generation and detection points were estimated and reduced with the assistance of computerized X‐scan translation techniques. Thickness measurements with uncertainties of less than 3% have been achieved.

A non-contact scanning system for laser ultrasonic defect imaging

A non-contact scanning system is described which is capable of producing two-dimensional ultrasonic images of defects in opaque materials. Lasers are used both for generation and detection of ultrasound. Short-duration high-power ( approximately 2 MW) laser pulses are used to generate ultrasound on the surface of a sample. Such ultrasound in the form of steps or pulses propagates through the sample and after interaction with defects are monitored by a 50 cm confocal Fabry-Perot interferometer. The interferometer uses back-scattered laser light from the samples surface, which can be diffuse in nature. System sensitivity is sufficient for applications on optically rough surfaces. Two-dimensional scans have been performed, leading to computer-generated images of artificial defects in aluminium.

An automatic stabilization system for a confocal Fabry-Perot interferometer used in the detection of laser-generated ultrasound

Abstract A laser confocal Fabry-Perot interferometer (CFPI) is described for the sensing of ultrasound in opaque solids having optically rough surfaces. Stabilization was required to eliminate the optical frequency drift arising from various sources in both the CFPI and the argon-ion laser. Measurements showed that conventional proportional control was inadequate when using feedback to the piezoelectrically-controlled interferometer mirror. Instead, a cascade control system was developed with a digital proportional-integral controller as the primary controller and an analogue proportional controller as the secondary controller. Two control loops were nested, with the secondary loop used to stabilize fast fluctuations and the primary loop used to eliminate the drift. When the interferometer mirror reaches the limit of its dynamic range, the intelligent digital controller was able to reset automatically the interferometer at a new operating point within its proper dynamic range. In this way, it is shown that the interferometer sensor can be used in a fully automatic way for on-line monitoring applications.

Through-transmission ultrasonic imaging of sub-surface defects using non-contact laser techniques

Abstract Through-transmission ultrasonic NDT measurements have been performed with a non-contact laser combination system. High power (~2 MW) laser pulses were used for the generation of longitudinal pulses in metallic samples. On reaching the far surface, ultrasound was detected with a 50 cm confocal Fabry-Perot interferometer. The interferometer analysed speckle from back-scattered laser light so that measurements could be made from samples with only a machine finish. Signal-to-noise ratios were sufficiently good for two-dimensional scans to be performed, producing optical images of artificial defects. For improved image quality, signals were normalised taking into account changes in sample surface reflectivity. Median filtering was also used. Results show that images of 4-mm diameter drill holes can be obtained with resolutions of ±0·5 mm.